Field Assessment of Abiotic Attenuation Rates using Chemical Reactivity Probes and Cryogenic Core Collection

Dr. Richard Johnson | Oregon Health and Science University

ER-2621

Objective

Abiotic attenuation is a key restoration process at many complex sites and, as a consequence, accurate measures of reaction rates are important for site decision-making. The ideal approach for assessing abiotic attenuation rates would be rapid, in situ measurements using the contaminant of concern. Unfortunately, in situ reaction rates for chlorinated solvents like trichloroethene (TCE) are too slow to be directly measured in situ, even under enhanced remediation conditions. Due to this, the standard approach for estimating these rates is laboratory batch tests. However, the timeframe for those experiments is typically years, limiting their use for decision-making and raising issues regarding stability of the core materials during those tests. A more rapid laboratory approach is to use structurally-similar surrogate compounds, whose attenuation behavior is similar to TCE across a broad range of abiotic mineral reactions, but whose rates are 100-1000 times faster than TCE. Another approach is to use redox-active dyes as probes, because their reaction rates are rapid enough to be used for in situ measurements.

The objective of this project is to examine options for estimating abiotic attenuation rates for chlorinated solvents (e.g., TCE) using a combination of cryogenically-collected core samples, well-characterized mineral phases, and a suite of reactants, including TCE, surrogate chlorinated compounds, and reactive dyes. Specific technical objectives include:

  1. Demonstrate the utility of reactivity probes (RPs) to estimate TCE abiotic attenuation rates over a broad range of geochemical conditions using well-characterized mineral phases;
  2. Evaluate the use of RPs (and TCE) to measure abiotic reaction rates in field samples collected with the cryogenic core method; and
  3. Integrate the data from the first two objectives to determine if one or more of the RPs can provide robust measures of TCE attenuation rates using cryogenically-collected core samples and/or in situ field tests (e.g., push-pull tests).

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Technical Approach

This project will evaluate a combination of techniques for estimating abiotic attenuation rates for chlorinated solvents. In the first task, the project team will correlate the reaction rates of TCE/tetrachloroethene (PCE) with those of surrogate chlorinated compounds and reactive dyes (chemical reactivity probes [CRPs]) across a broad range of mineral surfaces and geochemical conditions using laboratory batch experiments. In the second task, the project team will quantify the relationship between TCE/PCE and CRPs with the relative reaction rates of surrogates in field samples. This will determine the practicality of using RPs in a field screen context. The team will collect the field cores using cryogenic core collection, then place sub-samples in anoxic containers spiked with site groundwater TCE/PCE and/or CRPs. Finally, in the third task, the project team will use the chemical kinetic data collected from mineral and field samples in the first two tasks to estimate TCE reactivity in the laboratory and as a field screening tool. The team will develop correlation equations, and then use the redox-active dye CRPs to calculate reduction potentials for all experimental conditions. If successful, the values derived from reduction potential calculations will provide a second field screening approach for estimating the reduction rates of TCE/PCE.

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Benefits

The results of this research will provide a strong scientific basis for estimating subsurface abiotic attenuation rates. The combination of experiments with well-characterized mineral phases and field samples, as well as a suite of RPs, will yield a data set that allows different strategies for estimating attenuation rates to be rigorously compared. The experiments are designed to inform practical, scientifically-based decisions regarding how to best estimate abiotic attenuation rates under natural, enhanced, and sustained attenuation conditions. Additionally, this research will assess the potential value of collecting core samples frozen in situ as a means of preserving pore fluids and preventing oxygen exposure during sample recovery. (Anticipated Project Completion - 2019)

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Points of Contact

Principal Investigator

Dr. Richard Johnson

Oregon Health and Science University

Phone: 503-346-3432

Program Manager

Environmental Restoration

SERDP and ESTCP

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