A Field Method to Quantify Chlorinated Solvent Diffusion, Sorption, Abiotic and Biotic Degradation in Low-Permeability Zones

Science Center Objects

Strategic Environmental Research and Development Program project ER-2533

 

In chlorinated-solvent-contaminated fractured-sedimentary-rock aquifers, low-permeability (low-K) strata typically act as long-term or secondary sources of contamination to mobile groundwater in the high-permeability fractures. The fate of dissolved trichloroethene (TCE) in the low-K matrix is controlled by abiotic degradation, sorption, and diffusion in the matrix, and biodegradation reactions that occur principally in the fractures.

The U.S. Geological Survey and the University at Buffalo, with the support of the Strategic Environmental Research and Development Program (SERDP), the Toxic Substances Hydrology Program, and the U.S. Navy, are developing a field method capable of concurrently characterizing site-specific diffusion, sorption, and degradation of chlorinated volatile organic contaminants (CVOCs) in low-K zones.

Research Progress

Prototype Development and Testing

USGS scientists studying groundwater

USGS and University at Buffalo scientists injecting tracers to study diffusion.

(Credit: Daniel J. Goode, USGS. Public domain.)

At a well-characterized fractured-rock site, diffusive tracer tests were conducted in low-K intervals of open boreholes, open to un-fractured rock, isolated using a straddle-packer apparatus. CVOCs in the borehole fluid were initially removed by gas stripping, and concentrations gradually increased during diffusion out of the rock matrix. Conversely, tracers added to the borehole fluid gradually diffused into the rock matrix, and their concentrations in the borehole fluid decreased. Degradation reactions also contributed to decreases in parent compound concentrations and increases in product concentrations. Downhole components of the apparatus included Viton-clad straddle packers and closed-loop stainless steel tubing for fluid sampling. Bench-scale testing identified tubing inlet and outlet configurations that improved tracer mixing efficiency. Fluid samples from the isolated interval were collected using a peristaltic pump, and the volume of each sample was replaced with CVOC- and tracer-free borehole fluid to minimize pressure changes due to sampling. Prototypes were field-tested for approximately three months in 6-inch open boreholes at the former Naval Air Warfare Center, West Trenton, N.J., where TCE migrated from land surface into underlying fractured mudstones of the Newark Basin, and where hydraulic containment by pump-and-treat remediation, as well as natural attenuation by biodegradation, has been ongoing since the mid-1990s.

More information

Project ER-2533 at SERDP: Background, Objectives, Technical Approach, and Benefits.

Research Team 

Inverse Modeling to Estimate Sorption, Diffusion, and Reaction Rates and Coefficients

Schematic of tracer dye flow

Schematic of field test and use of modeling to estimate sorption, diffusion, and reaction rates.

(Credit: Richelle Allen-King, University at Buffalo. Public domain.)

Field measurements are being used in inverse solute transport modeling in the system to estimate the best-fit sorption coefficients, diffusion parameters, and degradation rates. The solute transport model uses the finite-difference method to solve a set of differential equations governing molecular diffusion, linear sorption, and first-order degradation of the VOC contaminant and its degradation products. Historical data of VOC concentrations are used as boundary condition at the wellbore to simulate VOC diffusion from the well into the rock. The model simulates concentration changes as VOC's diffuse from the rock into the wellbore and as tracers diffuse from the wellbore into the rock. Several in situ pilot tracer tests were conducted in boreholes in different low-transmissivity rock strata with different levels of contamination by TCE and cis-1,2-dichloroethene (cisDCE). In general, the results showed increases in TCE (blue in figure) and cisDCE (green) concentrations in the test sections with time that were used to estimate bulk diffusion rates and sorption coefficients. Concentrations of bromide (the inorganic tracer) decreased slowly with time and were used to estimate the rock matrix diffusion rate. Disappearance of trichlorofluoroethene (a TCE-analog, red) was used to provide independent estimates of VOC biodegradation rates.