2001. Geochemical investigation of three Permeable Reactive Barriers to assess impact of precipitation on performance and longevity

Abstract: A geochemical assessment was conducted at three permeable reactive barrier sites to evaluate performance and longevity issues. The assessment was carried out at former NAS Moffett Field (California), Dover AFB (Delaware), and former Lowry AFB (Colorado). These sites were selected because they differ in barrier design, contaminant types, hydrology, and geochemistry, and therefore represent a number of factors that could have bearing on long-term performance. Analysis of groundwater and iron cores were the primary means used to evaluate the potential for precipitate formation, which could affect both the surface reactivity of the iron as well as the hydraulic conductivity of the reactive cell. In addition, geochemical modeling with PHREEQC and Geochemist’s Workbench was used to simulate iron reactivity in each of the three groundwater types and assist in understanding precipitation sequences. The study focused on behavior of native inorganic constituents of the groundwaters, such as dissolved oxygen, calcium, magnesium, alkalinity, and sulfate, as potentially able to influence precipitation within the reactive cells. In addition, long-term accelerated column tests are currently underway using two different iron-groundwater systems which model the Moffett Field and Lowry barriers. Results of the column tests will assist in understanding precipitation kinetics and physicochemica