High Resolution Delineation of Contaminant Concentrations, Biogeochemical Processes, and Microbial Communities in Saturated Subsurface Environments

Dr. William Jackson | Texas Tech University

ER-2419

Objective

The ability to predict contaminant fate and transport in overburden aquifers is often limited by the intrinsic heterogeneity associated with the flow field, contaminant distribution, and coupled biotic and abiotic reactions. Processes occurring in low permeability zones are particularly important, as studies have demonstrated that contaminants residing in such materials can sustain groundwater plumes and impede overall contaminant attenuation. While the importance of identifying these processes in heterogeneous media has been well documented, there is currently no cost-effective tool for providing high resolution profiling of coupled contaminant, biogeochemical, and microbiological characteristics at the cm-scale.

This project will modify the design of equilibrium diffusion samplers (peepers) to provide high resolution delineation of the saturated subsurface with respect to distribution of contaminants (particularly chlorinated solvents), hydrogeologic conditions, geochemistry, and microbial community structure and activity. The primary objective is to develop and demonstrate a High Resolution Passive Profiler (HRPP) as a fine-scale delineation tool for the saturated subsurface. Focus will be placed on discerning contaminant, microbiological, and biogeochemical differences between low permeability and high permeability zones within heterogeneous or stratified media.

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Technical Approach

The HRPP will be designed to determine contaminant concentration and flux, groundwater velocity, microbial community structure, and potential for abiotic/biotic contaminant degradation in situ at the cm-scale along a vertical profile. Its development will consist of the following: (1) modifying the original peeper structure so that it can be inserted via direct push at total depths of at least 10 m; (2) incorporating the use of modified Bio-traps within the peeper intervals to evaluate microbial community structure and prevalence of critical biodegradative genes at the cm-scale; (3) utilizing granular activated carbon adsorbent to collect chlorinated solvents in each zone so that relative extents of contaminant degradation can be accessed via compound-specific stable isotope analysis; and (4) using conservative tracers within the peeper intervals to determine hydraulic and contaminant fluxes and also to facilitate determination of abiotic degradation rates. Tests and validation of the HRPP will be conducted in both laboratory flow cells and in a field trial.

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Interim Results

To date we have demonstrated in a combination of laboratory and field testing that the HRPP can successfully accomplish its goals including: evaluating chlorinated solvent concentrations and biogeochemical indicators (NO3-, NO2-, SO4), and community structure and activity at cm scale resolution. The HRPP prototype has been designed fabricated, and field tested. Structural analysis modeling and laboratory and field testing verify that the sampler is structurally strong enough to be inserted into saturated sediments using typical direct drive devices (e.g. GeoProbe). In a field trial the prototype was inserted to ~30ft below ground surface (BGS) without scoring with no structural deformation.

ER-2419 Graphic

Figure 1. Initial HRPP Design and Photograph of Stainless Prototype

In a field trial at Fort Dix, we installed two proto-type HRPP samplers using a standard Geoprobe at depths of 18 to 22 and 21 to 26ft BGS. Samplers were equilibrated for 21 days. After retrieval each HRPP produced duplicate data sets at 30cm intervals over a 4ft sampling depth. We demonstrated varying (200 to 100 µg/l) concentration profiles of cis-DCE even over the 4ft interval. As expected no nitrate and only minimal sulfate was present throughout the depth interval due to the strongly reducing conditions produced by ongoing substrate injections to stimulate reductive de-chlorination. Microbial analysis using the QuantArray procedure, demonstrates a diverse population of known dechlorinators. As expected the abundance and diversity of organisms varied between well samples and depth dependent HRPP samples. Finally, even with the relatively low concentrations of chlorinated solvents, we demonstrated that the HRPP was capable of collecting enough contaminant mass to successfully conduct compound-specific stable isotope analysis.

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Benefits

This project is expected to result in the development and field validation of a single tool that can be cost effectively and rapidly deployed to measure various parameters, including contaminant flux, groundwater velocity, degradative organisms, and the potential for abiotic/biotic contaminant degradation at the cm-scale in heterogeneous media. Development of samplers capable of producing characterization parameters with this level of resolution would be an advantage over existing methods and would lead to higher fidelity site models. Such a tool could also be easily employed for remedial design and assessing remedial technology effectiveness. (Anticipated Project Completion - 2019)

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Publications

Schneider, H., W.A. Jackson, K. Rainwater, D. Reible, S. Morse, P.B. Hatzinger, and U. Garza-Rubalcava. 2019. Estimation of Interstitial Velocity Using a Direct Drive High Resolution Passive Profiler. Groundwater, 10.1111/gwat.12874.

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Points of Contact

Principal Investigator

Dr. William Jackson

Texas Tech University

Program Manager

Environmental Restoration

SERDP and ESTCP

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