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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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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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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-
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
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with bromoform result from volatile organic compound remediation using in-situ
40
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,
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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.
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)
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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
320
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
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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
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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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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-
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containing minerals; however, human activity (beyond ISCO) may inadvertently
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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-
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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;
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
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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
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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
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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)
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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
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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)
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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
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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)
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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
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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.
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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.
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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).
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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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