Coupled Hydro-Biogeochemical Processes Controlling Cr Reductive

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Coupled Hydro-Biogeochemical Processes Controlling Cr Reductive Immobilization in Columbia River Hyporheic Zone Yuanyuan Liu, Fen Xu, and Chongxuan Liu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05099 • Publication Date (Web): 20 Dec 2016 Downloaded from http://pubs.acs.org on December 30, 2016

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Environmental Science & Technology

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Coupled Hydro-Biogeochemical Processes Controlling Cr Reductive

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Immobilization in Columbia River Hyporheic Zone

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Yuanyuan Liu1†, Fen Xu1, 2†, Chongxuan Liu1, 3*

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School of Environmental Studies, China University of Geosciences, Wuhan, 430074, China 3

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Pacific Northwest National Laboratory, Richland, WA 99354

School of Environmental Science and Engineering, Southern University of Science and

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Technology, Shenzhen, 518055, China

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Submit to Environmental Science & Technology

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*

Corresponding authors: Chongxuan Liu; Pacific Northwest National Laboratory, K8-96,

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Richland, WA 99354; (509)371-6350, (509)371-7370, Fax(509)371-6354; Email:

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[email protected], [email protected]

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†

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and Xu, F. contributed to the experimental part.

Author contributions: these authors contributed equally. Liu, Y. contributed to the modeling part

1 ACS Paragon Plus Environment


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ABSTRACT: An experiment and modeling study was conducted to investigate coupled hydro-

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biogeochemical processes controlling reductive immobilization of groundwater Cr in the

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hyporheic zone (HZ) at U.S. Department of Energy’s Hanford Site, where dynamic surface

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water-groundwater exchange occurs on a daily basis. Experiments were performed to calibrate

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kinetic models and the calibrated models were incorporated into a multicomponent reactive

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transport model to simulate Cr redox transformation and immobilization under field hydrological

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conditions. The results revealed that the rates of Cr(VI) reduction, Cr(III) accumulation, and

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Cr(VI) release to the river are mostly affected by dynamic sediment redox conditions represented

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by Fe(II) reactivity, which is controlled by its cyclic interaction with O2 carried by river water,

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microbial activities, and the supply and bioavailability of organic carbon (OC) that is present in

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the HZ and/or carried by transport. In addition, the HZ geophysical properties including

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hydraulic conductivity and the thickness of the top alluvial layer have a significant influence on

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Cr reactive transport and immobilization by controlling residence times for reactions, and the

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supply rates of O2, Cr, and OC into the HZ. The results provide important insights into the

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dynamic redox environments in the HZ that can reductively immobilize contaminants.


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1. INTRODUCTION

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Hyporheic zones (HZ) are groundwater and surface water mixing zones where various physical,

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chemical, and microbial processes can simultaneously occur.1-10 Microbial processes can

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enzymatically, or generate reductants such as Fe(II) and S(-II) to abiotically reduce redox

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sensitive metals and contaminants such as As, Cr, U, and Tc.10-21 Dissolved oxygen (DO) carried

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by river water can oxidize redox sensitive metals and contaminants6, 22, 23 or inhibit contaminant

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reduction by oxidizing chemical reductants such as Fe(II) and S(-II).21, 24 The supply of nutrients

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for microbial growth and activities,1, 5, 8 the delivery of chemical reactants such as oxygen and

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biogenic reductants,24 and the residence times for metal and contaminant reduction6, 25 are all

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affected by hydrodynamic processes that regulate groundwater and surface water mixing in the

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HZ. The contaminant transformation and reactive transport in the HZ are therefore complexly

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affected by hydrological, geochemical, and biogeochemical processes. Reactive transport models

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have been used to integrate these processes to describe contaminant behavior in the HZ.4 Classic

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models treat the HZ as a storage zone without considering hydro-biogeochemical process

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interactions.7, 26-29 Some recent models treat the HZ as a reactive transport zone and use

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multicomponent approaches to describe the reactive transport of N and C under simplified flow

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conditions.30-35 Despite these efforts, the coupling effect of hydro-biogeochemical processes on

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contaminant transformation and migration under dynamic flow conditions in the HZ has not been

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well-understood.4

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Chromium (Cr) is a common contaminant in soils, sediments, surface water, and

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groundwater.36-38 Cr(III) is a human nutrient at low concentration; however, Cr(VI) is toxic and

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potentially carcinogenic.39, 40 EPA has a drinking water maximum contaminant level of 100 μg/L

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for total Cr,41 and a criterion continuous concentration of 74 μg/L for Cr(III) and 11 μg/L for



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Cr(VI) for protection of aquatic lives.42 Cr exists in a mobile form as Cr(VI) in aerobic

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environments and an relatively immobile form as Cr(III) in anaerobic environments.14, 21 Cr(III)

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can be stabilized in aqueous phase if complexed with organic or carbonate ligands.43, 44 Cr(VI)

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can be reduced to Cr(III) abiotically by organic45-47 and inorganic reductants such as Fe(II) and

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S(-II),14, 48 and biotically through microbial respiration as a terminal electron acceptor.49-51 The

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reduced Cr is stable in the natural sediments under circumneutral pH conditions unless the

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sediments are rich in strong oxidants such as Mn(III/IV) oxides and hydrogen peroxide.21, 52

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Reduction of Cr(VI) to Cr(III) is a common approach for immobilizing Cr in environments.53, 54

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This study used Cr as an example to investigate multicomponent interactions between Cr,

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Fe, O, and organic carbon (OC) in HZ sediments and their effects on contaminant reactive

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transport and immobilization in the HZ. The sediments were from the Columbia River HZ in the

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US Department of Energy Hanford 300 Area, which located downstream of the 100 Area where

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several chromate plumes with a maximum Cr(VI) concentration of 38.5 μM55 are migrating

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toward the nearby Columbia River56, 57 (Figure S1A). More information about Columbia River,

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Cr(VI) in the river, and Cr(VI) plumes in the Hanford 100 Area is provided in the Supporting

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Information (SI). In this study, laboratory experiments were performed to derive biogeochemical

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kinetic models, which were then incorporated into a reactive transport model to simulate Cr, Fe,

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O, and OC interactions under field hydrological conditions. The modeling results were used to

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assess Cr reductive immobilization in the HZ and to estimate the rate of Cr(VI) discharge to the

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Columbia River. Sensitivity analysis was performed to estimate the uncertainty of the model and

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to identify the critical processes and factors that affect Cr(VI) reduction, Cr(III) accumulation,

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and Cr(VI) release to the river. The results provide important insights into the dynamic redox

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behavior in the HZ, and the role of the HZ as an ecotone for processing contaminants and as a


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natural redox barrier for reductively immobilizing Cr under dynamic hydrological conditions.

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2. MATERIALS AND METHODS

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2.1. Study Site and Materials. The location and hydrogeology of the study site are provided in

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SI. Briefly, the top 0.5 m alluvial layer of the HZ is the most restrictive zone for groundwater and

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surface water flow (effective conductivity of 0.018 m·h−1). It is the last sediment domain to

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remove Cr from groundwater before it discharges to the river.56 It is the focus area of this study

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and termed as the HZ layer hereafter.

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Sediments used in this study were frozen-collected (SI) from the HZ layer at 20 different

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locations in the 300 Area (Figure S1). The sediment properties were provided in SI. The frozen

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sediments were thawed and the

Coupled Hydro-Biogeochemical Processes Controlling Cr Reductive

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