- Program Areas
- Installation Energy and Water
- Environmental Restoration
- Munitions Response
- Resource Conservation and Resiliency
- Weapons Systems and Platforms
A Field Method to Quantify Chlorinated Solvent Diffusion, Sorption, Abiotic and Biotic Degradation in Low Permeability Zones
Dr. Richelle Allen-King | University at Buffalo
Mass transfer from low-permeability zones into more active flow paths is controlled by diffusion, sorption, and degradation in the low-permeability zones. The effects of these natural attenuation processes must be included in defensible predictions of long-term chlorinated volatile organic compound (CVOC) fate. Importantly, even very slow transformation reactions in the low-permeability zones can significantly reduce back diffusion. The rates of these processes depend on site-specific physical and biogeochemical subsurface properties.
The objective of this project is to develop and test a field method capable of concurrently quantifying site-specific CVOC diffusion and degradation rates and sorption coefficients in low-permeability zones. The project focuses on developing the method for application to trichloroethene (TCE) contamination in fine-grained sedimentary rocks, where sorption capacity is expected to be significant, reduced iron minerals that promote abiotic degradation are common, and biological activity in the matrix is expected to be minor due to small pore throat sizes. In evaluating the field method, the project will address the following technical questions:
- What is a suitable design for a borehole apparatus that can perturb borehole water quality to initiate a test, and then accurately quantify subsequent changes in the concentrations of TCE, its degradation products, and tracers, caused by fluxes between the low-permeability zone and the borehole?
- What combination of tracer properties, in situ monitoring strategies, and analysis procedures leads to a unique determination of diffusion and degradation rates and sorption coefficients while minimizing uncertainties?
The project team will develop a complete field and interpretive method for characterizing contaminant diffusion, sorption, and transformation processes in low-permeability zones. Task 1 includes development and field evaluation of a packer-based apparatus to isolate a low-permeability borehole interval, as well as the test method. The test will perturb water quality by removing TCE and its degradation products and adding selected conservative and reactive (sorbing and degrading) tracers. The apparatus will enable accurate monitoring of concentration changes during the few-weeks test duration following the perturbation. The team will evaluate the method by deploying the apparatus at a well-characterized field site in borehole intervals selected to encompass an order-of-magnitude range in the processes and properties of interest: TCE concentration, sorption distribution coefficient, abiotic degradation rate, and borehole biodegradation rate. Task 2 includes laboratory experiments that support development of the method. Task 2 also includes adaptation of a novel gas-phase sampling approach for TCE, degradation products, and a conservative gas tracer that will minimize the need to sample and replace water during a test. Experiments to fill knowledge gaps that would otherwise hinder field data interpretation are: determination of the sorption distribution coefficient ratios of the most important abiotic degradation products and the reactive tracer, and the relative degradation kinetic coefficients and product distributions for TCE and the reactive tracer. By focusing on the relative rates and coefficients, the project will maximize transferability of the results. Task 3 develops modeling codes and protocols for simulating the processes occurring during the borehole tests. The model will be used for test design and optimization, and for estimation of the parameters that characterize the attenuation processes. The measured concentrations of the contaminant, the conservative and reactive tracers, and degradation products will be inverted jointly to estimate these parameters. The method development process in these three tasks will iterate between field data collection experiments and simulation experiments.
This project will develop a method enabling site-specific determinations of the values of diffusion and sorption coefficients for TCE and its degradation products and the abiotic degradation rate of TCE in low-permeability zones. These site-specific values can be used within predictive models of site-scale contaminant fate, which are an important element in evaluating natural attenuation. Quantification of the rates of processes that control mass transfer from low-permeability zones into more active flow paths will enable more effective long-term management of groundwater contamination at sites that are dominated by the retention and slow release of contaminants from low-permeability materials. While method development in the project is focused on TCE in sedimentary rocks, the principles will be adaptable to other CVOCs and other low-permeability settings. (Anticipated Project Completion - 2018)