Stanford Synchrotron Radiation Lightsource

Biogeochemical responses to seasonal and synoptic hydrological changes (drought and flooding) profoundly influence groundwater quality across the Western U.S. Ebbing summer water tables expose sediments that were formerly water-saturated to air, causing oxidation of sulfides, dissolution of carbonates, and mobilization of dissolved organic carbon (DOC), nutrients, and contaminants. Declining snowpack, warmer summer temperatures, and drought, all of which depress groundwater levels, are expected to induce similar effects. Re-saturation of sediments occurs in spring when water tables rise concomitantly with meltwater-swollen rivers. Re-saturation releases a pulse of nutrients and contaminants and initiates a new suite of biogeochemical processes leading to precipitation of sulfides. This general release mechanism has been invoked to explain the longevity of U and metal contaminant plumes in the upper Colorado River Basin (CRB). Thus, because biogeochemistry is linked so strongly to moisture content, wet-dry cycling drives water quality.

Because groundwater discharge makes up a large fraction of surface water in Western watersheds, subsurface biogeochemical activity also mediates surface water quality. Biogeochemical reactions occurring along flow paths mediate water composition from infiltration or recharge points to discharge. Organic materials are particularly important in this context because biological activity is strongly enhanced in organic-enriched sediments, which are common in floodplains in the upper CRB. Organic-enriched sediments contain large inventories of C, S, Fe and contaminants, and exert outsized control on aquifer biogeochemistry and water quality.

The long-term (5 to 10-year) objective of this project is to address the overarching question: How do biogeochemical and transport processes in alluvial groundwater systems (bedrock to soil) couple to one-another and control water quality under hydrologically variable conditions? Over the next 3-year period, we will perform research to experimentally interrogate and model the coupling between hydrological and biogeochemical processes in shallow alluvial systems impacted by mining and ore processing activities. Quantitative process representations produced by this project will contribute to higher-scale water quality models. This project aligns with DOE-BER mission goals and investments in watershed function, hydrobiogeochemistry, and water quality.

The geographical center for this project is the upper CRB, for which a substantial biogeochemical and hydrologic literature exists that can guide our efforts. Uranium, molybdenum, and sulfate are persistent contaminants of concern at numerous DOE legacy processing sites across the upper CRB, including the former Riverton, WY ore processing site ( a focus of this project). Sediment-hosted Pb(II) and Zn(II), as well as Cd(II), and As(III)/(V) degrade water quality in the Ruby Range mining district (also a focus of the project) and others throughout the upper CRB.

Knowledge gaps. Chemical gradients are sharp, spatially variable, and transient in the capillary fringe. These qualities, as well as their impact on biogeochemistry, are challenging to model, but profoundly important to water quality. In spite of their importance, biogeochemical-hydrological couplings in transiently saturated zones have been largely overlooked. Little work has been done to develop quantitative process representations (reaction network models) extending down to the level of nutrient and contaminant mobilization, creating a substantial gap in water quality prediction capabilities. This project will define biogeochemical reaction networks operating under wet-dry cycles in shallow alluvial aquifers. We will further address the mechanisms by which transport and mixing link to these processes to drive water quality.

Overarching hypothesis: The overarching hypothesis that we will address is that redox cycling in the capillary fringe controls groundwater quality in shallow alluvial aquifers. Specifically, we posit that hydrologic forcing stimulates mixing, redox activity, solute transformations, and mineral precipitation/ dissolution. Moreover, reversing vertical transport of water due to ET and infiltration and lateral transport via advection, translocates nutrients and contaminants between soil, vadose and saturated zones. The direction of transport and suites of reactions that occur depend on the time of year and climate conditions.

Approach. To address these hypotheses, we will investigate the influence of wet-dry cycles on biogeochemical redox processes, mineral dissolution/precipitation and C mobilization in three subsurface zones, distinguished by moisture characteristics, that are ubiquitous in shallow alluvial systems: (i) soil (subtask 1), (ii) the capillary fringe (subtask 2), and (iii) the shallow aquifer (subtask 3). Subtask 3 explicitly examines coupling of biogeochemistry to unsaturated flow. Subtask 4 investigates nutrient and contaminant mobilization processes. The strategic aim of this organization is to enable a system-scale perspective, which is required to achieve our goal of developing quantitative reaction networks for coupled biogeochemical processes under variable hydrologic forcing. Reactive transport modeling will use these data to elucidate mechanisms of biogeochemical-transport coupling. The four hypothesis-driven subtasks are:

Subtask 1. Influence of rhizosphere processes on carbon dynamics and contaminant hosting solid phases. This subtask focuses on the question, "How do root-mediated processes influence biogeochemical cycles of relevance for contaminant mobility?" We will use laboratory flow reactor/rhizobox experiments to investigate mineral precipitation/dissolution and carbon transformation and export in the rhizosphere. Experimental results will be compared to field observations.

Subtask 2. Biogeochemical redox responses to hydrological transitions. Here we address the question, "What controls capillary fringe redox response to wet-dry cycles?" We expect organic carbon abundance to play a thresholding role and to mediate reduction rates, microbial community composition, and biogeochemical processes. We will collect porewater and sediments from laboratory flow reactor model systems and field sites, correlate solid/dissolved constituents to microbial community composition, and perform reactive transport modeling to understand these behaviors.

Subtask 3. Impact of hydrological-biogeochemical coupling on solute transport and groundwater biogeochemistry. We will address the question, "How do carbon and solutes transported from overlying redox-cycled sediments drive biogeochemical activity and water quality in underlying shallow aquifers?" We will investigate solute transport under saturated to partially saturated conditions in laboratory column experiments, and use reactive transport modeling to constrain their impact on redox processes.

Subtask 4. Mechanisms controlling release/uptake of nutrients and contaminants. Here we will address the question, "What are the molecular-scale mechanisms by which nutrients (P) and contaminants (U, Mo, Pb, Zn) are stored, transformed, and released in each of the aforementioned zones?" We expect that sediments will alternately import, export, and transform carbon, secondary mineral constituents, and contaminants. We will integrate spectroscopy and microscopy observations from model and field samples, laboratory measurements, and modeling to understand key mechanisms and reaction networks.

Integration of subtasks. The four subtasks strongly leverage one-another while allowing necessary experimental and scheduling flexibility. Because the subtasks are conceptually linked, advancements in any one area will simultaneously advance the others. We will use common experimental methodologies and team expertise to increase efficiency. Quantitative model representations developed in a given subtask will be explicitly utilized to model other subtasks.

Integrated experimental-modeling research plan. Our hypothesis-driven research plan will emphasize integration of robust experimental and modeling activities. Laboratory experiments will employ model systems to obtain porewater and sediment chemistry at high temporal (days) and spatial resolution (cm). Field sampling of porewater, gasses, sediments, and hydrologic parameters will be conducted to identify major processes, mobile species, biogeochemical hot spots/hot moments, and thus to ‘ground truth’ laboratory data and inform modeling activities.

Development of quantitative process representations. Reactive transport modeling will use experimental and field data to elucidate mechanisms of biogeochemical-transport coupling. We will develop consistent reaction networks containing compositional, kinetic, and thermodynamic frameworks of biogeochemical processes that can be coupled to transport processes to understand biogeochemical-hydrological coupling. We will work with the Berkeley Lab Watershed Function SFA to develop these representations so they can be incorporated into Scale Adaptive Watershed Simulation Capability (SAWaSC). We will similarly share process representations with the PNNL SFA and the general community.

Field sites. Redox cycling is expected to be influenced by climate-linked factors including: intensity of ET, direction and velocity of vertical and horizontal transport; the dominant form of precipitation; duration and frequency of spring/summer high-flow events and growing seasons; riparian vegetation composition; and the thickness of the capillary fringe. To better understand the collective impacts of these factors on redox cycling and linked biogeochemical processes, we will collect and analyze samples the semi-arid Riverton, WY floodplain site (Little Wind River, 1,500 m elevation), and head-watersheds of Slate River and Coal Creek in the Ruby Range mining district in Gunnison County, CO (3,000 m).