Under suitable conditions, monitored natural attenuation (MNA) can be an effective strategy for restoring aquifer systems contaminated with chlorinated solvents. However, a better understanding of factors controlling the long-term sustainability of natural attenuation (NA) along with methods to characterize and assess the potential for sustained MNA at chloroethene-contaminated sites are lacking. Technical guidance and predictive models are needed to quantify the effects of the complete range of attenuation mechanisms associated with chloroethene-contaminated sites, determine time frames for contaminant source depletion, and provide justifiable evidence and estimates on the sustainability of MNA during this time frame for a range of site conditions.

The objective of this project was to develop and test methods to assess the long-term effectiveness of MNA, capturing the full range of NA processes—diminishing source mass transfer rates, dilution and dispersion, biological and abiotic transformations, volatilization, and evapotranspiration.

Integrated protocols that balance the cost and level of effort with the need to minimize prediction uncertainty were developed for site characterization, monitoring, and modeling to assess the long-term sustainability of biological and physical attenuation processes. An assessment tool for site analyses was built upon further enhancements to SEAM3D (Sequential Electron Acceptor Model, 3D model), which was developed under the SERDP-funded project entitled Development of Simulators for In-Situ Remediation Evaluation, Design, and Operation (ER-1062). Modules were implemented to (1) predict chloroethene fate and transport as a function of redox condition and electron donor availability, (2) predict non-aqueous phase liquid (NAPL) source dissolution rate and natural depletion, and (3) predict organic chemical fluxes from groundwater due to natural processes. In addition, a test procedure to determine the amount and bioavailability of aquifer organic matter was developed. Field studies were conducted on a Department of Defense (DoD) site to assess the protocol. Model predictions and observations were compared, and long-term predictions with uncertainty analyses were performed.

The outcome of this project is a practical methodology to evaluate the feasibility and long-term sustainability of natural attenuation at chloroethene-contaminated sites while reducing cost and controlling risk. The technical guidance document and integrated protocols, including a comprehensive NA model, enable site managers to assess the biotic and abiotic natural attenuation processes at chloroethene-contaminated sites in conjunction with routine field tests and monitoring regimes. This project provides DoD with quantitative tools for predicting the time of remediation, the sustainability of natural attenuation processes within this time frame, and the degree of uncertainty associated with such results.