naturally occurring cr(vi) in groundwaterbaholmen/docs/enve290w/national... · l1608_c03.fm page 90...

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1-5667-0608-4/01/$0.00+$1.50© 2004 by CRC Press LLC

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Naturally Occurring Cr(VI) in Groundwater

CONTENTS

3.1 Naturally Occurring Cr(VI) in Groundwater, Including the Presidio in San Francisco Case Study ...............................................90 3.1.1 Introduction......................................................................................903.1.2 Examples of Naturally Occurring Cr(VI)

in Groundwater ...............................................................................913.1.2.1 Arid Alluvial basins in the

Southwest United States..................................................923.1.2.2 Natural Brines ...................................................................923.1.2.3 Chromite Ore Bodies........................................................933.1.2.4 Serpentinite Ultramatic Terrains ....................................93

3.1.3 The Erin Brockovich Effect: Hollywood and the Scientific Process.......................................................................94

3.1.4 Presidio in San Francisco Case Study..........................................963.1.4.1 Executive Summary..........................................................963.1.4.2 Introduction .......................................................................99

3.1.4.2.1 Site Background............................................1003.1.4.2.2 Previous Investigations................................1003.1.4.2.3 Potential Sources of Cr(VI) .........................1003.1.4.2.4 Approach for Presidio Investigation .........1003.1.4.2.5 Objectives .......................................................102

3.1.4.3 Geology, Hydrogeology, and Geochemistry ..............1023.1.4.3.1 Regional Geology .........................................1023.1.4.3.2 Presidio Hydrogeology................................1043.1.4.3.3 Groundwater Geochemistry in

Upland Areas ................................................1043.1.4.3.4 Chromium Geochemistry............................104

3.1.4.4 Investigative Methods....................................................1063.1.4.4.1 Field Methods ...............................................1063.1.4.4.2 Bedrock and Groundwat

Analytical Methods ......................................1093.1.4.4.3 Analytical Laboratories................................ 112

3.1.4.5 Leaching Test Procedures .............................................. 1133.1.4.6 Analytical Results ........................................................... 114

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Chromium(VI) Handbook

3.1.4.6.1 Bedrock Results............................................. 1153.1.4.6.2 Groundwater Results ................................... 1163.1.4.6.3 Leaching Test Results................................... 1183.1.4.6.4 Discussion of Results ...................................1253.1.4.6.5 Conclusions ...................................................129

3.1.5 Acknowledgments ........................................................................1303.1.6 Bibliography...................................................................................130

3.2 Cr(VI) Concentrations in Drinking Water Wellsand the Effects of Chlorination ...............................................................1333.2.1 Introduction....................................................................................1343.2.2 Methods and Materials ................................................................1343.2.3 Results and Discussion.................................................................1343.2.4 Bibliography...................................................................................137

3.1 Naturally Occurring Cr(VI) in Groundwater, Including the Presidio in San Francisco Case Study

Martin G. Steinpress

3.1.1 Introduction

Hexavalent chromium [Cr(VI)] in groundwater has generally been assumedto be anthropogenic (manmade) contamination, since it is used in a numberof industrial applications, including electroplating, tanning, industrialwater cooling, paper pulp production, and petroleum refining (Chapter 1).Trivalent chromium [Cr(III)], the most common form of chromium in thenatural environment, is highly insoluble and relatively immobile. However,Cr(VI) minerals have been found in nature (Chapter 2), and the ability ofmanganese dioxide to oxidize Cr(III) to Cr(VI) is well known (Bartlett andJames, 1979; Eary and Rai, 1987; Fendorf and Zasoski, 1992). The numberof occurrences of Cr(VI) is growing, particularly with statewide Cr(VI)sampling program with lower detection limits mandated for water suppliersby the California Department of Health Services (DHS) because of increas-ing health concerns.

This chapter grew out of the hypothesis that widespread Cr(VI) in ground-water at the Presidio in San Francisco (Figure 3.1.1) was geogenic (naturallyoccurring) as opposed to anthropogenic. The results of the subsequent inves-tigation and Technical Memorandum (TM), “Hexavalent Chromium in Ser-pentinite Bedrock and Groundwater in Upland Areas” (MontgomeryWatson, 1999b; Steinpress, 1998), are summarized in the case study inSection 3.1.4. The study culminated in the acknowledgment by the CaliforniaDepartment of Toxic Substances Control (DTSC) that “it appears that Cr(VI)occurs naturally in serpentinite bedrock and in water from some monitoring



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wells, including background locations screened in that bedrock” (DTSC,2000). During the course of the investigation, other examples of naturallyoccurring Cr(VI) in groundwater were identified and are also reviewed inthis chapter.

The impacts of the movie “Erin Brockovich” in the year 2000 are alsoconsidered in this chapter. Naturally occurring Cr(VI) can have as muchimpact on drinking water supplies as anthropogenic contamination, butremediation of background concentration are generally not feasible and thereis not a responsible party to fund cleanup. This leaves water suppliers andthe public in a quandary as to how to address the issue.

3.1.2 Examples of Naturally Occurring Cr(VI) in Groundwater

Naturally occurring Cr(VI) in groundwater has been identified in the fol-lowing geologic environments to date:

• Arid alluvial basins in the Southwest U.S.• Chromite ore bodies• Saline brines in evaporate basins• Serpentinite ultramafic terrains

Investigations of Cr(VI) in groundwater should also consider high tensile,low alloy (HTLA) steel well screens and casing, which have been in generaluse for many years (type 304 stainless steel is 18% chromium) (Roscoe Moss,2003).

FIGURE 3.1.1

The Presidio in San Francisco, looking south across the Golden Gate Bridge.


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3.1.2.1 Arid Alluvial basins in the Southwest United States

Cr(VI) of natural origin has been found in the groundwater of numerousalluvial basins in Arizona and adjacent parts of California, New Mexico, andNevada. This region includes portions of the Mojave Desert and Basin andRange Province. The first documentation of naturally occurring Cr(VI) ingroundwater was in Paradise Valley, Maricopa County, Arizona, where a U.S.Geological Survey study revealed concentrations up to 220 mg/l. Cr(VI) con-centrations are greater than 50 mg/l over a 40-square-mile area (Robertson,1975; 1991). The basin consists of Tertiary and Quaternary alluvium, and thereis a direct correlation between Cr(VI) concentration and the particle size of thealluvium (the fine-grained portion of the aquifer has the highest concentra-tions). The Cr(VI) is apparently present throughout the vertical extent of theaquifer. Eh and pH measurements of the groundwater indicated alkaline andoxidizing conditions, with many pH measurements in the 8 to 9 range. A directrelationship was also found between the pH and Cr(VI) concentrations. Anal-yses of drill cuttings did not detect any chromate [Cr(VI)] minerals or ions ina test well, including the minor sulfate minerals that were present.

A subsequent broader study that included a total of 436 samples in Arizonaand adjacent parts of California, Nevada, and New Mexico indicated a meanconcentration of 10.3 mg/l, standard deviation of 30.7, and a range of 0 to300 mg/l (Robertson, 1991). Silicate hydrolysis of volcanic ash and tuffs in thefine-grained alluvial deposits combined with low CO

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content was found tobe responsible for the elevated pH and the oxidation of Cr(III) to Cr(VI). Theabsence of ferrous iron, organic matter, or mafic minerals in the aquifer allowsthe water to retain its oxidizing and alkaline character. Basins in which ground-water has a longer residence time have higher Cr(VI) concentrations (concen-trations are inversely correlated with recharge rates) (Robertson, 1991).

More recently, naturally occurring Cr(VI) has been detected in water suppliesin several alluvial basins as a result of greater scrutiny inspired by the film“Erin Brockovich.” In the remote Cadiz Valley in the Mojave Desert of south-eastern California at the location of a proposed water storage project betweenMetropolitan Water District of Southern California (MWD) and Cadiz, Inc.,concentrations of 15 to 26 ppb (parts per billion) are present in native ground-water (MWD and Bureau of Land Management, 2001). U.S. Geological Surveysampling of selected public supply, domestic, and observation wells underlyinguncontaminated areas of the western Mojave Desert detected total dissolvedCr from less than the 0.8 detection limit to 60 ppb, and almost all of the Cr wasCr(VI) (Ball, 2002). In the eastern San Fernando Valley of Southern California,Cr(VI) is present at background concentrations of up to 0.01 mg/l upgradientof known sources in the vicinity of the Burbank Airport of volatile organiccompounds (VOCs) and metals contamination (Nagel, 1999).

3.1.2.2 Natural Brines

Both surface and subsurface natural brines found in saline lakes in closedbasins in the western Unites States have been found to have high pH

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conditions (8.9 to 10.1) (Truesdall and Jones, 1969), which are favorable forCr(VI). Cr(VI) minerals are well documented in evaporite deposits derivedfrom brines in the Atacama Desert, Chile (Eriksen, 1983). Chromium con-centrations in the natural brines are reported to range from 0.01 to 1 mg/kg (milligram per kilogram). Although not specified, such chromium con-centrations at a high pH are most likely Cr(VI).

3.1.2.3 Chromite Ore Bodies

A combined field and laboratory study of chromite bearing oxidized serpen-tinite rocks in India indicated the possibility of chromium mobilization fromchromite ores to water bodies (Godgul and Sahu, 1995). The authorsobserved that serpentinization is an intensely oxidizing process that createsalkaline pore waters that would promote oxidation of Cr(III). The studysuggested that mining practices enhance the rate and intensity of chromiummobilization (Godgul and Sahu, 1995), although acidic conditions from minewastes would tend to reduce chromium. Organic matter and iron-rich lateritedeposits also tend to reduce any released Cr(VI).

3.1.2.4 Serpentinite Ultramatic Terrains

Cr(VI) has also been detected in groundwater associated with serpentineterrains consisting of ultramatic rocks that do not contain known chromiteore bodies. Widespread Cr(VI) concentrations in groundwater detected inover 50% of the water supply wells downgradient of serpentinite terrains inDixon, Willows, Livermore, South San Francisco, and other California loca-tions support the interpretation that Cr(VI) is naturally occurring (Henrieet al., 2002; and this book). Background Cr(VI) in groundwater has beenreported at three locations in California:

• Lawrence Livermore National Laboratory• Vicinity of University of California (UC) Davis• Presidio in San Francisco

At Lawrence Livermore National Laboratory, experiments have been con-ducted to remove Cr(VI) from groundwater using anion-exchange resin. Eventhough Cr(VI) is believed to be naturally occurring, the concentrations of tensto hundreds of ppb still exceed the discharge standards for the groundwatertreated to remove VOCs (Torres, 1995). Leaching experiments are continuingto determine how Cr(VI) is being generated from soils (Ridley, 2002).

Cr(VI) in groundwater has also been documented in the vicinity of UCDavis in the Sacramento Valley (Chung et al., 2001). Concentrations up to0.2 mg/l have been reported from monitoring wells screened in alluvialdeposits allegedly contaminated by laboratory waste. However, Cr(VI) isalso present in off-site control wells and shallow upgradient wells. A studytested the inherent ability of the manganese dioxide-rich soils from drill

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cuttings to produce Cr(VI) from native Cr(III). All of the samples generatedCr(VI) with concentrations in the extracts ranging from 0.02 to 0.1 mg/l,which is not statistically different from the concentrations in the backgroundwells. The study concluded that hazardous concentrations of Cr(VI) can begenerated in the unsaturated zone and transferred to groundwater by anatural geogenic process, i.e., oxidation of native Cr(VI) by native manganeseoxides. It also suggested that other areas in the Sacramento and San Joaquinvalleys with similar alluvium might also have naturally occurring Cr(VI) ingroundwater (Chung et al., 2001).

At the Presidio in San Francisco, Cr(VI) became a contaminant of concernin the remedial investigation of past U.S. Army activities. Cr(VI) is oftenattributed to anthropogenic sources, since it is used in a number of indus-trial applications. However, numerous detections of Cr(VI) in several areasof the Presidio where no anthropogenic sources existed led to the devel-opment of an alternative hypothesis that Cr(VI) in groundwater could benaturally occurring. The case study included at the end of this chapterindicates that Cr(VI) is present in trace amounts in the serpentinite bedrockand can be leached from serpentinite, as demonstrated by leaching testsof drill cuttings. In addition, Cr(VI) concentrations in new backgroundwells located in pristine background areas are higher than in existingdowngradient wells. The results of the study lead to the development ofa conceptual model in which the source of Cr(VI) in the groundwater isthe serpentinite bedrock that underlies upland areas of the Presidio, whichwas acknowledged by DTSC (Steinpress et al., 1998; Montgomery Watson,1999a; DTSC, 2000).

3.1.3 The Erin Brockovich Effect: Hollywoodand the Scientific Process

In 1999, the movie “Erin Brockovich” was released that focused on a legalcase involving a cluster of cancers associated with groundwater contamina-tion from a Pacific Gas and Electric (PG&E) site in Hinkley, California. Themovie thrust the issue of Cr(VI) in drinking water into the public and polit-ical spotlight almost overnight. As a result, the high level of public concernboth overwhelmed and undermined the relatively slow-paced processes ofscientific investigation and regulatory evaluation. However, the concern alsohad the beneficial effect of spurring the California legislature to direct andprovide funding for regulators to speed up decisions related to the currenttotal chromium maximum contaminant level (MCL) and to consider a newMCL for Cr(VI).

The Groundwater Resources Association of California (GRA) held a sym-posium in Southern California on Cr(VI) in groundwater to bring the then-current state of scientific knowledge and some common sense to the epicen-ter of the controversy (GRA, 2001). The eastern San Fernando Valley, whichincludes one of the largest chlorinated solvent Superfund sites in the country,



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had suddenly received intense local and national media attention on Cr(VI)concerns when “Erin Brockovich” hit theaters. GRA’s symposium includednational and state experts who presented on all facets of this complex issue,including geochemical characteristics and distribution; risk, toxicology, andtesting; social, political, and legal issues; and regulatory approach and reme-diation.

Chromium commonly occurs as nontoxic, relatively immobile Cr(III). Untilrecently, toxic Cr(VI) was generally assumed to be a contaminant (anthro-pogenic) from industrial sites such as chrome plating facilities and coolingtowers. But chromium’s chemistry is complex, and it is increasingly beingrecognized that Cr(VI) also occurs naturally (geogenic). However, the inves-tigation, peer review, and publication of such occurrences are by necessitya methodical and time-consuming process.

The movie, increasing public alarm, and uncertainties with respect tohexavalent chromium’s toxicity and occurrence combined to create a block-buster environmental and scientific challenge. While the issue was ofnational interest, California drinking water regulators were propelled intothe limelight and were forced to sort through the risk dilemma with a dearthof directly applicable toxicological and water quality data.

The development and refinement of a drinking water standard for a spe-cific compound is a painstaking process that typically takes several years.The process begins with a risk assessment to develop a risk-based healthgoal. The United States Environmental Protection Agency (USEPA) does notconsider Cr(VI) to be a carcinogen by the ingestion route. The USEPA MCL(which also considers the technical and economic feasibility of attaining aprescribed level) of 0.1 mg/l for total chromium is therefore consideredprotective. On the other hand, California Environmental Protection Agency’s(Cal/EPA’s) Office of Environmental Health Hazard Assessment (OEHHA)completed an analysis of the data and considered Cr(VI) an oral carcinogen.In February 1999, that risk hypothesis led to the calculation of a healthprotective level (PHL) for Cr(VI) of 0.0002 mg/l and a public health goal(PHG) of 0.0025 mg/l for total chromium. This result led the DHS to reeval-uate the existing MCL of 0.05 mg/l for total chromium and consider a newMCL for Cr(VI). The PHGs are intended to be strictly advisory and are basedsolely on health considerations, whereas MCLs are legally enforceable drink-ing water standards. However, DHS must also evaluate the technical andeconomic feasibility for water systems when it is reevaluating an existingMCL or establishing a new MCL.

When “Erin Brockovich” premiered, the scientific process already under-way to evaluate the chromium MCL in light of the new PHG was short-circuited. In the movie, the Cr(VI) is contamination, and a deep-pocketcorporation was identified as the responsible party. In a typical Hollywoodplot, “good” triumphs over “evil,” and a hefty financial settlement compen-sates the plaintiffs. While there was some education of the public with respectto groundwater contamination, science largely took a back seat to the showylegal wrangling (as in the movie “A Civil Action”).

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In reality, many water suppliers, such as in the Southern California cities ofGlendale and Burbank, were caught between concerned members of the publicthat wanted cleanup to the lowest available regulatory standard or advisorylevel at any cost, and the technical and economic realities of pumping andtreating huge quantities of contaminated groundwater. Unfortunately, thePHG was been taken out of context by the media, in part due to difficultiesin conveying the distinction between PHGs and MCLs. As a consequence, thePHG become a

de facto

action level. Meanwhile, Cr(VI) testing resulting fromthe emergency directive from DHS (and also proactive interests) showed thatCr(VI) is popping up in numerous groundwater basins (including some “pris-tine” environments) in California. State and federal regulators faced challeng-ing decisions that demand scientific methods and common sense, and deeppockets will not be available to fund cleanup of naturally occurring Cr(VI).

Sometimes Hollywood and real life converge and have a mutually rein-forcing, long-lasting impact (e.g., “The China Syndrome” and Three MileIsland). Although the controversy over Cr(VI) taxed the scientific and regu-latory processes, the “Erin Brockovich” effect ultimately led to a better-informed public and resulted in a flurry of legislative activity and fundingfor groundwater protection. This accelerated our scientific understanding ofthe occurrence and distribution of chromium, the development of realisticregulatory standards (MCLs), and more effective remedial technologies. Wecan only hope that the scientific and regulatory communities can stay aheadof the screenwriters and be better prepared for the media and public responsewhen the next contaminant

du jour

hits the big screen.Subsequent to the release of the editorial views expressed above in

Ground-water

(Steinpress and Ward, 2001), the California legislature directed DHSto establish a primary drinking water standard for Cr(VI) by January 1, 2004,and to establish a secondary drinking water standard for Cr(VI) by July 1,2003 (Senate Bill 351, signed by the governor on October, 7, 2001). AnOEHHA review committee found no basis in either the epidemiological oranimal data published in the literature for concluding that orally ingestedCr(VI) is a carcinogen, citing the previously used study to be flawed. As aresult of finding no evidence of Cr(VI) being an oral carcinogen, OEHHAwithdrew the PHG of 0.0025 mg/l for total chromium on November 9, 2001.

3.1.4 Presidio in San Francisco Case Study

3.1.4.1 Executive Summary

During the U.S. Army’s Main Installation Remedial Investigation (RI) of thePresidio (Figure 3.1.2), hexavalent chromium [Cr(VI)] was detected ingroundwater samples from numerous monitoring wells. Due to its toxicity,the presence of Cr(VI) became a significant risk assessment issue in the RIand the Main Installation Feasibility Study (FS). This case study summarizesthe results of a study conducted to assess if Cr(VI) in groundwater at thePresidio in San Francisco could be attributed to natural sources.

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Cr(VI) is often attributed to anthropogenic sources, since it is used in anumber of industrial applications. However, numerous detections of Cr(VI)in several areas of the Presidio where no anthropogenic sources existed ledto the development of an alternative hypothesis that Cr(VI) in groundwatercould be naturally occurring (Figure 3.1.3). This hypothesis was based onthe following:

• Cr(VI) was reported in Presidio waste manifests; however, the Pre-sidio was primarily an Army training and debarkation point withlimited industrial operations (Dames and Moore, 1997a).

FIGURE 3.1.2

General location map.

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• Cr(VI) has been documented to occur naturally in several otherCalifornia groundwater basins associated with serpentinite (Torres,1995; Robertson, 1991; Henrie et al., 2002).

• Extensive areas of the Presidio are underlain by chromium-richserpentinite bedrock (Schlocker, 1974).

• Groundwater sampling and analyses confirmed that Cr(VI) waspresent upgradient as well as downgradient of several sites(USEPA, 1996; Montgomery Watson, 1998).

As a result, the Presidio stakeholders (Army, regulatory agencies, Pre-sidio Trust, National Park Service, and others) agreed to explore anddevelop multiple lines of evidence to address the issue of whether Cr(VI)in Presidio groundwater could be naturally occurring. Three undisturbedbackground locations were selected for obtaining groundwater and ser-pentinite bedrock samples for use in a Cr(VI) leaching study. The currentstudy was designed with the objective of developing the following threelines of evidence:

1. Background Bedrock Chemistry—Assesses Cr(VI) concentrations inbedrock samples collected at the three background drilling locations.

FIGURE 3.1.3

Presidio conceptual hydrogeologic model.

Uplands Bedrock LocationsUBR01 and UBR02

Inspiration PointSerpentinite Barrens

Not to Scale

Precipitation

VIEW TO SOUTHWEST



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2. Background Groundwater Geochemistry—Assesses Cr(VI) con-centrations and water chemistry in monitoring wells at the threedrilling locations.

3. Leaching Tests—To evaluate whether Cr(VI) can be generatedthrough oxidation of Cr(III) or leached from serpentinite bedrockthrough a series of laboratory leaching tests.

Serpentinite bedrock and groundwater samples were collected and the leach-ing tests were performed to address the above objectives.

The results of the current study found the following:

1. Cr(VI) was detected at concentrations of 0.06 and 0.46 mg/kg inthe serpentinite bedrock composites from two of the boreholes.

2. Cr(VI) was detected at concentrations of 0.0521 mg/l to 0.0983 mg/lin groundwater samples from the three new background wellsscreened in serpentinite.

3. Cr(VI) was detected in the leaching test containing serpentinitebedrock and distilled water. The leaching tests performed underexaggerated conditions also yielded Cr(VI) and varying waterchemistry parameters in the leachate over the 28-d test period.

These results indicate that Cr(VI) is present in trace amounts in the serpen-tinite bedrock and can be leached from serpentinite. In addition, Cr(VI)concentrations in the new background wells are higher than in existingdowngradient wells, and the concentration of Cr(VI) in groundwater is aresult of various complex and competing reactions. The above results led tothe conclusion that a source of Cr(VI) in the groundwater is the serpentinitebedrock that underlies upland areas of the Presidio.

Previous studies have demonstrated that complex competing factors influ-ence Cr(VI) concentrations in the environment (Bartlett, 1991). Geochemicaldata from the Presidio indicate an overall trend from oxidizing conditionsin the upland areas to reducing conditions beneath Crissy Field. In summary,the groundwater geochemistry of the upland areas favors Cr(VI) chemicalstability, whereas Crissy Field conditions favor Cr(VI) immobilization and/orreduction to Cr(III).

The results of this investigation have potential implications for future usesof Presidio groundwater as a potential drinking water supply. Any evaluationof the potential beneficial uses of groundwater or Cr(VI) mitigation must con-sider that there is a continual natural source of Cr(VI) from serpentinite bedrockand serpentinite-derived soils or sediments. Because Cr(VI) concentrations varyacross the Presidio, any groundwater mitigation/management plan should belocation-specific to address the natural sources of Cr(VI) in groundwater.

3.1.4.2 Introduction

Cr(VI) has been detected in both bedrock and groundwater samples in pre-vious investigations at the Presidio. This document summarizes the results


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of a study (Montgomery Watson, 1999b) conducted to assess if Cr(VI) in thegroundwater of the upland areas of the Presidio is naturally occurring.

3.1.4.2.1 Site Background

The Presidio is located in the City of San Francisco at the northern tip of theSan Francisco Peninsula and is bounded by San Francisco Bay on the northand the Pacific Ocean on the west (Figure 3.1.2). The topography ranges fromrolling upland hills (approximately 122 m above sea level) to coastal beachesalong the Pacific Ocean and the San Francisco Bay.

The Presidio was occupied by the U.S. Army in 1848 and has served as atraining, mobilization, and embarkation point during the Spanish AmericanWar and both World Wars, a medical debarkation center, and has providedcoastal defense for the San Francisco Bay area. In 1994, the Presidio was trans-ferred to the National Park Service (NPS) to become part of the Golden GateNational Recreation Area (Earth Tech, 1995). The Presidio Trust, established bythe U.S. Congress in 1996, manages the park in partnership with the NPS.

3.1.4.2.2 Previous Investigations

During the U.S. Army’s Main Installation RI of the Presidio (Dames & Moore,1997a), Cr(VI) was detected in groundwater samples collected from numerousmonitoring wells. Due to its toxicity, it became a significant risk assessmentissue in the Presidio RI and Feasibility Study (FS) (Dames and Moore, 1997a,b).Subsequent sampling and analyses also detected Cr(VI) in groundwater andbedrock samples throughout the Presidio (Table 3.1.1) (Steinpress et al., 1998).

3.1.4.2.3 Potential Sources of Cr(VI)

Cr(VI) is often attributed to anthropogenic sources, since it is used in anumber of industrial applications (including electroplating, tanning, indus-trial water cooling, paper pulp production, and petroleum refining [EPRI,1988]). However, the Presidio was primarily an Army training and debarkationpoint with limited industrial operations (Dames and Moore, 1997a). The lim-ited industrial operations at the Presidio primarily included maintenance andrepair of motor vehicles and aircraft. Industrial activities that include the useof chromium were not reported as past activities (Dames and Moore, 1997a),although Cr(VI) has been reported in Presidio waste manifests.

Cr(VI) has been reported to occur naturally in several environments(Robertson, 1975, 1991; Bartlett, 1991; James, 1996; Nagel, 1999), includingareas associated with serpentinite (Torres, 1995; Godgul and Sahu, 1995).Chromium-rich serpentinite bedrock and serpentine-derived soils underlieextensive areas of the Presidio, including the upland areas.

3.1.4.2.4 Approach for Presidio Investigation

Several previous investigations have detected Cr(VI) in Presidio bedrock,soil, and groundwater (Table 3.1.1). Based on these results, the approach forthe current Cr(VI) investigation at the Presidio was developed in a series of



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joint technical meetings (Montgomery Watson, 1999b). This working groupincluded representatives from the following regulatory agencies and stake-holders:

• U.S. Army and U.S. Army Corp of Engineers (USACE)• Department of Toxic Substance Control (DTSC)• U.S. Environmental Protection Agency (USEPA)• Presidio Trust• National Park Service • Restoration Advisory Board (RAB)

The objectives of the meetings were to:

• Develop an approach to assess if Cr(VI) could be naturally occurring.• Select the undisturbed background locations for the field investi-

gation.• Design the experimental conditions for a laboratory leaching study.

TABLE 3.1.1

Previous Hexavalent Chromium Investigations

Investigation/Site Results Reference

Main Installation RI Cr(VI) detected in groundwater upgradient ofseveral sites as well as in background bedrocksamples

Dames and Moore, 1997a

Presidio sites Cr(VI) detected in groundwater at Landfill 1, Battery Howe Wagner, Building 215 area, and El Polin Spring; the samples were requested by NPS and analyzed by USEPA’s National Enforcement Investigation Center (NEIC)

USEPA, 1996

Crissy Field and Inspiration Point

Soil samples, requested by NPS, contained detections of total chromium (3300 to 3900 mg/kg) and total nickel (1930 to 2300 mg/kg); Cr(VI) was not detected at a reporting limit of 21 mg/kg.

USEPA, 1997

Cr(VI) in Groundwater Technical Memorandum

Confirmed presence of Cr(VI) in groundwaterupgradient and downgradient of four uplandsites (i.e., Landfills 1 and 2, Battery HoweWagner, and Building 215 area)

Montgomery Watson, 1998

Building 1349 area Cr(VI) detected at two of the three monitoring wells, including an upgradient well partially screened in serpentinite

MontgomeryWatson, 1999b

Tennessee Hollow Cr(VI) detected in three of four piezometer wells installed by NPS as part of the Wetlands FS

Montgomery Watson, 1999b

Presidio Golf CourseClubhouse Excavation

Cr(VI) detected in all three serpentinite bedrock samples

Montgomery Watson, 1999b


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This effort resulted in an agreement by the working group that multiplelines of evidence pursued in the current study were necessary in order toassess if Cr(VI) is naturally occurring. In addition, the group selected andagreed upon the field program, leaching tests, and work schedule outlined inthe Final Work Plan (Montgomery Watson, 1999a). The three drilling locationsselected were in Tennessee Hollow, upgradient of Fill Site 1 and Landfill 2 andany other known anthropogenic sources of chromium (Figure 3.1.3).

3.1.4.2.5 Objectives

The objective of this study was to evaluate whether Cr(VI) in upland ground-water originates from serpentinite bedrock through the following lines ofevidence:

1. Background Bedrock Chemistry—Measures Cr(VI) concentrationsin bedrock samples collected at the three drilling locations.

2. Background Groundwater Geochemistry—Measures Cr(VI) anddissolved metals concentrations, as well as water chemistry param-eters in the monitoring wells installed at the three drilling locations.

3. Leaching Tests—To evaluate if Cr(VI) can be leached from serpen-tinite bedrock and/or generated through the oxidation of Cr(III)to Cr(VI) through a series of laboratory leaching tests.

The bedrock and groundwater analyses were performed to provide dataon upgradient Cr(VI) concentrations and groundwater geochemistry. Theleaching tests assess the relative influence of several chemical conditions andreactions that may be important to the chemically complex process of gen-erating Cr(VI) in groundwater.

3.1.4.3 Geology, Hydrogeology, and Geochemistry

The following section summarizes the geology, hydrogeology, and ground-water geochemistry of the Presidio upland area. A summary of the chromiumgeochemistry is also included in this section.

3.1.4.3.1 Regional Geology

The bedrock beneath the San Francisco peninsula is composed of ocean-floorultramafic basalts (magnesium- and iron-rich volcanic rocks) and sedimentsthat were stacked against and beneath the North American continent as aresult of subduction. This ancient complex terrain of subducted rocks, calledthe Franciscan Complex, has been deformed, heated, altered (i.e., physicaland/or chemical composition of the rock changed), and mixed as it wascrushed at the continental margin. This Mesozoic bedrock is overlain withQuaternary sediments that include dune sands, slope debris, bay mud, andartificial fill (Figure 3.1.4).

The Franciscan Complex that underlies most of the Presidio includes largeareas of the metamorphosed form of ultramafic rock known as serpentinite,



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shown in Figure 3.1.4. The geology of the Franciscan Complex has beensummarized in several documents [Schlocker, 1974; Wahrhaftig, 1989; theBasewide Groundwater Monitoring Plan (BGMP) (Montgomery Watson,1996); Main Installation RI (Dames and Moore, 1997a); and Steinpress, 1998].

Ultramafic rocks contain chromite, iron–magnesium oxides, and silicates.Ultramafic rocks typically contain 1000 to 3400 mg/kg chromium (Faust and

FIGURE 3.1.4

Generalized geology of the San Francisco peninsula.

Artificial Fill

Holocene Dune Sand

Colma Foundation

Merced Formation

Franciscan Bedrock(Roman Numerals DenoteStructural Blocks)

Strike-Slip Fault

Structural BoundaryBetween Franciscan Blocks(Approximate)

Lithologic Contact

Source: Modified from Wahrhaftig, 1989. 06-99 PR

MONTGOMERY WATSON

PRESIDIO OF SAN FRANCISCO

GENERALIZED GEOLOGYOF THE SAN FRANCISCO PENINSULA

0 1 2 3

SCALE IN KM

BayshoreSan BrunoFault

MercedFormation

CliffHouse

Land’s End

Baker Beach

Golden Gate

Fort PointFort Mason

TelegraphHill

YerbaBuena l.

Hunter’sPoint

LEGEND


104


Aly, 1981), and bedrock developed from serpentinite bedrock also have inher-ently high concentrations of chromium (Kabata-Pendias and Pendias, 1984).

During the Main Installation RI, the USEPA detected high concentrationsof chromium, nickel, and other metals in bedrock associated with Presidioserpentinite (Dames and Moore, 1997a; USEPA, 1996). As a result, the ser-pentinite in the geologic environment was considered a source of metalsfound in the Presidio groundwater.

3.1.4.3.2 Presidio Hydrogeology

The Presidio has two hydrogeologic environments:

• Upland areas• Crissy Field area

The upland areas are underlain by dune and marine sands resting on theFranciscan Complex bedrock. Groundwater in the upland areas generallyoccurs in a shallow water-bearing zone in Quaternary sediments and has anorthward regional flow direction toward San Francisco Bay beneath mostof the Presidio. The low-lying areas around Crissy Field consist of interbed-ded estuarine sands and bay muds. Groundwater below Crissy Field encom-passes the transition zone between groundwater recharged from the uplandareas and a tidally influenced seawater intrusion zone. The hydrogeologyin these two areas is discussed in detail in the Main Installation RI (Damesand Moore, 1997a), the Fuel Products Action Level Development Report(FPALDR) (Montgomery Watson, 1995), the Basewide Groundwater Moni-toring Plan (BGMP) (Montgomery Watson, 1996), and subsequent summarypublications (Steinpress, 1997; Steinpress et al., 1999). Table 3.1.2 includes asummary of the hydrogeologic units that overlie the Franciscan Complexbedrock in the Presidio upland areas, the focus of this investigation.

3.1.4.3.3 Groundwater Geochemistry in Upland Areas

The groundwater chemistry of the upland areas is mostly magnesium bicar-bonate-type, reflecting contact with the magnesium-rich serpentinite bed-rock (Montgomery Watson, 1996). Several of the wells also have sodiumchloride-type groundwater, indicative of marine sedimentary units withinthe Franciscan complex.

3.1.4.3.4 Chromium Geochemistry

Historical Eh and pH measurements indicate that the upland area is gener-ally an oxidizing environment with near neutral pH values (Dames andMoore, 1997a; Montgomery Watson, 1998). For chromium, the reduced oxi-dation state [Cr(III)] typically occurs in low to neutral pH (pH<7 to pH=7)and low Eh (reducing) conditions. Cr(VI) is the dominant oxidation state inalkaline (pH>7) and oxidizing (high Eh values) environments.



105

TAB

LE 3

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106


The concentration of Cr(VI) in groundwater is a function of several com-plex competing factors including:

• Rates of dissolution of chromium from the bedrock, either as Cr(III)or Cr(VI), or both

• Oxidation of Cr(III) to Cr(VI)• Reduction of Cr(VI) to Cr(III)• Sorption of Cr(III) to clay or organics• Precipitation of Cr(III) or Cr(VI)• Groundwater flow path

The two chromium oxidation states have very different mobilities in theenvironment due to differing solubility and adsorption characteristics. Cr(III)[e.g., Cr(OH)

3

] tends to be immobile because it is a cation that is sorbed tonegatively charged surfaces in the bedrock or aquifer material (Calder, 1988;McLean and Bledsoe, 1992). Cr(VI) is an oxyanion (e.g., HCrO

4–

or CrO

42–

)and is more mobile in groundwater because of higher solubility and lowadsorption to aquifer materials (Calder, 1988).

Two of the constituents known to oxidize Cr(III) to Cr(VI) in laboratorystudies are dissolved oxygen and manganese oxides (Eary and Rai, 1987).Cr(VI) can be reduced to Cr(III) in the presence of ferrous iron [Fe(II)],organic matter, and/or dissolved sulfides as groundwater enters a reducingand/or oxygen-poor zone (Calder, 1988).

3.1.4.4 Investigative Methods

This section describes the field methods, bedrock and groundwater sampleanalytical methods, and the leaching test procedures used in this investigation.

3.1.4.4.1 Field Methods

Field activities were conducted as proposed in the Final Work Plan (Mont-gomery Watson, 1999a). The methods are summarized below and a detaileddescription is provided in the TM (Montgomery Watson, 1999b).

3.1.4.4.1.1 Drilling, Core Extraction, and Lithologic Logging —

Three drill-ing locations upgradient of potential contaminant sources (e.g., landfills) inthe Tennessee Hollow drainage area were chosen for this investigation bythe stakeholders (Figure 3.1.3). Permits from the NPS and the UndergroundAlert Service (USA) were obtained before mobilizing to the drilling locations.

Drilling began on February 22, 1999 using a hollow stem auger (Figure 3.1.5).Continuous bedrock core samples were collected, logged, and photographedby the site geologist (Figure 3.1.6). The core was removed from the liners andthen placed in core boxes for temporary storage and future reference(Figure 3.1.7). The collected core was reviewed by the site geologist, projectmanager, and project chemist to select well screen intervals and the mostappropriate sampling intervals for the leaching test study. Boring logs and wellconstruction diagrams are included in the TM (Montgomery Watson, 1999b).



107

FIGURE 3.1.5

Hollow stem auger drilling at UBR01.

FIGURE 3.1.6

Undisturbed bedrock sample in core barrel at UBR02.


108


To minimize disturbance to the bedrock sample and preserve its innatechemical composition throughout the collection and handling activities, thesamples were placed in double polyethylene bags, stored on ice, and trans-ported under chain of custody to the analytical laboratory where they werestored at 4

±

2

°

C.

3.1.4.4.1.2 Groundwater Monitoring Well Construction, Development, andSampling —

Following completion of each borehole, the deep monitoringwells were installed in the borehole and the shallow wells were installed inthe immediate vicinity of the borehole. The bedrock borings at upland loca-tions UBR01 and UBR02 were drilled in or near outcrops of serpentinitebedrock and encountered groundwater in fractured serpentinite bedrock.The boring at UBR03 was drilled in Quaternary dune sands and encounteredsandy clay to sands of the Quaternary Colma Formation at 6.858 m belowground surface (bgs). The five wells constructed at three locations are sum-marized in Table 3.1.3.

Monitoring wells UBR01GW01, UBR02GW01, and UBR02GW02 were ini-tially sampled on March 4, 1999 after being developed using only a bailerand centrifugal pump to provide rapid data necessary for the leaching testsetup. Because the pH and Eh measurements taken during development

FIGURE 3.1.7

Boxed serpentinite core at UBR02.



109

were different than those of other groundwater monitoring wells in theupland areas, the wells were redeveloped between March 16–18, 1999 usingprotocols specified in the BGMP (Montgomery Watson, 1996). The monitor-ing wells were sampled during March 22–23, 1999 according to protocolsspecified in the BGMP (Montgomery Watson, 1996), which include purginga minimum of three well volumes and sampling with a bailer equipped witha bottom-emptying device. Samples for the analyses of dissolved metals[including Cr(VI)] were filtered in the field using a 0.45

µ

m in-line filter.Monitoring well development and purge logs are presented in the TM (Mont-gomery Watson, 1999b).

3.1.4.4.2 Bedrock and Groundwater Analytical Methods

The analytical methods are described in the approved Final Work Plan(Montgomery Watson, 1999a) and summarized below.

3.1.4.4.2.1 Bedrock Samples —

The bedrock core from each of the boringswas composited based on their similarity in mineralogy and degree of weath-ering to produce one composite sample for each boring. The sample depthsused for the three bedrock composites were as follows:

• UBR01: 11.125, 12.192, 13.411, and 14.326 m BGS• UBR02: 6.706, 8.382, and 9.754 m BGS• UBR03: 20.726 and 21.336 m BGS

To generate the composite samples, individual bedrock samples from thecontinuous cores from the three locations were each crushed and homoge-nized using a riffle splitter (Figure 3.1.8). After homogenization, the sampleswere passed through a 4-mm sieve (Figure 3.1.9).

The three bedrock composite samples were analyzed for the parametersidentified in Table 3.1.4.

TABLE 3.1.3

Upland Bedrock Monitoring Wells

Location Well NameScreened

Interval (m) Rationale

UBR01 UBR01GW01 13.72 to 16.76 Screened in serpentinite fracture zoneUBR02 UBR02GW01 10.06 to 14.63 Screened in deep fracture zone in

serpentiniteUBR02GW02 7.32 to 8.84 Screened in shallow fracture zone in

serpentiniteUBR03 UBR03GW01 15.24 to 19.81 Screened in deep Colma Formation

UBR03GW02 9.45 to 10.97 Screened in Colma Formation above low permeability sandy clay


110


FIGURE 3.1.8

Homogenization of bedrock sample using a riffle splitter.

FIGURE 3.1.9

Sieving a bedrock composite sample using a 4-mm sieve.



111

The quick oxidation test is a qualitative measure of the samples’ capacityto oxidize soluble Cr(III) to Cr(VI). The easily reducible manganese test isan aggressive method for measuring the amount of reducible manganeseoxide compounds in the bedrock that may be available to oxidize Cr(III) toCr(VI). Detailed descriptions of the analytical methods are included in theTM (Montgomery Watson, 1999b).

Several individual bedrock samples were also collected at discrete intervalsthroughout the bedrock core. These samples were collected based on distinctfeatures observed in the bedrock. Table 3.1.5 lists the individual bedrocksamples, their distinct features, and the analyses performed.

TABLE 3.1.4

Laboratory Analyses for Bedrock Samples

Parameter Analytical Method Laboratory

Cr(VI) 7199/DI Extraction QuanterraCr(VI) 7196A/3060A PrimaTotal Metals

a

6010B QuanterraTotal Organic Carbon Modified 9060 QuanterraTotal Organic Carbon Walkley-Black PrimaQuick Oxidation Test Bartlett and James, 1979 PrimaEasily Reducible Manganese Gambrell, 1996 PrimaPercent Moisture D2216 QuanterrapH — PrimaEh — Prima

a

Al, Ag, As, Ba, Be, Ca, Cd, Cr, Co, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, Sb,Se, Si, Ti, Tl, V, and Zn.

TABLE 3.1.5

Individual Bedrock Samples

Drilling Location

Depth of Sample

(m BGS) Distinctive Feature Analyses

UBR01 10.36 Strong black staining Cr(VI), easily reducible manganese, and quick oxidation test

UBR01 7.32 Slightly weathered serpentinite

Cr(VI), easily reducible manganese, and quick oxidation test

UBR02 14.33 Strong iron-oxide staining

Cr(VI) and total metals

a

UBR02 14.63 White calcite deposits

Total metals

a

UBR03 10.06 Slight black staining Cr(VI) and easily reducible manganese

a

Al, Ag, As, Ba, Be, Ca, Cd, Cr, Co, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, Sb, Se, Si, Ti, Tl, V, and Zn.


112


3.1.4.4.2.2 Groundwater Samples —

Groundwater samples were analyzedfor the parameters identified in Table 3.1.6.

Reporting limits for all analytical methods are presented in the TM (Mont-gomery Watson, 1999b).

Field measurements were also conducted for the groundwater samplesusing a Yellow Springs Instruments (YSI) multimeter equipped with an in-line flow-through cell. These measurements include:

• Dissolved oxygen• Oxidation-reduction potential (ORP)• pH• Turbidity• Specific conductance• Temperature• Salinity

The field measurement data can be found in the groundwater purge logsin the TM (Montgomery Watson, 1999b).

3.1.4.4.3 Analytical Laboratories

The laboratory analyses of the groundwater and leachate samples wereperformed by Quanterra Environmental Services, an analytical laboratoryvalidated by the USACE Missouri River District and certified by the Stateof California to perform the requested analyses. The leaching tests andselected analyses associated with the bedrock samples were performed byPrima Environmental, a treatability laboratory in Sacramento, CA.

TABLE 3.1.6

Laboratory Analyses for Groundwater

and Leachate Samples

ParameterAnalytical

Method

Dissolved metals

a

6010B Cr(VI) 7199 Alkalinity 310.1 pH 9040 Chloride 300.0 Fluoride 340.2 Nitrate/nitrite 353.2 Sulfate 300.0 Total dissolved solids 160.1

a

Al, Ag, As, Ba, Be, Ca, Cd, Cr, Co, Cu, Fe, K, Mg,Mn, Na, Ni, Pb, Sb, Se, Si, Ti, Tl, V, and Zn.

Note:

All laboratory analyses were performed byQuanterra Environmental Services.



113

3.1.4.5 Leaching Test Procedures

Based on the analytical results described in Section 3.1.4.4.2 for the three bed-rock composite samples, composite UBR01 was selected for use in the leachingtests. The sample was crushed to increase the surface area of the bedrock inorder to increase the chemical reaction rates and reduce the time of the tests.

The leaching tests consisted of six batch tests conducted in duplicate, usinga series of 14 bottles per test (Table 3.1.7). Test 1 was a blank and containeddeionized (DI) water only.

Groundwater collected from monitoring well UBR01GW01 was used asthe leaching solution in Test 2 of the leaching test. Due to the already elevatedCr(VI) concentration (0.0654 mg/l) in UBR01GW01, DI water was used inTest 3 and the exaggerated tests (e.g., Tests 4, 5, and 6) to allow any increasesin Cr(VI) concentrations during the 28-d test period to be distinguished.

As described in the Final Work Plan (Montgomery Watson, 1999a), Tests 4,5, and 6 were performed under intentionally exaggerated conditions [i.e.,excess concentrations of oxidants or soluble Cr(III)] to provide information onthe time scale and limiting factors that may influence the oxidation of Cr(III)to Cr(VI) and its release into the groundwater. Hydrogen peroxide, manganesedioxide, and chromium chloride were added to the bottles in Tests 4, 5, and 6,

TABLE 3.1.7

Leaching Test Conditions

Test No.

LeachingSolution Bedrock

Other Additions Rationale

1 125 ml of DI water None None Blank2 125 ml of groundwater

sample UBR01GW0125 g None To monitor if Cr(VI) can be

leached or oxidized from bedrock with the site groundwater

3 125 ml of DI water 25 g None To monitor if Cr(VI) can be leached or oxidized independent of groundwater chemistry

4 125 ml of 5% H2O2a 25 g None To enhance Cr(III)

oxidation to Cr(VI) by adding a strong oxidant

5 125 ml of DI water 25 g 2.5 g (29 mmole) of MnO2

b

To enhance Cr(III) oxidation to Cr(VI) by adding an oxidant

6 125 ml of 2 mmole/l CrCl3

c

25 g None To determine if the solubility of Cr(III) is the limiting factor in the oxidation of Cr(III) to Cr(VI)

a Prepared by diluting 333 mL of 30% H2O2 to 2 l with DI water.b JT Baker, “Baker Analyzed” manganese dioxide powder.c Prepared by dissolving 1.067 g CrCl3 ⋅ 6H2O in 2 l of DI water.

Au: Insertion Ok?


114 Chromium(VI) Handbook

respectively, to achieve these different exaggerated conditions, as described indetail in the TM (Montgomery Watson, 1999b). The bottles containing theleaching mixture were continuously agitated on a shaker table (Figure 3.1.10).The leaching tests were performed over a 28-d period and sampled at 1 h, 4h, 24 h, 7 d, 14 d, 21 d, and 28 d.

At each sampling interval, two duplicate bottles from each of the testconditions were removed from the shaker table and sacrificed for analyses.The leachate was filtered and analyzed for the following parameters:

• Selected dissolved metals (Cr, Fe, Mg, Mn, Ni)• Cr(VI)• pH• Eh

At the 1-h and 28-d sampling intervals, the leachates were also analyzedfor the complete suite of dissolved metals (Table 3.1.6), and the leachedUBR01 bedrock composite was analyzed for Cr(VI), easily reducible manga-nese, and the quick oxidation test.

3.1.4.6 Analytical Results

Samples collected for this Cr(VI) study were analyzed for a variety of param-eters (listed above) to assess if Cr(VI) could be leached and/or generated fromthe serpentinite bedrock. These analytical results are described in the following

FIGURE 3.1.10Leaching test bottle setup on shaker table.


Naturally Occurring Cr(VI) in Groundwater 115

three subsections. A discussion of the overall trends demonstrated by theanalytical data is presented in Section 5.0.

3.1.4.6.1 Bedrock ResultsKey Points can be summarized as follows:

• Cr(VI) detected in serpentinite bedrock.• Bedrock has the intrinsic ability to oxidize Cr(III) to Cr(VI).• Bedrock extracts are slightly alkaline and exhibit oxidizing condi-

tions.

All three of the serpentinite bedrock composites (UBR01, UBR02, andUBR03) contained detectable quantities of Cr(VI) as analyzed by the alka-line digestion method (Table 3.1.8). These results confirm bedrock Cr(VI)in the Presidio Golf Course excavation samples and RI (Dames and Moore,1997a).

Two of the bedrock composites, UBR01 and UBR02, also demonstrated theability to oxidize dissolved Cr(III) to Cr(VI) (as seen in the quick oxidationtest results). These results suggest that Cr(VI) could be generated from theserpentinite bedrock if soluble Cr(III) is present.

The Eh and pH values for the three bedrock composites indicate alkaline(pH above 8) and oxidizing (large positive Eh values) conditions. There werealso relatively low concentrations of reducing agents, such as total organiccarbon.

Bedrock composite UBR01 was used in the leaching tests due to relativelylower concentrations of extracted Cr(VI) and higher dissolved chromium,quick oxidation test and easily reducible manganese results than in the othertwo bedrock composites.

Before the start of the leaching tests, the UBR01 bedrock composite wasreanalyzed for Cr(VI), easily reducible manganese, and the quick oxidation

TABLE 3.1.8

Bedrock Composite Analytical Results

Bedrock Composite IDAnalytical Parameters UBR01 UBR02 UBR03

Cr(VI) by alkaline digestion (mg/kg) 0.064 0.463 0.231Chromium (mg/kg) 1,160 371 83.5Manganese (mg/kg) 595 671 216Quick oxidation test (mg/kg) 1.60 1.01 <0.08Total org carbon (mg/kg) 329 452 897Easily reduced Mn (mg/kg) 136 102 15pH 8.85 8.00 8.44Eh (mV)a 454 421 441

a Eh values are corrected for reference electrode and temperature.

Au: Please check the sentence for clarity.



test to confirm that the composition of the bedrock had not changed betweenthe initial analyses and the leaching test setup (Table 3.1.9). The original andreanalyzed results showed that the chemical composition of the bedrockcomposite did not change significantly before the beginning of the leachingtest. Because Cr(VI), by alkaline digestion results, is very close to the report-ing limit (0.05 mg/kg), the difference between these results is not consideredsignificant.

Several individual bedrock samples were also collected at discrete intervalsfrom the bedrock core based on distinctive features. The most notable resultis that Cr(VI) was detected in the samples (Table 3.1.10).

3.1.4.6.2 Groundwater ResultsKey Points can be summarized as follows:

• Cr(VI) detected in groundwater from all three monitoring wellsscreened in serpentinite bedrock.

• Groundwater from monitoring wells screened in serpentinite bed-rock showed slightly alkaline and oxidizing conditions.

Monitoring wells UBR01GW01, UBR02GW01, and UBR02GW02 were ini-tially sampled on March 4, 1999 after being developed with only a bailer

TABLE 3.1.9

Ubr01 Bedrock Composite Analyses

Analytical ParametersUBR01 (Analysis on

3/1/99)UBR01 (Reanalysis

on 3/15/99)

Cr(VI) by alkaline digestion (mg/kg) 0.064 0.127Quick oxidation test (mg/kg) 1.60 1.67Easily reducible manganese (mg/kg) 136 137

TABLE 3.1.10

Individual Bedrock Sample Results

Drilling Location

Depth of Sample(ft BGS) Analysis

Result(mg/kg)

UBR01 24 Cr(VI) 0.540Easily reducible manganese 601Quick oxidation test 3.90

UBR01 34 Cr(VI) 0.602Easily reducible manganese 1235Quick oxidation test 13.21

UBR02 47 Cr(VI) 0.0104UBR03 33 Cr(VI) 0.0264

Easily reducible manganese 891



and centrifugal pump. Because the pH and Eh measurements were differentfrom groundwater in most other upland areas, the wells were redevelopedusing a surge block and submersible pump to purge additional sedimentand groundwater as per the BGMP (Montgomery Watson, 1996). The wellswere resampled on March 22, 1999 and yielded Eh values more typical ofthe upland areas, but the pH values remained relatively high (Table 3.1.11).The original low Eh measurements were unexplained but are consideredartifacts of the well installation. The monitoring wells at UBR03 were notsampled on March 4, 1999 due to high turbidity that could interfere withthe analyses.

The analytical results for dissolved metals and general water chemistryparameters (i.e., alkalinity, anions, and total dissolved solids) were consistentbetween the two sampling events.

Cr(VI) was detected in four out of the five monitoring wells installed atthe background locations (Table 3.1.12). Wells screened in the serpentinitebedrock (UBR01GW01, UBR02GW01, and UBR02GW02) contained higherCr(VI) concentrations than monitoring wells screened in the Colma Forma-tion (UBR03GW01 and UBR03GW02).

The Cr(VI) concentrations from all five wells strongly correlated (R2 = 0.99)with the dissolved chromium concentrations (Figure 3.1.11), showing that

TABLE 3.1.11

Summary of Groundwater pH and Eh

Sample IDCollected 3/4/99

Collected 3/22/99 and 3/23/99

pH Eha (mV) pH Eha (mV)

UBR01GW01 8.32 –24 8.31 423UBR02GW01 9.14 –15 9.18 348UBR02GW02 8.33 –33 8.36 308UBR03GW01 NS NS 7.53 464UBR03GW02 NS NS 6.62 422

a Eh values are corrected for reference electrode and temperature.

Note: NS stands for not sampled.

TABLE 3.1.12

Groundwater Analytical Results

Analytical Parameter

Monitoring WellsUBR01GW01

UBR02GW01

UBR02 GW02

UBR03GW01

UBR03GW02

Cr(VI) (mg/l) 0.0654 0.0521 0.0983 <0.00050 0.00064Dissolved chromium (mg/l) 0.066 0.049 0.100 <0.010 <0.010Dissolved manganese (mg/l) 0.013 <0.015 0.010 0.280 0.059pH 8.31 9.18 8.36 7.53 6.62Eh (mV)a 423 348 308 464 422

a Eh values are CORRECTED for reference electrode and temperature.



the chromium was predominantly in the hexavalent form. Monitoring wellsfrom locations UBR01 and UBR02 also have slightly alkaline (pH>7) andoxidizing conditions (large positive Eh values).

Groundwater samples collected from wells at location UBR03 had pHvalues that were closer to neutral as well as higher concentrations of dis-solved manganese, indicating different groundwater chemistry.

The geochemistry of groundwater samples from monitoring wells atUBR01 and UBR03 are similar to the magnesium bicarbonate patterns seenin other upland area wells. Monitoring wells at UBR02 have higher concen-trations of sodium and chloride, a pattern more typical of groundwaterassociated with marine sedimentary units within the Franciscan Complex.High pH values associated with groundwater from the three upland bedrockwells are consistent with those typically found in serpentinite terrains(Kruckeberg, 1985).

3.1.4.6.3 Leaching Test ResultsKey Points can be summarized as follows:

• Cr(VI) detected in Test 3 (DI water + bedrock composite) indicatesthat Cr(VI) can be leached from the bedrock.

• Cr(VI) concentrations in bedrock and groundwater are a result ofcompeting reactions (i.e., dissolution and precipitation).

• Presence of oxidants and soluble Cr(III) is important in the complexchemical reactions governing Cr(VI) generation.

The leaching tests consisted of six different test conditions, performed induplicate, analyzed over the 28-d test period (Table 3.1.13).

Results are presented as averages of the two duplicates for each of the testconditions in the following sections. Groundwater collected from monitoringwell UBR01GW01 was used as the leaching solution in Test 2 of the leaching

FIGURE 3.1.11Correlation between Cr(VI) and dissolved chromium.

0

50

100

0 25 50 75 100

Cr(VI) (µg/L)

Dis

solv

ed C

hrom

ium

(µg

/L)



test. Due to the Cr(VI) concentration (0.0654 mg/l) present in UBR01GW01,a leaching solution of DI water was used in the exaggerated tests (i.e., Tests4, 5, and 6) in order to distinguish any increases in Cr(VI) concentrationsduring the 28-d test period.

3.1.4.6.3.1 Test 1—DI Water — Test 1 served as a blank control to monitorfor the possible contamination sources from the DI water or during theanalyses of the leaching test. This test condition contained DI water only (nobedrock composite). Dissolved metals and Cr(VI) were not detected abovereporting limits during the leaching test, indicating that the DI water didnot contribute to Cr(VI) concentrations in the leaching tests.

3.1.4.6.3.2 Test 2—Groundwater + Bedrock — Leaching Test 2 consisted ofa combination of site groundwater [with an initial Cr(VI) concentration of0.0654 mg/l] and bedrock samples from location UBR01. The objective of thistest was to evaluate whether the groundwater affects the leaching or oxidationof Cr(VI) from the bedrock.

The results show that Cr(VI) concentrations declined during the test periodand the leachate contained a final Cr(VI) concentration of 0.0052 mg/l. Thisoverall decrease in the leachate Cr(VI) concentration coincided with anincrease in the Cr(VI) concentration in the remaining leached bedrock(Figure 3.1.12).

The leached bedrock also demonstrated an overall decrease in the abilityto oxidize Cr(III) to Cr(VI), as shown in the quick oxidation test results(Table 3.1.14).

The inverse relationship between Cr(VI) concentrations in the leachate andthe leached bedrock suggests that precipitation and dissolution are the likelycontrols on the Cr(VI) concentrations in the bedrock and groundwater. Thiscould be due in part to the increased surface area of the bedrock matrixproviding additional active sites for sorption of Cr(VI). Other studies have

TABLE 3.1.13

Leaching Test Conditions

Test No.

Leaching Solution Bedrock

Other Additions

1 125 ml of DI water None None2 125 ml of groundwater sample

UBR01GW0125 g None

3 125 ml of DI water 25 g None4 125 ml of 5% H2O2

a 25 g None5 125 ml of DI water 25 g 2.5 g (29 mmole)

of MnO2b

6 125 ml of 2 mmole/l CrCl3c 25 g None

a Prepared by diluting 333 mL of 30% H2O2 to 2 l with DI water.b JT Baker, “Baker Analyzed” manganese dioxide powder.c Prepared by dissolving 1.067 g CrCl3 ⋅ 6H2O in 2 l of DI water.



also shown that increased Cr(VI) soil concentrations inhibit Cr(III) oxidation(Fendorf and Zasoski, 1992).

3.1.4.6.3.3 Test 3—DI Water + Bedrock — Test 3 consisted of DI watercombined with the bedrock composite. The objective of this test was toevaluate if Cr(VI) could be leached from the bedrock by a leaching solutionof DI water.

The results indicate that Cr(VI) was leached from the bedrock, as shownby Cr(VI) concentrations in the leachate (0.00125 to 0.0054 mg/l) during the28-d test period. The leachate Cr(VI) concentration peaked at 0.0054 mg/lat day 14 and then decreased to 0.00125 mg/l by day 28 (Figure 3.1.13).

Over the test period, there was a slight increase in the leached bedrockCr(VI) concentration (Table 3.1.15), indicating that the Cr(VI) precipitatedout of solution and sorbed to the bedrock.

The Cr(VI) concentrations in the leachate and leached bedrock were closeto the reporting limits (0.0005 mg/l and 0.05 mg/kg, respectively). Due tothe analytical uncertainties at concentrations close to the reporting limits, itis not possible to assess whether these differences are significant. The quickoxidation test results decreased over the 28-d testing period. This suggeststhat as Cr(VI) was adsorbed onto the bedrock, it may have blocked sites onthe bedrock that would have been available to oxidize the soluble Cr(III)added as part of the quick oxidation test.

FIGURE 3.1.12Test 2 Cr(VI) concentrations.

TABLE 3.1.14

Leached Bedrock Analytical Results for Test 2

Analytical ParameterSampling Interval

1 h 28 d

Cr(VI) by alkaline digestion (mg/kg) 0.09 0.238Quick oxidation test (mg/kg) 1.32 0.24

0

20

40

60

80

Sampling Interval

Leac

hed

Bed

rock

(mg/

kg)

Leac

hate

(µg

/L)

0

0.1

0.2

0.3Cr(VI) inLeachate

Cr(VI) inLeachedBedrock

1 Hour 28 Days



3.1.4.6.3.4 Test 4—DI Water + Bedrock + Hydrogen Peroxide — Test 4 con-sisted of the addition of the oxidant hydrogen peroxide (H2O2) in a mixtureof DI water and the bedrock composite. This test was performed to evaluateif Cr(III) in the bedrock could be oxidized to Cr(VI) by adding H2O2.

The results of Test 4 showed increased Cr(VI) concentrations in the leachate(Figure 3.1.14) relative to Test 3 (DI water and bedrock) concentrations. Theinitial high Cr(VI) concentration was followed by a decrease that stabilized

FIGURE 3.1.13Test 3 leachate Cr(VI) concentrations.

TABLE 3.1.15

Leached Bedrock Analytical Results for Test 3

Analytical ParameterSampling Interval

1 h 28 d

Cr(VI) by alkaline digestion (mg/kg) 0.093 0.155Quick oxidation test (mg/kg) 1.66 0.89

FIGURE 3.1.14Test 4 leachate Cr(VI) concentrations.

0

1

2

3

4

5

6

1hour

24hours

14days

28days

Sampling Interval

Cr(

VI)

(µg

/L)

0

100200

300

400500

600

1

hour

4

hours

24

hours

7

days

14

days

21

days

28

days

Sampling Interval

Cr(

VI)

(µg

/L)



at approximately 0.140 mg/l. These concentrations were much higher thanthe results of Test 3 (Figure 3.1.13), indicating that more Cr(VI) may begenerated if suitable oxidizing agents are available.

Cr(VI) concentrations and the quick oxidation test results also decreasedin the leached bedrock (Figure 3.1.15). It has been previously noted that theaddition of H2O2 can act as an oxidant or reductant under different geochem-ical conditions (James, 1999).

3.1.4.6.3.5 Test 5—DI Water + Bedrock + Manganese Dioxide — Test 5 con-sisted of manganese dioxide (MnO2) added to a mixture of DI water andbedrock. The objective of Test 5 was to evaluate if Cr(III) in the bedrock canbe oxidized to Cr(VI) by MnO2, since MnO2 is known to oxidize Cr(III) toCr(VI) (Fendorf and Zasoski, 1992).

The test results (Figure 3.1.16) did not indicate a significant increase inleachate Cr(VI) concentrations compared to Test 3 (DI water and bedrock).However, the Test 5 Cr(VI) concentrations did not decrease over the lastsampling intervals, as was seen in Test 3.

A summary of dissolved metals [Cr(VI), chromium, and manganese] forthis leaching test are presented in Table 3.1.16.

FIGURE 3.1.15Test 4 Cr(VI) and quick oxidation test results.

FIGURE 3.1.16Tests 3 and 5 leachate Cr(VI) concentrations.

0

1

2

Sampling Interval

Qui

ck O

xida

tion

Tes

t (m

g/kg

)

0

0.3

0.6

Cr(

VI)

(m

g/kg

) Cr(VI)

QuickOxidationTest

1 Hour 28 Days

0123456

1

hour

1

day

14

days

28

days

Sampling Interval

Cr(

VI)

(µg

/L)

Test 3

Test 5



It was expected that the dissolved manganese concentration in the leachatewould have increased with the addition of MnO2 because manganese oxidesare reduced to Mn(II) as Cr(III) is oxidized to Cr(VI). However, the dissolvedMn(II) concentration in the leachate remained below the reporting limitthroughout the 28-day test period (Table 3.16).

The concentration of easily reducible manganese in the leached bedrockincreased (Figure 3.1.17), but Cr(VI) and dissolved Mn concentrations in theleachate did not. This indicates that the easily reducible manganese testoverestimates the amount of manganese oxides readily available for oxidiz-ing Cr(III) to Cr(VI). This test could measure other forms of manganese thatare insoluble but do not reduce to Mn(II) in the presence of Cr(III). Inaddition, a laboratory reagent grade form of MnO2, which is crystalline andnot very soluble, was used for this test. In retrospect, a freshly precipitatedform of MnO2 may have been more conducive for the oxidation of Cr(III) toCr(VI).

3.1.4.6.3.6 Test 6—DI Water + Bedrock + Chromium Chloride — Test 6 con-sisted of adding soluble Cr(III) [in the form of chromium chloride (CrCl3 )] toa mixture of DI water and bedrock. The objective of Test 6 was to evaluateif the bedrock could oxidize soluble Cr(III) to Cr(VI), similar to the quick

TABLE 3.1.16

Test 5—dissolved Metals in Leachate


Sampling Interval1 h 4 h 24 h 7 d 14 d 21 d 28 d

Cr (mg/l) 0.018 0.0069 0.00525 0.0082 0.0074 0.0204 0.0067Cr(VI) (mg/l) 0.0049 0.0046 0.0051 0.00465 0.00515 0.057 0.0052Mn (mg/l) <0.015 <0.015 <0.015 <0.015 <0.015 <0.015 <0.015

FIGURE 3.1.17Test 5 easily reducible manganese in leached bedrock.

0

2000

4000

6000

8000

10000

12000

14000

1 hour 28 days

Sampling Interval

Mn (

mg/k

g)



oxidation test. This test is valuable in assessing not only the possible oxida-tion of Cr(III) to Cr(VI) but also the stability of Cr(VI).

The CrC13 solution that was added in this test contained some Cr(VI)(Table 3.1.17), which may have contributed to the Cr(VI) concentration mea-sured in the leachate as described below.

The 1-h sampling interval Cr(VI) data were qualified as potentially biaseddue to the discrepancy between the duplicate samples. This data point maynot truly represent an increased Cr(VI) concentration. Since the Cr(VI) con-centrations from the remaining six sampling intervals (Table 3.1.18) wereless than or very similar to the Cr(VI) concentration in the CrC13 solution,it is inconclusive as to whether the Cr(VI) in the leachate was oxidized fromCr(III) or was an artifact of the Cr(VI) concentration of the CrC13 solution.

The leachate pH values for Test 6 were lower than the other leachate tests,most likely due to the addition of the CrCl3 solution. The lower pH couldhave caused metals (e.g., manganese) to become more soluble, explainingthe initial increase in dissolved manganese (Figure 3.1.18) compared to man-ganese concentrations in Test 3 (DI water and bedrock).

The leached bedrock lost some of its ability to oxidize soluble Cr(III) (e.g.,quick oxidation test results) and showed decreased Cr(VI) concentrationsover the 28-day testing period (Figure 3.1.19).

3.1.4.6.3.7 Data Quality — The analytical results were reviewed and val-idated according to EPA Functional Guidelines (USEPA, 1994a,b) and theChemical Data Acquisition Plan/Field Sampling Plan (Montgomery Watson,1994). All analytical data met data quality objectives and are consideredusable.

The leaching tests were performed in duplicate (two samples per samplinginterval per test condition). Relative percent differences (RPDs) between thesample and its associated duplicate outside the acceptance limit (40%) were

TABLE 3.1.17

Test 6 Crcl3 Solution

Dissolved Chromium (mg/l)

87.200

Cr(VI) (mg/l) 0.222pH 3.6

TABLE 3.1.18

Test 6—Cr(VI) in Leachate


Sampling Interval1 h 4 h 24 h 7 d 14 d 21 d 28 d

Cr(VI) (mg/l) 1900a 0.137 0.091 0.154 0.257 0.180 0.138

a Qualified data; potentially biased.



qualified to indicate a potential bias. Table 3.1.19 lists the percentage ofsamples considered comparable with their associate duplicates.

3.1.4.6.4 Discussion of ResultsThis section discusses the groundwater analytical results in the Presidioupland areas and associated geochemical trends.

FIGURE 3.1.18Test 6 dissolved manganese in leachate.

FIGURE 3.1.19Test 6 Cr(VI) and quick oxidation test results.

0

100

200

300

400

500

600

1

hour

24

hours

14

days

28

days

Sampling Interval

Mn

(µg/

L)

0

5

10

15

20

Sampling Interval

Cr(

VI)

(m

g/kg

)

0

0.3

0.6

0.9

1.2Q

uick

Oxi

datio

n T

est (

mg/

kg) Cr(VI)

Quick OxidationTest

1 Hour 28 Days



3.1.4.6.4.1 Groundwater Analytical Results in Upland Areas — The ground-water analytical results from the new background wells in Tennessee Hol-low and other existing monitoring wells screened in serpentinite at PresidioRI sites are plotted on the Eh-pH diagram in Figure 3.1.20 using the field

TABLE 3.1.19

Sample and Duplicate Comparison

Analytical ParametersNumber of Analyses

Number of RPDs <40%

Percent Acceptable

Dissolved chromium 84 79 94Dissolved iron 84 84 100Dissolved magnesium 84 84 100Dissolved manganese 84 78 93Dissolved nickel 84 84 100Cr(VI) 84 79 94pH 84 84 100Eh 84 84 100Cr(VI) by alkaline digestion 20 18 90Easily reducible manganese 20 20 100Quick oxidation test 20 19 95

FIGURE 3.1.20

+800

+600

+400

+200

0

−2006 7 8 9 10 11 12

pH

937GW12 (<0.5)

LF02GW10 (<0.5)

LF07GW02 (<0.5)

UBR02GW02 (98.3)

HWGW04 (7.4)UBR02GW01 (52.1)UBR03GW01 (<0.5)

UBR01GW01 (65.4)HWGW01 (122)

1349MW03 (69)LF02GW04 (0.72)

1349MW02 (43)HWGW05 (<0.5)

UBR03GW02 (0.64) Cr(VI)

Cr(III)

Eh

(mV

)

Au: PleaseProvide theFigure 3.1.20Caption.



Eh (i.e., Eh = ORP measurement + 200 mV as per the YSI instruction manual)and pH measurements. Eh-pH diagrams are used to illustrate geochemicalconditions, namely the chemical equilibrium and speciation of multivalentelements (e.g., chromium) under a range of pH and Eh conditions. Themethods used to construct the Eh-pH diagrams are described in the TM(Montgomery Watson, 1999b). The concentration of Cr(VI) detected in eachof the wells is shown in parenthesis next to the well identification number.Samples collected from the new background wells screened in serpentiniteat locations UBR01 and UBR02 are near or in the Cr(VI) stability field (shownin yellow), indicating that Cr(VI) is stable in the high pH and Eh environ-ments present at these locations. The pH values of the background wellsfrom locations UBR01 and UBR02 were significantly higher than the neutral(pH 7) values typical of previous investigations at the Presidio. It appearsthat the high pH in groundwater samples from the three upland wells (e.g.,pH 8.5 to 9.3) is associated with the serpentinite bedrock in the upland areas,in contrast to downgradient areas overlain by Quaternary sedimentary units.This pH data are consistent with high pH values (up to 11.8) observed inother serpentinite terrains (Kruckeberg, 1985).

Samples collected from the UBR03 wells screened in Colma Formation sed-iments fall within the same general pH and Eh range typical of most otherPresidio upland monitoring wells, including those screened in serpentinite,which have Cr(VI) concentrations from <0.0005 to 0.122 mg/l (Table 3.1.20).This suggests that pH and Eh alone are not the only factors in producingconditions conducive to Cr(VI) generation. Rather, there are other constituentsand chemical reactions in both the groundwater and the bedrock that arecontrolling the valence state and concentration of chromium. Dissolution/precipitation reactions, availability and form of oxidants and reductants, andthe buffering capacity of the bedrock also affect the concentration and oxida-tion state of chromium in the groundwater.

TABLE 3.1.20

Cr(VI) Concentration in Monitoring Wells Screened in Serpentinite

Monitoring Well IDCr(VI) Concentration

(mg/l)

UBR01GW01 0.0654UBR02GW01 0.0521UBR02GW02 0.0983LF02GW02 <0.0005LF02GW04 0.00072HWGW01 0.122HWGW04 0.0074HWGW05 <0.00051349MW02 0.0431349MW03 0.069



The groundwater sample results for Cr(VI) from the five new backgroundwells are shown in Figure 3.1.21. All of the samples collected from the newbackground wells screened in serpentinite (e.g., UBR01 and UBR02 wells)had detectable concentrations of Cr(VI), indicating that Cr(VI) is apparentlyoriginating from the serpentinite bedrock. This is further supported by thefact that Cr(VI) has been detected in several other monitoring wells screenedin the serpentinite bedrock at the Presidio (Table 3.1.20).

The two monitoring wells in which Cr(VI) was not detected (LF02GW02and HWGW05) had detections of dissolved iron. Since dissolved iron [Fe(II)]is a known reductant of Cr(VI), it would be expected that Cr(VI) would notbe present in the groundwater.

The results also indicate a strong correlation (R2 = 0.98) between the dis-solved Cr and Cr(VI) concentrations in the groundwater from wells screenedin serpentinite. This strong correlation between Cr(VI) and dissolved Crdemonstrates that virtually all of the chromium in groundwater from thesewells is in the hexavalent state.

3.1.4.6.4.2 Geochemical Trends in Tennessee Hollow — Several trends areapparent when viewing Cr(VI), Eh, and pH in a regional perspective.Groundwater analytical results from the new background wells screened in

FIGURE 3.1.21Upland area conceptual model and Cr(VI) distribution.

MONTGOMERY WATSON

PRESIDIO OF SAN FRANCISCO

UPLAND AREACONCEPTUAL MODEL

AND Cr(VI) DISTRIBUTION05-99 PR

Notes:

Concentration of Cr(VI)by ion chromatography in µg/L.LF01 and LF02 results from January 1998.UBR results from March 22, 1998.Not To Scale

Fracture Zones

VIEW TO SOUTHInspiration PointSerpentiniteBarrens

Au: Please provide figure caption.



serpentinite have higher Cr(VI) concentrations than those found downgra-dient in the Fill Site 1 and Landfill 2 areas (Figure 3.1.21).

The groundwater flows from the upland areas towards Crissy Field withgenerally decreasing Cr(VI) concentrations (Figure 3.1.3). This pattern isconsistent with Cr(VI) being leached and/or oxidized from the serpentinitebedrock, which remains stable in the upland areas due to the highly oxidiz-ing environment (i.e., high pH and Eh). As groundwater flows north throughCrissy Field toward the San Francisco Bay, it encounters the organic-rich,reducing sediments (including bay muds) which provide conditions suitablefor reducing Cr(VI) to Cr(III) [i.e., low dissolved oxygen and the presenceof dissolved Fe(II)] (Abu-Saba and Flegal, 1996). There is an overall north-ward decrease in Cr(VI) concentrations and pH values from the new uplandbedrock wells (UBR01 and UBR02) through Fill Site 1 and Landfill 2, Ten-nessee Hollow (TH), the Building 215 area, and finally to Crissy Field. Ehmeasurements typically vary more at the Presidio because of the variabilityof oxidation/reduction couples, and do not show as clear a trend as pH.However, low dissolved oxygen and high dissolved iron associated with theorganic-rich bay muds beneath Crissy Field indicate conditions unfavorablefor the occurrence of Cr(VI) (Montgomery Watson, 1998).

3.1.4.6.5 ConclusionsThe Presidio investigation developed and assessed multiple lines of evidenceto determine if Cr(VI) in the Presidio’s groundwater could be attributed tonatural sources. Bedrock and groundwater samples were collected at threebackground locations and used in a laboratory leaching study for this inves-tigation. The three undisturbed drilling locations near the head of the Ten-nessee Hollow watershed were selected by a stakeholder committee thatincluded technical experts and other representatives of the Army, USACE,Presidio Trust, NPS, DTSC, USEPA, and the RAB.

The bedrock and groundwater samples were analyzed to evaluate back-ground bedrock chemistry and groundwater geochemistry. The leachingtests assessed whether Cr(VI) could be leached from the serpentinite bedrockor generated through the oxidation of Cr(III). The results of the bedrock,groundwater, and leaching test analyses indicate the following:

• Cr(VI) is present in trace amounts in the natural geologic environ-ment (i.e., serpentine bedrock).

• Cr(VI) concentrations in groundwater from the new monitoringwells screened in the serpentinite bedrock were higher than exist-ing downgradient wells.

• Cr(VI) is generated or leached from the serpentinite bedrock byvarious complex chemical reactions.

These results support the conclusion that the serpentinite bedrock is asource of Cr(VI) detected in upland groundwaters. The elevated pH and



positive Eh of the groundwater from the new background wells screened inserpentinite bedrock indicate that geochemical conditions are favorable forthe presence of Cr(VI) in groundwater. Cr(VI) in groundwater from wellsdowngradient in Tennessee Hollow indicates that it persists in the oxidizedconditions of the upland water table aquifer. Geochemical data demonstratea change from oxidizing conditions in the upland areas to reducing condi-tions further downgradient in the Crissy Field area. The groundwatergeochemistry of the Presidio upland areas favors Cr(VI) stability, whereasthe groundwater geochemistry in the Crissy Field area favors Cr(VI) immo-bilization and/or reduction of Cr(VI) to Cr(III).

The results of this investigation also have potential implications for futuregroundwater use at the Presidio and similar environments. Any evaluationof groundwater use or Cr(VI) mitigation must consider that there is a con-tinual natural source of Cr(VI) from the serpentinite bedrock and serpen-tinite-derived soils and sediment, so traditional remediation is inappropriate.

This investigation culminated in the acknowledgment by the CaliforniaDepartment of Toxic Substances Control (DTSC) that “it appears that Cr(VI)occurs naturally in serpentinite bedrock and in water from some monitoringwells, including background locations screened in that bedrock” (DTSC, 2000).Coincidentally, the Presidio Trust assumed responsibility for environmentalremediation of the Presidio from the U.S. Army at the conclusion of the study.

3.1.5 Acknowledgments

The technical memorandum “Hexavalent Chromium in Serpentinite Bedrockand Groundwater in Upland Areas” (Montgomery Watson, 1999b) was pre-pared by Montgomery Watson (now MWH) under the direction of the Pre-sidio in San Francisco Base Realignment and Closure (BRAC) EnvironmentalCoordinator, David M. Wilkins. The following project staff with MontgomeryWatson (unless otherwise noted) provided technical leadership, review, andother contributions: —Bruce Handel (Technical Manager, USACE); Roger C.Henderson, P.E. (Technical Lead, USACE); Jessica Hardy (TechnicalReviewer, USACE); Melih Ozbilgin, Ph.D. (Program Manager); Martin Stein-press, R.G., C.H.G. (Project Manager and Hydrogeologist); Marla L. Miller(Project Chemist); Leslie R. Typrin, M.S. (Project Environmental Scientist);Technical Advisory Committee comprised Gregory E. Little, R.G., John S.Porcella, P.E., Jim V. Rouse, R.G., and William R. Mabey, Ph.D; LaboratoryLeach Tests were conducted at PRIMA Environmental (Sacramento, CA).

3.1.6 Bibliography

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Ball, J.W., 2002, Occurrence of Hexavalent Chromium in Ground Water in the Western Partof the Mojave Desert, California, Geological Society of America Denver AnnualMeeting, Paper No. 197–2.

Bartlett, R.J., 1991, Chromium cycling in soils and water: links, gaps, and methods,Environ. Health Perspect., 92, 17–24.

Bartlett, R. and James, B., 1979, Behavior of chromium in soils: III oxidation, J. Environ.Qual., 8, 1, 31–35.

Calder, L.M., 1988, Chromium contamination in groundwater, in Nriogen, J.O. andNieboel, E., Eds., Chromium in the Natural and Human Environments, Vol. 20,John Wiley and Sons, New York, pp. 215–229.

Chung, J., Burau, R.G., and Zasoski, R.J., 2001, Chromate generation by chromatedepleted subsurface materials, Water, Air, Soil Pollut., 128, 407–417.

Dames and Moore, 1997a, Final Remedial Investigation Report, Presidio Main Instal-lation, Presidio of San Francisco, California, Report prepared for the U.S.Army Environmental Center (USAEC).

Dames and Moore, 1997b, Final Feasibility Study Report, Presidio Main Installation,Presidio of San Francisco, California, Report prepared for USAEC.

Department of Toxic Substances Control (DTSC), 2000, Letter from Henry Chui, P.E.,Office of Military Facilities, to Ms. Sharron Reackhof, Presidio Trust, 2 p.

Earth Tech, Inc. (Earth Tech), 1995, Base Realignment and Closure (BRAC) Cleanup Plan,Presidio of San Francisco, California, Report prepared for USAEC, version 2 (final).

Eary, L.E. and Rai, P., 1987, Kinetics of chromium(III) oxidation to chromium(VI) byreaction with manganese and dioxides, Environ. Sci. Technol., 21, 1187–1193.

Electric Power Research Institute (EPRI), 1988, Chromium Reactions in Geologic Mate-rials, Report prepared by Battelle, Pacific Northwest Laboratories for EPRI, PaloAlto, California, EPRI EA–5741, Interim Report.

Eriksen, G.E., 1983, The Chilean nitrate deposits, Am. Sci., 71, 366–374.Faust, S.D. and Aly, M.A., 1981, Chemistry of Natural Waters, Ann Arbor Science, Ann

Arbor, MI.Fendorf, S.E. and Zasoski, R.J., 1992, Chromium(III) oxidation by δ-MnO2, part 1,

characterization, Environ. Sci. Technol., 26, 1, 79–85.Garrells, R.M. and Christ, C.L., 1965, Solutions, Minerals, and Equilibria, Harper, New York.Godgul, G. and Sahu, K.C., 1995, Chromium contamination from chromite mine,

Environ. Geol., 25, 4, 251–257.Groundwater Resources Association of California, 2001, Symposium on Hexavalent

Chromium in Groundwater, Glendale, CA, http://www.grac.org/hex_binders.html.

Henrie, T.D., Simion, V., Auckly, C., and Weber, J.V., 2002, Chromium 6+ Concentrationsin Drinking Water Wells and the Effects of Chlorination, Poster Presentation, Spring2002 Conference of the CA-NV American Water Works Association section.

James, B.R., 1996, The challenge of remediating chromium-contaminated soil, Envi-ron. Sci. Technol., 30, 6, 248–251.

James, B.R., 1999, personal communication.Kabata-Pendias, A. and Pendias, H., 1984, Trace Elements in Soils and Plants, 2nd ed.,

CRC Press, Boca Raton, FL.Kruckeberg, A.R., 1985, California Serpentines: Flora, Vegetation, Geology, Soils, and

Management Problems, University of California Press, Berkeley, CA.McLean, J.E. and Bledsoe, B.E., 1992, Behavior of Metals in Soil, Groundwater Issue, U.S.

Environmental Protection Agency (USEPA), Robert S. Kerr, Environmental Re-search Laboratory, EPA/540/S-92/018, Ada, OK, 25 p.

Au: Should it be "Presidio of San Fran-cisco" or " Pre-sidio in San Francisco"? Please check.

Au: Is this the editor name for the report? Please specify.



Montgomery Watson, 1994, Chemical Data Acquisition Plan/Field Sampling Plan (CDAP/FSP), Investigation of Various Underground Storage Tanks Sites, Presidio of SanFrancisco, California, Report prepared for the USACE, Sacramento District,Sacramento, CA.

Montgomery Watson, 1995, Fuel Products Action Level Development Report (FPALDR),Presidio of San Francisco, California, Report prepared for the USACE, Sacra-mento District, Sacramento, CA.

Montgomery Watson, 1996, Draft Basewide Groundwater Monitoring Plan (BGMP),Presidio of San Francisco, California, Report prepared for the USACE, Sacra-mento District, Sacramento, CA.

Montgomery Watson, 1998, Technical Memorandum: Hexavalent Chromium in Ground-water, Presidio of San Francisco, California, Report prepared for the USACE,Sacramento District, Sacramento, CA.

Montgomery Watson, 1999a, Final Letter Work Plan for Hexavalent Chromium Investi-gation in Upland Areas, Presidio of San Francisco, California, Report preparedfor the USACE, Sacramento District, Sacramento, CA.

Montgomery Watson, 1999b, Technical Memorandum: Hexavalent Chromium in Serpen-tinite Bedrock and Groundwater in Upland Areas, Report prepared for the USACE,Sacramento District, Sacramento, CA.

Metropolitan Water District of Southern California (MWD) and Bureau of LandManagement, 2001, Cadiz Groundwater Storage and Dry-Year Supply Pro-gram, Final EIR/EIS response to Comments.

Nagel, R., 1999, personal communication.Ridley, M., 2002, personal communication.Robertson, F.N., 1975, Hexavalent chromium in the groundwater in Paradise Valley,

Arizona, Groundwater, 13, 516-527.Robertson, F.N., 1991, Geochemistry of Grand Water in Alluvial Basins of Arizona and

Adjacent Parts of Nevada, New Mexico, and California, U.S. Geological SurveyProfessional Paper 1406-C, 89 p.

Roscoe Moss Company, 2003, A guide to water well casing and screen selection,http://www.roscoemoss.com.

Schlocker, J., 1974, Geology of the San Francisco North Quadrangle, California, U.S. Geo-logical Survey Professional Paper 782, Washington, DC, 109 p.

Steinpress, M.G. and Ward, A.C., 2001, The scientific process and Hollywood: thecase of hexavalent chromium in groundwater, Guest Editorial in Ground Water(AGWSE Journal), 39, 3, 321 p.

Steinpress, M.G., Miller, M., Ozbilgin, M., Henderson, R., and Handel, B., 1999, Hy-drostratigraphic controls on groundwater geochemistry and remediation at thePresidio of San Francisco, Cordilleran Section Centennial Meeting, Berkeley, CA.June 2–4, Geological Society of America, Abstract with Programs, pp. A to 98.

Steinpress, M.G., 1998, Transformation of the Presidio of San Francisco: hydrogeologyand environmental restoration, in Jacobs, J.A. and Bertucci, P.F., Eds., Hydroge-ology of the Northern San Francisco Bay Area Field Trip Guidebook, GroundwaterResources Association of California, Seventh Annual Meeting.

Steinpress, M.G., Miller, M., Little, G., Ozbilgin, M., Mabey, R.V., Henderson, R.,Handel, B., and Wilkins, D., 1998, Hexavalent chromium in groundwater at thePresidio of San Francisco: anthropogenic or naturally occurring? Seventh AnnualMeeting on California Groundwater Effective and Efficient Usage for the Year 2000and Beyond, October 22 and 23, 1998, California Groundwater Resources Asso-ciation, Walnut Creek, CA, 23 p.



Steinpress, M.G., 1997, Geology and hydrogeology of the Presidio, in Barnes, N.L.,Ed., Hydrogeology and Environmental Restoration at the Presidio of San Francisco,Association of Engineering Geologists San Francisco Section Spring Field Trip.

Torres, R.A., 1995, Removing Hexavalent Chromium from Subsurface Waters with Anion-Exchange Resin, Lawrence Livermore National Laboratory, UCRL-ID-114369, 12 p.

Truesdall, A.H. and Jones, B.F., 1969, Ion association in natural brines, Chem. Geol.,4, 51–62.

U.S. Environmental Protection Agency (USEPA), 1994a, Contract Laboratory Pro-gram (CLP) National Functional Guidelines for Organic Data Review, EPA540/R-94/012.

U.S. Environmental Protection Agency (USEPA), 1994b, Contract Laboratory Program(CLP) National Guidelines for Inorganic Data Review, EPA 540/R-94/012.

U.S. Environmental Protection Agency (USEPA), 1996, Analytical Results—HexavalentChromium Results for Samples from Presidio Army Base, San Francisco, California,National Enforcement Investigations Center (NEIC) Project Q53.

U.S. Environmental Protection Agency (USEPA), 1997, Analytical Results—ElementalConstituent Results for Soil Samples from Presidio Army Base, San Francisco, Cali-fornia, National Enforcement Investigations Center (NEIC) Project Q53.

Wahrhaftig, C., 1989, Geology of San Francisco and vicinity, American GeophysicalUnion Field Trip Guidebook, T105, 69 p.

3.2 Cr(VI) Concentrations in Drinking Water Wells and the Effects of Chlorination

Tarrah D. Henrie, Veronica Simion, Chet Auckly, and Jeannette V. Weber

ABSTRACT The California Water Service Company (Cal Water) con-ducted a one-year study on the occurrence of hexavalent chromium[Cr(VI)] in groundwater and the effects of chlorination on speciation. Dueto recent health concerns raised by the media and the public, the Cali-fornia Department of Health Services instituted a statewide Cr(VI) study.The State requires monitoring Cr(VI) concentrations in the water fromwells, where total chromium concentrations have previously been above0.0025 mg/l. Cr(VI) was found in the northern portion of the CentralValley and in the Bay Area. The majority of total chromium was Cr(VI)and not trivalent chromium [Cr(III)] as previously thought by publichealth professionals. Cr(III) is much less soluble than the chromate anion(CrO4

2–), so this finding is not surprising. There was little difference inCr(VI) concentration in chlorinated and unchlorinated water. It was notconfirmed whether the chromium was from anthropogenic sources ornaturally occurring.

Au: Please provide names of pub-lisher and location, if necessary.



3.2.1 Introduction

The California Water Service Company owns and operates 25 districts andprovides drinking water in over 70 communities. Many of these districts usegroundwater only, others use surface water only, and some are served by acombination of ground water and surface water. During the first year of thisstudy, the districts were prioritized according to chromium concentrationand only sources with more than 0.0025 mg/l of chromium were sampledduring 2001. Over 500 samples were collected from more than 80 ground-water wells throughout California.

Total chromium is made up of chromium in primarily two oxidation states:trivalent chromium [Cr(III)] and hexavalent chromium [Cr(VI)]. While mostCr(III) compounds are insoluble at pH 5 and Eh>0.8 V, and are of low toxicity,Cr(VI) is more soluble and carcinogenic when inhaled. Cr(VI) has receivedmuch media attention, because the California Public Health Goal (PHG,0.0002 mg/l) is lower than the California maximum contaminant level (MCL,0.05 mg/l). The PHG is set by the California Environmental Protectionagency, which is a nonenforceable level below which there is no expectedhealth effect. The MCL is the enforceable limit for a contaminant

3.2.2 Methods and Materials

Samples were collected from 76 wells before and after chlorination. Two roundsof sampling were performed with a gap of six months. Until 2001, the chromiumdetection limit for reporting (DLR) was 0.001 mg/l. Because our internal detec-tion limit had in the past been as low as 0.001 mg/l, we were able to use thesedata to classify the wells as less than 0.0025 mg/l or greater than 0.002 mg/l.

Total chromium was analyzed by Cal Water’s in-house certified laboratoryin San Jose, California, by EPA method 200.8. BSK Analytical Laboratory inFresno, California, analyzed Cr(VI) by EPA method 218.6. In order to observethe 24 h hold-time for Cr(VI), the samples were collected after 11:00 a.m. andshipped with blue ice, overnight, to BSK. The samples were analyzed in themorning when received.

3.2.3 Results and Discussion

Table 3.2.1 lists the districts that receive groundwater along with the numberof active wells and the range of total chromium concentration. The DLR forthe chromium data shown is 0.010 mg/l. Based on our lower internal detec-tion limit, wells were classified as below a concentration of 0.0025 mg/l orabove it. Bakersfield, King City, and Oroville did not have any wells thatcontained more than 0.0025 mg/l of total chromium, and thus were notsampled during 2001. In other districts more than 50% of the wells haddetectable chromium (Dixon, Willows, Livermore, and South San Francisco).

The results from all of the districts were grouped in order to draw mean-ingful conclusions on a statewide basis. Figure 3.2.1 shows a linear correlationbetween total chromium and Cr(VI). Although the correlation coefficient is

Au: Table 3.2.1missing.



only 0.83, the graph does demonstrate that the concentration of Cr(VI) can bereasonably estimated from the total chromium level.

Note that the slope of the line is 0.818, which indicates a higher proportionof Cr(VI) than previously thought. The amount of Cr(VI) ranged betweenno detection up to 100%. Based on a single investigation, in 1999, the Officeof Environmental Health and Hazard Assessment (OEHHA) estimated thatCr(VI) comprised about 7% of the total chromium (DHS, 2001).

Cr(III) is not very soluble under normal groundwater conditions, becauseinsoluble solid Cr(OH)3 and Cr2O3 form even under slightly acidic conditions(Rai et al., 1987). In aerobic aquifers, with alkaline pH, Cr(VI) is more com-mon (Evanko and Dzombak, 1997).

Figure 3.2.2 shows chromium concentrations in chlorinated and unchlori-nated water. A linear regression fits the data well and yields a correlationcoefficient of 0.95. The slope of the line is 0.99, demonstrating that Cr(VI)concentrations are essentially unchanged by chlorination. Though there wasno expectation that total chromium concentration would change with chlo-rination, it was plotted for reference, and is similar to the Cr(VI) data.

The valence state of chromium can be changed through several redox reac-tions. Cr(VI) can be transformed under anaerobic conditions to Cr(III) byreaction with iron (Fe2+), sulfur (S2–), and organic matter (Batchelor et al., 1998).

FIGURE 3.2.1Total chromium versus Cr(VI) in 12 California communities.

35

30

25

20

15

10

5

00 5 10

Total Cr (µg/L)

Total Cr versus Cr(VI)

y = 0.818x – 0.90r2 = 0.83

95% Confidence Interval

Cr(

VI)

(µg

/L)

15 20 25 30 35



The reduction reaction with iron is fast in water, but not quite as fast as in soilenvironments (Batchelor et al.,1998). Cr(III) can be converted to Cr(VI) byreaction with manganese oxides and hydroxides (Bartlett and James, 1979).Cr(III) can also be oxidized to Cr(VI) by molecular oxygen. The oxidationreaction of Cr(III) by oxygen, shown in Equation 3.2.1, is very slow (Kent,2001). This reaction is pH dependent, forming Cr(III) at low pH.

4Cr3+ + 3O2 + 10H2O ↔ 4CrO42− + 20H+ (3.2.1)

The Department of Health Services required that the samples be takenfrom each source twice, with a gap of six months. The data would establishwhether or not Cr(VI) concentrations vary seasonally. Figure 3.2.3 showsdata from the first quarter plotted against data from the third quarter at thesame location. Interestingly, Cr(VI) concentrations do not appear to vary (theslope is 1.02), however, total chromium concentrations may vary seasonally;they were slightly higher in the third quarter.

Chromium can be naturally occurring or it can be the result of anthropo-genic activities. High concentrations of naturally occurring chromium ingroundwater have been found near Baltimore, Maryland and are probablythe result of serpentine parent material, which often contains some chro-mium. Serpentine [Mg3Si2O5(OH)4] is the state mineral of California and canbe found in the Coast Range as well as in the Sierra Nevada (Alt andHyndman, 1995).

FIGURE 3.2.2Chromium in chlorinated and unchlorinated water in 12 California communities.

0

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25

20

15

10

5

0

10 20 30

Chromium (µg/L) in unchlorinated water

Total CrCr(VI)

Tota

l Cr

(µg/

L) in

chl

orin

ated

wat

er



3.2.4 Bibliography

Alt, D.D. and Hyndman, D.W., 1995, Roadside Geology of Northern California, MountainPress , Missoula, MT.

Batchelor, B., Schlautman, M., Hwang, I., and Wang, R., 1998, Kinetics of Chromium(VI)Reduction by Ferrous Iron, Amarillo National Resource Center for PlutoniumReport ANRCP-1998-13.

Bartlett, R. and James, B., 1979, Behavior of chromium in soils: III oxidation, J. Environ.Qual., 8, 31–35.

California Department of Health Services (DHS), 2001, Hexavalent chromium[chromium-6] in drinking water, http://www.dhs.ca.gov/org/ps/ddwem/chemicals/chromium6/cr+6index.htm.

Evanko, C.R. and Dzombak, D.A., 1997, Remediation of Metals-Contaminated Soils andGroundwater. GroundWater Remediation Technologies Analysis Center Tech-nology Evaluation Report TE-97-01.

Kent, D.B., 2001, Processes Influencing the Distribution, Fate, and Transport of Chromiumin Ground Water,. Groundwater Resources Association of California HexavalentChromium in Groundwater Seminar.

Rai, D., Sass, B.M., and Moore D.A., 1987, Chromium(III) hydrolysis constants andsolubility of chromium(III) hydroxide, Inorg. Chem., 26, 345–349.

FIGURE 3.2.3Chromium in the first and third quarter in 12 California communities.

35

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25

20

15

10

5

00 5 10 15 20 25 30

Chromium (µg/L) First Quarter

Total Cr in unchlorinated water

Total Cry = 1.24x – 0.026r2 = 0.94

Cr(VI) in unchlorinated water

Tota

l Cr

(µg/

L) T

hird

Qua

rter

Cr(VI)y = 1.02x – 0.02r2 = 0.95


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