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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

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Environmental Science & Technology

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EST Article

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Hexavalent Chromium Sources and Distribution in California Groundwater

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

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Fendorf1*

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Earth System Science Dept. Geological Sciences Dept.

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Stanford University, Stanford, CA 94305. USA

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

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ACS Paragon Plus Environment


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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wells, and a greater number of people throughout California.

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INTRODUCTION

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

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

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

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

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

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

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

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

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

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

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jeopardizes water quality.1,8

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

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

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

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

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

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

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

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

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

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soils of California.1,11

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

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

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

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

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

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

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

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

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

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1 17–20

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

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

.

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

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

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

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

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

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

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metamorphically

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

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

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

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

derived

equivalent

rocks.24

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Concomitantly,


higher

Cr(VI)

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

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

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

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

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

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

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

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

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needed to better predict groundwater contamination and inform water management

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decisions.

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

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

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

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variables, including groundwater chemical constituents, groundwater level, sediment

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

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

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

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

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

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

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

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

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

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

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

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

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undisturbed, natural processes.

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METHODS

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Drinking Water Standard and Data Source

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

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

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

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

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

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levels.27

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Data used are from the GeoTracker and GeoTracker Groundwater Ambient

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

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

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

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

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

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

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

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

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

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further discussion.

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Statistical Analyses: Correlations and Hierarchical Clustering

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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values underwent a logarithmic transformation before the computation of correlation

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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Statistical Analyses: Correlations and Mapping

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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considered separately. Each isomer was considered separately when calculating

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Spearman’s correlation coefficients with Cr(VI).

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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Information, Tables S1 - S8).

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

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

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

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

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

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RESULTS AND DISCUSSION

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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study unit.32

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Groundwater Cr(VI) Arising from Industrial Activities

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Hexavalent chromium in groundwater monitoring wells largely originates from

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

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

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

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

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

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

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

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

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

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relationships between groundwater constituents.

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

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

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

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

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

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

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

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

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

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

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

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23

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

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

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

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

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

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

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

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

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

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

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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

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

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

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

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

319

remediating groundwaters contaminated with chlorinated solvents, including

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

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

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

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

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

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

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

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

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

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

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

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

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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

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

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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)

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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

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

345

extensive monitoring following the injection of any oxidants.

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347

Groundwater Cr(VI) Arising from Natural Sources

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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

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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;

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

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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

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

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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


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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



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


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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



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


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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



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

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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


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)


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


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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



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


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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



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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.

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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

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TOC Art

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1 EST Article 1 2 Hexavalent Chromium Sources ... - ACS Publications

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