Prediction of Groundwater Quality Improvement Down-Gradient of In Situ Permeable Treatment Barriers and Fully-Remediated Source Zones. ESTCP Cost and Performance Report

Prepared in cooperation with CH2M HILL, Tempe, AZ. Environmental Security Technology Certification Program.

In situ permeable treatment barriers (PTB) are designed so that contaminated groundwater flows through an engineered treatment zone within which contaminants are eliminated or the concentrations are significantly reduced. These systems are often considered for the containment of dissolved groundwater contaminant plumes or for controlling the discharge and larger scale impact of dissolved contaminants from source zones to aquifers. The performance of a PTB is typically judged by short-term changes in groundwater concentrations with time within the treatment zone and also in wells located some distance downgradient. The objectives of this project were to: a) propose a practicable approach that can be used to project reasonable order-of-magnitude estimates of groundwater quality improvements with time downgradient of a PTB, b) conduct detailed monitoring and characterization downgradient of a well-understood PTB site, and c) illustrate and reflect on the use of the proposed approach for the PTB system studied in this project. These objectives were met by this demonstration project. A PTB is installed to prevent further downgradient discharge of impacted groundwater and to meet prescribed numerical standards for groundwater cleanup downgradient of the PTB. When selecting a PTB system, it is critical that all stakeholders understand how groundwater quality changes will occur with time and distance downgradient of the PTB and how long it might take to achieve standards at different distances downgradient of the PTB. Detailed monitoring and characterization of groundwater concentration changes with time downgradient of a full-scale methyl tert-butyl ether (MTBE) biobarrier PTB system were conducted at the Naval Base Ventura County (NBVC). The recommended site-specific assessment approach for PTB systems is one that focuses on characterization of vertical variations in horizontal hydraulic conductivity.