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Geochemical Triggers of Arsenic Mobilization During Managed Aquifer Recharge Sarah Fakhreddine, Jessica Dittmar, Don Phipps, Jason Dadakis, and Scott Fendorf Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b01140 • Publication Date (Web): 09 Jun 2015 Downloaded from http://pubs.acs.org on June 18, 2015
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Geochemical Triggers of Arsenic Mobilization During Managed Aquifer Recharge
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Sarah Fakhreddine1, Jessica Dittmar1, Don Phipps2, Jason Dadakis2, and Scott Fendorf1*
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1Earth System Science Department, Stanford University, Stanford, CA 94305
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*Corresponding Author: Email:
[email protected] Phone: 650‐723‐5238
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Abstract
2Orange County Water District, 18700 Ward Street, Fountain Valley, CA 92708
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Mobilization of arsenic and other trace metal contaminants during managed aquifer
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recharge (MAR) poses a challenge to maintaining local groundwater quality, and thus also
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to ensuring the viability of aquifer storage and recovery techniques. Arsenic release from
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sediments into solution has occurred during purified recycled water recharge of shallow
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aquifers within Orange County, CA. Accordingly, we examine the geochemical processes
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controlling As desorption and mobilization from shallow, aerated sediments underlying
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MAR infiltration basins. Further, we conducted a series of batch and column experiments to
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evaluate recharge water chemistries that minimize the propensity of As desorption from
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the aquifer sediments. Within the shallow Orange County Groundwater Basin sediments,
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the divalent cations Ca2+ and Mg2+ are critical for limiting arsenic desorption; they promote
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As (as arsenate) adsorption to the phyllosilicate clay minerals of the aquifer. While native
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groundwater contains adequate concentrations of dissolved Ca2+ and Mg2+, these cations
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are not present at sufficient concentrations during recharge of highly purified recycled
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water. Subsequently, the absence of dissolved Ca2+ and Mg2+ displaces As from the
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sediments into solution. Increasing dosages of common water treatment amendments
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including quicklime (Ca(OH)2) and dolomitic lime (ca. CaO·MgO) provides recharge water
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with higher concentrations of Ca2+ and Mg2+ ions and subsequently decreases the release of
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As during infiltration.
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Introduction
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Managed Aquifer Recharge (MAR) is a viable option for both enhancing local
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groundwater resources and storing water for later usage. However, the addition of
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recharge water can alter native geochemistry and result in the mobilization of trace
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element contaminants. Arsenic is a ubiquitous contaminant of subsurface sediments and
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consequently can pose a risk to groundwater quality. Thus, an understanding of the various
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processes controlling arsenic mobilization during MAR is critical for preventing the
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degradation of recharge water quality. Furthermore, a better understanding of the
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geochemical interactions at the sediment‐water interface can dictate water treatment
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decisions and provide a basis for adjusting recharge water chemistry to minimize dissolved
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As concentrations during aquifer recharge.
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numerous sites and can be attributed to various geochemical mechanisms as reviewed in
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Neil et al.1 In general, common processes triggering arsenic release to groundwater can be
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categorized into (i) ligand exchange (ex. competitive displacement of arsenate by
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phosphate), (ii) desorption at alkaline pHs (>8.5), (iii) reduction of arsenate into the more
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labile arsenite species, and (iv) dissolution of arsenic bearing mineral phases as a result of
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a shift in redox conditions.2–4 For example, the injection of oxygenated recharge water into
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previously anoxic aquifers has resulted in the oxidative dissolution of arsenic‐bearing
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sulfidic minerals (primarily arsenian pyrite and arsenopyrite) and the subsequent release
The release of naturally occurring arsenic during MAR has been reported at
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of dissolved arsenic in MAR sites in the Netherlands, Australia and Florida, USA.5–12
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Instances of non‐redox related As desorption include an MAR site in the San Joaquin Valley,
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California, where the infiltration of high quality recharge water into the underlying shallow
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aerobic aquifer induced changes in pH resulting in the desorption of a labile fraction of
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As.13 Additionally, the exchange of competitive ligands, including phosphate ions, was
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reported as a mechanism of As release during aquifer recharge at sites in South Australia
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and Northern China.9,14
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This study focuses on MAR in Orange County, CA where highly purified recycled
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water of low ionic strength is used to augment local groundwater supplies by infiltration
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via surface recharge basins as part of Orange County Water District’s (OCWD) Groundwater
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Replenishment System (GWRS). Arsenic resides in the shallow aquifer sediments of the
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Orange County Groundwater Basin underlying the recharge basins, and infiltration of
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GWRS water has resulted in transient spikes in As concentrations as measured in nearby
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groundwater monitoring wells that dissipate with distance and successive recharge pulses.
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Since GWRS water contains no detectable As prior to infiltration, observed spikes in
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groundwater concentration suggest that a labile As source resides within the sediments
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and releases As into the pore water following infiltration of high purity recharge water. The
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ambient groundwater in the shallow aquifer is oxygenated and has an oxidative reductive
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potential (ORP) comparable to that of the infiltrating GWRS water, indicating that redox
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mechanisms are unlikely to cause the observed desorption of As. Further, the high purity
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recharge water does not provide a source of competitive ions and does not cause shifts to
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alkaline pHs. The release of As during infiltration in Orange County indicates that MAR
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processes can trigger mechanisms of As release which are not traditionally implicated in
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field‐scale systems. The objectives of our study were therefore to (i) identify the aqueous
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geochemical causal mechanism of arsenic mobilization and (ii) define recharge water
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amendments to minimize dissolved concentrations of this contaminant.
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We used a series of batch and column experiments of sediments collected near
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infiltration basins in Orange County, CA to test various triggers of arsenic release, including
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alterations in ionic strength and composition. Furthermore, we assess potential modifiers
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to GWRS water prior to recharge including the impact of saturating influent column water
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with quicklime (Ca(OH)2), dolomitic lime (ca. CaO·MgO), gypsum (CaSO4·2H2O) and
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combinations of the three amendments. In sum, our results provide a basis for
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understanding arsenic release from MAR and for modifying recharge water chemistry to
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minimize dissolved concentrations of this contaminant.
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Materials and methods
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Field site.
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Orange County Water District’s (OCWD) Groundwater Replenishment System
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(GWRS) is the world’s largest advanced treatment system for potable reuse with a current
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production capacity of 70 million gallons per day (MGD) and an anticipated expansion to
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100 MGD.15 Secondary‐treated wastewater is collected from the Orange County Sanitation
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District (OCSD) and further treated using microfiltration, reverse osmosis, and ultraviolet
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light with hydrogen peroxide addition for advanced oxidation to produce a high quality
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recharge supply for MAR. Prior to recharge into the aquifer, the water is stabilized in order
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to protect the distribution system (e.g., pipelines) using partial decarbonation and
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quicklime (Ca(OH)2) addition; the typical composition of post‐treatment purified recycled
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water and pre‐recharge groundwater composition are provided in Table 1 (an expanded
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version is provided in Supporting Information). The GWRS recharge water is discharged
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into a series of surface recharge basins, and infiltration of GWRS water has resulted in
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increased As concentrations in nearby groundwater. Prior to infiltration, the ambient
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groundwater and recharge water contain no detectable As (
