identifying sources of groundwater in the lower colorado ...arizona, guay (2001) applied...

Identifying sources of groundwater in the lower Colorado River valley,USA, with d18O, dD, and 3H: implications for river water accounting

Bradley E. Guay · Christopher J. Eastoe · R. Bassett ·Austin Long

Abstract Isotope measurements (d18O, dD, 3H) indicategroundwater origin in the Lower Colorado River Valley(LCRV) and provide an alternative, or supplement, to theUS Bureau of Reclamation’s proposed “accounting sur-face” method. The accounting surface method uses a hy-draulic criterion to identify certain wells away from theflood plain that will eventually yield mainstream Col-orado River water. New isotope data for 5 surface-waterand 18 groundwater sites around Topock Marsh, Arizona,are compared with river-water data (1974–2002) from 11sites between Utah and Mexico and with groundwaterdata from previous LCRV studies. Three groundwatersources are repeatedly identified in the LCRV: (1) localrecharge derived from precipitation, usually winter rain,plots slightly below the global meteoric water line(GMWL) and has dD values that are 20‰ greater thanthose of recent river water; (2) “older” (pre-1950) upperbasin river-water plots on or near the GMWL, distinctfrom local rainfall and recent river water; and (3) recent(post-1950) Colorado River water, including TopockMarsh samples, plots below the GMWL along an evap-oration trend. Large floods, as in 1983, complicate in-terpretation by routing less evaporated upper basin water

into the LCRV; however, tritium content can indicate theage of a water. River-water tritium has declined steadilyfrom its peak of 716 TU in 1967 to about 11 TU in 2002.Mixtures of all three groundwater sources are common.

R�sum� Les mesures isotopiques (d18O, dD, 3H) in-diquent les origine de l’eaux souterraines dans la Vall�ede la Rivi�re du Bas Colorado (LCRV) et sont unealternative, ou un suppl�ment, � la m�thode des bilanshydrologiques propos�e par du «US Bureau of Reclama-tion». Cette m�thode de bilan hydrologique utilise uncrit�re hydraulique permettant d’identifier certains puitshors de la plaine d’inondation qui pomperaient une partnon n�gligeable de leur eau dans la rivi�re Colorado. Denouvelles donn�es isotopiques provenant de 5 sites d’eaude surface et 18 d’eaux souterraines autour de TopockMarsh en Arizona, sont compar�es avec les donn�es(1974–2000) de 11 sites localis�s entre Utah et Mexico,ainsi que des donn�es d’autres �tudes sur la LCRV. Cessources d’eaux souterraines sont identifi�es � plusieursreprises dans la LCRV: (1) la recharge locale d�rivantdes pr�cipitations, g�n�ralement les pluies hivernales, seretrouvent l�g�rement sous la ligne d’eau m�t�oritiqueglobale (GMWL) et poss�de des valeurs de dD 20% su-p�rieures aux valeurs des eaux r�centes de la rivi�re; (2)les eaux vieilles (pre-1950) du bassin sup�rieur de la ri-vi�re poss�dent une valeurs tr�s proches de la GMWL,distinctes des valeurs de la pluie locale et des eaux r�-centes de la rivi�re; et (3) les eaux r�centes (post-1950) dela Rivi�re Colorado, incluant les �chantillons de TopockMarsh, se positionnent � c�t� de la GMWL sur une droited’�vaporation. Les grandes inondations, par exemple cellede 1983, compliquent l’interpr�tation en reprenant dans laLCRV moins d’eaux marqu�es comme �vapor�es etprovenant du bassin sup�rieur; par ailleurs le pic de tri-tium est descendu de 716 TU en 1967 � 11 TU en 2002.Les m�langes de ces trois sources sont assez fr�quentes.

Resumen Mediciones isot�picas (d18O, dD, 3H) indicancual es el origen del agua subterr�nea en el Valle Bajo delR�o Colorado (LCRV) y aportan una alternativa, o com-plemento, para el m�todo “superficie de conteo” pro-puesto por el Bur� de Reclamaci�n de Estados Unidos. Elm�todo superficie de conteo utiliza un criterio hidr�ulicopara identificar ciertos pozos alejados de la planicie deinundaci�n que eventualmente producir�n agua a partir de

Received: 29 November 2002 / Accepted: 21 February 2004Published online: 7 May 2004

� Springer-Verlag 2004

B. E. Guay ())US Army Corps of Engineers,Buffalo District,1776 Niagara Street, Buffalo, NY 14207-3199, USAe-mail: [email protected]

C. J. EastoeDepartment of Geosciences,University of Arizona,Tucson, AZ 85721, USA

R. BassettGeochemical Technologies Corp.,3760 Vance St., Suite 200, Wheat Ridge, CO 80033, USA

A. LongDepartment of Geosciences and Departmentof Hydrology and Water Resources,University of Arizona,Tucson, AZ 85721, USA

Hydrogeology Journal (2006) 14:146–158 DOI 10.1007/s10040-004-0334-4

la corriente principal del R�o Colorado. Los nuevos datosisot�picos para 18 sitios de agua subterr�nea y 5 sitios deagua superficial cerca de los Pantanos Topock, Arizona,se comparan con datos de agua de r�o (1974–2002) pro-venientes de 11 sitios localizados entre Utah y M�xico, ycon datos de aguas subterr�neas de estudios previos rea-lizados en el LCRV. Se identifican reiteradamente tresfuentes de aguas subterr�neas en el LCRV: (1) recargalocal derivada de precipitaci�n, generalmente lluvia deinvierno, cuya composici�n cae ligeramente por debajo dela l�nea de agua mete�rica global (GMWL) y tiene valoresdD que son 20‰ mayores que los reportados para aguade r�o reciente; (2) el agua de r�o “m�s vieja” (pre-1950)de la cuenca alta cuya composici�n cae sobre o cerca dela GMWL, diferente de la lluvia local y del agua de r�oreciente; (3) agua reciente (post-1950) del R�o Colorado,incluyendo muestras de los Pantanos Topock, con com-posici�n por debajo de la GMWL a lo largo de una ten-dencia a la evaporaci�n. Inundaciones grandes, como en1983, complican la interpretaci�n al transmitir menosagua evaporada de la cuenca alta hacia el LCRV; sinembargo, el contenido de tritio puede indicar la edad delagua. El contenido de tritio en agua de r�o ha disminuidoconstantemente desde la concentraci�n pico de 716 TU en1967 a cerca de 11 TU en 2002. Es comffln que existamezclas de las tres fuentes de agua subterr�nea.

Keywords d18O, dD, 3H · Groundwater · LowerColorado River · Stable isotopes · Water accounting

Introduction

Stable hydrogen and oxygen isotopes and tritium areuseful for establishing the origin of groundwater in areaswhere waters of different origin, age, and evolution arepresent. This is the setting in the Lower Colorado RiverBasin (LCRB) of the United States, where snow-meltfrom regions of higher altitude and latitude produce riverwater that eventually mixes with tributary water, orgroundwater derived from local rainfall (Fig. 1). Distin-guishing water origin is increasingly important for man-aging this over-allocated river system, and the applicationof environmental isotopes is recognized as a promisingtool for dealing with legal and water accounting disputes.

One such dispute involves the accounting of ColoradoRiver water in the LCRB, as required by court decree(Arizona vs. California; US Supreme Court 1964). TheSecretary of the Interior (Secretary) and its agency, theUS Bureau of Reclamation (Reclamation), must deter-mine and report the diversions, return flows, and con-sumptive use of water from the mainstream. Consumptiveuse is calculated as the difference of water diversionand return flow. Article I of the decree also defines con-sumptive use to include, “water drawn from the main-stream by underground pumping.” Reclamation has pre-sumed that wells located on the flood plain and certainother wells on the surrounding alluvial terraces yield riverwater (Wilson and Owen-Joyce 1994). The authors stated

“no method was available for identifying wells [outsidethe flood plain] that yield water that will be replaced bywater from the river and wells that yield water that will bereplaced by water from precipitation or inflow from ad-jacent tributary valleys.” There are several thousand ofthese wells in Arizona and California, yet their with-drawal is expected to be a small portion of overall con-sumptive use (Jeff Addiego, USBR, Water AccountingTeam, personal communication 2003).

In 1994, and with technical assistance from the USGeological Survey (USGS), Reclamation proposed an“accounting surface” method to address the wells outsidethe flood plain. The method relies on a hydraulic criterionthat required the delineation of the river aquifer (thesubsurface strata presumed hydraulically connected to themainstream) and the accounting surface (the unconfinedstatic water table within the river aquifer). Wells that havea static water-level elevation equal to or below the ac-counting surface are presumed to yield water that will bereplaced by water from the river (Wilson and Owen-Joyce1994). The river aquifer boundary and accounting surface

Fig. 1 Map of Colorado River watershed showing the LowerColorado River Basin (shaded) and study area, Topock Marsh,Arizona. Circled numbers are approximate sampling sites for riverwater (Table 1)

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elevation is depicted in a series of 19 plates that covernearly 500 km from Utah to Mexico. Because no oneis entitled to use Colorado River water without a con-tract with the Secretary (US Congress 1948), many wellowners will, for the first time, need to purchase a contractwhile they are still available.

While the proposed changes to river-water accountinghave several operational advantages for Reclamation, oneforeseeable criticism is that the method does not providedirect evidence that a well yields mainstream water. Manywell owners are concerned by the approach, and welcomean alternative method that directly confirms the with-drawal of mainstream water or better yet, quantifies thepercentage of mainstream water in a mixed-source well.

In a recent hydrologic investigation of Topock Marsh,Arizona, Guay (2001) applied environmental isotopes(18O, 2H or deuterium or D, and 3H or tritium) to inves-tigate the sources of groundwater surrounding an im-pounded wetland located on the flood plain of the LowerColorado River (Figs. 1 and 2). Not surprisingly, recentriver water was the primary source to wells on the floodplain, but wells outside the flood plain showed differentisotopic compositions. A review of previous isotopestudies along the Lower Colorado River showed a re-curring data pattern that suggested their use as a primaryindication of water source.

In this paper, new environmental isotope data (d18O,dD, and 3H) for waters in the southern Mohave Valley andthe Colorado River are compared with earlier findings. It

is shown that d18O–dD data, in some cases supported bytritium and geohydrologic data, provide a direct indica-tion of a water’s source. Groundwater is identified asoriginating from local precipitation, “older” (pre-1950,probably pre-dam) river water, and recent (post-1950)river water, with the latter approximately the legal equiv-alent of mainstream water. The objective of this paper isto interpret the recurring pattern of environmental isotopedata and establish their use for identifying groundwatersources in the LCRV. If supported, the approach providesan alternative, or supplement, to the accounting surfacemethod now being considered for river-water accounting.

Site Description

The Colorado River falls some 3,700 m from the RockyMountains of Wyoming and Colorado to the deserts ofCalifornia and Arizona, flowing nearly 2,400 km throughparts of seven US states (Fig. 1), before emptying intothe Gulf of California in Mexico (Nathanson 1978). Acanyon section near Lee’s Ferry, Arizona, has provided aconvenient division between the Upper and Lower Col-orado River Basins. Below Hoover Dam, the river passesthrough a series of constricted bedrock canyons and widealluvial flood plains. This paper concerns the LowerColorado River Valley (LCRV), which refers to the river-corridor area between Hoover and the Mexico border.

The climate is arid along the LCRV; for instance, thecities of Needles, California and Yuma, Arizona receiveabout 119 mm (4.7”) and 74 mm (2.9”) rainfall per year,respectively (WRCC 1997). The precipitation occurs asrainfall in nearly equal amounts during the summer(convective) and winter (cyclonic) seasons (Hely and Peck1964; Pyke 1972). Snowfall is rare within the LCRV, butit occurs at upper elevations north of Davis Dam.

Historically, the natural flows in the Colorado Rivervaried tremendously, with peak flows occurring in Aprilto June (USBR 1996). This late-spring snowmelt fromUpper Basin tributaries is the primary source (~70%) ofriver water entering the LCRV (Bruce Williams, USBRengineer, personal communication 1999). Hoover Damand several subsequent river engineering projects since1935 have regulated river flow. Hydropower operationsgenerally store spring runoff and increase outflows duringthe summer season. During some anomalous weatherevents, such as the strong El Nio-Southern Oscillationevent in 1993 and hurricane Nora in the fall of 1997,heavy precipitation in the LCRB can influence river flowsfor days or even weeks. In 1993, for example, the BillWilliams River discharge into the Colorado River in-creased from <0.3 to 184 m3/s in only a few days (USGS2000). Overall, river flows decrease southward fromHoover Dam to Mexico because of consumptive uses ofriver water (Owen-Joyce and Raymond 1996).

Mohave Valley extends 64 km between Davis Damand Topock Gorge (Fig. 2). Before dams, the river me-andered and annually flooded backwaters areas, such asthe former oxbow of Topock Marsh (Guay 2001). The

Fig. 2 Map of Mohave Valley and Topock Marsh sampling sites(Tables 2 and 3). The river is the border between states. Geohy-drologic units are summarized from Metzger and Loeltz (1973):Younger Alluvium (includes flood plain), Older Alluviums (ter-races), and bedrock. Two inlet canals gravity feed Colorado Riverwater into the marsh. The marsh is impounded but flow issoutherly. Topock Marsh (SN 12–15) sites are surface-water, allother sites are groundwater

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pre-dam range in river stage at Needles, California wasnearly 5 m, although in a few years it was about 8 m(Metzger and Loeltz 1973). The decline in river flowscaused by dams eventually required the river be chan-nelized and diked in the 1950s, with most of the modernflood plain on the Arizona side of the River (Fig. 2).Today, nearly one-quarter of the entire flood plain, mostlynorth of Topock Marsh, is agricultural land irrigated withColorado River water.

The US Fish and Wildlife Service (USFWS) have man-aged Topock Marsh since 1941 as a refuge and breedingground for migratory birds and other wildlife (USFWS1994). The former riverine backwater was converted into a13-km-long impounded wetland in the mid-1960s with theconstruction of an inlet canal and south dike (Guay 2001).Today there are two unlined inlet canals that feed Col-orado River water directly into the northern half of themarsh. Water not lost to evaporation or seepage from themarsh reenters the river through an outlet structure on thesouth dike. Figure 2 shows the majority of sampling sitesare on the flood plain, with a few sites located on thehigher elevation older alluvial deposits.

The hydrology of the LCRV is dominated by the Col-orado River; in fact, it provides 96% of the annual watersupply and virtually all the recharge to the river aquifer(Wilson and Owen-Joyce 1994). Water budget estimates(Owen-Joyce and Raymond 1996; Metzger and Loeltz1973; USDOI 2000; Guay 2001) and groundwater contourdata (Metzger and Loeltz 1973) in Mohave Valley suggestthe river loses water to the groundwater reservoir. Forexample, during a modulated river flow period (1950–1966), the river annually lost about 2% of its flow in thevalley (Metzger and Loeltz 1973). Quantification of trib-utary inflow is recognized to be a difficult problem(Owen-Joyce and Raymond 1996), however, the averagecombined surface-water and groundwater inflow to theriver from Mohave Valley tributaries and adjacent basinsis estimated to be 3.4�107 m3 (Owen-Joyce 1987), orabout 0.3% of the typical river flow entering the valley(Metzger and Loeltz 1973). Together, these data suggestthe discharge from the flood plain reservoir is almost2.3% (river loss plus tributary inflows) of the mainstreamflow, which is caused mainly by natural phreatophyte andcrop evapotranspiration (ET), and open-water evaporation(Owen-Joyce and Raymond 1996; USDOI 2000). Mostflood plain vegetation can easily reach the water table,which is generally less than 3 m below the ground surface(Metzger and Loeltz 1973; Guay 2001). Flood plain sed-iments are very permeable, commonly with high trans-missivity values (e.g., 10,788 m2/day; Metzger and Loeltz1973). River stage varies several feet during daily andseasonal cycles and has been shown to have a direct effecton the water-table elevation (Guay 2001). In summary,Mohave Valley is a losing river reach where ColoradoRiver water is later lost to ET and open water evaporation.

Previous Work

Owen-Joyce and Raymond (1996) and Wilson and Owen-Joyce (1994) have reviewed published reports and papersdescribing the geology, groundwater resources, waterquality, and water-accounting methods along the lowerColorado River. Metzger and Loeltz (1973) performed adetailed geohydrology study of the Needles area. Theyused a comparison of chemical constituents in river andwell water to support their conclusion that the ColoradoRiver was the dominant source of groundwater rechargein the basin. Several wells, however, appeared to yieldmixtures of river water and “local recharge.” The authorsacknowledged the limitations of major-ion chemistry foridentifying water source and provided two exampleswhere unrelated waters (river and non-river) had nearlyidentical solute concentrations. The primary approach thatestablishes Colorado River water as the dominant sourceof recharge in the LCRV is a water balance (Metzger andLoeltz 1973; Owen-Joyce and Raymond 1996; US De-partment of the Interior 2000). These results and relatedstudies (Owen-Joyce 1987; Wilson and Owen-Joyce1994) suggest that the sum of hydrologic componentsargues against significant groundwater discharge to theriver, inferring that recharge to the alluvium by localprecipitation must be negligible in the LCRV.

Isotope data (d18O, dD, and in some cases 3H) wereused to identify water sources and soil water processes inthe LCRV (Robertson 1991), in Mexicali Valley (Payneet al. 1979), and in upper Mohave Valley (Wyman 1997).Collectively, these studies identified groundwater thatoriginated from local precipitation, from recent (i.e., post-dam) Colorado River water, and from older (pre-dam)Colorado River water. In contrast, Payne et al. (1979)identified local groundwater whose source was the GilaRiver. Payne and Robertson relied on hydrologic andgeochemical data to support their findings. The benefitsof environmental tracers (e.g., 3H) over Darcy’s Law andwater balance approaches for determining water move-ment in desert soils of the American Southwest have beendiscussed by Phillips (1994).

Isotope data for precipitation in the LCRV were pre-sented by Friedman et al. (1992). They found that theaverage dD values for summer and winter rainfall be-tween 1982 and 1989 in Needles were –47 and –71‰,respectively, and the difference decreases southwardalong the river, where in Yuma the summer and winterrain are equivalent (–55‰). Smith et al. (1992) andGleason et al. (1994) found that the majority of deeperwells and perennial springs in southeastern California hadwaters that were more depleted in deuterium (lower dD)than the lightest winter precipitation reported by Fried-man et al. (1992). They postulated that the isotopicallylight water dated from an earlier period, possibly the latePleistocene, with a different climatic regime and lowerdD in precipitation. However, Davisson et al. (1999)disagreed, and using dD, d18O and 14C data, they sug-gested that isotopically light water from higher latitudes is

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flowing as groundwater into southeastern Nevada alongelongate north–south grabens.

Data and Analytical Methods

Table 1 provides sampling and isotope data for ColoradoRiver water only (1974–2002), listed geographically fromnorth to south. Tables 2 and 3 provide similar data forTopock Marsh and other southern Mohave Valley sites(1996–1998). The site number (SN) identifies samplelocations on Figs. 1 and 2.

All stable isotope data are reported in notation, whered ¼ R

Rstd� 1� �

� 1000o=oo; and R represents either 18O/16Oor D/H ratio of the sample. Rstd is the isotope ratio ofVSMOW (Vienna standard mean ocean water) or SMOWin the case of older measurements. The data are fromseveral laboratories and were measured using differenttechniques, but are comparable. For this study d18O, dD,and tritium values were measured at the University ofArizona’s Geoscience Laboratory of Isotope Geochemis-try. d18O and dD were determined on a Finnigan Delta-Smass spectrometer with automated CO2 equilibration andCr reduction attachments. Analytical precisions (1s) forthese techniques are 0.08‰ and 0.8‰ for d18O and dD,respectively. Some early dD values were measured usingthe Zn reduction technique with an analytical precision(1s) of 1.8‰. Data from other laboratories, where re-ported, have analytical precisions (1s) of 0.05 to 0.1‰ ford18O and 0.4 to 0.5‰ for dD (Payne et al. 1979; Friedmanet al. 1992; Smith et al. 1992; Gleason et al. 1994). Forthis study, tritium values were measured by liquid scin-tillation counting on electrolytically enriched water in aQuantulus 1220 Spectrophotometer, with a detection limitof 0.7 TU for 8-fold enrichment and 1,500 min ofcounting. Available precisions for other tritium data ran-ged from €5 to €0.6 TU, or typically €4% of the reportedvalue.

Results

Colorado River Water Entering the LCRB (SN 1–3)River water entering the LCRB was sampled at the Col-orado–Utah state line, Cisco (Utah), and at Lee’s Ferry,Arizona (Fig. 1, Table 1). Figure 3 shows that the d18O–dD pairs of river water upstream of Glen Canyon Dam(SN 1,2) plot on or near the Global Meteoric Water Line(GMWL, see Craig 1961). The average d18O and dDvalue is –16.2 and –122‰, respectively. Note that thesedata were collected during the high-flow 1984–1987 pe-riod (USBR 2000) and may be depleted in 18O and deu-terium compared to water from other times.

Lower Colorado River Water (SN 3–11)Water below Glen Canyon Dam (Lee’s Ferry) divergesfrom the GMWL along an evaporation trend (Fig. 3).Figure 4 illustrates the temporal and spatial variability in

the dD values of river water. d18O values were not de-termined for many of these samples. The dD valuesare arbitrarily grouped by decade and plotted with re-spect to distance (km) downstream of Glen Canyon Dam.Most samples were taken downstream of Hoover Dam(575 km) in areas that experience extreme evaporation.Several data points in Fig. 4 are offset slightly for plotclarity.

Colorado River Tritium DataCompiled tritium data from several river locations areplotted against time in Fig. 5. Under normal conditions,the sample location should not affect a river tritium valuebecause the transit time of water released from HooverDam to Imperial Dam is estimated to be less than 10 days(Bruce Williams, USBR engineer, personal communi-cation 1999). Further, tritium is a conservative tracer;changes in tritium content due to evaporation are smallrelative to measurement errors. Tritium levels declinedexponentially until about 1990 before leveling off at about11 TU (1998–2002).

Topock Marsh and Surrounding ShallowGroundwater (SN 12–20)Figure 6a is a d18O–dD plot of surface-water samplesfrom Topock Marsh (SN 12–15). Each sample site rep-resents a quadrant of the marsh. Recent (1996–1998)Colorado River water sampled at nearby Needles (SN-7)is also plotted. The marsh is clearly subject to strongevaporation (~2 m/year; Guay 2001). A regression ofthese data defines the Topock Marsh Evaporation Line(TMEL), a reference used throughout this work.

Figure 6b shows d18O–dD data for shallow ground-water from the flood plain around Topock Marsh. Rep-resented are four shallow monitoring wells (SN 16–19)and 3-Mile Lake (SN-20), which is an isolated seepagelake recharged by groundwater (i.e., river water). Acomparison of Fig. 6a, b shows the relationship of con-temporary river water, evaporated river water, and shal-low groundwater.

Southern Mohave Valley Groundwater (SN 21–35)Figure 7 shows the isotope composition of other ground-water samples from southern Mohave Valley in relation torecent Colorado River water (1996–1998). Also depict-ed are the approximate d18O–dD values for winter andsummer rain in Needles, California (Friedman et al. 1992)and two composite rainfall event samples.

SN-21 is a private residential well located 300 m eastof the marsh on an older alluvium terrace. The Refugeirrigation well (SN 22) is located 200 m east of the river,but is used infrequently. SN-23 is a residential well andSN-24 is a high-yield irrigation well, but both are situatedon the modern flood plain and within a few hundredmeters of the river or irrigated fields. All four samplesplot along the TMEL.

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Table 1 Colorado River isotope data and sample information

Sitenumber(SN)

Local identifier Sample date Data source River kilometerbelow GlenCanyon Dam

d18O ‰ dD ‰ Tritium unit (TU)

1 Colorado–UtahState line

12/04/84 USGSa - –16.5 –12301/24/85 “ - –16.0 –11903/20/85 “ - –16.6 –12504/03/85 “ - –16.3 –12507/10/85 “ - –17.1 –12511/05/85 “ - –16.4 –12203/25/86 “ - –16.3 –12105/13/86 “ - –16.4 –12307/15/86 “ - –16.7 –12208/19/86 “ - –16.0 –11810/29/86 “ - –16.1 –12112/16/86 “ - –16.5 –12202/25/87 “ - –16.3 –12104/21/87 “ - –16.5 –12306/23/87 “ - –16.7 –12308/25/87 “ - –15.5 –116

2 Cisco, UT 11/19/84 “ - –16.4 –12103/19/85 “ - –16.1 –11904/22/85 “ - –16.1 –11605/20/85 “ - –16.4 –12007/23/85 “ - –16.0 –11809/03/85 “ - –15.9 –11911/19/85 “ - –16.4 –12101/22/86 “ - –16.4 –12003/24/86 “ - –16.0 –12005/19/86 “ - –16.2 –12006/23/86 “ - –16.6 –12108/19/86 “ - –15.8 –11811/20/86 “ - –15.8 –11803/25/87 “ - –15.9 –11804/20/87 “ - –15.6 –11605/21/87 “ - –16.4 –12006/25/87 “ - –16.0 –11707/22/87 “ - –15.9 –11808/19/87 “ - –15.9 –118

3 Lee’s Ferry, AZ 11/07/84 “ 8 –14.7 –11301/04/85 “ 8 –14.9 –11704/12/85 Friedmanb 8 –12005/08/85 USGSa 8 –15.6 –11906/11/85 Friedmanb 8 –11807/03/85 USGSa 8 –15.1 –11509/03/85 “ 8 –15.2 –11510/20/85 Friedmanb 8 –11411/05/85 USGSa 8 –15.1 –11301/08/86 “ 8 –15.0 –11505/09/86 “ 8 –15.3 –11607/02/86 “ 8 –15.0 –11408/27/86 “ 8 –14.8 –114

4 Lake Mead (inflow) 10/04/80 (Robertson 1991) 483 –12.9 –10610/05/84 Friedmanb 483 –11503/19/85 “ 483 –11510/19/85 “ 483 –11503/22/96 Eastoec 483 –10.8 –93 13.71997 Wymand 483 –12.7 –102

5 Lake Mead(outflow)

10/04/84 Friedmanb 576 –10704/01/85 “ 576 –10907/02/85 “ 576 –107

6 Laughlin, NV 1991 Wymand 687 –12.6 –981991 “ 687 –12.7 –981992 “ 687 –12.4 –1011993 “ 687 –12.0 –981995 “ 687 –12.0 –991997 “ 687 –12.5 –99

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The three municipal supply wells of Needles (SN 25–27) are located along the western margin of the modernflood plain. The wells are cased to about 19 m butscreened to 39 m below the ground surface. Five samplesfrom these wells plot near or to the lower left of averagerecent Colorado River. Also annotated are the tritiumconcentrations measured in four samples, which rangefrom 0.9 to 28.4 TU.

Only two rain events were sampled during this study(SN-28). The isotope results “bracket” the average winterrainfall value. Individual precipitation events most likelyvary widely in d18O and dD, but mixing in groundwater

attenuates the variability. This phenomenon is well doc-umented in the Tucson basin (Kalin 1994).

Water from two springs (SN 29,30) and an artesianwell (SN-31) discharge several hundred feet above theriver and therefore cannot derive from river water.These, along with four other well samples from WarmSprings and Sacramento Wash drainage areas (SN 32–35), cluster in the domain of local winter precipitationand are isotopically distinct from mainstream ColoradoRiver water.

Table 1 (continued)

Sitenumber(SN)

Local identifier Sample date Data source River kilometerbelow GlenCanyon Dam

d18O ‰ dD ‰ Tritium unit (TU)

7 Needles, CA 07/08/94 Eastoec 730 –12.0 –93 16.409/01/94 “ 730 16.010/22/94 “ 730 –11.8 –99 16.606/19/98 (Guay 2001) 730 –13.1 –103 11.7

Needles, CA (southof)

08/05/96 “ 735 –12.3 –9601/22/97 “ 735 –12.2 –9805/15/97 “ 735 –12.6 –10009/25/97 “ 735 –12.8 –9912/30/97 “ 751 –12.6 –9804/15/98 “ 751 –13.0 –99

8 Parker Dam (out-flow)

10/03/80 USGSe 817 –12.5 –10404/28/93 Eastoec 817 14.107/09/94 “ 817 –11.6 –92 16.708/03/95 “ 817 –10.9 –90 14.104/01/98 “ 817 –12.8 –101 12.7

Parker, AZ 09/30/84 Friedmanb 840 –11010/05/84 “ 840 –113

9 Imperial Dam(inflow)

10/04/80 (Robertson,1991)

1,047 –12.5 –103 80

11/12/97 USGSe 1,047 –12.1 –10005/14/97 “ 1,047 –12.1 –9802/12/97 “ 1,047 –12.0 –9812/17/97 “ 1,047 –12.2 –10012/18/97 “ 1,047 –12.0 –9608/20/97 “ 1,047 –12.2 –9708/26/97 “ 1,047 –12.2 –10011/13/97 “ 1,047 –12.0 –99 331,441,716,613,

531,428,347,291, 250,211,181,151,132,114,97,83,79,69,58,48,39,34,28g

02/18/97 “ 1,047 –12.0 –9605/29/97 “ 1,047 –12.1 –10006/26/97 “ 1,047 –12.3 –98

10 Imperial Dam(outflow)

1965–1987 USBRf 1,04905/11/02 Eastoec 1,089 11.0

11 Mexicali Valley 12/16/74 (Payne et al.1979)

1,110 –12.2 –101 177

12/16/74 “ 1,163 –12.2 –99 17412/16/74 “ 1,165 –12.2 –9812/16/74 “ 1,167 –12.0 –9904/30/76 “ 1,110 –12.3 –9905/31/76 “ 1,110 –12.1 –99

a USGS, Tyler Coplen, personal contact (fax data), Reston, VA, +1-703-6485862, 3 June 1998. Database is USGS-NASQAN, samplingsites corresponding to in-house Site 841 (Cisco, UT), Site 206 (CO-UT State line), and Site 130 (Lee’s Ferry, AZ)b Irving Friedman, personal contact (fax data), USGS Lakewood, CO, +1-303-2367888, 10 Feb 1998c Christopher Eastoe, Dept. of Geosciences, Laboratory of Isotope Geochemistry, Univ. of Arizona, +1-520-6211638d Richard Wyman, Final report to Mohave County Water Authority, Wyman Engineering Consultants, Boulder City, NV, +1-702-2931098, 10 Nov 1997e USGS, Cheryl Parten, personal contact (fax data), USGS national water information system, Phoenix, AZ, +1-602-3793088, 1997–1998f USBR, James Setmire, personal contact email data (e-mail: [email protected]), USBR, CA, +1-909-6955310, 2 Sept1999. Fig. 29 in Setmire et al. (1993) plotted 1977–1988 datag Bold text includes Colorado River water tritium values for consecutive years (1965–1987)

152


Discussion

Earlier isotope data sets from the LCRV (Payne et al.1979; Robertson 1991; Wyman 1997) have characteristicssimilar to those presented in this study. One notable dif-ference is that the evaporation trend in earlier studiesplots below the TMEL. Nonetheless, all the studies haveidentified, where present, the same groundwater sourcesand their mixtures. As with this study, the source desig-nations have been based principally on the relative plot-ting positions of d18O–dD pairs, but were supported tovarying degrees by tritium, hydrologic, and geochemicaldata.

Colorado River Water Entering the LCRBThe d18O–dD values of river water upstream of LakePowell at present, and probably in the recent past, plotclose to the GMWL (Fig. 3). This plotting position is thesignature of upper basin snowmelt that has undergonelittle evaporation. It is significant because some ground-water samples in the LCRV plot near this position (seePayne et al. 1979; Wyman 1997). Local snowfall mightproduce similar values, but such events are rare. “Older”river water was probably recharged during spring floodsbefore the closure of Hoover Dam in 1936 (USBR 2000).Thus the residence time of some groundwater near the

flood plain is expected to be more than 70 years, which isalso confirmed by tritium data (see below).

Lower Colorado River WaterBelow Glen Canyon Dam, the d18O–dD values of riverwater increase to the right of the GMWL along anevaporation trend (see Figs. 3 and 6a). The slope of theline is typically between 5 and 6 (Robertson 1991; Set-mire et al. 1993). A statistical regression of availabled18O–dD pairs for the river (SN 3–11) and Topock Marsh(SN 12–15) yields slopes of 5.6 and 5.1, respectively.

Data in Fig. 4 suggest several trends despite obviousgaps. First, the two-decade range of dD at certain loca-tions, e.g., below Parker Dam (817–840 km) and aboveLake Mead (483 km), was about 25‰. One cause was themid-1980s high flow period (USBR 2000), where iso-topically depleted melt-water from the upper basin ap-parently filled the entire river system. Extreme localrunoff events, such as flooding of the Bill Williams River(1993, 1995) and Gila River (1984), could possibly haveincreased the range. The evaporative losses in LakesMead and Powell appear to cause the dD value to increaseby about 5 to 7‰ in each lake. Conversely, for periods ofless than a decade, the variability in dD at certain loca-tions (e.g., SN-6) has been quite small (<10‰). It isspeculation, but large variability appears to be caused by

Table 2 Surface-water and shallow groundwater isotope data and sample information. Italics are the estimated values

Sitenumber(SN)

Local identifier Sampledate

Datasource

Surface-water(SW), ground-water (GW)

d18O ‰ dD ‰ Welltype

Approx.surfaceelevation

Staticwaterelevation

(m m.s.l.) (m m.s.l.)

12 Topock Marsh(north dike)

08/05/96 (Guay2001)

SW –10.4 –87

05/15/97 “ “ –12.4 –9509/25/97 “ “ –12.5 –9712/30/97 “ “ –8.5 –7704/15/98 “ “ –12.7 –98

13 Topock Marsh(5-Mile NE)

05/15/97 “ “ –12.4 –97

14 Topock Marsh(Beal ditch)

09/25/97 “ “ –9.2 –8204/15/98 “ “ –9.4 –81

15 Topock Marsh (outlet) 08/01/96 “ “ –4.4 –5901/26/97 “ “ –2.5 –4105/15/97 “ “ –7.6 –7509/25/97 “ “ –6.9 –7212/30/97 “ “ –7.0 –7004/15/98 “ “ –7.4 –71

16 Monitoring Well 1 05/15/97 “ GW –8.5 –80 2” PVC 140 13812/30/97 “ “ –8.2 –7504/15/98 “ “ –8.3 –75

17 Monitoring Well 2 05/15/97 “ “ –10.8 –95 “ 140 13812/30/97 “ “ –10.9 –9104/15/98 “ “ –11.0 –97

18 Monitoring Well 3 05/15/97 “ “ –11.3 –95 “ 140 13912/30/97 “ “ –10.9 –9004/15/98 “ “ –10.9 –84

19 Monitoring Well 4 05/15/97 “ “ –12.7 –102 “ 140 13812/30/97 “ “ –12.4 –9804/15/98 “ “ –11.4 –94

20 3-Mile Lake 08/02/96 “ GW/SW –10.4 –8901/26/97 “ “ –11.8 –95

153


Fig. 3 Plot of average and original d18O–dD pairs (1984–1987) inriver water entering the Lower Colorado River Basin (data sourcesgiven in Table 1)

Fig. 4 Colorado River dD values (1974–1998) sorted by decadeand plotted by river-kilometer below Glen Canyon Dam. Note,some nearly identical values have been shifted slightly (<€0.5‰and €15 km) for better illustration (data sources given in Table 1).An approximate trend line is added for reference

Fig. 5 Tritium concentrations (1965–2002) in lower ColoradoRiver samples. Data sources and locations are given in Table 1

Tab

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154


infrequent high flow events, while river management orseasonal evaporation may control smaller variability.Nonetheless, Colorado River water is isotopically distinc-tive because of its evaporation trend.

Colorado River TritiumThe 2002 river-water sample indicates a river tritiumvalue near 11 TU (Table 1). The average annual tritium inTucson precipitation has been about 5 TU since 1992,indicating complete removal of bomb tritium from theatmosphere (Eastoe et al. 1997). The lagged decrease inColorado River tritium probably results from the retentionof bomb tritium in Lakes Powell and Mead, which storeabout 5 years of river discharge (USBR 1996).

More significantly, tritium behaves as a conservativetracer, meaning its movement is not slowed or decreasedin concentration by interaction with the solid phase and isnot produced in the soil (Phillips 1994). Thus, if the tri-tium content of groundwater is near 11 TU, it suggestsrecharge by recent river water. Conversely, if the tritium

concentration is negligible (<1 TU), the water pre-datesthe atmospheric detonation of thermo-nuclear devices,which is, before 1950. A few groundwater samples havetritium levels that exceed the levels in today’s river water(e.g., SN-27); these waters are probable mixtures thatcontain some bomb-induced tritium.

Topock Marsh and Surrounding ShallowGroundwaterThe Colorado River is essentially the only source of in-flow into Topock Marsh (Owen-Joyce 1987; Guay 2001).Figure 6a clearly shows that as marsh (i.e., river) water isevaporated, the samples plot along an evaporation line(TMEL) that passes through the field of recent riverwater. Values of d18O and dD increase in the direction offlow from the inlet (SN-12) to the outlet (SN-15) duringeach sampling period (e.g., 5/15/97).

The majority of groundwater samples from shallowmonitoring wells (SN 16–19) lie on or near the TMEL,suggesting an evaporated river-derived source (Fig. 6b).This is not surprising given their shallow depth and lo-cation between the river and marsh. Evaporation ofgroundwater occurs in low-lying seepage areas and as aresult of capillary rise in the fine-grained alluvium. Al-ternatively, these data may indicate recharge by evapo-rated marsh-water. Plant transpiration does not appear todeflect d18O–dD pairs off the TMEL (see Busch et al.1992). Guay (2001) noted significant water uptake byphreatophytes in the vicinity of SN-16 and SN-18.

Points lying below the TMEL cannot be derived fromcontemporary marsh or river water; these probably re-present the contribution of earlier river water with lowerinitial d18O and dD. The SN-18 sample (4/15/98) plots

Fig. 6 a Plot of d18O–dD pairs (1996–1998) in Topock Marsh andnearby Colorado River. Regression of evaporated marsh-watersamples (triangles) defines the Topock Marsh Evaporation Line(TMEL) with slope 5.1. b Plot of d18O–dD pairs (1996–1998) in theshallow groundwater near Topock Marsh (Table 2). SN 16–19 aremonitoring wells. SN-20 is a seepage lake (see text). Recent Col-orado River water (SN-7) added for reference

Fig. 7 Plot of d18O–dD pairs (1996–1998) in groundwater andrainfall events in southern Mohave Valley (Table 3). The averagevalue for recent Colorado River water (SN-7) and estimates ofwinter and summer rain in Needles (after Friedman et al. 1992) areplotted for reference. City of Needles wells (open triangle) and SN-29 are annotated with tritium concentrations (TU). Note SN-33 hasbeen offset from SN-35 for plotting purposes

155


above the TMEL because recent rain had entered the wellthrough a leaky well-cover. SN-20 samples from 3-MileLake confirm a recent river source that is isotopicallysimilar to the less evaporated inlet waters at TopockMarsh (SN-12).

Southern Mohave Valley GroundwaterMarsh and recent river water are the likely source water atSN-21 and SN-22, respectively. This result was unex-pected at SN-21 because the well owner estimated thewell bottom to be above the marsh elevation. SN-23 andSN-24 contained water that is comparatively depleted in18O and D. These samples are interpreted as river-watermixtures, probably containing a less evaporated riverwater, possibly recharged when the area was inundatedfor several weeks during the 1983 flood.

The City of Needles wells (SN 25–27) had isotopevalues that suggest variable source waters. SN-27 resem-bled recent river water in plotting position, but containedsome residual bomb tritium (28.4 TU), indicating a mix-ture with older, post-1960 river water. Both samples fromSN-26 were relatively depleted in 18O, again suggesting amixture with “older” river water. Perhaps more signifi-cant, the summer sample had a tritium level (18.3 TU) ator slightly above river water, but the winter sample indi-cated a pre-bomb (0.9 TU) river water source. The SN-25summer d18O–dD pair and tritium content (15.2 TU)typified mid-1990s river water, yet the winter plottingposition (d18O= –13.8‰, dD= –99‰) was again sugges-tive of a mixture. These sites were also inundated duringthe 1983 flood. The composition of Needles well-watermay be influenced seasonally by the river elevation.Overall, groundwater within the modern flood plain prob-ably has been overprinted by generations of river waterand complex mixtures have resulted.

Water from four well or spring sites (SN 29–32) clearlyoriginated from local precipitation. All sites are located�8 km from the modern flood plain, and, most impor-tantly, their water elevations are significantly (>100 m)above the adjacent river elevation (~ 139 m above m.s.l.).Further, the d18O–dD pairs cluster in the domain of winterprecipitation as estimated by Friedman et al. (1992). Inthe case of SN-30, the negligible tritium content (<0.6€0.3 TU) suggests a pre-bomb age. Other wells (SN 33–35)located nearer the river also appear to be recharged bylocal precipitation. The Topock Well (SN-34) stable iso-tope values (d18O= –9.9‰, dD= –74‰) were unexpectedin an active pumping well so close to (<200 m) the river,and its specific conductance (1,900 �S/cm) value wastypical of more saline groundwater from the flood plain.

By combining all the data and interpretations from theisotope studies in the LCRV, and using data of Smith et al.(1992) and Friedman et al. (1992) for modern precipitationand groundwater in southeastern California, a recurringdata pattern emerges. Figure 8 shows the data “fields” thatindicate the relative plotting position of d18O–dD valuesfor waters in the LCRV. This simple interpretive frame-work includes plausible and commonly observed water

types and reflects the relatively uncomplicated hydrologyof this and other desert river systems; that is, a river whosefeatures include a dominant upper basin source (snow-melt), few contributing rivers, mixing and evaporation inlarge reservoirs, and isotopically dissimilar local precipi-tation (usually rainfall).

Field 1. Local precipitationFigure 8 depicts the fields of winter (1W) and summer(1S) precipitation in a Mohave Valley location. The actualplotting positions will vary with latitude and elevation.Since both winter and summer rain undergoes significantevaporation, their fields are offset slightly from theGMWL. 1W is significant because winter rainfall is gen-erally the source of tributary recharge in the LCRV (Smithet al. 1992). Therefore, the isotopic make-up of watersfrom wells and springs away from influence of the Col-orado River should have d18O–dD values that cluster near1W. The age of tributary water can be readily indicated aspre- or post-bomb using the tritium content.

Field 2. Ancient local precipitation or “fossil water”Smith et al. (1992) noted that the majority of southeasternCalifornia groundwater was considerably depleted indeuterium relative to the lightest (winter) precipitation.They hypothesize that such water most likely originatedas late Pleistocene precipitation. Pleistocene- to Holo-cene-aged waters have been identified in other locationsin the Southwestern USA (Davisson and Criss 1993;Phillips 1994). Groundwater with apparent 14C ages ofPleistocene age in Sacramento Valley is depleted in 18Oby ~2‰ relative to the local mean precipitation values(Davisson and Criss 1995). In the LCRV, late Pleistoceneriver water was probably isotopically lighter than what ishere designated as pre-bomb (probably pre-dam) river

Fig. 8 Schematic d18O–dD plot showing the proposed fields ofdifferent groundwater types in Mohave Valley and other locationsalong the LCRV: Field 1 modern precipitation (1S summer rain,1W winter rain); field 2 ancient precipitation or fossil water; field 3“older” and upper basin river water; field 3b mixture of 3 and 4;field 4 recent river water

156


water, but we have no data confirming its presence inMohave Valley. Nonetheless, this field represents aplausible water type.

Field 3. Upper basin river water (older river water)Field 3 represents the relatively unevaporated upper basinriver water entering the LCRB. Before control structureswere emplaced (pre-1936), annual floods recharged theriver aquifer system with waters of this kind. Ground-water that plots in field 3 and have a negligible tritiumconcentration are interpreted as older river water, possiblypre-dam. Field 3b represents a commonly observed mix-ture (e.g., Payne et al. 1979).

Field 4. Recent Lower Colorado River waterField 4 represents evaporated recent river water from theLCRV. This field exhibits temporal and spatial variabil-ity. The evaporation trend has a slope of 5 to 6. As dis-cussed above, most of the ground- and surface-waters inMohave Valley, especially within the modern flood plainboundary, can be explained in terms of field 4 and insome cases field 3. Field 4 waters are expected to havetritium concentrations near 11 TU.

Conclusion

Stable hydrogen and oxygen isotopes and tritium can es-tablish the origin of groundwater in the Lower ColoradoRiver Valley (LCRV). Isotope data, especially whensupported by geochemical and hydrologic data, can iden-tify mainstream water and locally recharged (tributary)water. The unusual robustness of isotope data from theLCRV derives from the river basin’s physical geographyand modern hydraulic controls. The groundwater sourcesidentified in this study — locally recharged rainfall (trib-utary water), “older” Colorado River water, and recentriver water — concur with previous studies. Precise agedetermination of older river water and tributary water willrequire more complex analytical techniques. Mixtures ofall groundwater types are common, but criteria can beestablished that estimate the percentages of various watersources in a sample.

Wells outside the flood plain probably represent acomparatively small consumptive use of river water, butthe large number of well owners do represent a politicalforce that might oppose the accounting surface methodand leave a gap in river water accounting. Disagreementsover water accounting have led to protracted legal battlesbetween LCRB states. At a minimum, well owners willwant to determine whether their well actually withdrawsmainstream water. Without isotope data, the accountingsurface falls short because it can only demonstrate thephysical potential for water movement from the river to-wards the well. Conventional geochemical data are usefulbut rarely provide a direct indication of a water’s source.Isotope data can and do resolve water resource disputes. Inaddition, long–term monitoring of isotope values ingroundwater will signal basin-scale shifts in the boundary

between tributary– and river–derived groundwater. Suchinformation could be used to promote water conservationand resource planning in the rapidly growing river com-munities. The analytical costs and interpretive criterianeed further evaluation. For now, though, isotope dataappear to provide an alternative, or at the very least, asupplement to the hydraulic approach being proposed bythe Bureau of Reclamation.

References

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identifying sources of groundwater in the lower colorado ...arizona, guay (2001) applied...

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