small scale spatial occurrence trends of arsenic in...

SCERP W-03-4, Alberto Barud-Zubillaga - 1 -

SMALL-SCALE SPATIAL OCCURRENCE TRENDS OF ARSENIC IN THE GROUND-WATER RESOURCES OF THE PASO DEL NORTE REGION PROJECT NUMBER: W-03-4 ALBERTO BARUD-ZUBILLAGA, UNIVERSITY OF TEXAS AT EL PASO ALFREDO GRANADOS-OLIVAS, UNIVERSIDAD AUTÓNOMA DE CIUDAD JUÁREZ PATRICK GURIAN, DREXEL UNIVERSITY ELIA BENITEZ MARQUEZ, UNIVERSITY OF TEXAS AT EL PASO PHILLIP GOODELL, UNIVERSITY OF TEXAS AT EL PASO

INTRODUCTION High arsenic concentrations in drinking water have been linked to diverse types of cancer and to other serious diseases (Berg 2001, Siegel 2002). To reduce this potential health risk, in 2001 the US Environmental Protection Agency (USEPA) proposed lowering the maximum level of arsenic permitted in drinking water from 50 to 10 micrograms per liter (µg/l) to be in effect by February of 2006. However according to the Norma Oficial Mexicana (Spanish for Mexican official rule) the maximum contaminant level will be lowered from 50 to 25 µg/l in 5 µg/l increments from 2001 to 2005. In the Paso Del Norte Region, the United States and Mexico share the groundwater resources, where approximately 20% of water supply wells exceed the new US arsenic standard. This work focused in the analysis of the small-scale variability of arsenic to better understand the factors associated with it in groundwater. Archival data on arsenic and related parameters were used as well as additional sampling of groundwater and aquifer materials to identify the geochemical conditions associated with high arsenic in the groundwater. RESEARCH OBJECTIVES The main goal of this investigation was to better understand the source, behavior and distribution of Arsenic in ground-water in the Paso del Norte Region so that public water system suppliers can take proper actions for the new maximum contaminant level for the US and Mexico. In addition to the overall understanding of arsenic in the area, this research provided a geographic information system (GIS) containing the entire database acquired during the investigation. Some of the parameters contained in the database are arsenic (As) concentration levels, chemical and physical parameters, and exact global positioning system (GPS) location. Some other deliverables from this research are a series of digital and paper maps that will show the distribution of arsenic in the Paso del Norte Region, a digital tri-dimensional model of the distribution of arsenic in the study area, and at least one peer-reviewed publication in a national or international journal.


RESEARCH METHODOLOGY/APPROACHES Location of Study area The Paso Del Norte Region is an urbanized region of the Rio Grande Valley located at the boundary between the U.S. states of Texas and New Mexico and the Mexican state of Chihuahua (Figure 1). The region includes the cities of El Paso, Texas, and Ciudad Juárez, Mexico. It is located in the northern Chihuahuan Desert and has a subtropical arid climate (Fisher and Mullican1990). Rainfall averaged 7.8 inches and temperature 63.4 °F (from – 8 °F to 109 °F) during the period from 1960 to 1980 (Fisher and Mullican1990). The Rio Grande River, the Franklin Mountains and the Sierra Juárez geographically characterize the region. The Rio Grande River serves of natural border between the U.S. and Mexico. This study case will focus in the drinking water supply to these two cities whose total population exceeds two million people. Hydrogeology The Paso del Norte Region as refer in this report consists of El Paso and Ciudad Juárez which are located directly across from each other on opposite sides of the U.S.-Mexico border. The three main drinking water sources for El Paso are the Hueco Bolson aquifer (which provides 30% of El Paso’s annual supply), the Mesilla Bolson aquifer (20% of annual supply), and the Rio Grande river (50% of annual supply) (EPWU 2002). Ciudad Juárez relies entirely on the Hueco Bolson for its drinking water supply. There are two main aquifers: the Hueco Bolson, which is located east of the Franklin Mountains and the Sierra Juárez and the Mesilla Bolson, which is located west of the Franklin Mountains and west of Sierra Juárez but is also know as the Conejos Medanos aquifer (Figure 1). There is a third aquifer which serves as a buffer zone between the Rio Grande and the two aquifers mentioned above. This is the Rio Grande alluvium aquifer. It basically mimics the course of the river. The first two aquifers are filled with Tertiary and Quaternary alluvial unconsolidated sediments, are not hydraulically connected, and drain to the Rio Grande River. The Hueco Bolson aquifer underlies both cities and is crossed by the Rio Grande River from west to east. Therefore, the geochemical composition of the Hueco Bolson was expected to vary only modestly between the two cities. However, during the course of this work, a confining layer (that is partially characterized here) has been found to separate high arsenic water in the southeastern portion of the Paso del Norte region (known as the “Lower Valley” as it includes the downstream portion of the Rio Grande Valley) from lower-arsenic waters in the remainder, major part of the Hueco Bolson. The identification of this confining layer has changed the previous understanding of the Hueco as a thoroughly homogeneous, unconfined aquifer (WRRI 1997). Both aquifers, the Hueco and the Mesilla, have on average high Cl and K concentrations, compared to neighboring aquifers and also compared to aquifers from across the U.S in which the predominant rocks are carbonate, gypsum, limestone, etc, as shown in Table 1. This table has been modified from Eby 2004 and the arsenic values from the USGS website. Table 1 is expanded in Table 2 for the Hueco and the


Mesilla. Table 2 shows averages of historical data for the Mesilla and Hueco bolsons where the Mesilla has lower levels of most parameters than the Hueco Bolson except for sulfate and total dissolved solids (TDS). The Hueco basin fill belongs to the Fort Hancock and Camp Rise formations of Quaternary-Tertiary age. The Hueco is a basin that is considered a homogeneous basin (WRRI 1997). Some limited recharge in the western portion of the basin is due to runoff from the Franklin Mountains. Eight shallow wells (out of 266) drain water from the river (EPWU 2003). The main hydrologic characteristics for the two basins are listed in Table 3 (WRRI 1997 report). The Mesilla Bolson fills, from which the fresh water comes, are from the Quaternary-Tertiary ages (middle Pleistocene and Oligocene epochs), Santa Fe groups (Deep, Middle and Upper). These groups are unconsolidated alluvium from both nearby mountains and distant sources outside the basin (Bexfield 2001). The Mesilla basin is neither homogeneous in composition nor in hydrologic parameters. (WRRI 1997). The two principal mechanisms of recharge to the Mesilla are seepage from the Rio Grande and deep percolation of irrigation water (WRRI 1997) at the rural areas. Twenty-five out of 58 wells (43%) in the Mesilla Bolson are directly hydraulically connected with the Rio Grande River. Most probably with the shallower wells which are in the 100-200 ft range.

Arsenic Geochemistry and related processes Competitive Desorption On the solid phase of ferric hydroxides (≡FeH2AsO4) some anions may compete with AsO4

-3 for sorption sites. If an anion displaces an arsenic molecule, the arsenic will desorbs from the solid phase and pass into the aqueous phase, increasing the dissolved arsenic concentration. The molecules that usually occur in groundwater and compete with arsenic oxide are: CO3

2-, SiO2 -2, PO4 3-, OH- which may be strongly or

weakly adsorbed to iron hydroxides but because they occur in orders of magnitude higher concentrations than arsenic concentrations, produce a considerable competitive effect (Holm 2002). Reductive Dissolution When either iron or manganese is reduced they become more soluble and may release arsenic ions adsorbed to them as shown by the following reactions: Fe(III) less soluble Fe(II) more soluble As(V) more adsorbed As(III) less strongly sorbed Table 2 shows the Mesilla and Hueco bolsons descriptive statistics for some trace, most major ions, well depth and pH. Units are ppm unless otherwise indicated. On Table 3 you can see the hydraulic parameters for the Hueco and Mesilla Basins taken from WRRI 1997 report.


The final concentrations of both metals (Fe and As) have increased in solution. Thus a positive correlation between arsenic and iron or manganese would support this mechanism. Reduced environments generally occur deep in the aquifer or when microorganisms promote the reduction. Dissolved oxygen (DO) is expected to be very low or absent and oxidized species of almost all molecules are expected to be scarce (more or less depending of the reduced degree of the environment). At Bangladesh and West Bengal, reductive dissolution has been found to be the principal process mobilizing arsenic from the river delta material into the water after organic decomposition (Mc Arthur 2002). Organic material buried for thousands of years may give rise to this process and such organic material may be present in sediments deposited by the Rio Grande. Evaporative Concentration The evaporation of water from the recharge or from the unconfined aquifers will concentrate the ions including arsenic and dissolved solids in the water. Evaporation also raises the pH as it increases alkalinity by concentrating CaCO3. Higher pH promotes the competitive desorption of arsenic from iron hydroxide (Montoya and Gurian 2003, 2004). Upwelling Excessive pumping causes water to flow toward the well, and if the unit is confined (or partially confined) the flow upward may occur in important amounts. If some fault is close to the well, the suction will bring the deeper, more mineralized waters to the well also. The minerals and dissolved ions will be different from those usually found in that region of the aquifer (Baxfield 2001). As with the evaporative concentration mechanism, such mineralized waters will have high alkalinity and high pH that promotes the arsenic desorption. Sources of Information About 97% of the information used in this work was from archival databases. All the information gathered was from four different sources: Archives from the EPWU, archives from the JMAS and new data acquired from samples collected during the project. The historical data provided by the El Paso Water Utilities (EPWU) included information on 14 different major ions and 17 trace elements including arsenic at 266 drinking and non-drinking water supply wells. The Junta Municipal de Agua y Saneamiento (JMAS) data included information for Ciudad Juárez on concentrations of major ions but no arsenic data. Another source was acquired from a dataset from the Texas Water Development Board of about 250 drinking and non-drinking water wells for the El Paso area. Because of the different nature of these databases and some degree of overlap between the EPWU and the TWDB databases, the databases were analyzed separately rather than being pooled into a single dataset.


Approximately 3% of the data used in this study was from sample collection and chemical analyses designed to fill gaps in the archival databases. Additional groundwater samples were taken in Ciudad Juárez and analyzed for arsenic because the archival databases had no information on arsenic concentration on the Mexican side of the border. Some of these groundwater samples were analyzed for dissolved oxygen and other field parameters such as temperature, pH, and electrical conductivity to fill in for some of the gaps on the original data. Only 21 additional samples were collected from the Hueco Bolson (13 samples) and the Mesilla Bolson (8 samples). They were collected as a quality control measure mainly to validate the archival information. Arsenic in groundwater is believed to be controlled primarily by interactions between ground water and solid phase aquifer materials. As the archival data did not contain information on the solid phase aquifer materials, a set of well cuttings was analyzed for arsenic, iron and total organic carbon (TOC). Correlations between solution and solid phase arsenic (as found by Robertson 1989) would suggest a local source (or sink) for arsenic mobilized into the water, while the lack of such a correlations would indicate that mobilization mechanisms operate over a larger spatial scale. Methodology Groundwater samples were collected between November 2003 and December 2004. A total of 105 water samples were collected. Eighty-four of these samples were collected from drinking water wells property of the JMAS in Juárez located within the boundaries of the Hueco Bolson, 13 from the Hueco Bolson in El Paso and the 8 left from the Mesilla Bolson in El Paso. Water samples were sent either to the New Mexico State University (NMSU) Soil, Water and Air Testing (SWAT) Laboratory located in Las Cruces, New Mexico or Assaigai Laboratories in El Paso. Both laboratories utilized the ICP method EPA 200.8. All water samples were collected during normal operation of the wells and taken before chlorination unless otherwise indicted. The water was allowed to flow for about two to five minutes before sampling. Samples were collected in 200 ml plastic containers, which were thoroughly rinsed and filled up without headspace. For some of the samples collected in Ciudad Juárez, field parameters such as pH, temperature, electrical conductivity, and salinity were measured. Fifteen soil samples of cuttings from wells drilled by the El Paso Water Utilities (EPWU) years back are archived at the Geology Department of the University of Texas at El Paso. 1.5g of material from each of the 15 well cores samples were taken and analyzed for arsenic and iron. Eleven of these samples were also analyzed for Total organic carbon (TOC). Each 1.5 grams sample was thoroughly sieved to 75 microns size (standard sieve #200) before analysis.


PROBLEMS/ISSUES ENCOUNTERED Problems/Issues: According to the UACJ, the historical data available for analysis from the Mexican side consist of chemical data from 1965 through 1999 that was collected and released by the JMAS. This dataset lacks of arsenic concentrations, therefore we decided to sample a lot more samples than originally planned. We also proposed to do a speciation analysis for specific wells whenever we suspected that anthropogenic contribution was an issue. Because the EPWU had already a speciation analysis we decided to invest our resources in getting arsenic data for the wells in Juárez instead of the speciation analysis. Moreover, sampling and analyzing these results took us more time than previously thought due to the time-consuming effort of going out to several wells and sample them, not to mention the trouble of acquiring the proper permits to access the facilities. RESEARCH FINDINGS T-tests were performed to compare the mean arsenic concentrations of the archival data with the corresponding values obtained for samples collected as part of this project. Separate tests were conducted for the Hueco and the Mesilla Bolsons. For the El Paso samples, values from both the archives and the sampling were available at the same wells, which allowed a paired samples t-test to be used. For the Juárez samples archival information was not available so independent sample t-tests were used to compare the means values with the El Paso section of the Hueco Bolson. The t-test for difference of means did not show a significant difference between samples from this study and archival values in the Hueco Bolson. For the Mesilla Bolson, the sample set showed slightly higher arsenic concentrations than the EPWU archives. A t-test for paired samples shows that the maximum credible percentage of difference between the two data sets is about 2%. This confirms the accuracy of the archival data used in this study. The mean solid-phase concentrations obtained from the analysis of the well cuttings are shown in Table 4. A significant correlation was found between solid-phase Fe and As (R2= 0.557, P-value .025) (Table 5). This correlation suggests that arsenic is often associated with iron in the solid phase and supports previous researcher that aqueous arsenic concentrations are controlled largely by sorption-desorption reactions with iron hydroxide (Robertson 1989). The water sampling in the Juárez part of the Hueco, specifically in the Lower Valley, showed noticeable differences from arsenic concentrations in the archival data for the Hueco. This appears to indicate that the confined portion of the Hueco Bolson found in the Lower Valley is much higher in arsenic than the major unconfined aquifer. Not only were the arsenic concentrations (Table 6) out of the confidence interval for the Hueco Bolson archival observations but these samples from the Juárez Lower Valley wells located in the confined aquifer also showed intriguing field observations such as 1) a difference up to ten degrees Celsius (18o F) higher temperature than any other


observation (Table 6) and 2) a strong negative correlation between dissolved oxygen and arsenic (Figure 5). The largest observations for arsenic in the Hueco Bolson were 70 ppb and 86.6 ppb from two Juárez artesian wells. However these two wells are out of the arsenic concentration map shown in Figure 2 as there is no continuous data between the urbanized area and these two wells. These wells are located at the most southeastern end of the Juárez valley close to the small town of El Millón across from Tornillo Texas about 40 miles from downtown El Paso. Data from these two wells however were considered for the statistical analysis. The pH and temperature were strongly positively correlated between themselves and with arsenic in these Lower Valley wells (Figures 3 and 4). The two highest arsenic observations were measured in the lowest DO wells, and the odor of sulfide in one of those is an additional indication of the role of reducing conditions as a mobilization mechanism in the Lower Valley semi-confined wells. Figures 3, 4 and 5 and Table 6 partially characterize this confined unit of the Hueco Bolson in the Lower Valley. EPWU archives show for the Lower Valley confined wells on the U.S. side of the border, the average arsenic concentration was about 12 ppb compared to 3 ppb in the unconfined neighboring wells. In the whole of the Hueco, the average arsenic concentration in the archival observations is less than 8 ppb. Eastoe’s (2004) study of the Juárez Lower Valley wells found a Global Meteoric Water (GMW) isotopic signature in these Lower Valley wells that distinguishes them from their neighbors as well as from the major part of the Hueco with clearly evaporated waters. Finally in the WRRI report (1997), conductivity logs show the thick clay layers characteristic of aquitards (semi-impervious layers) at El Millón location inside the Juárez Lower Valley region. In the Mesilla Bolson, the most significant positive correlations with arsenic were found for pH and depth; the most significant negative for Cl, TDS, hardness, Mn, Ca, Mg, Na, SO4 and electrical conductivity (EC) (Table 7). These trends reflect the influence of the Rio Grande River on shallow wells in the Mesilla Bolson. The river water is generally low in arsenic concentration but high in dissolved solids. The low arsenic concentrations are common for aerobic surface waters where arsenic is present in the oxidized form arsenic(V), which can be readily removed from the aqueous phase by sorption to a variety of minerals, such as iron and aluminum oxides. The high salinity of the Rio Grande in this region can be explained by evaporative concentration of the river as it flows through an arid region and by the high salinity of return flows from agricultural irrigation. Table 1 shows higher average sulfate concentrations in the Mesilla than in most of the other aquifers, Stiff diagrams from the Water Resources Research Institute (WRRI) 1997 report support this result. The positive correlation between arsenic and pH may also be due to the influence of river water on the aquifer as the ground water that is not influenced by the river would be expected to have higher alkalinity and higher pH. This correlation may also be due to higher pH causing desorption of arsenic from hydroxide solids (Montoya and Gurian 2004). Thus this correlation supports the competitive desorption mechanism of


mobilization. Evaporative concentration may also play a role by concentrating both the arsenic and the alkalinity in the groundwater. A speciation analysis (EPWU 2003) found that arsenic is present as both As(III) and As(V) but with at least half and usually closer to two-thirds of the arsenic present as As(V). This indicates that conditions are at least moderately reducing in the high arsenic wells. Thus reductive dissolution may play a role in mobilizing arsenic. However, it does not result in the dissolution of enough iron to produce detectable concentrations in the aqueous phase. It may be that the dissolution of relatively small amounts of iron on the outer surface of the aquifer materials is sufficient to mobilize ppb levels of arsenic into the aqueous phase. Even if iron is not dissolved, the reduction of arsenic(V) to arsenic(III) reduces the affinity of the arsenic for the hydroxide solids. Nitrate levels in some wells in the southwestern parts of El Paso County exceed the 10 mg/L drinking water standard (WRRI 1997) with concentrations reaching 126 mg/L (EPWU 2003) (Table 2). The average in the last 75 years is 7.4 mg/L for the Hueco basin (EPWU 2003). The threat that high nitrates imply is an important health issue beyond the scope of this work. In the Hueco Bolson, the most significant positive correlations with arsenic were with: pH, Na, CO3, SO4, Cl, PO4, TDS and electrical conductivity (EC) (Table 7); the most significant negative correlations with depth, Mg, HCO3 and NO3. The positive correlations with pH, carbonate, sulfate, and phosphate support competitive desorption as a mechanism of arsenic mobilization. The negative correlation with bicarbonate may be due to confounding with pH (low pH would favor bicarbonate over carbonate but would also lead to low arsenic by favoring sorption of arsenic to iron hydroxides). Other mechanisms may also play a role in arsenic mobilization. Eastoe’s (2004) found strong evidence of evaporated waters in the Hueco, indicating that evaporation may contribute to arsenic mobilization. The positive correlation with sodium and negative correlation with magnesium suggest that more cation-exchanged waters have higher arsenic concentrations. More cation-exchanged waters would be expected have been exposed to more minerals and have had more opportunities to encounter arsenic-bearing minerals. In the set of nine observations from the confined aquifer in the Lower Valley suburbs of Juárez, the high and significant correlation of As with pH (R_Square = 0.98, p-value < 0.001, Figure 4) support competitive desorption as a mobilization mechanisms, while the negative correlation between DO indicates a possible role for reductive processes in mobilizing arsenic (Figure 5). The correlation with temperature may indicate an influence from geothermal waters. CONCLUSIONS In the US side of the Mesilla Bolson, the distribution of arsenic is largely controlled by depth, with low arsenic water from the Rio Grande River overlying semi-confined, deeper and older water with higher arsenic concentrations. Evaporative concentrations,


competitive desorption, and reductive dissolution may also influence the arsenic concentrations but less dramatically than the distinction between the young river water and the older, deeper water. The likely source of arsenic in the deeper aquifer is the Santa Fe group of rock, which is of volcanic origin. Volcanic rocks are often enriched in arsenic (Siegel 2002) and may release arsenic over time, particularly under high pH conditions. In the most urbanized area of the Hueco Bolson (where the Rio Grande river is canalized), arsenic concentration seems to progressively increase following the river course possibly due to the lack of recharge from the river water causing a deterioration of the quality of water. Lack of recharge, high pumping levels, evaporative concentration, reductive dissolution, and competitive desorption are all compatible with the data observed. These mechanisms may operate together or separately, and their relative importance may vary throughout the aquifer. Further downriver, a confining unit appears to exist in the Lower Valley occupying both sides of the border. A comprehensive study of this aquifer is warranted since the presence of a confining layer may significantly impact overall evaluations of the ground water resources of the region. The average arsenic in these Lower Valley, confined wells is 12 ppb (ranging from 1.5 to 87 ppb). A significant negative correlation with dissolved oxygen indicates that reductive dissolution may contribute to high arsenic concentrations in this area. The noticeable difference between wells in the confined Lower Valley and their neighbors (where the average arsenic concentrations are 12 and 3 ppb respectively), as well as the leaky confinement condition in the Mesilla basin where the average arsenic is 13 ppb, suggest that in confined units (or partially confined), the arsenic is more likely to dissolve or desorbs from the solid phase than in unconfined hydrologic units, possibly because the confining layer contributes to reducing conditions (clay and silt confining layers often have high organic content). RECOMMENDATIONS FOR FURTHER RESEARCH Recommendations can be divided in two geographic areas: the Mesilla Bolson or Conejos-Medanos aquifer and the Hueco Bolson. Each aquifer has very different hydrogeologic characteristics that will be better addressed separately. For the Mesilla it is recommended to expand the research for areas closer to downtown El Paso as most of this study was focused on drinking water wells located at the far northern tip of El Paso County. It will be useful to include any type of wells such as monitoring wells, private wells and other use other than drinking wells. As noticed on Figure 2, no reliable concentration map was generated as no data was obtained from this area. Moreover the geospatial analysis of the Canutillo well field will be better presented if it is segmented by depth. This will certainly happen in the near future from our team. For the Hueco Bolson it is suggested to go further away from the urban area. This will help filling in the gaps between fairly low arsenic values at recharging areas to very hi values close to the southeastern edge of the aquifer, where most probably there is recharge but also an interaction with geothermal sources. Eastoe and Hibbs are


working on dating these waters. By analyzing these two datasets conclusions surely will be more robust. RESEARCH BENEFITS This study was designed to assist the EPWU and the JMAS in understanding the source mobility and distribution of arsenic in groundwater. Said this, the utilities will be in better shape to make the appropriate decisions and provide their users with water with quality in compliance with their laws. For the scientific community, this research provided and extensive knowledge on the behavior of arsenic in the region. In addition several students at undergraduate, masters and doctoral level were involved, letting them have experience with a real problem example in the geohydrology area. ACKNOWLEDGMENTS This work was mainly sponsored by the Southwest Center for Research and Policy through a cooperative agreement with the U.S. Environmental Protection Agency. SCERP can be contacted for further information through www.scerp.org and [email protected]. Special thanks to Model Institutions for Excellence (MIE) Program at UTEP and Universidad Metropolitana in Puerto Rico (UMET) for partial funding to allowing Ms. Milagros Cruz Cruz participating at her internship experience. To Ms. Elia Benitez-Marquez for partially contributing with grant money to expand the research. To the EPWU personal, in particular to Scott Rinhert and Eric Bangs for their active participation during sampling and proving the team with the data needed. To the Junta Municipal de Agua y Saneamiento of Ciudad Juárez for their support during collection of water samples mainly Antonio, Ing. Rascón, and Ing. Manuel Herrera under the Administration of Ing. Acosta del Val. And to Dr. Jorge Gardea-Torresdey for allowing the team developing partial laboratory analyses at the Environmental Chemistry Laboratory at UTEP. REFERENCES Baxfield, L M, 2001 Occurrence and sources of arsenic in ground water of the middle Rio Grande basin, central New Mexico, M.S. Thesis, Hydrology, New Mexico Institute of Mining and Technology, Socorro, New Mexico. Berg, M, Tran HC, Nguyen TC, Pham HV, Schertenleib R and Giger W, 2001 Arsenic contamination of groundwater and drinking water in Vietnam: a Human health threat, Environment Sc. and Tech, Vol 35, No 13. Eastoe, Ch, 2004, Isotopes in the Hueco presentation EPWU Hueco Bolson/Rio Grande Aquifer Meeting, El Paso, TX. EPWU 2003, El Paso Water Utilities report on line at http://www.epwu.org


Eby G N, 2004 Principles of Environmental Geochemistry, Brooks Cole Fisher R S. and Mullican W. F. 1990, Integration of ground-water and vadose-zone geochemistry to investigate hydrochemical evolution: a case study in arid lands of the northern Chihuahuan Desert, Trans-Pecos Texas, Bureau of Economic Geology, University of Texas at Austin. Holm, T R, 2002 Effects of CO3 ,HCO3 , Si, and PO4 on Arsenic sorption to HFO Journal AWWA 94:4, April. Mc Arthur, JM, 2002, A Layman’ guide to arsenic pollution of groundwater in Bangladesh and west Bengal, London Arsenic Group http://www.es.ucl.ac.uk/research/lag/as Montoya, T. and P.L. Gurian, 2003, Numerical Solution of a Chemical Equilibrium Model of Arsenic Sorption to Ferric Hydroxide. Proceedings of the Modeling and Simulation Workshop of the International Test and Evaluation Association. Montoya, T. and P.L. Gurian, 2004, Modeling Arsenic Removal by Coagulation with Ferric Salts: Effects of pH and Dosage. Proceedings of the Texas Water 2004 Conference, Arlington, TX. Robertson F. N, 1989, Arsenic in ground water under oxidizing conditions, south west United States. Environmental Geochemistry and Health, v.11, p171-186 Siegel, F R, 2002, Environmental Geochemistry of Potentially toxic Metals, Springer. Smedley P.L. and Kinniburgh D.G, 2002, A review of the source, behavior and distribution of arsenic in natural waters, Applied Geochemistry 17, p.517 WRRI, 1997, Trans-International Boundary Binational Report, New Mexico State University, Las Cruces, New Mexico

APPENDIX A. Official Financial Expenditures Report


APPENDIX B. Proposed paper


GEOHYDROLOGIC INFLUENCES ON ARSENIC OCCURRENCE IN THE GROUNDWATER IN EL PASO, TEXAS Milagros Cruz Cruz1 and Alberto Barud-Zubillaga2 1 Project Model Institution for Excellence, Universidad Metropolitana, San Juan, Puerto Rico 2Center for Environmental Resource Management, The University of Texas at El Paso, El Paso Texas Abstract - Occurrence of arsenic in natural water sources has received significant attention during recent years. Arsenic in drinking water can cause a variety of human health problems, including skin and circulatory diseases, and even risks of skin, bladder, lung, liver and kidney cancer. The new Maximum Contaminant Level (MCL) for arsenic of 10 ppb goes into effect on January 2006. This MCL affects the El Paso Water Utilities (EPWU), the municipal drinking water supplier for most of the El Paso County, Texas. Although arsenic in the area might be attributed to natural and anthropogenic activity, this research focuses on naturally occurring factors. The most common geochemical environments in the El Paso area that might contribute to elevated arsenic concentrations are: basin-fill deposits of alluvial-lacustrine origin, volcanic deposits, and geothermal systems. Data from the Texas Water Development Board were compiled and analyzed to understand the occurrence and distribution of arsenic as well as other chemical constituents related to arsenic. It is well known that arsenic is pH-dependent and is influenced by the occurrence of some constituents like sulfide, iron oxide, chloride and others. According to this data analysis, no relation was found with these constituents, but some relation was found with the stratigraphy of the Rio Grande River Basin and the groundwater flow pattern. Keywords: groundwater, arsenic, occurrence, constituents Introduction The occurrence and mobility of arsenic in groundwater and its associated health risks to humans are some of the greatest challenges of supplying safe drinking water to El Paso residents. Arsenic content in drinking water have been reported in some wells of the Hueco and Mesilla Bolsons, the two of the three current sources of drinking water for El Paso County (El Paso Water Utilities 2002). These readings exceed the new MCL of 10 ppb by considerable amounts as in most of the Western US (Welch et al., 2000). The results of clinical findings for arsenic poisoning from drinking contaminated water show that long-term exposure may lead to various diseases such as conjunctivitis, hyperkeratosis, hyperpigmentation, cardiovascular diseases, disturbances in the peripheral vascular and nervous systems, skin cancer, gangrene, leucomelonisis, non pitting, swelling, hepatomegaly and splenomegaly (Kiping, 1977; WHO (World Health Organization), 1981; Pershagen, 1983). High concentrations of arsenic in drinking water can also result in an increase in stillbirths and spontaneous abortions (Csanady and Straub, 1995).



Arsenic can be a naturally occurring trace element in soil, rocks, sediment, natural water and organisms, but rarely occurs in a free state. It is largely found in combination with sulfur, oxygen and iron (Brewstar, 1994; Chatterjee, 1994). Occurrence of arsenic in groundwater depends on the local geology, hydrology and geochemical characteristics of the aquifer materials (Bhattarchrya et al., 1997). Additionally, the geochemical characteristics of the aquifer material and their interactions with water also play an important role in controlling the retention and mobility of arsenic within the subsurface (Jain and Ali, 2000), and this can be enhanced by anthropogenic activities, such as mining, smelting operations (García-Sánchez and Alvarez-Ayuso., 2003) and landfill run-off (Welch et al., 1988; Korte and Fernando, 1991). In the upper Midwest of the United States, quaternary glacial sediments appear to be associated with high arsenic concentrations in groundwater (Welch, et al., 2000). Arsenic release from iron oxide appears to be the most common cause of widespread arsenic concentrations exceeding 10 ppb in groundwater (Welch, et al., 2000). This can occur as a consequence of different geochemical conditions, including release of arsenic to groundwater through reaction of iron oxide with either natural or anthropogenic organic carbon. Iron oxide can release arsenic to alkaline water, such as in some alkaline aquifers of the Western United States (Welch, et al., 2000). Redox conditions in an aquifer also play an important role in determining the mobility of arsenic (Jain and Ali, 2000). The oxidation of some mineral species, especially sulfide minerals (García-Sánchez and Alvarez-Ayuso., 2003), causes arsenic to become soluble and might enter the aquifer environment through drainage water. It is necessary to consider if any anthropogenic activity, such as pumping rate or the land use pattern affect the redox conditions of the aquifer, because it is of interest in ascertaining the primary sources of arsenic content in groundwater. Nevertheless, some studies have reported that the release of arsenic was pH-dependent and was related to the total iron and free iron oxides in the sediments (Jain and Ali, 2000). Consequently, the objective is to understand the geographic distribution of arsenic in El Paso, Texas and its geological influences. This research presents a compilation and analysis of data for arsenic occurrence in the Hueco and Mesilla Bolsons and the Rio Grande aquifer. It studies the data in relation to the geologic evolution of the aquifer, its sediment type and groundwater flow patterns. Results should improve the identification of mechanisms of arsenic release to groundwater to provide a framework that guides the placement of new water wells if necessary to comply with the new rule. Study Area Location - El Paso, Texas lies in the Southwestern United States and covers an area of more than 5,589 square kilometers. The Rio Grande aquifer system in westernmost Texas corresponds to the eastern part of the Southwest alluvial basins aquifer system (Ryder, 1996), which is a large system of aquifers in alluvial basins in the Southwestern United States and Mexico (Fig. 1). A small portion of the Mesilla Bolson is in Western El Paso County at the west of the Franklin Mountains and underlaying the Rio Grande Aquifer on the west. The Hueco Bolson is situated in parts of New Mexico, Texas and Mexico. In Texas, the Northern part of the bolson



lies between the Franklin Mountains on the west and the Hueco Mountains on the east (Ryder, 1996). The Hueco and Mesilla Bolsons are the primary source of water for El Paso County and neighboring areas, where precipitation is sparse 20.32 – 30.48 centimeters annually (Ryder, 1996). Although a large volume of water is stored in the basin deposits, pumping easily exceeds natural recharge and leads to long-term depletion of stored water (Ryder, 1996). Currently, El Paso Water Utilities has 120 wells located in the Hueco Bolson. By 2002, 84 of the wells were active and pumped 48,292,047.42 cubic meters (El Paso Water Utilities, 2002). Most of the pumping is from the Mesa-Nevins and Airport well fields, which are located in the middle section of the Hueco Bolson aquifer. EPWU wells in the Mesilla Bolson are located in the Canutillo area and supply the west side of El Paso. For 2002, 18 wells were operating and pumped 27865588.19 cubic meters. In 2003, it is expected that these wells will operate to supply west-side demands. With the new United States Environmental Protection Agency Maximum Contaminant Level (MCL) for arsenic of 10 ppb, some new sample analysis has been conducted and readings up to 10 ppb have been reported in 64 wells located in the Hueco and Mesilla Bolsons.

Figure 1. Study area showing aquifers in the Paso del Norte Geohydrological settings - The Cenozoic Era is the period that runs from 65 million years ago to present, and it represents a time of important changes in the El Paso, Texas area in terms of the current hydrogeology. The Rio Grande aquifer system is in the Basin and Range Physiographic Province. Vertical movements along block faults have resulted in structurally high mountain ranges that trend south and southeast and are separated by structurally low parallel basins (Ryder, 1996). The basin areas are filled with thick sequences of clastic sediments that have eroded from the adjacent highlands. The basin deposits are of the late Tertiary and



Quaternary ages and consist mostly of clay, silt, sand, and gravel (Fig. 2) (Collins and Raney, 1991). The alluvial deposits of the Mesilla Bolson are predominantly coarse grained around the margins of the basin and fine grained near the basin center, which are not studied in this investigation. The Rio Grande alluvium is part of the Mesilla Bolson alluvial aquifer; it overlies the older basin fill, from which it cannot be easily distinguished. The total thickness of the unconsolidated deposits in this aquifer is estimated to be at least 609.6 meters, and the thickness of the Rio Grande alluvium is 45.72 meters or less (Ryder, 1996). The water chemistry in the shallower part of the Mesilla Bolson aquifer is influenced by the quality of the water in the Rio Grande. The water in the shallowest part of the aquifer is generally more mineralized than that in the deepest part (Ryder, 1996). Concentrations of dissolved solids in the shallower groundwater are as much as several thousand milligrams per liter, whereas water from the deeper part of the aquifer commonly has dissolved solids concentrations that are less than 300 mg/l. The depth of freshwater extends to as much as 426.72 meters below land surface. Water in the southern half of the basin deposits is more mineralized than elsewhere (Ryder, 1996). This could be due, in part, to the narrow valley outlet at downtown El Paso between Sierra Juárez to the south and the southern tip of the Franklin Mountains that restricts groundwater outflow and prevents the flushing of water with greater dissolved solids concentrations. The aquifer receives recharge by infiltration of runoff around the basin margins, and from seepage from the Rio Grande, ephemeral streams, canals, and excess irrigation water (Ryder, 1996).



Era System Stratigraphic Unit Lithology Hydrogeologic

unit Quaternary Rio Grande alluvium

and alluvium of tributary streams

Gravel, sand, silt and clay deposited by the Rio Grande and its tributaries; up to 60.96 meters thick at some locations. Locally contains caliche.

C

enoz

oic

Quaternary and Tertiary

Basin-fill deposits Gravel, sand, silt and clay deposited by the ancestral Rio Grande or streams local to individual basins; commonly 304.80 meters thick, up to 2,743.20 meters thick in Hueco Bolson.

Rio Grande Aquifer System

Quaternary Volcanic/clastic and volcanic deposits

Reworked tots and alluvial deposits that consist almost exclusive of volcanic/clastic deposits interbedded with ash-flow tots; up to 1828.80 meters thick in the southern Salt Bolson.

Figure 2. Cenozoic Stratigraphy The deposits that compose the Hueco Bolson alluvial aquifer include the Rio Grande alluvium along the Rio Grande River, which is probably no more than 60.96 meters thick. The unconsolidated alluvial deposits in the Hueco Basin consist of gravel, sand, silt, and clay (Ryder, 1996). Mostly basin-fill clastic deposits characterize the Cenozoic stratigraphy of the aquifer, although local intrusive and extrusive igneous rocks also exist (Collins and Raney, 1991). Little is known about most of the Tertiary basin-fill sequence in the Hueco Bolson because outcrops are mostly limited to the upper ten meters and because few stratigraphic data have been reported from drill holes (Collins and Raney, 1991). The aquifer contains about 12,334,818,375 cubic meters of freshwater in an approximately 18.13 square kilometers (Ryder, 1996). An additional large amount of slightly saline water is available in deposits that underlie and adjoin the freshwater-bearing deposits to the east. The rapid and limited



recharge to the aquifer by runoff from the Franklin Mountains into alluvial-fan deposits makes this a favorable area for groundwater development (Ryder, 1996). Under natural conditions, groundwater flow is toward the Rio Grande and in a down-valley direction. In developed areas, groundwater moves toward pumping centers. Natural hydraulic gradients have been reversed in intensively pumped areas and water in the shallow alluvium moves downward across local confining units to replenish water that is pumped from deeper zones (Fig. 3) (Ryder, 1996). This new flow pattern concentrates the groundwater to the middle section of the Hueco Bolson. The basin fill in the southeastern part of the Hueco Bolson is mostly fine grained and probably consists largely of playa deposits. Field data suggests that the thickness of the deposits in this area ranges from 304.8 to 914.4 meters. Sand and gravel are substantial only in the upper 60.96 to 121.92 meters, which includes the Rio Grande alluvium (Ryder, 1996). Groundwater generally becomes more mineralized with depth from the northern part of the basin toward the southeast. Water with less than 1,000 mg/l of dissolved solids is contained in deposits that are more than 121.92 meters thick in the vicinity of El Paso, but freshwater diminishes rapidly toward the southeast (Ryder, 1996). Although water with dissolved solids concentrations of less than 1,000 mg/l is desired for most public and industrial uses, water with greater concentrations is acceptable for such uses as livestock watering and irrigation. The southeastern part of the Hueco Bolson alluvial aquifer is a valuable source of water for these purposes. METHODOLOGY Arsenic occurrence - Occurrence of arsenic in natural water depends on the local geology, hydrology and geochemical characteristics of the aquifer materials (Jain and Ali, 2000). Consequently, a geological profile was developed for the aquifers located in the limits of the EL Paso County using data and references from the United States Geological Survey (USGS). The constituents related to the occurrence of arsenic where identified, among them the following where recognized in the study area: iron, manganese, sulfate, copper, phosphate, chloride, silica, carbonate, calcium, sodium, magnesium, bicarbonate and total dissolved solids. Databases from the Texas Water Development Board Publications Catalog was obtained to determine the presence of arsenic in the study area. Available information contains groundwater data in character delimited text format and PDF reports for El Paso County. Using Microsoft Excel the several databases were imported and reformatted to process them properly. These databases contained Water Quality, Infrequent Constituent Water Quality data and general well information. Each database was edited according to its data dictionary, which are also available at the TWDB website. The reformatted databases were imported into ESRI ArcView 3.3 software. Tables were merged generating an ArcView shapefile containing 347 data points (samples) with arsenic content for the El Paso area. Mentioned data points (samples) where distributed among 216 wells as follows: 16 wells in the Rio Grande Aquifer, 25 wells in the Mesilla Bolson and 175 wells in the Hueco Bolson. The new database was plotted in an ArcView GIS view, overlaying an existing GIS laying with the geologic map of El Paso generated by the University of Texas at Austin (UT Austin) (Collins and Raney, 2000). Other layers where added as well for references among them an El Paso County street map and a geological faults map, to find the spatial relationship between faults, geology of the area and the occurrence of arsenic. Databases were



exported back to Excel to identify linear relationships between occurrence of arsenic and each constituent. Lastly an existing EPWU model of the groundwater flow patterns was used to establish a relationship between the occurrence of arsenic and the groundwater flow patterns in the Hueco Bolson. Results Data analysis - The average concentration of arsenic according to the data was 8.83 ppb. A maximum concentration of 39 ppb was found in the state well number 4915607 located in the Hueco Bolson and sampled on October 4, 1978. Some constituents related to the occurrence of arsenic according to the reviewed literature were also analyzed, such as: iron, total dissolved solids, manganese, copper, sulfate, chloride, magnesium, silica, bicarbonate, carbonate and sodium. As shown in Table 1 average concentrations exceeding the MCL were found on most of the USEPA regulated constituents related to arsenic occurrence. However, as opposed to what was expected, no significant correlation was found between arsenic content and the constituents. The average pH of groundwater in the study area was 7.62, which is slightly alkaline, suggesting desorption from iron hydroxides may contribute to arsenic mobilization. Geohydrological analysis - The natural occurrence of arsenic is closely related to the Cenozoic stratigraphy of the Rio Grande Aquifer System. During the Tertiary period mountain-building forces were in action and bodies of molten magma moved onto the Earth’s crust. These magma bodies cooled into the crust forming andesite plutons. Some have been exposed by later erosion, like the Campus andesite and Cristo Rey Plutons, to name a few. Andesite is a common extrusive igneous rock composed predominantly of plagioclase feldspar, with the typical ferromagnesian component being amphibole or biotite. These two are silicate minerals, and silica is a constituent that is closely linked to the occurrence of arsenic. The accumulation of the basin-fill deposits starts around the last part of the Tertiary and the beginning of Quaternary period, when a new system of stresses began. These were extensional, or pull-apart stresses and they generated many of the local features. One product of the tensional forces and rifting was the formation of various elongate basins, separated one from the other by mountains. Examples include the Mesilla Bolson, the Franklin Mountains, the Hueco Bolson and the Hueco Mountains. Gravel, sand, silt and clay were deposited by the ancestral Rio Grande or local streams to each of the individual basins. Desert sand, blown into the area from the southwest, is another constituent of the loose unconsolidated sediment of the basin fill. Basaltic volcanic activity also took place. The basaltic magma arose deep in the crust, rose to the surface and erupted to form cinder cone volcanoes and lava flows that dot the surface of the Mesilla Bolson. These fine-grained rocks are mostly of calcium-rich plagioclase and pyroxene, with smaller amounts of olivine and amphibole. These last stages of geologic activity added to the sediment constituents that are closely related to the occurrence of arsenic such as calcium, carbonates, phosphorus, iron, sodium, magnesium and others. The occurrence of these constituents is clearly observed in the data analysis. There is no clear evidence that indicates that the structural geology of the study area has a relation with the arsenic occurrence. One



hydrological influence for the occurrence of arsenic is the flow pattern of the groundwater. In the area of the Hueco Bolson the natural flow pattern has been altered toward the cones of depressions mainly from the Town, Water Plant, Lower Valley and Cielo Vista well fields. It is important to mention that according to the analysis done with the GIS database, the concentration of arsenic in the groundwater increases in the same direction of the flow pattern for the Town, Water Plant, Lower Valley and Cielo Vista areas. However, more sample analysis is needed to confirm this statement. For the Mesilla Bolson the arsenic concentration increases substantially to the southern portion of the aquifer. This can be a result of the narrow valley outlet at downtown El Paso between Sierra Juárez to the south and the southern tip of the Franklin Mountains that restricts groundwater outflow and prevents the flushing of water with greater dissolved solids concentrations. Consequently, the high concentration of minerals results in high arsenic concentrations. Conclusions As a result of the compilation and analysis of arsenic occurrence data and of its related constituents, the following conclusions were obtained. Constituents that are related to the occurrence of arsenic were added through the geologic evolution of the basin-fill deposits during the Cenozoic Era. Nevertheless, there is no statistically significant relationship between the occurrence of arsenic and the occurrence of related constituents as in other parts of the world. It is noteworthy that the new altered groundwater flow pattern has some effects in arsenic concentrations in the Hueco Bolson, which also tie with the main areas of water pumping. On the other hand, the groundwater in the Mesilla Bolson has high arsenic concentrations in the Southern portion of the aquifer, most probable as a result of the to narrow valley outlet at downtown El Paso between the Sierra Juárez, Cristo Rey and the Southern tip of the Franklin Mountains. Acknowledgements – The authors wish to thank SCERP for funding this project and to the MIE Project at UMET and UTEP for provided the opportunity to participate on this research. Patrick L. Gurian, PhD. and Elia Marquez, M.S. provided excellent cooperation. Manuel A. Ramos provided statistical and GIS support.



Literature cited

Bhattacharya P., Chartterjee D. and Jacks G. 1997. Occurrence of arsenic contaminated groundwater in alluvial aquifers from Delta Plains, Eastern India: options for safe drinking water supply. Water Resources. Dev. 13, pp 79-92.

Brewstar M. D. 1994. Removing arsenic from contaminated water. Water Environ. and Tech 4, pp 54-57. Chartterjee A. 1994. Ground Water Arsenic Contamination in Residential Area and Surroundings of P.N. Mitra Lane, Behala, Culcutta, due to Industrial Effluent Discharge. Ph.D. thesis, Jadavpur University, Calcutta, India. Csanady M. and Straub I. 1995. Health damage due to pollution in Hungary. In Proceedings of the Rome Symposium, September, 1994, IAHS Publ. No. 233, pp 1-11. Collins, E.W. and Raney, J.A., 1991. Tertiary and quaternary structure and the paleotectonics of the Hueco Basin, Trans-Pecos Texas and Chihuahua, Mexico. Geological Circular 91-2, The University of Texas at Austin. El Paso Water Utilities, 2002. García-Sánchez, A. And Alvarez-Ayuso, E., 2003. Arsenic in soils and waters and its relation to geology and mining activities (Salamanca Province, Spain). Journal of Geochemical Exploration 4108 (2003) pp 1-11. Jain, C.K. and Ali, I., 2000. Arsenic: Occurrence, toxicity and speciation techniques. Hydrology. Wat. Res. Vol. 34, No. 17, pp 4304-4312. Kiping M.D. 1977. Arsenic, the Chemical Environment, Environment and Man, Vol. 6 eds J. Lenihan and W. Fletcher, pp 93-110, Glasglow. Korte N.E. and Fernando Q. 1991. A review of arsenic (III) in groundwater. Critical Review of Environmental Control 21, pp 1-11. Pershagen G. 1983. The Epidemiology of Human Arsenic Exposure, ed. B. A. Fowler, pp 199-221. Elsevier, Amsterdam. Ryder, P.D., 1996. Rio Grande Aquifer System, Groundwater Atlas of the US, USGS. Chapter C, HA 730-E. Smedley, P.L. and Kinniburgh, D.G., 2001. A review of the source, behavior and distribution of arsenic in natural waters. Applied Geochemistry 17 (2002) pp 517-568 Welch, A.H., Lico M.S. and Hughes J.L. 1988. Arsenic in groundwater of the Western United States. Groundwater 26, pp 333-347.



Welch, A.H., Westjohn, D.B., Helsel, D.R., and Wanty, R.B., 2000, Arsenic in ground water of the United States-- occurrence and geochemistry: Ground Water. v.38 no.4, pp 589-604. WHO (World Health Organization), 1981. Environmental Health Criteria, 18: Arsenic. World Health Organization, Geneva. Table 1: Arsenic occurrence related to the concentrations of the related constituents

Constituent Min (ppb) Mean (ppb) Max (ppb) MCL (ppb) Arsenic 0 9 39 10 Manganese 2.3 97 1,250 50 Iron 1 202 4,385 300 Copper 2.50 25 441 1,000 Sulfate 11,000 265,275 4,240,000 250,000 Chloride 17,382 347,526 11,400,000 250,000 TDS 0 951,247 11,141,000 500,000 Silica 0 24,111 66,000 50 Calcium 2,000 69,705 940,000 300 Magnesium 250 20,865 404,000 - Sodium 0 246,159 3,400,000 - Carbonate 0 1228 26,400 - Bicarbonate 45,000 190,649 808,000 - Phosphate 45 74 470 -

- There is not an MCL standard for this specific constituent.

APPENDIX C. NGWA 2005 Paper


Geochemistry of Groundwater Arsenic in the El Paso region: a Case Study By Elia B. Marquez, University of Texas at El Paso, Patrick L. Gurian, Department of Civil, Architectural, and Environmental Engineering, Drexel University, Alberto Barud-Zubillaga, Center for Environmental Resource Management, University of Texas at El Paso, Philip Goodell, Department of Geology, University of Texas at El Paso. Abstract The new Maximum Contaminant Level (MCL) of 10 µg/l for arsenic is currently exceeded in many ground water supplies across the nation. However, in many areas the small-scale spatial occurrence trends and geochemical controls on arsenic in ground water are not well understood. In this work, the El Paso region, in which approximately 20% of the water supply wells exceed the new arsenic standard, is analyzed as a case study to better understand the factors associated with high arsenic levels in ground water. The spatial distribution of arsenic concentrations in ground water of the Hueco and Mesilla Basins that supply water to the El Paso region was evaluated by using statistical analysis with data from over 300 wells for the last 35 years. In the Hueco Basin positive and significant correlations between arsenic and pH, carbonate, phosphate and sulfate were found which suggests that competitive desorption of arsenic from iron hydroxides at high pH may be a factor contributing to arsenic mobilization. A positive correlation between solid phase iron and arsenic also supports this mechanism. Evaporation and upwelling from deeper waters are mechanisms that may produce the high solute concentrations that lead to competitive desorption. Also in the Hueco, the positive correlations with sodium, negative with Mg, and slightly negative with Ca indicate that more heavily cation-exchanged waters have higher arsenic levels, possibly because they are older and have had more opportunities to dissolve arsenic. The average concentration of arsenic was 9 µg/L in the Hueco, while for the Mesilla Basin the average was 14 µg/L. The higher levels in the Mesilla basin may be due to the presence of volcanic sediments in the Santa Fe group in the Mesilla Basin, as previous research has indicated volcanic rocks often contain elevated amounts of arsenic. The arsenic concentrations increase with well depth for the Mesilla basin, but not in the Hueco Basin. The lower arsenic concentrations found in shallow ground water in the Mesilla Basin are probably the result of dilution from surface water as these wells are under the influence of the Rio Grande River. Speciation analyses in the Mesilla show the proportion of As(III) to total-As ranging between 0% to 48%. While the oxidized form As(V) is predominant, the presence of As(III) indicates that reductive processes likely contribute somewhat to mobilization in the Mesilla basin.



1. INTRODUCTION High arsenic concentrations in drinking water have been linked to diverse types of cancer and to other serious diseases (Berg 2001, Siegel 2002). To reduce this potential health risk, in 2001 the US Environmental Protection Agency (USEPA) lowered the maximum level of arsenic permitted in drinking water from 50 to 10 micrograms per liter (µg/l). In this work, the Paso Del Norte region, in which approximately 20% of water supply wells exceed the new arsenic standard, is analyzed to better understand the factors associated with high arsenic levels in ground water. Archival data on arsenic and related parameters are used as well as additional sampling of groundwater and aquifer materials to identify the geochemical conditions associated with high arsenic in the groundwater. 2. LOCATION AND HYDROLOGY The Paso Del Norte Region is an urbanized region of the Rio Grande Valley located at the intersection of the U.S. states of Texas and New Mexico and the Mexican state of Chihuahua (Figure 1). The region includes the cities of El Paso, Texas, and Ciudad Juárez, Mexico. It is located in the northern Chihuahuan Desert and has a subtropical arid climate (Fisher and Mullican1990). Rainfall averaged 7.8 inches and temperature 63.4 °F (from – 8 °F to 109 °F) during the period from 1960 to 1980 (Fisher and Mullican1990). The Rio Grande River and the Franklin Mountains geographically characterize the region. The Rio Grande River serves of natural border between the U.S. and Mexico. There are two aquifers: the Hueco Bolson, which is located east of the Franklin Mountains and the Mesilla Bolson, which is located west of the Franklin Mountains (Figure 1). These aquifers are filled with Tertiary and Quaternary alluvial unconsolidated sediments, are not hydraulically connected, and drain to the Rio Grande River. El Paso and Ciudad Juárez are located directly across from each other on opposite sides of the U.S.-Mexico border. This study case will focus in the drinking water supply to these two cities whose total population exceeds two million people.



Figure 1. Paso del Norte region including southwest part of Texas, southern New Mexico and northern Chihuahua Mexico. (Modified from National Atlas of the United States 2005) The three main drinking water sources of El Paso are the Hueco Bolson aquifer (which provides 30% of El Paso’s annual supply), the Mesilla Bolson aquifer (20% of annual supply), and the Rio Grande river (50% of annual supply) (EPWU 2002). Ciudad Juárez relies on the Hueco Bolson for 100% of its drinking water supply. The Hueco Bolson aquifer underlies both cities and is crossed by the Rio Grande River from west to east. Therefore, the chemical composition of the Hueco groundwater was expected to vary only modestly between the two cities. However, during the course of this work, a confining layer (that is partially characterized here) has been found to separate high arsenic water in the southeastern portion of the Paso del Norte region (known as the “Lower Valley” as it includes the downstream portion of the Rio Grande Valley) from lower-arsenic waters in the remainder, major part of the Hueco Bolson. The identification of this confining layer has changed the previous understanding of the Hueco as a thoroughly homogeneous, unconfined aquifer (WRRI 1997). The two basins, the Hueco and the Mesilla, have on average high Cl and K concentrations, compared to neighboring aquifers and also compared to aquifers from across the U.S in which the predominant rocks are carbonate, gypsum, limestone, etc, Table 1 (Modified from Eby 2004 and the arsenic values from the USGS website map). Table 1 is expanded in Table 2 for the Hueco and the Mesilla. The Mesilla Bolson has lower levels of most parameters than the Hueco Bolson except for sulfate and total dissolved solids (TDS).



Table 1. Major ions in eight US aquifers with dominant rock types (modified from Eby 2004). The units are parts per million (ppm), except for As: ppb.

Aquifer__dominant rock type

Ca

Mg Na K Cl SO4

HCO3 SIO2 TDS

As* ppb

NM_gypsum

636 43 17

NA 24

1570 143 29

2480 18

FL_carbonate 34

5.6 3.2

0.5 4.5 2.4 124 12 NA 5

PA_carbonate 83 17 8.5

6.3 17 27 279 NA NA 3

AL_limestone 46

4.2 1.5

0.8 3.5 4.0 146 8 222 1

MO_sandstone 3

7.4 857

2.4 71 1.6 2080 16

3098 25

NM_rhyolite 6.5

1.1 38 2 17 15 77 103 222 11

Hueco Bolson 59

14.3 212

9.2 310 114 141 29 840 9

Mesilla Bolson 29

5.2 158

4.1 115 168 117 31 603 14

As* averages based upon USGS web site’s map. NA- not available 2.1 The Mesilla Aquifer characteristics The main hydrologic characteristics for the two basins are listed in Table 3 (WRRI 1997 report). The Mesilla Basin fills, from which the fresh water comes, are from the Quaternary-Tertiary ages (middle Pleistocene and Oligocene epochs), Santa Fe groups (Deep, Middle and Upper). These groups are unconsolidated alluvium from both nearby mountains and distant sources outside the basin (Bexfield 2001). The Mesilla basin is neither homogeneous in composition nor in hydrologic parameters. (WRRI 1997). The two principal mechanisms of recharge to the Mesilla are seepage from the Rio Grande and deep percolation of irrigation water (WRRI 1997). 25 out of 58 wells (43%) in the Mesilla Bolson are directly hydraulically connected with the Rio Grande River. 2.2 The Hueco Aquifer characteristics The Hueco basin fill belongs to the Fort Hancock and Camp Rise formations of Quaternary-Tertiary age. The Hueco is a basin that is considered a homogeneous basin (WRRI 1997). Some limited recharge in the western portion of the basin is due to runoff from the Franklin Mountains. 8 shallow wells (out of 266) drain water from the river (EPWU 2003).



3. ARSENIC GEOCHEMISTRY 3.1 Competitive Desorption On the solid phase of ferric hydroxides (≡FeH2AsO4) some anions may compete with AsO4

-3 for sorption sites. If an anion displaces an arsenic molecule, the arsenic will desorb from the solid phase and pass into the aqueous phase, increasing the dissolved arsenic concentration. The molecules that usually occur in groundwater and compete with arsenic oxide are: CO3

2-, SiO2 -2, PO4 3-, OH- which may be strongly or weakly

adsorbed to iron hydroxides but because they occur in orders of magnitude higher concentrations than arsenic concentrations, produce a considerable competitive effect (Holm 2002). 3.2 Reductive Dissolution When iron or manganese are reduced they become more soluble and may release arsenic ions adsorbed to them as shown by the following reactions: Fe(III) less soluble Fe(II) more soluble As(V) more adsorbed As(III) less strongly sorbed Table 2. The Mesilla and Hueco Basins descriptive statistics for some trace, most major ions, well depth and pH. (Units are ppm unless otherwise indicated). From this study’s sampling pooled with EPWU archives.



Mesilla As ppb pH depth ft Fe ppb Mn ppb SiO2 Ca Mg Na KMean 14 8 454 242 142 31 29 5 158 4

Std. Deviation 7 1 276 253 205 8 30 8 98 3Minimum 4 7 40 18 2 5 2 0 65 1Maximum 41 11 1208 940 769 53 149 74 691 12

Hueco As ppb pH depth ft Fe ppb Mn ppb SiO2 Ca Mg Na KMean 9 8 579 487 91 29 59 14 212 9

Std. Deviation 4 0 201 2156 294 10 105 24 265 4Minimum 1 6 59 6 1 6 4 0 28 3Maximum 25 10 1330 24000 2700 145 2260 476 5220 54

Mesilla HCO3 CO3 SO4 Cl NO3 PO4 TDS Hardness EC uSMean 117 6 168 115 2 0 603 91 983

Std. Deviation 69 14 110 127 3 0 375 100 592Minimum 0 0 9 26 0 0 190 5 400Maximum 416 100 600 800 13 1 2180 678 3720

Hueco HCO3 CO3 SO4 Cl NO3 PO4 TDS Hardness EC uSMean 141 2 114 310 6 0 840 207 1417

Std. Deviation 50 4 104 616 5 1 1109 358 1871Minimum 0 0 1 18 0 0 241 22 406Maximum 383 25 1290 12900 43 9 22000 7600 33100

Table 3. Hydraulic parameters for the Hueco and Mesilla Basins. From WRRI 1997 report.

AquiferArea (acres)

Thickness (ft)

H conductivity (ft/day)

Transmissivity ft2/d

Storage coefficient

Mesilla 210 000 200- 2500 1- 100 2600-20000 0.0004Hueco 346 000 200 - 10500 6 -100 4000-28000 .093 -.0003 The final concentrations of both metals (Fe and As) have increased in solution. Thus a positive correlation between arsenic and iron or manganese would support this mechanism. Reduced environments generally occur deep in the aquifer or when microorganisms promote the reduction. Dissolved oxygen (DO) is expected to be very low or absent and oxidized species of almost all molecules are expected to be scarce (more or less depending of the reduced degree of the environment). At Bangladesh and West Bengal, reductive dissolution has been found to be the principal process mobilizing arsenic from the river delta material into the water after organic decomposition (Mc Arthur 2002). Organic material buried for thousands of years may give rise to this process and such organic material may be present in sediments deposited by the Rio Grande.



3.3 Evaporative Concentration The evaporation of water from the recharge or from the unconfined aquifers will concentrate the ions including arsenic and dissolved solids in the water. Evaporation also raises the pH as it increases alkalinity by concentrating CaCO3. Higher pH promotes the competitive desorption of arsenic from iron hydroxide (Montoya and Gurian 2003, 2004). 3.4 Upwelling Excessive pumping causes water to flow toward the well, and if the unit is confined (or partially confined) the flow upward may occur in important amounts. If some fault is close to the well, the suction will bring the deeper, more mineralized waters to the well also. The minerals and dissolved ions will be different from those usually found in that region of the aquifer (Baxfield 2001). As with the evaporative concentration mechanism, such mineralized waters will have high alkalinity and high pH that promotes the arsenic desorption.



4. INFORMATION SOURCES Approximately 97% of the information used in this work was from archival databases. A database provided by the El Paso Water Utilities (EPWU) included information on the concentrations of 14 different major ions and 17 trace elements at 266 drinking and non-drinking water supply wells. For Ciudad Juárez data on concentrations of major ions were obtained from the archives of the Junta Municipal de Aguas y Saneamiento (JMAS). The JMAS archives did not include information on arsenic concentrations. A data set from the Texas Water Development Bureau of about 250 drinking and non-drinking water wells was also used. Because of the different nature of these databases and some degree of overlap between the EPWU and the TWDB databases, the databases were analyzed separately rather than being pooled. Approximately 3% of the data used in this study was from sample collection and chemical analyses designed to fill gaps in the archival databases. Additional groundwater samples were taken in Ciudad Juárez and analyzed for arsenic because the archival databases had no information on arsenic concentration on the Mexican side of the border. These groundwater samples were also analyzed for dissolved oxygen and temperature because these parameters were not included in the archival data. Because a great deal of archival data on the U.S. side of the border was available from the EPWU, only 21 additional samples were collected (13 from the Hueco and 8 from the Mesilla) mainly to validate the archival information. However for Ciudad Juárez, no archival information was found for arsenic, and the bulk of the additional ground water sampling (83 samples) was conducted in the portion of the Hueco Bolson underlying Ciudad Juárez. Arsenic in groundwater is believed to be controlled primarily by interactions between ground water and solid phase aquifer materials. As the archival data did not contain information on the solid phase aquifer materials, a set of well cuttings was analyzed for arsenic, iron and total organic carbon (TOC). Correlations between solution and solid phase arsenic (as found by Robertson 1989) would suggest a local source (or sink) for arsenic mobilized into the water, while the lack of such a correlations would indicate that mobilization mechanisms operate over a larger spatial scale. 5. METHODS OF COLLECTION AND PREPARATION Groundwater samples were collected from November 2003 to December 2004. A total of 104 wells sampled. Water was analyzed for arsenic by a contract laboratory (the NMSU SWAT laboratory) using ICP method EPA 200.8. In the Hueco Bolson a total of 96 water samples were collected in the field mainly in Ciudad Juárez.



The water samples were collected when the wells were operating and were drawn at the wellhead, before chlorination. The water was allowed to flow for about three minutes before sample collection. Samples were collected in 200 ml plastic containers, which were thoroughly rinsed and filled without headspace. For a small set of samples collected in Ciudad Juárez, the pH, temperature, electroconductivity, and salinity were measured in the field. The samples were analyzed within 10 days of the collection date. Cuttings from the wells drilled by the El Paso Water Utilities (EPWU) have been archived in the Geology Department of the University of Texas at El Paso. 1.5g material from each of the 15 well cores samples was analyzed for arsenic and iron. Total organic carbon (TOC) was analyzed in 11 wells. From the whole 15 samples, 11 where from high (11 ppb to 27ppb) groundwater arsenic and 4 wells where groundwater arsenic was lower than 6 ppb. Each 1.5 grams sample was thoroughly sieved to 75 microns size (standard sieve # 200) before analysis. 6. RESULTS AND DISCUSSION T-tests were performed to compare the mean arsenic concentrations of the archival data with the corresponding values obtained for samples collected as part of this project. Separate tests were conducted for the Hueco and the Mesilla Bolsons. For the El Paso samples, values from both the archives and the sampling were available at the same wells, which allowed a paired samples t-test to be used. For the Juárez samples archival information was not available so independent sample t-tests were used to compare the means values with the El Paso section of the Hueco Bolson. The t-test for difference of means did not show a significant difference between samples from this study and archival values in the Hueco Bolson. For the Mesilla Bolson, the sample set showed slightly higher arsenic concentrations than the EPWU archives. A t-test for paired samples show that the maximum credible percentage of difference between the two data sets is about 2%. This confirms the accuracy of the archival data used in this study. The mean solid-phase concentrations obtained from the analysis of the well cuttings are shown in Table 4. A significant correlation was found between solid-phase Fe and As (R2= 0.557, P-value .025) (Table 5). This correlation suggests that arsenic is often associated with iron in the solid phase and supports previous researcher that aqueous arsenic concentrations are controlled largely by sorption-desorption reactions with iron hydroxide (Robertson 1989).



Table 4. Wells core sample results from this study. Note that the average solid-phase arsenic modestly exceeds the mean crustal value of 2ppm (Siegel 2002). As aqueous corresponds to the water concentration in the archival data.

As ppb aqueous

As ppmsolid phase

Fe ppm solid phase

TOC_% solid phase

Solid sample Depth (ft)

Mean 12.2 4.1 7124 0.5 308 Minimum < 0.5 0.0 1820 0.0 100 Maximum 26.8 10.1 19700 1.4 540

Table 5. Wells core samples Pearson correlation coefficients from this study.

Aqueous As

Solid phase As

Solid phase Fe

Solid phase TOC

Solid phase As [ppm] .279

Solid phase Fe [ppm] .274 0.557*

Solid phase TOC [%] .311 -.024 .337

Depth of core sample [ft]

-.317 .206 .149 -.047

* Correlation is significant at the 0.05 level (2-tailed). The map in Figure 2 shows the average arsenic levels in the EPWU wells for the last 20 years averaged by well. The intermediate and deep wells in the Mesilla Bolson show high arsenic concentrations, as do a cluster of well in the southeastern Hueco Bolson, including five in the Lower Valley. Arsenic concentrations are low in the region immediately to the east of the Franklin Mountains, which is believed to receive recharge from storm water flows from the mountains (WRRI 1997). The water sampling in the Juárez part of the Hueco, specifically in the Lower Valley, showed noticeable differences from arsenic concentrations in the archival data for the Hueco. This appears to indicate that the confined portion of the Hueco Bolson found in the Lower Valley is much higher in arsenic than the major unconfined aquifer. Not only were the arsenic concentrations (Table 6) out of the confidence interval for the Hueco Bolson archival observations but these samples from the Juárez Lower Valley wells located in the confined aquifer also showed intriguing field observations such as 1) a difference up to ten degrees



Celsius (18o F) higher temperature than any other observation (Table 6) and 2) a strong negative correlation between dissolved oxygen and arsenic (Figure 5). Indeed the largest observations for arsenic in the whole Hueco Bolson were 70 ppb and 86.6 ppb in two Juárez artesian wells. The pH and temperature were strongly positively correlated between themselves and with arsenic in these Lower Valley wells (Figures 3 and 4). The two highest arsenic observations were measured in the lowest DO wells, and the odor of sulfide in one of those is an additional indication of the role of reducing conditions as a mobilization mechanism in the Lower Valley semi-confined wells. Figures 3, 4 and 5 and Table 6 partially characterize this confined unit of the Hueco Bolson in the Lower Valley. EPWU archives show for the Lower Valley confined wells on the U.S. side of the border, the average arsenic concentration was about 12 ppb compared to 3 ppb in the unconfined neighboring wells. In the whole of the Hueco, the average arsenic concentration in the archival observations is less than 8 ppb. Eastoe’s (2004) study of the Juárez Lower Valley wells found a Global Meteoric Water (GMW) isotopic signature in these Lower Valley wells that distinguishes them from their neighbors as well as from the major part of the Hueco with clearly evaporated waters. Finally in the WRRI report (1997), conductivity logs show the thick clay layers characteristic of aquitards (semi-impervious layers) at El Millon location inside the Juárez Lower Valley region. Table 6. Juárez nine well water samples in the Hueco Lower Valley from this study.

AqueousAs ppb PH

Temperature C

EC µS

DO ppm

Mean 16.56 7.98 24.90 1406 3.17

Median 9.30 7.80 23.30 1153 3.03

Minimum 0.90 7.66 18.60 667 0.10 Maximum 86.60 8.80 35.70

2313 6.70

In the Mesilla Bolson, the most significant positive correlations with arsenic were found for pH and depth; the most significant negative for Cl, TDS, hardness, Mn, Ca, Mg, Na, SO4 and electroconductivity (EC) (Table 7). These trends reflect the influence of the Rio Grande River on shallow wells in the Mesilla Bolson. The river water is generally low in arsenic concentration but high in dissolved solids.



The low arsenic concentrations are common for aerobic surface waters where arsenic is present in the oxidized form arsenic(V), which can be readily removed from the aqueous phase by sorption to a variety of minerals, such as iron and aluminum oxides. The high salinity of the Rio Grande in this region can be explained by evaporative concentration of the river as it flows through an arid region and by the high salinity of return flows from agricultural irrigation. Table 1 shows higher average sulfate concentrations in the Mesilla than in most of the other aquifers, Stiff diagrams from the Water Resources Research Institute (WRRI) 1997 report support this result. The positive correlation between arsenic and pH may also be due to the influence of river water on the aquifer as the ground water that is not influenced by the river would be expected to have higher alkalinity and higher pH. This correlation may also be due to higher pH causing desorption of arsenic from hydroxide solids (Montoya and Gurian 2004). Thus this correlation supports the competitive desorption mechanism of mobilization. Evaporative concentration may also play a role by concentrating both the arsenic and the alkalinity in the groundwater. A speciation analysis (EPWU 2003) found that arsenic is present as both As(III) and As(V) but with at least half and usually closer to two-thirds of the arsenic present as As(V). This indicates that conditions are at least moderately reducing in the high arsenic wells. Thus reductive dissolution may play a role in mobilizing arsenic. However, it does not result in the dissolution of enough iron to produce detectable concentrations in the aqueous phase. It may be that the dissolution of relatively small amounts of iron on the outer surface of the aquifer materials is sufficient to mobilize ppb levels of arsenic into the aqueous phase. Even if iron is not dissolved, the reduction of arsenic(V) to arsenic(III) reduces the affinity of the arsenic for the hydroxide solids. Nitrate levels in some wells in the southwestern parts of El Paso County exceed the 10 mg/L drinking water standard (WRRI 1997) with concentrations reaching 126 mg/L (EPWU 2003) (Table 2). The average in the last 75 years is 7.4 mg/L for the Hueco basin (EPWU 2003). The threat that high nitrates imply is an important health issue beyond the scope of this work. In the Hueco Bolson, the most significant positive correlations with arsenic were with: pH, Na, CO3, SO4, Cl, PO4, TDS and electroconductivity (EC) (Table 7); the most significant negative correlations with depth, Mg, HCO3 and NO3. The positive correlations with pH, carbonate, sulfate, and phosphate support competitive desorption as a mechanism of arsenic mobilization. The negative correlation with bicarbonate may be due to confounding with pH (low pH would favor bicarbonate over carbonate but would also lead to low arsenic by favoring sorption of arsenic to iron hydroxides). Other mechanisms may also play a role in arsenic mobilization. Eastoe’s (2004) found strong evidence of evaporated waters in the Hueco, indicating that evaporation may contribute to arsenic mobilization. The positive correlation with



sodium and negative correlation with magnesium suggest that more cation-exchanged waters have higher arsenic concentrations. More cation-exchanged waters would be expected have been exposed to more minerals and have had more opportunities to encounter arsenic-bearing minerals. In the set of nine observations from the confined aquifer in the Lower Valley suburbs of Juárez, the high and significant correlation of As with pH (R_Square = 0.98, p-value < 0.001, Figure 4) support competitive desorption as a mobilization mechanisms, while the negative correlation between DO indicates a possible role for reductive processes in mobilizing arsenic (Figure 5). The correlation with temperature may indicate an influence from geothermal waters.

Figure 2. Map with the arsenic concentrations in the El Paso city wells. (Modified from EPWU 2003 and UT Austin). Red circles (largest): 22 to 32 ppb, Blue: 14.7 to 22ppb, Purple: 10.6 to 14.7, Light Blue: 8 to 10.5 ppb, Pink and Black smallest dots: less than 8ppb.



20 25 30 35

Temperature C

7.50

8.00

8.50

9.00

pH

R-Square = 0.94p value = 0.006

Figure 3. Scatter plot of pH versus temperature for 9 well water samples from Ciudad Juárez inside the HuecoBolson.

7.75 8.00 8.25 8.50 8.75

pH

0

25

50

75

As

ppb

R-Square = 0.98p value < 0.001

Figure 4. Scatter plot of aqueous arsenic versus pH. 9 wells sampled at Juárez city. This as well as the scatter plot of arsenic and temperature for nine Juárez observations, show a significant positive correlation coefficient supporting desorption/dissolution from the solid phase.



0 2 4 6

DO ppm

-50

0

50

100

As

ppb

R-Square = 0.32p value = 0.009

Figure 5. Scatter plot of arsenic with DO for the Juárez wells in Lower valley.

Table 7. Spearman’s (Non Parametric) Correlation coefficients for the Hueco and Mesilla Bolsons. (Observations averaged by depth from this study pooled with EPWU archives).

pH Fe Mn SiO2 Ca Mg NaHueco .388(**) -0.041 -0.118 -0.052 -0.101 - .318(**) .444(**)Mesilla .326(*) 0.002 - .638(**) -0.13 - .503(**) - .477(**) - .362(*)

HCO3 SO4 Cl NO3 PO4 TDS HardnessHueco - .366(**) 0.237(**) .313(**) - .42(**) 0.190(*) .293(**) -0.139Mesilla -0.296 - .412(**) - .511(**) -0.019 -0.067 - .407(**) - .492(**)

Depth CO3 K ECHueco - .293(**) .232(**) -0.054 .279(**)Mesilla .426(**) 0.339 -0.264 - .351(*)

*Correlation is significant at the 0.05 level (2-tailed). ** Correlation is significant at the 0.0 level (2-tailed).



7. CONCLUSIONS In the Mesilla aquifer, the distribution of arsenic is largely controlled by depth, with low arsenic water from the Rio Grande River overlying semi-confined, deeper and older water with higher arsenic concentrations. Evaporative concentration, competitive desorption, and reductive dissolution may also influence the arsenic concentrations but less dramatically than the distinction between the young river water and the older, deeper water. The likely source of arsenic in the deeper aquifer is the Santa Fe group of rock, which is of volcanic origin. Volcanic rocks are often enriched in arsenic (Siegel 2002) and may release arsenic over time, particularly under high pH conditions. In the Hueco Bolson, a confining unit appears to exist in the Lower Valley occupying both sides of the border (defined by the Rio Grande River). A comprehensive study of this aquifer is warranted since the presence of a confining layer may significantly impact overall evaluations of the ground water resources of the region. The average arsenic in these Lower Valley, confined wells is 12 ppb (ranging from 1.5 to 87 ppb). A significant negative correlation with dissolved oxygen indicates that reductive dissolution may contribute to high arsenic concentrations in this area. The noticeable difference between wells in the confined Lower Valley and their neighbors (where the average arsenic concentrations are 12 and 3 ppb respectively), as well as the leaky confinement condition in the Mesilla basin where the average arsenic is 13 ppb, suggest that in confined units (or partially confined), the arsenic is more likely to dissolve or desorbs from the solid phase than in unconfined hydrologic units, possibly because the confining layer contributes to reducing conditions (clay and silt confining layers often have high organic content).

In the Hueco aquifer, evaporative concentration, reductive dissolution, and competitive desorption are all compatible with the data observed. These mechanisms may operate together or separately, and their relative importance may vary throughout the aquifer. 8. REFERENCES

Baxfield, L M, 2001 Occurrence and sources of arsenic in ground water of the middle Rio Grande basin, central New Mexico, M.S. Thesis, Hydrology, New Mexico Institute of Mining and Technology, Socorro, New Mexico.

Berg, M, Tran HC, Nguyen TC, Pham HV, Schertenleib R and Giger W, 2001

Arsenic contamination of groundwater and drinking water in Vietnam: a Human health threat,Environment Sc. and Tech, Vol

35, No 13. Eastoe, Ch, 2004, Isotopes in the Hueco presentation EPWU Hueco Bolson/Rio

Grande Aquifer Meeting, El Paso, TX. EPWU 2003, El Paso Water Utilities report on line at http://www.epwu.org



Eby G N, 2004 Principles of Environmental Geochemistry, Brooks Cole Fisher R S. and Mullican W. F,1990, Integration of ground-water and vadose-

zone geochemistry to investigate hydrochemical evolution: a case study in arid lands of the northern Chihuahuan Desert, Trans-Pecos Texas, Bureau of Economic Geology, University of Texas at Austin.

Holm, T R, 2002 Effects of CO3 ,HCO3 , Si, and PO4 on Arsenic sorption to HFO

Journal AWWA 94:4, April. Mc Arthur, JM, 2002, A Layman’ guide to arsenic pollution of groundwater in

Bangladesh and west Bengal , London Arsenic Group http://www.es.ucl.ac.uk/research/lag/as

Montoya, T. and P.L. Gurian, 2003, Numerical Solution of a Chemical Equilibrium Model of Arsenic Sorption to Ferric Hydroxide, Proceedings of the Modeling and Simulation Workshop of the International Test and Evaluation Association.

Montoya, T. and P.L. Gurian, 2004, Modeling Arsenic Removal by Coagulation

with Ferric Salts: Effects of pH and Dosage, Proceedings of the Texas Water 2004 Conference, Arlington, TX.

Robertson F. N, 1989, Arsenic in ground water under oxidizing conditions, south

west United States. Environmental Geochemistry and Health, v.11, p171-186 Siegel, F R, 2002, Environmental Geochemistry of Potentially toxic Metals,

Springer. Smedley P.L. and Kinniburgh D.G, 2002, A review of the source, behavior and

distribution of arsenic in natural waters, Applied Geochemistry 17, p.517 WRRI, 1997, Trans-International Boundary Binational Report, New Mexico State

University, Las Cruces, New Mexico

APPENDIX D. UACJ Technical Report


SCERP FINAL REPORT March, 2005 Small-scale spatial occurrence trends of arsenic in the ground-water resources of the

Paso del Norte Region

A collaborative research project of the Universidad Autónoma de Cd. Juárez and the University of Texas at El Paso UACJ Principal Investigator: Alfredo Granados Olivas, Ph.D. Professor of Hydrology-GIS Director Coordinador del Centro de Información Geográfica-CIG UTEP Principal Investigator: M.S. Alberto Barud Zubillaga Program Coordinator/Manager Center for Environmental Resource Management CIG Research Team Members: Ing. Hugo Luis Rojas V. (GIS Master) Miguel Dominguez Acosta (Professor) Carlos Martinez Piña (Grad Student) Nora Reyes (Under Grad Student) SCERP FY2003 Applied Environmental Research Program



Acknowledgments We would like to express our appreciation for the active involvement in this project by the staff of the Junta Municipal de Agua y Saneamiento (JMAS). In particular we would like to express our appreciation to Ing. Mario Boisselier Perea, President of JMAS; to M.S. Manuel Herrera Mercado, Chief of the Technical Department of JMAS; to M.S. Ezequiel Rascon, Geohydrology Department and Ing. Javier Gomez Hydrogeoquemistry Department of JMAS. Also, we like to acknowledge the support of the head and technical staff at the Environmental Lab located at the Civil and Environmental Engineering Department from the Institute of Engineering and Technology (IIT) of UACJ. Special thanks are due to M.S. Angelina Dominguez Chicas, Coordinator for the Environmental Engineering Program and Environmental Lab at IIT-UACJ and to the technical staff working on this program. Finally, we would like to express our gratitude to the Southwest Center for Environmental Research and Policy (SCERP) for having provided the financial support for this undertaking. Small-scale spatial occurrence trends of arsenic in the ground-water resources of the

Paso del Norte Region



ABSTRACT Ciudad Juárez, Chihuahua located at the central region of the Transboundary Paso del Norte region shares a common border with El Paso, Texas, which together are considered as one of the most rapid growing communities along the Mexico and U.S. border region. At this region a dramatic increase in the last 20 years of inhabitants has been a demographic challenge to policy makers where, population growth has been considered as one of the highest annual demographic rates on the Mexican Republic (~4 %), with approximately 1.3 million people just on the City of Juárez.. This binational metropolitan region demands infrastructure, such as the need for housing, water, sewage, flood control, recreational areas and others related to pursuing a good quality of life. With this population growth, water resources play a major role for the sustainable development argument at the border community. Presently, the only source of potable water for the city of Juárez relies on the aquifer formations located at the transboundary Hueco Bolson. Out of more than 180 deep wells that provide water for domestic and industrial purposes on the urban area of Juárez, some have been taken out of production due to the deterioration of its ground water quality, where salinization and other solutes are the main problem within the Juárez water wells. In this document we address the situation related to groundwater quality at the Mexican side of this shared watershed, focusing at the geospatial distribution of small-scale arsenic trends while using GIS technology evaluating the geospatial distribution of Arsenic concentrations at the sampled sites. Sixty-one deep wells that supply potable water to the city of Juárez where sampled and tested for Arsenic and sent to a US lab where they where analyzed for this and other chemical elements. Results show that 17 wells (~28 %) exceeded the allowed limits established by the Mexican authorities (0.025 mg/l) and the World Health Organization limits of 10 ppb. Some of these wells are corresponding with wells that historically have showed a continuous deterioration of its water quality and there is no specific correlation with the geomorphologic features found where the well fields are located. Most wells have and average of 200 m depth of drilling and well screens are located at different depths were the most productive lithologic column was found at the time of drilling. Among these lithological arrangements, several clay lenses where identified from the geologic cuttings and are mixed with the semi-confined productive aquifer layers located at the Hueco Bolson deposits. It is recommended that these wells exceeding quality specifications should be brought out of production, until a resampling process takes place to confirm these exceeding values. GIS technology has enhanced the analysis and the conceptual modeling of three dimensional aquifer formations helped on identifying the main productive layers of the aquifer, delineating the geospatial distribution of the Arsenic concentrations proving to be an important asset to evaluate potential risks of contamination to the Hueco Bolson Aquifer.



INTRODUCTION The Paso del Norte region has been one of the most dynamic and fastest growing areas along the border territory between Mexico and the United States. Within the Ciudad Juárez, Chihuahua and El Paso, Texas area, the Paso del Norte Region has been considered as the biggest metropolitan area on the border where human settlements have increased on the last 20 years with close to 2 million, (Fig. 1). Similar to other border communities between Mexico and the United States, Ciudad Juárez, Chihuahua has experienced a process of rapid concentration of population and fast urbanization that is manifested throughout the constant deterioration of natural resources (IMIP, 2002). This growth of the city and population has not been without problems where the increasing needs for water resources is a challenge for administrators and policy makers (Fig. 2). More than 1.3 million people are settled on this city and urban growth has increase year by year where its urban infrastructure has manifested a deficit on pavement, housing, transportation, recreational areas, and other items related to a urbanized region. In regards to water resources, its main source of water is ground water extracted from the aquifer formations at the transboundary Hueco Bolson, a Tertiary and Qaternary basin fill aquifer that spans the international border. The well fields located on the geopolitical extension of the City of Juárez are close to 270 deep wells located at different geomorphic formations within the urban areas on the watershed, out of which approximately 180 are continuously pumping ground water throughout all year to cover the city demand (Fig. 3). Intense pumping of water to cover domestic, industrial and other need of water has increase over time and its geochemical evolution has been deteriorating on its quality and cones of depression have manifested a concentration on hydraulic head located at sites where intense pumping is taking place. Overpumping on this aquifer system has resulted in drawdown of the water table, encroachment of brackish groundwater, and the early retirement of wells, (Hibbs, et al., 2001). Some wells have shown an increase on TDS concentrations and other chemicals throughout time that have a geospatial correlation with observed cones of depression manifested due to intense overpumping of water wells on the city, (Fig. 4). Most of the drawdowns near municipal wellfields vary from 15 m to approximately 55 m product of the last 100 years of pumping, and cones of depression are mostly concentrated on the downtown area of this Ciudad Juárez, Chih., - El Paso, Tx., metroplex where the oldest wells are located (Fig. 5). Problems with ground water quality are product of overdevelopment of the aquifer. TDS and other chemical constituents present on distributed potable water are part of natural dissolve solids on water, which have increase in concentration throughout the years, (Del Hierro, 2004). Arsenic concentrations identified as one of the potential natural contaminants to ground water resources, can be naturally found on soil deposits that conform the present aquifer formations and when ground water gets in contact with lithologic formations that have arsenic deposits, this ground water can end up on the household faucets, creating a potential hazard for human life due to its carcinogenic risk (Fig. 6). The World Health Organization (WHO) establishes a maximum concentration of Arsenic in ground water of 10 ppb in order to comply with standards and reduce health risks in humans.



This paper addresses the spatial mapping of small-scale Arsenic occurrence in the ground-water resources of the Paso del Norte Region as potential sources of contamination to available ground water resources on the Hueco Bolson. Risks to human health will be analyzed while evaluating patterns of concentration distribution as they relate to geomorphic formations of the confined and unconfined aquifer layers in the urban areas of Ciudad Juárez, Chih., Mexico. MATERIALS AND METHODS In order to select the sampling wells, a geospatial analysis was done while using GIS coverages where well fields and urban infrastructure (ie., transportation, roads, land use, etc..), where located within the urban areas of Ciudad Juárez. These data bases included the X/Y coordinate locations of water wells, and the Junta Municipal de Agua y Saneamiento (JMAS) which is the authority and responsible for providing water to the community, provided lithologic information on water wells and the logistics to visit all the selected wells for sampling. Samples where taken accordingly with Standard Methods procedures and transported to a US lab to do the analysis on the water samples. Analysis for pH and Electrical Conductivity, as well as, temperature, and flow where taken at the field with a field sampling kit, (Fig. 7). Project team members worked with digital coverages generated at the Geographic Information Center (CIG) at UACJ related to several natural resources coverages where used (i.e., topography, geomorphology, surface and ground water hydrology). As these products were developed and field data captured, they were electronically shared via the Internet for peers review and further discussion at several personal meetings held during the project progression. To accomplish the objectives on the project, the GIS research center at UACJ used a selection of GIS software packages to proceed analyzing the digital information. All digital products were designed to be utilized with ESRI’s GIS products such as ArcInfo®, ArcView®, ArcGIS®, ArcMap®, and ArcExplorer® as well as other compatible software products, such as the Spatial Analyst tool. All of the above themes were represented by figures such as points, lines or polygons within the GIS system with digital data added to each of the coverages, having the advantage of been a set of georeference data with spatial attributes attached to each one of these themes. A data dictionary and metadata files were developed for all spatial components following the FGDC standards, where database attributes were documented to make available a cross-reference and definition for those values listed in the attribute tables. As these databases and attribute information increased, data dictionary was also modified and updated. This coverage was used as a base map to “drop” on top of this digital map the rest of the coverages without merging, such as the geology, climate, vegetation, and soils maps. These maps where digitally loaded into a Trimble XM GPS unit with ArcPAD software, allowing the visualization of well fields and land use while verifying the polygons and ground truthing these water wells whereas using the Mobile map utility, which has a Pathfinder Pocket GPS receiver by Trimble.



RESULTS AND DISCUSSION Sixty one wells where sampled on the urban area of Ciudad Juárez and 17 (~28 %) of these wells exceeded the WHO limits of 10 ppb and the Mexican standards of 0.025 mg/l. Results show values of Arsenic ranging from 1.7 ppb at well JMAS-13 RR to 23.3 ppb at well JMAS-145 (Fig. 8). Geospatial distribution of Arsenic concentration on the aquifer formation has a geomorphic relationship with sedimentary deposits since most of the wells that exceeded established limits where located at the area of influence found at the flood plain of the Rio Bravo/Rio Grande. In contrast, wells with the lowest concentrations appear to be near the alluvial fans sloping from the Sierra de Juárez located at the west side of the urbanized area (Fig. 9), which are the most dynamic aquifer systems were residence time of ground water recharge has been estimated to be of about 45 years, as opposed to the wells located at the floodplain areas, where residence time has been estimated to be of about 3000 years at the deeper semi-confined aquifer formations (Eastoe, et al., 2004; Hibbs, et al., 2004). Quality of ground water has deteriorated throughout time as observed on the Stiff Diagrams where Cations and Anions concentrations has increase (Fig. 10). Samples taken from 1986 to 1990 showed a good quality of water at this well, as compared to continuous 5 year periods from 1991-1995 and 1996-1999, where concentration of these ions increased, deteriorating the water quality at well ID 134, (other stiff diagrams related to Table 1 can be observed on Annex 1). In regards to Arsenic presence, Well ID JMAS 134 had a concentration of 18.10 ppb as observed on Table 1. The list presented at Table 1 shows the results of the 17 wells that exceeded the allowable limits of Arsenic, as well as, pH, Electric Conductivity, Temperature, well yield and TDS. The geospatial distribution of these exceeding wells is observed in figure 11, where most of wells are located at the dominant floodplain area of the Rio Bravo, where sedimentary deposits had a surface hydrology control by pre-development of Elephan Butte dam on the recent geologic time.



Table 1 Well ID’s exceeding Arsenic limits

Arsenic water analysis from SWAT and Assaigai Laboratories

Well_ID Arsenic (ppb) pH EC_rel

Temp (ºC)

Flow (lps)

Salinity (TDS)

JMAS 145 23.30 8.02 1368 24.2 NA 700

JMAS 130 22.20 8.48 969 23.8 58 NA

JMAS 134 18.10 7.94 1492 25 NA 800

JMAS 121 17.4 8.06 1055.00 21.60 52.00 500

JMAS 218 15.70 NA NA NA NA NA JMAS 47 R 15.60 8.19 1062 23.8 NA 500

JMAS 212 15.30 NA NA NA NA NA

JMAS 163 14.20 7.92 1330 22.7 48 700 JMAS 19 R 14.00 7.81 1615 22.5 45 NA JMAS 8 Za 13.20 8.19 827 23.6 47 400

JMAS R1 11.70 NA NA NA NA NA

JMAS 11R 11.4 8.18 1031.00 22.30 48.00 500

JMAS 198 11.30 8.01 1410 27.3 49 700 JMAS 115 R 10.50 8.42 889 23.3 52 NA

JMAS 117 10.50 7.89 1280 23.4 45 700

JMAS 17R 10.40 7.86 992 23.9 NA NA

JMAS 110 10.30 8.19 774 24.6 NA 400 The later is shown on three-dimensional models developed from stratigraphic columns of lithologic information conforming the aquifer deposits (Fig. 12). These sedimentary deposits show heterogeneity on sedimentary packages of floodplain load where finer materials are unevenly distributed along the identified geomorphic features; nevertheless, clay sediments are dominant on the east side of upper cross section on figure 12, where wells are located along the floodplain of Rio Bravo. These sediments have been transported on the recent geologic time from headwaters of the watershed, where other type of regional and local geologic materials conform the outcrops of the topography, where volcanic outcrops are present.



CONCLUSIONS While the attempt of this study was to identify the presence of potential risks of ground water contamination due to small-scale Arsenic concentrations on water wells located on the aquifer formations at the Hueco Bolson, while evaluating its geospatial distribution using GIS tools, as well as, to identify its correlation with geomorphic units comprising the aquifer formations on this transboundary aquifer, we have found that, since there are several potential sources of naturally formed Arsenic on the area, its geospatial distribution could be linked to the geologic parent materials of sedimentary deposits which comprise the stratigraphic presence of its lithologic arrangements controlled by the identified geomorphic units. Most wells that exceeded the allowable limits of Arsenic concentrations where located at the mapped floodplain of the Rio Bravo, where sedimentary packages conforming the semi-confined aquifer formations are predominantly comprised of clay and fine grain materials, in contrast with the original hypothesis that estimated high concentration of Arsenic where cones of depression where present due to overpumping of aquifer, the regions where cones of depression where present had no wells exceeding the allowed limits of Arsenic concentration. On the Mexican side of the area of study, the lack of a proper understanding of the sedimentary packages of aquifer formations is one future work that can help on a better understanding of small-scale trend of Arsenic presence on ground water resources at this transboundary aquifer formation. It is also clear that, the assets that Geographic Information Systems aids provided to the project a more complete overview, allowing research team members a geospatial evaluation creating databases from the areas under investigation where the highest risks where mapped and allowing team members to concentrate on the sampling processes taken as part of the field work. Proper action is recommended to prevent risks to the community by revisiting the wells that produced positive results on small-scale Arsenic trends on ground water resources, as well as other sites that where not sampled on this project, by getting out of production these wells tested positive, until further studies are done.



REFERENCES Del Hierro Ochoa, J.C., 2004. Comportamiento Hidrogeoquímico del Bolsón del Hueco, en el Área Urbana de Ciudad Juárez, Chihuahua, México. 1965-1999. Unpublish M.S. Thesis. Maestria en Ingenieria Ambiental y Ecosistemas. Departamento de Ingenieria Civil y Ambiental. Universidad Autonoma de Ciudad Juárez. Cd. Juárez, Chihuahua. Diciembre, 2004. ppg 106. Eastoe, Christopher, Granados-Olivas, Alfredo, Hogan, James, Bangs, Eric, 2003. Isotopes (O, H, Tritium) as indicators of water movement near the international boundary in the El Paso (Texas)-Ciudad Juárez (Chihuahua) metropolitan área. Seattle 2003 GSA Annual Meeting & Exposition. Geoscience Horizons, Seattle, Washington, 2003. Geological Society of America Hibbs, Barry, Bill Hutchison, Chris Eastoe, Mercedes Merino, Alfredo Granados, 2004. Isotopic and Numerical Analysis of Transboundary Groundwater Flow – El Paso/Juárez Area. XXXIII IAH & 7º ALHSUD Congress "Groundwater Flow Understanding: from local to regional scales", Zacatecas between October 11- 15, 2004. International Association of Hydrogeology. ISBN:970-32-1749-4 Hibbs, B., Phillips, F., Hogan, J., Eastoe, C., Hawley, J., Granados, A., and Hutchison, B., 2003, Hydrogeologic and Isotopic Study of the Groundwater Resources of the Hueco Bolson Aquifer El Paso/Juárez Area: Hydrological Science and Technology, v. 19, no. 1-4, p. 109-119. IMIP, 2003. Plan Sectorial de Agua Pluvial. Instituto Municipal de Investigaciones y Planeacion;Comision Nacional del Agua (CNA); Gobierno del Estado de Chihuahua and Municipio de Ciudad Juárez. Ciudad Juárez, Chih., Mayo, 2002. IMIP, 2002. Plan de Desarrollo Urbano de Ciudad Juárez. Instituto Municipal de Investigaciones y Planeacion; SEDESOL and Gobierno del Estado de Chihuahua. Ciudad Juárez, Chih., Julio, 2002. Ochoa Cunningham, Francisco, 2004. Personal communication to discuss the design and survey to build this hydraulic infrastructure which he was responsible for the supervision and construction of these diques. Rascon, Ezequiel, 2004. Internal publication from JMAS presented at the IBWC-CILA meeting on Hueco Bolson Transboundary Studies hosted by UNESCO in El Paso, Tx., 2004



Figure 1. The Paso del Norte Región



Figure 2. Historic trend of ground water pumping at the Hueco Bolsón (m3/year) Source: Rascon, et al., 2004

0

25000000

50000000

75000000

100000000

125000000

150000000

175000000

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Año

Volu

men

ext

raid

o

(m3/

año)

El Paso y Fuerte BlissCiudad Juárez



Figure 3. Well fields located at the urban area of Ciudad Juárez



Figure 4. Historical trend of TDS concentration from different wells in Ciudad Juárez Source: Hibbs, et al., 2003

Well JMAS ID 7R

400

600

800

1000

1200

1970 1975 1980 1985 1990 1995 1999

TDS (mg/l)

Well JMAS ID 23

400600800

100012001400

1970 1975 1980 1985 1990 1995 1999

TDS (mg/l)

Well ID JMAS 37

0

500

1000

1500

1975 1980 1985 1990 1995 1999

TDS (mg/l)



Praxedis G. Guerrero(San Ignacio)

Guadalupe D. B.

Barreales

Dr. Por firio Parra(Caseta)

Tres Jacales(El Millón)

Tres Jacales

Jesus Carranza

San Agustin

San Isidro

Loma Blanca

Socor ro

Clint

Fabens

El Alamo

Los Pat itos

El Nido

GLORIA A DI OS

DESIERTO

Samalayuca

350000 360000 370000 380000 390000 400000

3460

000

3470

000

3480

000

3490

000

3500

000

3510

000

3520

000

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000

3540

000



Figure 5. Water table depletion in meters from the period of 1902 till 2000 Source: Rascon, 2004

Figure 6. Conceptual model of well fields and aquifer formations with flow direction and lithology. Source: Rascón, 2004.

DIAGRAMA CONCEPTUAL DEL FUNCIONAMIENTO HIDRAULICO

ALUVION RIO GRANDE

DEPOSITOS DE BOLSON

EVAPOTRANSPIRACION

FLUJOS SUBTERRANEOSDE SALIDA

FLUJOS SUBTERRANEOSDE ENTRADA

BOMBEO

RETORNO DE RIEGO

RECARGA POR EL RIO BRAVO



Figure 7. Sampling at well fields



Figure 8. Geospatial distribution of Arsenic on the urban area of Ciudad Juárez (ppb)



Figure 9. Geomorphic units and geospatial distribution of Arsenic (ppb)

Figure 10. Stiff Diagram of well JMAS-134



Figure 11. Geospatial location of wells exceeding Arsenic concentration



Figure 12. 3D cross section of aquifer formations at Hueco Bolson

ANNEX 1 Stiff Diagrams



Annex 2 Sampling Results

Arsenic water analysis from SWAT and Assaigai Laboratories

Well_num Arsenic (ppb) pH EC_rel

Temp ºC Flow L/s Salinity TDS

JMAS 145 23.30 8.02 1368 24.2 NA 700

JMAS 130 22.20 8.48 969 23.8 58 NA

JMAS 134 18.10 7.94 1492 25 NA 800

JMAS 121 17.4 8.06 1055.00 21.60 52.00 500


JMAS 47 R 15.60 8.19 1062 23.8 NA 500


JMAS 163 14.20 7.92 1330 22.7 48 700

JMAS 19 R 14.00 7.81 1615 22.5 45 NA

JMAS 8 Za 13.20 8.19 827 23.6 47 400

JMAS R1 11.70 NA NA NA NA NA

JMAS 11R 11.4 8.18 1031.00 22.30 48.00 500

JMAS 198 11.30 8.01 1410 27.3 49 700

JMAS 115 R 10.50 8.42 889 23.3 52 NA

JMAS 117 10.50 7.89 1280 23.4 45 700

JMAS 17R 10.40 7.86 992 23.9 NA NA



JMAS 110 10.30 8.19 774 24.6 NA 400

MXP004 9.80 7.79 1093 24.7 NA NA

JMAS 193 9.30 8.13 673 NA 52 NA

JMAS 191 8.9 8.48 609.00 25.60 52.00 300

JMAS 187 8.70 8.22 574 27.2 51 300

JMAS 205 8.60 8 732 26.2 NA 300

JMAS Z4 8.60 8.31 1085 NA 44 NA

JMAS 160 8 8.30 672.00 24.60 38.00 300

JMAS 219 8.00 7.99 866 24.9 50 400

JMAS 160T 7.8 8.30 672.00 24.60 38.00 300

JMAS 183 7.50 8.1 512 27.3 31 200

JMAS 202 7.50 8.15 577 26.8 67 300

JMAS 171 7.4 8.26 541.00 23.50 60.00 300

JMAS 222 7.20 8.05 622 26.1 38 300

JMAS 149 7 8.36 314.90 24.20 55.00 200

JMAS 188 7.00 8.07 524 27 53 200

JMAS 67 R 7.00 7.97 1399 23.3 NA 700

JMAS 150 6.9 8.20 708.00 22.30 51.00 400

JMAS 95 6.8 8.13 537.00 24.30 50.00 300

JMAS 133 6.60 7.99 1032 22.8 NA 500

JMAS 113 6.5 7.79 1.43 20.70 40.00 700

JMAS 176 6.40 7.92 1029 26.4 55 500

JMAS 136 R 6.10 8.17 513 26.2 43 200

JMAS 161 6.10 8.02 982 NA 26.5 600

JMAS 5Z 6.00 8.02 693 24.6 53 300

JMAS 138 5.90 8.07 530 26.8 41 200

JMAS 42 R 5.90 7.6 1742 23.3 NA 900

JMAS 182 5.8 8.34 742.00 21.50 55.00 400

JMAS 99R 5.5 8.53 552.00 23.90 50.00 NA



JMAS 88 5.30 8.06 811 23.7 40 400

JMAS 94 R 5.20 7.99 813 25.6 39 400

JMAS 98R 5 8.07 676.00 22.80 34.00 300

JMAS 48 4.90 7.62 1785 21.9 60 NA

MXP003 4.90 7.8 1153 23.3 NA NA

JMAS 72R 4.4 8.11 573.00 22.30 50.00 300

JMAS 119 4 7.71 1424.00 15.10 53.00 700

JMAS 9R 3.70 7.72 1765 21.9 NA 1000

JMAS 58 3.5 8.12 1015.00 21.10 84.00 500

JMAS 63 R 3.30 8.06 771 23.8 NA 400

JMAS 49 3.20 7.86 764 21.9 NA 400

JMAS 23 3.10 7.58 1763 22.5 NA 900

JMAS 5 B 3.10 7.72 1115 21.8 NA 600

JMAS 84 3.1 8.13 705.00 21.10 59.00 400

JMAS 62 2.80 7.66 1604 20.5 NA NA

JMAS 33 2.2 7.93 1127.00 19.70 59.00 600

JMAS 12 1.90 7.63 1883 21.9 NA 1000

JMAS 15 R 1.90 7.67 1188 21.2 NA 600

JMAS 13RR 1.7 7.66 2412.00 21.10 34.00 1200

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