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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. Typically, expectations for groundwater concentration changes with time are based on a single site-wide average linear groundwater velocity estimate. For example, clean groundwater would be expected to be observed from 0 ft to 365 ft downgradient of a PTB after one year at a site having a 1 ft/day average linear groundwater velocity. Previous ESTCP-sponsored studies have concluded that this approach does not agree well with observations at PTB sites and that a better understanding of the subsequent improvements in downgradient groundwater quality with time is needed. Realistic projections of how the downgradient concentrations will change with time are important, or else incorrect performance conclusions might be drawn in the short term, leading to premature abandonment of the PTB technology and unnecessary investment in other remedial options.
This project demonstrated a new approach for projecting improvements in groundwater quality downgradient of PTBs over time. While complementary to conventional site characterization practices, it emphasizes characterization of vertical variations in horizontal hydraulic conductivity. This can be accomplished by a number of approaches, including the use of direct push tools. This project also demonstrated the use of a simple spreadsheet-based tool that facilitates projections of groundwater quality with time, using the horizontal hydraulic conductivity distribution with depth.
The objectives of this project were to (1) 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, (2) conduct detailed monitoring and characterization downgradient of a well-understood PTB site, and (3) illustrate and reflect on the use of the proposed approach for the PTB system studied in this project.
Detailed monitoring and characterization of groundwater concentration changes with time downgradient of a full-scale MTBE biobarrier PTB system were conducted at the Naval Base Ventura County (NBVC) in Port Hueneme, California. Collection of depth-discrete dissolved MTBE and hydraulic conductivity data along several transects parallel and perpendicular to the plume axis indicated that movement of clean water downgradient from the NBVC PTB reflected variations in horizontal groundwater velocity. These findings further demonstrated that use of a single site-wide estimate of groundwater velocity for that site would lead to unreasonably low predicted concentrations at shallower depths and unreasonably high predicted concentrations at deeper depths based on the hydrogeology of the NBVC site.
The recommended site-specific assessment approach for PTB systems (or fully remediated source zones) is one that focuses on characterization of vertical variations in horizontal hydraulic conductivity. This can be done at most sites through coring followed by laboratory tests or by using in-situ discrete pumping tests, both of which were demonstrated at the NBVC site. Using this information along with hydraulic gradient data, well construction information (i.e., screened interval data), pretreatment concentrations, treatment zone concentration data, and estimates of downgradient groundwater quality change with time can be produced, assuming that horizontal groundwater flow is the dominant dissolved chemical transport mechanism. A spreadsheet-based tool (DGCHANGE v1.0) was developed to help users perform these calculations and better visualize the projected concentration versus time behavior in the aquifer and at the wells.