Hexavalent Chromium Sources and Distribution in ... - ACS Publications

5 oxygenated, high pH (greater than 8) groundwater promote Cr(III) oxidation and ... under drought conditions.25,26 The complexities governing the fat...
0 downloads 0 Views 4MB Size
Subscriber access provided by UNIV OF DURHAM

Environmental Processes

Hexavalent Chromium Sources and Distribution in California Groundwater Debra M. Hausladen, Annika Alexander-Ozinskas, Cynthia N. McClain, and Scott Fendorf Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06627 • Publication Date (Web): 27 Jun 2018 Downloaded from http://pubs.acs.org on June 28, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34

Environmental Science & Technology

1

EST Article

2 3

Hexavalent Chromium Sources and Distribution in California Groundwater

4 5 6

Debra M. Hausladen1, Annika Alexander-Ozinskas1, Cynthia McClain2, and Scott

7

Fendorf1*

8 9 10

1

Earth System Science Dept. Geological Sciences Dept.

2

Stanford University, Stanford, CA 94305. USA

11 12 13 14 15

*corresponding author. Email: [email protected]; Phone: (650) 723-5238

16 17 18 19 20 21 22 23 24 25 26 27 28

1

ACS Paragon Plus Environment

Environmental Science & Technology

29 30 31 32

ABSTRACT Groundwater resources in California represent a confluence of high-risk factors

33

for hexavalent chromium contamination as a result of industrial activities, natural

34

geology, and, potentially, land use. Here, we examine state-wide links in California

35

between groundwater Cr(VI) concentrations and chemicals that provide signatures for

36

source attribution. In environmental monitoring wells, Cr(VI) had the highest co-

37

occurrence and also clustered with 1,4-dioxane and several chlorinated hydrocarbons

38

indicative of the metal plating industry. Additionally, hotspots of Cr(VI) co-occurring

39

with bromoform result from volatile organic compound remediation using in-situ

40

chemical oxidation that inadvertently oxidizes naturally occurring Cr(III). In

41

groundwater supply wells which are typically free of industrial inputs, Cr(VI) correlates

42

with DDE, vanadium, ammonia and clusters with nitrate and dissolved oxygen,

43

suggesting potential links between agricultural activities and Cr(VI). Specific controls

44

on Cr(VI) vary substantially by region: from metal plating industry around Los Angeles

45

and the San Francisco Bay areas, to natural redox conditions along flow paths in the

46

Mojave Desert, to correlations with agricultural practices in the Central Valley of

47

California. While industrial uses of Cr will lead to the most acute cases of groundwater

48

Cr(VI) contamination, oxidation of naturally-occurring Cr affects a larger area, more

49

wells, and a greater number of people throughout California.

50

2

ACS Paragon Plus Environment

Page 2 of 34

Page 3 of 34

Environmental Science & Technology

51 52 53

INTRODUCTION

54

within the surface environment in two predominant oxidation states, Cr(III) and

55

Cr(VI).1,2 Anthropogenic activities including metal plating and alloying, leather

56

tanning, anti-corrosion of industrial cooling waters, and wood preservative treatment

57

can lead to elevated Cr concentrations in the environment. The two oxidation states of

58

Cr have appreciably different chemistries and human toxicities. Chromium(III) is

59

benign and needed in small concentrations for human nutrition; it forms a trivalent

60

cation (or hydrolysis product) that binds strongly to mineral surfaces and forms a metal

61

hydroxide of limited solubility.3 By contrast, Cr(VI) is typically present as the chromate

62

oxyanion, HxCrO4x-2, and is a known carcinogen when exposure occurs via inhalation4

63

and potentially by ingestion5–7; it also tends to be mobile in the environment and thus

64

jeopardizes water quality.1,8

Chromium is the twenty-first most abundant element in Earth’s crust and exists

65

Although industrial uses of Cr will lead to the most acute cases of groundwater

66

Cr(VI) contamination, oxidation of naturally-occurring Cr (both as a result of natural

67

processes and anthropogenic activity) may affect a larger area, more wells, and a greater

68

number of people. Chromium occurs naturally in the Earth’s crust (100 mg kg-1) and at

69

elevated concentrations in mafic and ultramafic rock (200 and 2,400 mg kg-1,

70

respectively) that occur near oceanic and continental plate margins.9,10 Weathering of

71

ultramafic bedrock or its metamorphic derivatives (e.g., serpentinite) produce soils

72

enriched in Cr, with concentrations as high as 10,000 mg kg-1 reported in serpentine

73

soils of California.1,11

3

ACS Paragon Plus Environment

Environmental Science & Technology

Page 4 of 34

74

Molecular oxygen is a thermodynamically viable oxidant of Cr(III) but the

75

reaction is kinetically limited (i.e., slow) at pH < 9.12 Manganese(III/IV) oxides, by

76

contrast, are facile oxidants of Cr(III) and are thus considered the dominant means by

77

which Cr(VI) is naturally generated from Cr(III) in soils and sediments.1,11 However,

78

oxidation of rock-derived Cr(III) is limited by minerals of low solubility10 and is likely

79

to occur by co-deposition of Cr(III) and Mn(III/IV) minerals.11,13,14 Oxidation may also

80

occur in groundwater recharged slowly (thousands of years) along high pH (greater than

81

8), oxic flow paths, such as in parts of the Mojave Desert.15,16 Concentrations of Cr(VI)

82

in groundwater attributed to natural oxidation of geogenic Cr(III) can exceed 50 μg L-

83

1 17–20

84

Cr(III) by human activities such as in-situ chemical oxidation (ISCO) of chlorinated

85

solvents where oxidants such as permanganate or persulfate are used.21–23

.

Higher Cr(VI) concentrations may result from oxidation of naturally occurring

86

The spatial distribution of groundwater Cr(VI) derived from oxidation of

87

geogenic Cr(III) appears to have many controls, with the Central Valley of California

88

serving as prime example of a region where Cr(VI) distribution is governed by multiple

89

factors. Soils and sediments in western Sacramento Valley (the northern portion of

90

California’s Central Valley) are enriched in Cr and ultramafic-derived or

91

metamorphically

92

concentrations were observed in wells in western regions than wells in the eastern

93

Valley;15 Manning et al.15 suggest that distance from ultramafic outcrops is a dominant

94

control on Cr in groundwater. Further, environmental tracers of groundwater history

95

suggest that redox dynamics of the unsaturated zone and long residence times of

derived

equivalent

rocks.24

4

Concomitantly,

ACS Paragon Plus Environment

higher

Cr(VI)

Page 5 of 34

Environmental Science & Technology

96

oxygenated, high pH (greater than 8) groundwater promote Cr(III) oxidation and

97

minimize adsorption of Cr(VI), optimizing Cr(VI) mobilization into groundwater.15

98

If oxidation of Cr(III) solids is promoted by vacillating redox conditions within

99

the vadose zone, human activities such as groundwater pumping and increased nutrient

100

content may accelerate Cr(VI) generation. Groundwater extraction for irrigation in the

101

Central Valley has exceeded 12 million acre-feet in dry years and is projected to grow

102

under drought conditions.25,26 The complexities governing the fate of Cr have not yet

103

been resolved, and further understanding of processes resulting in Cr(VI) generation is

104

needed to better predict groundwater contamination and inform water management

105

decisions.

106

There exists an ever-growing database of Cr(VI) concentrations in California

107

groundwater, providing a database large enough for rigorous geostatistical analysis of

108

groundwater constituents. In parallel, there are growing databases of other related

109

variables, including groundwater chemical constituents, groundwater level, sediment

110

composition, and measures of land subsidence, which increase our ability to find links

111

between groundwater Cr(VI) concentrations and potentially controlling variables.

112

Accordingly, we hypothesize that sources of Cr(VI) can be determined based on specific

113

chemical signatures. Our objectives of this study were therefore to determine

114

relationships between Cr(VI) and other groundwater contaminants that serve as source

115

identifiers, to hierarchically cluster chemicals that co-occur with Cr in groundwater, and

116

to map spatial distributions of Cr(VI) in California with a particular focus on the Central

117

Valley of California. These analyses are used to assess the contributions of three primary

118

Cr(VI) sources: (1) anthropogenic Cr(VI) resulting from industrial pollution, (2)

5

ACS Paragon Plus Environment

Environmental Science & Technology

119

injection of anthropogenic oxidants of naturally-occurring Cr(III), and (3) agricultural

120

activities that may enhance oxidation of naturally-occurring Cr(III). Chromium(VI)

121

concentrations resulting outside of these three influences are considered to form through

122

undisturbed, natural processes.

123 124

METHODS

125

Drinking Water Standard and Data Source

126

As a consequence of hazards posed by Cr(VI), in July 2014, California set a state

127

maximum contaminant level (MCL) for Cr(VI) in drinking water of 10 μg L-1. The

128

Cr(VI) MCL was in place until September 2017 before being suspended by a court

129

ruling for failing to consider the economic feasibility of compliance. As new MCL

130

regulation is pending adoption, 10 𝜇g L-1 will be used herein to define hazardous Cr(VI)

131

levels.27

132

Data used are from the GeoTracker and GeoTracker Groundwater Ambient

133

Monitoring and Assessment Program (GAMA) databases.28,29 Values are reported for

134

wells spanning the state of California (Figure 1; Supporting Information Figure S1) over

135

the last half century, with most of the sampling occurring after the year 2000

136

(Supporting Information, Figure S2). Within the dataset, some chemicals were not

137

reported broadly enough to make robust statistical relationships. Chlorate, a disinfection

138

byproduct and a naturally occurring compound in arid soils and sediments, correlated

139

with Cr(VI) but was reported only for three well clusters in the Central Valley.30

140

Similarly, measurements of bromacil, an herbicide correlated with Cr(VI), were only

6

ACS Paragon Plus Environment

Page 6 of 34

Page 7 of 34

Environmental Science & Technology

141

reported for three locations over time. We thus omitted both chlorate and bromacil from

142

further discussion.

143 144

Statistical Analyses: Correlations and Hierarchical Clustering

145

Statistical and spatial analyses were done using R (Version 3.3.1) and ArcMap

146

(Version 10.3: Figure 4; Version 10.4.1: Figure 1c and 5; Version 10.5.1 for all other

147

graphics and analyses). In order to account for differences between acute and

148

background Cr(VI) concentrations, the dataset was separated into two parts: (i)

149

environmental monitoring wells (henceforth “monitoring wells”) regulated by the

150

California Water Board, and (ii) groundwater supply wells (henceforth “supply wells”)

151

reported by the California Department of Public Health, Department of Water

152

Resources, or the United States Geological Survey. The former are wells installed to

153

monitor contamination at sites that were previously, currently, or will potentially be

154

undergoing remediation, while the latter wells are used for drinking water and irrigation

155

and represent background Cr(VI) concentrations for this study. 120,032 monitoring

156

wells and 84,410 supply wells are included in the GAMA database; of these, 5,073

157

monitoring wells and 10,642 supply wells report Cr(VI) concentrations. The dataset

158

does not include well-depth for either class of wells. With time-series included, 47,253

159

unique measurements of Cr(VI) concentration from monitoring wells and 31,800 from

160

supply wells were included in our statistical analysis. Monitoring wells are installed

161

principally at contaminated sites and thus can bias the results. Nevertheless, a significant

162

portion of the monitoring wells do not show Cr(VI) concentrations greater or equal to

163

10 𝜇g L-1 (1335 wells), and a high number of monitoring wells reside in counties outside

7

ACS Paragon Plus Environment

Environmental Science & Technology

164

of the San Francisco Bay area and the Los Angeles metropolitan area, diminishing the

165

likelihood for an inherent bias that would result from just measuring wells contaminated

166

with Cr(VI). Even so, statewide fingerprinting of Cr(VI) sources using co-occurring

167

groundwater constituents is likely biased by the distribution of monitoring wells,

168

especially given the very small fraction of monitoring wells (4%) which report Cr(VI)

169

concentrations. Thus, the decision to monitor Cr(VI) concentrations, often being linked

170

to anthropogenic sources of contamination, may lead to inherent biases within the

171

dataset. Because the spatial distribution of wells across the state is not uniform (Figure

172

1; Supporting Information, Figure S1), the statistical results may not be representative

173

of all regions of California but do cover the preponderance of the state.

174

Pearson’s and Spearman’s correlation coefficients were calculated for every pair

175

of chemicals in both datasets and used to hierarchically cluster variables in a

176

dendrogram. A buffer with a 1 km radius was generated in ArcMap around each well

177

with a reported value of Cr(VI). The average values of chemicals within each buffer

178

zone were used to find Spearman’s and Pearson’s correlations in statistical computing

179

program R. In order to transform data to normality, zero values were excluded and all

180

values underwent a logarithmic transformation before the computation of correlation

181

coefficients. Although complex relationships between Cr(VI) and other groundwater

182

constituents are expected, linear correlations were investigated as a first-order

183

approximation. Pearson’s correlation coefficient is a measure of linearity from -1 to 1,

184

while Spearman’s rank correlation coefficient is a measure of monotonicity from -1 to

185

1. Pearson’s and Spearman’s rank-order correlations were calculated, but due to the

186

positive skew of the data, only Spearman’s rank correlations are listed in Table 2. Both

8

ACS Paragon Plus Environment

Page 8 of 34

Page 9 of 34

Environmental Science & Technology

187

methods were used in order to discern different patterns of relationship between

188

chemicals, e.g., the variance in clustering results between the Pearson’s and Spearman’s

189

dendrograms for supply and monitoring wells (Supporting Information, Figures S3-S6).

190

However, the assumption of normality is not met for all groundwater constituents, and

191

thus Pearson’s correlations are purely comparative. Further, correlation analysis

192

conducted on a statewide-scale may mask relationships between Cr(VI) and other

193

chemicals that emerge at a local-scale (e.g., county or smaller) analysis of the data.

194

Chemicals were paired when they were measured in the same month within the same

195

well cluster (i.e., all wells within a 1 km radius). An uncertainty in the quantification of

196

reported values based on the detection limit of the method used by the reporting

197

laboratory was not possible as analytical methods are not included in GAMA’s database.

198

However, this study focuses on the strongest relationships between Cr(VI) and other

199

chemicals, and thus errors associated with uncertainty in detection limits are minimal.

200 201

Statistical Analyses: Correlations and Mapping

202

As Cr(VI) distribution throughout the Central Valley is poorly described by any

203

industrial fingerprinting, an analysis of correlation between Cr(VI) concentrations in

204

groundwater supply wells and proximity to ultramafic outcrops was performed for this

205

area. Ordinary least squares was used to find a correlation between the average reported

206

Cr(VI) concentration in each buffer zone and its distance from the nearest ultramafic

207

outcrop, which was computed using the Near tool (ArcGIS 10.3, 2014). Optimized Hot

208

Spot Analysis (ArcGIS 10.3, 2014) was used to group the residuals into statistically

9

ACS Paragon Plus Environment

Environmental Science & Technology

209

significant high and low clusters, which were then used to perform Ordinary Kriging

210

with the Geostatistical Analyst extension to create a smooth interpolated surface.

211

To investigate state-wide links between groundwater Cr(VI) concentrations and

212

chemicals that offer signatures of origin, we mapped the co-occurrence of Cr(VI) with

213

three different chemical compound classes in both monitoring and supply wells: (1)

214

volatile organic compounds (representing industrial Cr), (2) bromoform (representing

215

ISCO), and (3) nitrate (representing agricultural activity). For each of these chemicals

216

observed above threshold values within 1 km and 1 year of Cr(VI) concentrations  10

217

g L-1, the chemical concentration along with the corresponding Cr(VI) concentration

218

was recorded. Threshold values of chemicals were set near federal and state maximum

219

contaminant levels (MCL) when possible. The US EPA Drinking Water Screening

220

Level was used for 1,4-dioxane.31 We used thresholds for nitrate ≥ 45 mg L-1 (or 10 mg

221

L-1 as N), bromoform ≥ 1 µg L-1 (n = 15899), 1,4-dioxane concentrations ≥ 0.67 µg L-1

222

(n = 31,187 of 64,819), tetrachloroethene (PCE) and trichloroethene (TCE) both ≥ 5

223

µg L-1 (n = 91,575 of 750,385 and n = 124,263 of 771,819 observations, respectively).

224

Due to the size of recorded 1,1- and 1,2-dichloroethene (DCE) concentrations (over 2

225

million samples), we set a higher threshold of 100 µg L-1 (37,802 of 2,061,053

226

observations; n = 23,761 cis-1,2-DCE, n = 3,331 trans-1,2-DCE, and n = 10,710 1,1-

227

DCE). After pairing these observations with wells exhibiting Cr(VI) concentrations ≥

228

10 µg L-1, 178,304 observations represent 1,2-DCE and 166,568 observations represent

229

1,1-DCE. Both isomers of 1,2-DCE (cis-1,2-DCE and trans-1,2-DCE), often found at

230

contaminated sites as a degradation product of TCE, were grouped together for spatial

231

analysis. 1,1-DCE, which often reflects a different source (e.g., semiconductor device

10

ACS Paragon Plus Environment

Page 10 of 34

Page 11 of 34

Environmental Science & Technology

232

fabrication, polymerization of vinyl chloride, acrylonitrile, and acrylates), was

233

considered separately. Each isomer was considered separately when calculating

234

Spearman’s correlation coefficients with Cr(VI).

235

The Near tool (ArcGIS, Version 10.5.1) was used to calculate the distance from

236

the well locations for selected chemicals to the nearest well having Cr(VI)

237

concentrations ≥ 10 µg L-1 . All well-pairs that met the threshold concentrations within

238

1 km were kept, and many chemical observations corresponded with multiple sites

239

and/or time points for wells with elevated Cr(VI) concentrations (≥ 10 μg L-1). Three-

240

dimensional data visualization was performed for selected chemical classes by using a

241

kernel function to fit a smoothly tapered surface to each point. In order to avoid over

242

weighting areas with a high density of observations (e.g., a well with a long history of

243

time-resolved data), a fishnet of rectangular cells (1 km2) was created for the entire state

244

of California and the maximum Cr(VI) value was chosen for each cell using a spatial

245

join. ArcScene 10.4.1 was used for 3D rendering of output rasters (Figure 1b; Figure 2).

246

Qualitative 3D images are accompanied by numeric values aggregated by county. For

247

all well pairs, the concentration of each chemical and elevated Cr(VI) ( 10 μg L-1) was

248

averaged by a unique well-site identifier before being averaged by county (Supporting

249

Information, Tables S1 - S8).

250

Box plots created in R were used to visualize the distribution of Cr(VI) within

251

supply wells throughout the Central Valley (Figure 3). For this purpose, the Valley was

252

divided into five regions based on surface hydrologic features: the western Sacramento

253

Valley, eastern Sacramento Valley, western San Joaquin Valley, eastern San Joaquin

254

Valley, and southern Valley region that lies outside of the connected river system.

11

ACS Paragon Plus Environment

Environmental Science & Technology

255 256

RESULTS AND DISCUSSION

257

Hundreds of supply wells across California have exceeded Cr(VI)

258

concentrations of 10 g L-1 in the past several years (Figure 1, Table 1; Supporting

259

Information, Figure S1). Indeed, 780 of 10,642 sampled public supply wells have Cr(VI)

260

concentrations greater than 10 µg L-1 for at least one measured sample. However, within

261

the studied dataset, monitoring wells have the highest reported concentrations of Cr(VI),

262

reaching 2.9 g L-1 in groundwater. Monitoring wells are regulated by the California

263

Water Board with 26% (1,335 of 5,073 wells) showing Cr(VI) concentrations above 10

264

µg L-1; in some cases, they have acute Cr(VI) contamination with concentrations in

265

excess of 1,000 µg L-1 (Table 1). Only ca. 7% of supply wells have average Cr(VI)

266

concentrations that exceed 10 g L-1 (780 of 10,642 wells), while 15% percent of all

267

Cr(VI) measurements within the supply wells are greater than 10 g L-1 (5,663 of 37,002

268

samples) (Table 1); this may reflect, in part, more frequent monitoring requirements for

269

wells with known Cr(VI) concentrations above the MCL. The state-wide spatial

270

distribution of Cr(VI) in groundwater highlights the contribution from urban centers

271

where monitoring wells predominate (Figure 1). While monitoring constraints and

272

defined priority areas spatially bias the data, the GAMA program has taken measures to

273

promote a statistically-consistent basis for comparing chemical concentrations in supply

274

wells within different study units. The GAMA program combines the highest priority

275

basins into 35 study units and subsequently samples 60 - 120 public supply wells in an

276

attempt to provide a spatially-unbiased assessment of groundwater quality within each

277

study unit.32

12

ACS Paragon Plus Environment

Page 12 of 34

Page 13 of 34

Environmental Science & Technology

278 279

Groundwater Cr(VI) Arising from Industrial Activities

280

Hexavalent chromium in groundwater monitoring wells largely originates from

281

direct anthropogenic activities, either from Cr(VI) used for industrial purposes or from

282

oxidation of naturally-occurring Cr(III) by oxidants injected for organic solvent clean-

283

up (as seen by the high co-occurrence of volatile organic compounds and bromoform

284

concentrations with Cr(VI), as illustrated in Figure 2). Chemicals closest to having a

285

monotonic relationship with Cr(VI) are 1,4-dioxane, 1,1-dichloroethene (1,1-DCE), and

286

bromoform; hierarchical clustering further reveals relationships between Cr(VI) and

287

both nitrate and dissolved oxygen (Supporting Information, Figures S3). Due to the

288

large spatial-scale over which the correlation analysis is conducted, some correlations

289

may be masked—geostatistical analysis at local scales may reveal more detailed

290

relationships between groundwater constituents.

291

The high-degree of correlation between Cr(VI) and chlorinated solvents (e.g.,

292

1,1-DCE, 1,1-DCA), their byproducts, and solvent stabilizers, such as 1,4-dioxane, is

293

indicative of pollution from metal manufacturing, cleaning, and surface preparation

294

prior to chrome plating, where these solvents are used in sequence as degreasers and

295

anti-corrosive agents prior to chromic acid treatment.33,34 Fourteen percent of

296

monitoring wells reporting Cr(VI) concentrations ≥ 10 μg L-1 (686 of 5,073 wells) co-

297

occur with 1,2-dichloroethene (cis- and trans-1,2-DCE) concentrations exceeding 100

298

μg L-1 (Figure 2; Table 1). A similar, but narrower, distribution of co-occurrence is seen

299

between 1,1-DCE and Cr(VI) (data not shown). Unintentional oxidation of naturally-

300

occurring Cr(III) by ISCO, used to remediate volatile organic compounds, likely

13

ACS Paragon Plus Environment

Environmental Science & Technology

301

explains the high correlation with some of the organic chemicals such as bromoform.21–

302

23

303

with Cr(VI) concentrations ≥10 g L-1 correlate with this organic compound (Figure 2;

304

Table 1; and Supporting Information, Table S3). The higher co-occurrence of

305

bromoform with Cr(VI) in monitoring wells relative to supply wells suggests that the

306

co-occurrence is related to industrial Cr(VI) contamination. Although bromoform is one

307

of the trihalomethanes known to occur as a disinfection byproduct in drinking water

308

where organic compounds react with bromide35, it can also be formed by reaction of

309

bromide with ozone.36 Within the dataset, monitoring wells show spikes in Cr(VI) co-

310

occurring with spikes in bromoform after ISCO by ozone sparging, supporting the

311

hypothesis that ISCO may be a relevant pathway contributing to Cr(VI) contamination.

312

However, it is also possible that bromoform is a byproduct of septic systems.

313

Permanganate, also used for ISCO, cannot be distinguished from natural Mn

314

concentrations within the database (or persulfate from sulfate). However, Cr(VI)

315

generation has been seen following ozone sparging for remediation of petroleum

316

hydrocarbons within Cr(III)-rich sediment.37,38

Indeed, 17% of supply wells (132 of 780) and 54% of monitoring wells (718 of 1335)

317

In areas with high naturally occurring Cr(III) minerals, such as Winters,

318

Woodland, Dixon, Esparto, San Jose, and Watsonville (Figure 4), the use of ISCO for

319

remediating groundwaters contaminated with chlorinated solvents, including

320

contamination by perchloroethylene (PCE), trichloroethylene (TCE), and 1,2-

321

dichloroethylene (DCE), appears to have contributed Cr(VI) concentrations up to

322

millions of μg L-1 (Figure 2; Supporting Information, Tables S4-S7). Although cleanup

323

reports for many wells with the highest reported Cr(VI) concentrations in the GAMA

14

ACS Paragon Plus Environment

Page 14 of 34

Page 15 of 34

Environmental Science & Technology

324

database reveal that these spikes occurred only after ISCO injections, a quantitative

325

statewide comparison of industrial sources in relation to ISCO is not yet available. The

326

attributed source of Cr(VI) at any individual site in the GAMA dataset can only be found

327

in associated cleanup reports for monitoring wells; this information is not yet included

328

in downloadable datasets (GeoTracker GAMA, 2016).28 Elevated metal concentrations

329

have recently prevented the closure of a number of ISCO-treated sites.39 Despite the

330

uncertainty surrounding the processes controlling the release of metals, only 21% of

331

historical ISCO sites (19 out of 89 case studies) have monitored metals.39 Chromium

332

was impacted by all of the oxidant chemistries selected to represent a range of ISCO

333

treatments. Further, the release of Cr(VI) has been reported as a concern at ISCO sites

334

where persulfate22, permanganate40, or hydrogen peroxide were used for treatment.21,41

335

While Cr concentrations often attenuate following ISCO treatment, there is no evidence

336

of Cr recovery to concentrations below (pre-2018 California) MCL values post-

337

treatment. Even for persulfate treated sites, which have been shown to release less

338

Cr(VI) than permanganate treatment schemes, average Cr(VI) groundwater

339

concentrations increase by an order of magnitude (0.7 to 78.2 µg L-1) even six months

340

after treatment (with significantly higher short-term increases).39 Chromium(VI)

341

concentrations have been reported as high as 3 mg L-1 post field treatment;39 a survey

342

of 23 ISCO treatment sites showed not just elevated metal concentrations at over half

343

the sites but also the migration of metals outside the treatment area.39,42 It is therefore

344

important to screen soils and sediments for metals before ISCO treatment and to conduct

345

extensive monitoring following the injection of any oxidants.

346

15

ACS Paragon Plus Environment

Environmental Science & Technology

347

Groundwater Cr(VI) Arising from Natural Sources

348

Supply wells typically have minimal direct impact from industrial processes and instead

349

represent Cr(VI) generated from weathering and oxidation of naturally-occurring Cr-

350

containing minerals; however, human activity (beyond ISCO) may inadvertently

351

accelerate oxidation of Cr(III) to Cr(VI). Vanadium(III), like Cr(III), is present in

352

ultramafic-derived minerals,43 and the correlation between V(V) and Cr(VI) here may

353

be due to similar weathering and oxidation processes. However, V and Cr may also co-

354

occur as phosphate fertilizer impurities.44 Supporting a geogenic origin, Wright and

355

Belitz45 found high concentrations of V in groundwater in agricultural areas as well as

356

in oxic and alkaline groundwater of California beyond regions receiving fertilizer input;

357

Izbicki et al.20 similarly showed that Cr(VI) is also associated with oxic and alkaline

358

groundwater. In combination, the results of Wright and Belitz45 combined with Izbicki

359

et al.20 suggest common sources of Cr and V to California groundwater independent of

360

fertilizer application. Spatial analysis of Cr(VI) distribution in supply wells of

361

California’s Central Valley (Figures 3 and 4) illustrates greater groundwater

362

concentrations on the west side of the Valley, consistent with previous

363

observations.15,24,46 Using geochemical cluster analyses, Morrison et al.24,47 argued that

364

western Central Valley soils were more extensively derived from ultramafic material

365

than their eastern, silicic counterparts. Using the GAMA dataset, Manning et al.15 also

366

noted greater Cr(VI) concentrations in groundwater across the west side of the

367

Sacramento Valley and posited that distance from ultramafic outcrops was a primary

368

control on Cr(VI) in groundwater. Indeed, the highest concentrations of Cr(VI) in

369

groundwater within the Central Valley occur in the western region of both the

16

ACS Paragon Plus Environment

Page 16 of 34

Page 17 of 34

Environmental Science & Technology

370

Sacramento and San Joaquin Valleys (Figure 3) and generally correlate with ultramafic

371

outcrops (or their metamorphic derivatives). However, while distance from ultramafic

372

outcrops is highly correlated with certain areas of the western Central Valley, it does

373

not explain the entire Valley, or the entire state (Figure 4).48,49 Hundreds of wells with

374

average Cr(VI) concentrations greater than 10 μg L-1 are many tens of (or greater)

375

kilometers away from the nearest ultramafic outcrop, while hundreds of wells that are

376

much closer to ultramafic outcrops are, on average, below the detection limit. Alluvial

377

transport (or other material transport processes) may re-distribute Cr(III)-bearing solids,

378

as observed throughout the western region of the Central Valley, and negates the

379

correlation between ultramafic outcrops and Cr(VI) production. Moreover, Morrison et

380

al.50 suggests that chemical weathering and sediment transport of Cr(III)-bearing

381

minerals results in a more soluble form of Cr(III), which may account for a

382

disproportionate fraction of Cr(VI) generation.

383

Despite the fact that Cr concentrations within serpentine soils did not correspond

384

to variations in soil pH,51 it is important to consider that serpentine soils often have

385

lower pH values than their aquifer counterparts.52 In addition to oxidation processes, an

386

important outcome of pH is its control on Cr(VI) adsorption. Alkaline conditions restrict

387

Cr(VI) adsorption and further promote its dissolved concentrations and migration.53 The

388

distinct processes that generate Cr(VI) from geogenic forms of Cr(III) remain

389

unresolved, but within the literature there appears to be a strong link to pH, dissolved

390

oxygen, unsaturated zone thickness, and Mn dynamics, specifically co-locating Mn-

391

oxide minerals with Cr(III) solids.15,54 Our findings suggest that while industrial uses of

392

Cr lead to the most acute cases of groundwater Cr(VI) contamination, oxidation of

17

ACS Paragon Plus Environment

Environmental Science & Technology

393

naturally-occurring Cr, both due to natural processes and anthropogenic activity (e.g.,

394

land-use change, ISCO), likely affects a much larger area of California and threatens

395

the water supply of a greater number of people.

396 397

Potential Agricultural Links

398

Many of the chemicals positively correlated with Cr(VI) in the supply well

399

dataset are related to agriculture. Agriculture-related compounds with the strongest

400

positive correlations with Cr(VI) are ammonia, nitrate, and DDE (Supporting

401

Information, Figures S5); they may co-occur with Cr(VI) through direct inputs (e.g., Cr-

402

bearing fertilizer) or via soil processes that enhance Cr(III) oxidation and/or Cr(VI)

403

desorption. For example, ammonia and nitrate, which correlate with Cr(VI) (Figure 2;

404

Supporting Information, Table S8), may contribute to Cr(VI) generation/release through

405

acid or base generation upon nitrification or ammonification, respectively. Laboratory

406

incubation experiments show a significant increase in Cr(VI) production rates for HCl-

407

amended soils indicating that soil acidification, resulting from processes such as

408

nitrification of ammonium in fertilizers, may affect Cr-redox cycling.46 Subsequent

409

experiments show, in fact, an increase in Cr(VI) generation associated with acidification

410

resulting from nitrification of (NH4)2SO4, although not as much as predicted by HCl

411

additions.55 Indeed, the GAMA dataset illustrates a correlation between nitrate and

412

Cr(VI) in both the monitoring and supply wells (Figure 2e-f, Table 2), consistent with

413

previous studies;15,46,54,56,57 however, whether this is a causal mechanism or a correlated

414

phenomenon is not yet understood. Nitrate and Cr(VI) may co-occur in shallow aquifers

415

as a result of high nitrogen loading, fertilization, and increased recharge through the

18

ACS Paragon Plus Environment

Page 18 of 34

Page 19 of 34

Environmental Science & Technology

416

vadose zone by irrigation. Further, nitrate may enhance58 or interfere with59 Cr(VI)

417

reduction, leading to correlations between nitrate and Cr(VI) in groundwater. It is

418

nevertheless clear that high nitrate alone does not induce high Cr(VI) concentrations, as

419

noted within Figure 2. Nitrate may also be associated with septic discharge, and the

420

counties outside of the Central Valley (or counties less than ~100 acres of crop cover)

421

with high rates of co-occurrence of nitrate with Cr(VI), in fact, correspond to the

422

counties with the highest population density (Figure S7). Legacy land-use effects may

423

also contribute to the co-occurrence of Cr(VI) with nitrate in the Los Angeles, which

424

was an agricultural center of North America until the 1950s.60 A limitation of the current

425

GAMA dataset is that the depth of supply-wells is unknown and so a trend between

426

Cr(VI) and nitrate with depth cannot be examined using the complete dataset. However,

427

Manning et al.15 found that older, deeper groundwater had lower concentrations of

428

nitrate, even when it had elevated concentrations of Cr(VI).

429

Irrigation coupled with groundwater extraction increases the propensity for

430

downward migration of compounds through soils and underlying sediments, as is well

431

illustrated for nitrate, and thus may serve as an additional link between agriculture and

432

groundwater Cr(VI) concentrations. The simplest link with irrigation is therefore that

433

Cr(VI) generated within soils or near-surface sediments may be transported down into

434

underlying aquifers more rapidly due to increased recharge. More importantly, however,

435

may be a change (or acceleration) of Mn cycling within the vadose zone, leading to

436

greater interaction between Cr(III) minerals and Mn(III/IV)-bearing oxides—the result

437

being generation of Cr(VI), as also suggested by Manning et al.15. Fluctuating water

438

content may lead to Mn(III/IV) reduction during wetting (or water-table rebound)

19

ACS Paragon Plus Environment

Environmental Science & Technology

439

periods, followed by oxidative re-distribution of Mn-oxides during drying (or water-

440

table drawdown). Concentration of Mn(III/IV)-oxides proximal to Cr(III) bearing

441

minerals may result with more extensive wetting-drying cycles and would then lead to

442

Cr(VI) generation, as described in Hausladen and Fendorf (2017).61

443

Data for a robust statistical correlation between groundwater pumping or

444

irrigation and groundwater Cr(VI) concentrations are not yet available. The area

445

equipped for irrigation does, however, show the overlay with groundwater Cr(VI)

446

concentrations (Figure 5). Similar to nitrate, there are many areas outside of the

447

irrigation zone where Cr(VI) results in high groundwater concentrations from natural

448

origins. Thus, conclusive links between groundwater pumping, irrigation, and

449

groundwater Cr(VI) concentrations awaits further study.

450 451

Multiple Cr(VI) Sources

452

Chemical signatures associated with elevated Cr(VI) differ between wells

453

associated with contaminated sites (monitoring wells) and those used for drinking water

454

(supply wells). Chromium(VI) hotspots surrounding industrialized centers highlight the

455

legacy of anthropogenic contamination. The high correlation between peak Cr(VI)

456

concentrations and compounds indicative of metal manufacturing, cleaning, and chrome

457

plating tie this industrial legacy to the most acute cases of groundwater contamination.

458

Furthermore, oxidative remediation efforts targeting chlorinated solvents likely

459

compound Cr(VI) contamination by enhancing transformation of native Cr(III) in

460

aquifer sediments, noticeable especially in monitoring wells. In wells free of industrial

461

inputs, where contamination levels are generally much lower, Cr(VI) concentrations

20

ACS Paragon Plus Environment

Page 20 of 34

Page 21 of 34

Environmental Science & Technology

462

most closely correlate with compounds reflecting agricultural activity (e.g., nitrate). The

463

Central Valley illustrates the strongest link between agriculture activity and oxidation

464

of naturally-occurring Cr(III). Further work may be able to substantiate whether a

465

process-level response of Cr(VI) generation to agricultural practices exists—at present

466

there is no confirmative, direct evidence for causation. Importantly, the spatial

467

distribution of Cr(VI) throughout California reflects regional controls: from metal

468

plating industry around the two major metropoles to natural redox conditions along slow

469

flow paths in the Mojave to land-use change increasing transport through the vadose

470

zone of the Central Valley.

471 472 473 474 475 476 477 478 479 480 481 482 483

ACKNOWLEDGEMENTS

484 485 486 487 488 489 490 491

SUPPORTING INFORMATION

This research was supported in part by a National Science Foundation Graduate Research Fellowship under Grant No. DGE-114747 and by the US Department of Energy (DOE) Office of Science, Office of Biological and Environmental Research (BER), through the Subsurface Biogeochemistry Program (Award Number DESC0016544) and SLAC National Accelerator Laboratory scientific focus area (SFA) (Contract No. DE-AC02-76SF00515). We are very grateful to David Medieros and Ana Rivera, at the Stanford Geospatial Center, for their assistance with ArcGIS analyses. We also thank Ellery Wulczyn and Kris Sankaran for their invaluable statistical analyses.

Map of reported Cr(VI) concentrations for supply and monitoring wells within the GeoTracker GAMA dataset (Figure S1), Frequency histogram of Cr(VI) (Figure S2), Spearman’s and Pearson’s correlations and hierarchical clustering for monitoring and supply wells (Figure S3 – S6), Regional population and crop coverage statistics (Figure S7), Summary statistics for Cr(VI) and suite of groundwater constituents measured throughout California at both monitoring and supply wells (Table S1 – S8)

21

ACS Paragon Plus Environment

Environmental Science & Technology

492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534

REFERENCES (1) Bartlett, R. J; James, B. R. Behavior of Chromium in Soils: III. Oxidation. J. Environ. Qual. 1979, 8 (1), 31–35. (2) Ball, J. W.; Nordstrom, D. K. Critical Evaluation and Selection of Standard State Thermodynamic Properties for Chromium Metal and Its Aqueous Ions, Hydrolysis Species, Oxides, and Hydroxides. J. Chem. Eng. Data 1998, 43 (6), 895–918. (3) Fendorf, S. E. Surface Reactions of Chromium in Soils and Water. Geoderma 1995, 67 (1), 55–71. (4) Chromium, Nickel and Welding: Views and Experts Opinions of an IARC Working Group on the Evaluation of Carcinogenic Risks to Humans, Which Met in Lyon 5 ‐ 13 June 1989; International Agency for Research on Cancer, International Agency for Research on Cancer, Eds.; IARC monographs on the evaluation of carcinogenic risks to humans; Lyon, 1990. (5) Costa, M. Toxicity and Carcinogenicity of Cr(VI) in Animal Models and Humans. Crit. Rev. Toxicol. 1997, 27 (5), 431–442. (6) Welling, R.; Beaumont, J. J.; Petersen, S. J.; Alexeeff, G. V.; Steinmaus, C. Chromium VI and Stomach Cancer: A Meta-Analysis of the Current Epidemiological Evidence. Occup. Environ. Med. 2015, 72 (2), 151–159. (7) Sun, H.; Brocato, J.; Costa, M. Oral Chromium Exposure and Toxicity. Curr. Environ. Health Rep. 2015, 2 (3), 295–303. (8) Beaumont, J. J.; Sedman, R. M.; Reynolds, S. D.; Sherman, C. D.; Li, L.-H.; Howd, R. A.; Sandy, M. S.; Zeise, L.; Alexeeff, G. V. Cancer Mortality in a Chinese Population Exposed to Hexavalent Chromium in Drinking Water: Epidemiology 2008, 19 (1), 12–23. (9) Nriagu, J. O.; Nieboer, E. Chromium in the Natural and Human Environments; John Wiley & Sons, 1988. (10) Oze, C.; Bird, D. K.; Fendorf, S. Genesis of Hexavalent Chromium from Natural Sources in Soil and Groundwater. Proc. Natl. Acad. Sci. 2007, 104 (16), 6544– 6549. (11) Fandeur, D.; Juillot, F.; Morin, G.; Olivi, L.; Cognigni, A.; Webb, S. M.; Ambrosi, J.-P.; Fritsch, E.; Guyot, F.; Brown, J., Gordon E. XANES Evidence for Oxidation of Cr(III) to Cr(VI) by Mn-Oxides in a Lateritic Regolith Developed on Serpentinized Ultramafic Rocks of New Caledonia. Environ. Sci. Technol. 2009, 43 (19), 7384–7390. (12) Eary, L. E.; Rai, D. Kinetics of Chromium (III) Oxidation to Chromium (VI) by Reaction with Manganese Dioxide. Environ. Sci. Technol. 1987, 21 (12), 1187–1193. (13) Garnier, J.; Quantin, C.; Guimarães, E. M.; Vantelon, D.; Montargès-Pelletier, E.; Becquer, T. Cr(VI) Genesis and Dynamics in Ferralsols Developed from Ultramafic Rocks: The Case of NiquelâNdia, Brazil. Geoderma 2013, 193–194, 256–264.

22

ACS Paragon Plus Environment

Page 22 of 34

Page 23 of 34

Environmental Science & Technology

535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578

(14) McClain, C. N.; Fendorf, S.; Webb, S. M.; Maher, K. Quantifying Cr(VI) Production and Export from Serpentine Soil of the California Coast Range. Environ. Sci. Technol. 2017, 51 (1), 141–149. (15) Manning, A. H.; Mills, C. T.; Morrison, J. M.; Ball, L. B. Insights into Controls on Hexavalent Chromium in Groundwater Provided by Environmental Tracers, Sacramento Valley, California, USA. Appl. Geochem. 2015, 62, 186–199. (16) Ball, J. W.; Izbicki, J. A. Occurrence of Hexavalent Chromium in Ground Water in the Western Mojave Desert, California. Appl. Geochem. 2004, 19 (7), 1123–1135. (17) Mills, C. T.; Morrison, J. M.; Goldhaber, M. B.; Ellefsen, K. J. Chromium(VI) Generation in Vadose Zone Soils and Alluvial Sediments of the Southwestern Sacramento Valley, California: A Potential Source of Geogenic Cr(VI) to Groundwater. Appl. Geochem. 2011, 26 (8), 1488–1501. (18) Gonzalez, A. R.; Ndung’u, K.; Flegal, A. R. Natural Occurrence of Hexavalent Chromium in the Aromas Red Sands Aquifer, California. Environ. Sci. Technol. 2005, 39 (15), 5505–5511. (19) Ndung’u, K.; Friedrich, S.; Gonzalez, A. R.; Flegal, A. R. Chromium Oxidation by Manganese (Hydr)Oxides in a California Aquifer. Appl. Geochem. 2010, 25 (3), 377–381. (20) Izbicki, J. A.; Ball, J. W.; Bullen, T. D.; Sutley, S. J. Chromium, Chromium Isotopes and Selected Trace Elements, Western Mojave Desert, USA. Appl. Geochem. 2008, 23 (5), 1325–1352. (21) Rock, M. L.; James, B. R.; Helz, G. R. Hydrogen Peroxide Effects on Chromium Oxidation State and Solubility in Four Diverse, Chromium-Enriched Soils. Environ. Sci. Technol. 2001, 35 (20), 4054–4059. (22) Kaur, K.; Crimi, M. Release of Chromium from Soils with Persulfate Chemical Oxidation. Groundwater 2014, 52 (5), 748–755. (23) ITRC (Interstate Technology In Situ Chemical Oxidation Team Regulatory Council). Technical and Regulatory Guidance for In Situ Chemical Oxidation of Contaminated Soil and Groundwater; Second Edition ISCO-2; Washington, DC, 2005; p 172. (24) Morrison, J. M.; Goldhaber, M. B.; Lee, L.; Holloway, J. M.; Wanty, R. B.; Wolf, R. E.; Ranville, J. F. A Regional-Scale Study of Chromium and Nickel in Soils of Northern California, USA. Appl. Geochem. 2009, 24 (8), 1500–1511. (25) Hanson, R. T.; Flint, A. L.; Flint, L. E.; Faunt, C. C.; Schmid, W.; Dettinger, M. D.; Leake, S. A.; Cayan, D. R. Integrated Simulation of Consumptive Use and Land Subsidence in the Central Valley, California, for the Past and for a Future Subject to Urbanization and Climate Change, Paper Presented at the Eighth International Symposium on Land Subsidence (EISOLS), Queretaro, Mexico. IAHS Publ 2010, 339, 467–471. (26) Scanlon, B. R.; Faunt, C. C.; Longuevergne, L.; Reedy, R. C.; Alley, W. M.; McGuire, V. L.; McMahon, P. B. Groundwater Depletion and Sustainability of Irrigation in the US High Plains and Central Valley. Proc. Natl. Acad. Sci. 2012, 109 (24), 9320–9325.

23

ACS Paragon Plus Environment

Environmental Science & Technology

579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623

(27) California State Water Resources Control Board. Chromium-6 Drinking Water MCL https://www.waterboards.ca.gov/drinking_water/certlic/drinkingwater/C hromium6.html (accessed Dec 17, 2017). (28) California State Water Resources Control Board. GeoTracker GAMA. http://geotracker.waterboards.ca.gov/gama (accessed Jul 14, 2016). (29) United States Geological Survey. Groundwater Ambient Monitoring and Assessment Program (GAMA) https://ca.water.usgs.gov/projects/gama/index.html (accessed Mar 17, 2018). (30) Rao, B.; Hatzinger, P. B.; Böhlke, J. K.; Sturchio, N. C.; Andraski, B. J.; Eckardt, F. D.; Jackson, W. A. Natural Chlorate in the Environment: Application of a New IC-ESI/MS/MS Method with a Cl 18 O 3 - Internal Standard. Environ. Sci. Technol. 2010, 44 (22), 8429–8434. (31) Environmental Protection Agency. Technical Fact Sheet ‐ 1,4‐Dioxane; EPA 505-F-14-011; 2014. (32) United States Geological Survey; California State Water Resources Control Board. California Water Science Center - Groundwater Ambient Monitoring and Assessment (GAMA) Program https://ca.water.usgs.gov/projects/gama/index.html (accessed Mar 17, 2018). (33) Baral, A.; Engelken, R. D. Chromium-Based Regulations and Greening in Metal Finishing Industries in the USA. Environ. Sci. Policy 2002, 5 (2), 121– 133. (34) Eichinger, E.; Osborne, J.; Van Cleave, T. Hexavalent Chromium Elimination: An Aerospace Industry Progress Report. Met. Finish. 1997, 95 (3), 363840– 41. (35) Chow, A. T.; Dahlgren, R. A.; Harrison, J. A. Watershed Sources of Disinfection Byproduct Precursors in the Sacramento and San Joaquin Rivers, California. Environ. Sci. Technol. 2007, 41 (22), 7645–7652. (36) Hoigné, J. The Chemistry of Ozone in Water. In Process Technologies for Water Treatment; Stucki, S., Ed.; Springer US: Boston, MA, 1988; pp 121–141. (37) Flora, T.; Regional Water Board. State Water Resources Control Board GeoTracker https://geotracker.waterboards.ca.gov/profile_report.asp?global_id=T0608 590392 (accessed Mar 20, 2018). (38) Oze, C. J.; LaForce, M. J.; Wentworth, C. M.; Hanson, R. T.; Bird, D. K.; Coleman, R. G. Chromium Geochemistry of Serpentinous Sediment in the Willow Core, Santa Clara County, CA; Open-File Report 03–251; USGS, 2003; p 24. (39) Impacts on Groundwater Quality Following the Application of ISCO: Understanding the Cause of and Designing Mitigation for Metals Mobilization; Strategic Environmental Research and Development Program Final Report SERDP ER-2132; Department of Defense, 2015; p 282. (40) Crimi, M. L.; Siegrist, R. L. Geochemical Effects on Metals Following Permanganate Oxidation of DNAPLs. Ground Water 2003, 41 (4), 458–469. 24

ACS Paragon Plus Environment

Page 24 of 34

Page 25 of 34

Environmental Science & Technology

624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668

(41) Villa, R. D.; Trovó, A. G.; Nogueira, R. F. P. Environmental Implications of Soil Remediation Using the Fenton Process. Chemosphere 2008, 71 (1), 43–50. (42) Siegrist, R. L.; Crimi, M. L.; Petri, B.; Simpkin, T.; Palaia, T.; Krembs, F.; Munakata-Marr, J.; Illangasekare, T.; Ng, G.; Singletary, M.; et al. In Situ Chemical Oxidation for Groundwater Remediation: Site Specific Engineering and Technology Application.; ER-0623 Final Report; 2009. (43) Schwertmann, U.; Pfab, G. Structural Vanadium and Chromium in Lateritic Iron Oxides: Genetic Implications. Geochim. Cosmochim. Acta 1996, 60 (21), 4279–4283. (44) Molina, M.; Aburto, F.; Calderón, R.; Cazanga, M.; Escudey, M. Trace Element Composition of Selected Fertilizers Used in Chile: Phosphorus Fertilizers as a Source of Long-Term Soil Contamination. Soil Sediment Contam. Int. J. 2009, 18 (4), 497–511. (45) Wright, M. T.; Belitz, K. Factors Controlling the Regional Distribution of Vanadium in Groundwater. Ground Water 2010, 48 (4), 515–525. (46) Mills, C. T.; Morrison, J. M.; Goldhaber, M. B.; Ellefsen, K. J. Chromium(VI) Generation in Vadose Zone Soils and Alluvial Sediments of the Southwestern Sacramento Valley, California: A Potential Source of Geogenic Cr(VI) to Groundwater. Appl. Geochem. 2011, 26 (8), 1488–1501. (47) Morrison, J. M.; Goldhaber, M. B.; Ellefsen, K. J.; Mills, C. T. Cluster Analysis of a Regional-Scale Soil Geochemical Dataset in Northern California. Appl. Geochem. 2011, 26, S105–S107. (48) Oakeshott, G. Geology and Mineral Deposits of San Fernando Quadrangle. Los Angel. Cty. Calif. Calif. Div. Mines Geol. Bull. 1958, 172. (49) Ehlig, P. L. Causes of Distribution of Pelona, Rand, and Orocopia Schist along the San Andreas and Garlock Faults. Stanf. Univ. Publ. Geol. Soc. 1968, 11, 294–306. (50) Morrison, J. M.; Goldhaber, M. B.; Mills, C. T.; Breit, G. N.; Hooper, R. L.; Holloway, J. M.; Diehl, S. F.; Ranville, J. F. Weathering and Transport of Chromium and Nickel from Serpentinite in the Coast Range Ophiolite to the Sacramento Valley, California, USA. Appl. Geochem. 2015, 61, 72–86. (51) Oze, C.; Fendorf, S.; Bird, D. K.; Coleman, R. G. Chromium Geochemistry in Serpentinized Ultramafic Rocks and Serpentine Soils from the Franciscan Complex of California. Am. J. Sci. 2004, 304 (1), 67–101. (52) Oze, C.; Fendorf, S.; Bird, D. K.; Coleman, R. G. Chromium Geochemistry of Serpentine Soils. Int. Geol. Rev. 2004, 46 (2), 97–126. (53) Rai, D.; Eary, L. E.; Zachara, J. M. Environmental Chemistry of Chromium. Sci. Total Environ. 1989, 86 (1–2), 15–23. (54) Izbicki, J. A.; Wright, M. T.; Seymour, W. A.; McCleskey, R. B.; Fram, M. S.; Belitz, K.; Esser, B. K. Cr(VI) Occurrence and Geochemistry in Water from Public-Supply Wells in California. Appl. Geochem. 2015, 63, 203–217. (55) Mills, C. T.; Goldhaber, M. B. Laboratory Investigations of the Effects of Nitrification-Induced Acidification on Cr Cycling in Vadose Zone Material Partially Derived from Ultramafic Rocks. Sci. Total Environ. 2012, 435–436, 363–373. 25

ACS Paragon Plus Environment

Environmental Science & Technology

669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686

(56) Dawson, B. J. M.; Bennett V, G. L.; Belitz, K. Ground‐Water Quality Data in the Southern Sacramento Valley, California, 2005—Results from the California GAMA Program; Data Series 285; USGS, 2008. (57) Spalding, R. F.; Exner, M. E. Occurrence of Nitrate in Groundwater--A Review. J. Environ. Qual. 1993, 22, 392–402. (58) Han, R.; Geller, J. T.; Yang, L.; Brodie, E. L.; Chakraborty, R.; Larsen, J. T.; Beller, H. R. Physiological and Transcriptional Studies of Cr(VI) Reduction under Aerobic and Denitrifying Conditions by an Aquifer-Derived Pseudomonad. Environ. Sci. Technol. 2010, 44 (19), 7491–7497. (59) Chovanec, P.; Sparacino-Watkins, C.; Zhang, N.; Basu, P.; Stolz, J. F. Microbial Reduction of Chromate in the Presence of Nitrate by Three Nitrate Respiring Organisms. Front. Microbiol. 2012, 3. (60) Surls, R.; Gerber, J. B. From Cows to Concrete: The Rise and Fall of Farming in Los Angeles; Angel City Press: Santa Monica, California, 2016. (61) Hausladen, D. M.; Fendorf, S. Hexavalent Chromium Generation within Naturally Structured Soils and Sediments. Environ. Sci. Technol. 2017, 51 (4), 2058–2067.

26

ACS Paragon Plus Environment

Page 26 of 34

Page 27 of 34

Environmental Science & Technology

Table 1. Summary values and statistics for groundwater monitoring and supply wells for all recorded sites (# wells) over time (# observations), along with average concentrations of signature chemicals that correlate with Cr(VI) concentrations ≥ 10 𝜇g L-1. The first row summarizes wells having detectable Cr(VI) concentrations; all other values and statistics are for wells reporting Cr(VI) concentrations ≥ 10 𝜇g L-1. Chemical concentrations are in μg L-1 except for nitrate (in mg L-1). SE = standard error.

Groundwater Supply Wells # # x chemical Cr(VI) # wells observations mean (± SE) mean (± SE) wells Cr(VI)(detectable) 10642 37002 - 5073 2 (±3) Cr(VI) ≥ 10 780 5663 - 17 (±5) 1335 Bromoform 132 1569 718 5 (±6) 17 (±8) Nitrate 75 142 70 (±19) 21 (±8) 556 1,4-Dioxane 45 47099 16 (±2) 14 (±11) 494 PCE 84 157536 24 (±38) 15 (±5) 947 TCE 77 180623 17 (±25) 15 (±6) 953 1,2-DCE - - - - 686







1

ACS Paragon Plus Environment

Environmental Monitoring Wells # x chemical Cr(VI) observations mean (± SE) mean (± SE) 50666 - 294 (±5648) 8811 - 1828 (±14522) 190334 71 (±533) 2919 (±26688) 1809 130 (±310) 1865 (±24884) 543764 215 (±510) 4108 (±27441) 507618 491 (±1196) 1944 (±15051) 949252 1171 (±1865) 1792 (±13924) 178304 894 (±3556) 3093 (±17533)

Environmental Science & Technology

Table 2. Spearman’s rho correlation coefficients for Cr(VI) relative to other chemical constituents in California groundwater over the past 15 years considering supply wells within a 1 km radius. Chemical Spearman's rho 4,4-DDE 0.2073686 p-value < 2.2e-16 Ammonia-N, Ammonium-N -0.2273531 p-value < 2.2e-16 Nitrate 0.3945855 p-value < 2.2e-16 Vanadium 0.4738548 p-value < 2.2e-16



2

ACS Paragon Plus Environment

Page 28 of 34

Page 29 of 34

Environmental Science & Technology



A

B

C [Cr(VI)] (!g L-1-1) ) [Cr(VI)] (!g L 10 >100 10 >100

Glenn: 8 wells Sutter: 2 wells Sacramento: 9 wells

Yolo: 28 wells Solano: 8 wells

Merced: 13 wells

Alameda: 2 wells San Mateo: 2 wells

Monterey: 9 wells

Santa Cruz: 7 wells

Santa Barbara: 8 wells

Kern: 8 wells San Bernardino: 35 wells

2000 1500 1000 0

500

Frequency Frequency

27200

D

Riverside: 42 wells

27700 27700

Los Angeles: 23 wells

0

20

40

60

80

100

120

140

160

180

200

Cr (VI) (!g L ) Cr(VI) (mg L-1)-1

Figure 1. (A) Study area and locations of monitoring (orange) and supply (purple) wells for all reported Cr(VI) concentrations (as of July 2016). (B) Normalized kernel density of mean Cr(VI) concentrations per square kilometer in California groundwater from supply and monitoring wells. Supply wells having (C) Cr(VI) concentrations above 10 µg L-1 for the 5-year period between 2009 and 2014 (dot size scales proportionally to Cr(VI) concentration) and (D) frequency distribution for 15-year period from 2000-2015.



3

ACS Paragon Plus Environment



Environmental Science & Technology

Cr(VI) correlating with VOCs

VOCs

C

Bromoform

E

Nitrate

Cr(VI) correlating with bromoform

D

Cr(VI) correlating with nitrate

F

Figure 2. Normalized kernel densities of (A) maximum concentrations for four volatile organic compounds (PCE, TCE, 1,2-DCE, 1,4-dioxane), (C) maximum bromoform concentrations, and (E) maximum nitrate concentrations for all groundwater monitoring and supply wells that occur within 1 km of elevated Cr(VI) concentrations (>10 μg L-1). (B, D, F) Normalized kernel densities of groundwater Cr(VI) concentrations corresponding to maximum concentrations of chemicals in panels A, C, and E, respectively.



4

ACS Paragon Plus Environment

Page 30 of 34

Page 31 of 34

Environmental Science & Technology

n = 1198

n = 848 n = 2317 n = 3008

n = 1631

Figure 3. Groundwater Cr(VI) concentrations within 5 regions of the Central Valley: western Sacramento Valley (WSV), eastern Sacramento Valley (ESV), western San Joaquin Valley (WSJ), eastern San Joaquin Valley (ESJ), and the southern region (SOU).



5

ACS Paragon Plus Environment

Environmental Science & Technology

Page 32 of 34

Kriged correlation between Cr(VI) in supply wells and proximity to ultramafic outcrops Significantly uncorrelated

Not Significant

Significantly correlated Major N-S rivers

Esparto !Woodland Winters ! Dixon !

Ultramafic outcrops

!

!

San Jose

!

Watsonville

¯ 0

35

70

140 Kilometers

Figure 4. Kriged correlation between Cr(VI) concentrations in groundwater supply wells and proximity to ultramafic outcrops within the Central Valley of California. Red zones indicate high degree of positive correlation between distance from ultramafic outcrops and Cr(VI) concentrations in groundwater.



6

ACS Paragon Plus Environment

Page 33 of 34

Environmental Science & Technology

Cr(VI) in µg L-1 10.00 - 50.00 50.01 – 100.00 100.01 – 407.00

Woodland Dixon

San Jose

Figure 5. Area in California equipped for irrigation in 2005, ultramafic outcrops, and groundwater supply wells exceeding 10 µg L-1 Cr(VI) between the years 2000 and 2015. Irrigation cells are approximately 10 km . Areas of mafic rock in Southern California (e.g., Pelona Schist, Orocopia Schist) not pictured. 2



7

ACS Paragon Plus Environment

Environmental Science & Technology

TOC Art







8

ACS Paragon Plus Environment

Page 34 of 34