Background/Objectives: Ebullition is a process in which gas is produced and subsequently released from sediments and into the overlying water column. Gas production in sediments is typically biogenic in origin due to the anaerobic biodegradation of sediment organic matter by microorganisms.
Ebullition can potentially contribute to the transport of dissolved and pure phase contaminants from the sediment column to the overlying water body. Ebullition-facilitated NAPL transport is of particular concern for design of a cap treatment layer, whereas ebullition pressures and release in general can disrupt cap physical stability.
Approach/Activities: Bench-scale tests were conducted to estimate gas production rates and ebullition potential following removal of shallow sediments. Sediment samples were collected from two horizons: a shallow/surficial fill unit and the underlying alluvial deposits. Vessels containing site water and sediment samples were assembled and incubated under anaerobic conditions at two temperatures corresponding to the average groundwater temperature at the site (13 °Celsius) and the maximum surface water temperature (28 °Celsius). Negative controls consisting of sand and site water or sand and deionized water were incubated with sample vessels. The volume of gas produced in each vessel was collected and quantified over a period of 4 weeks.
Results/Lessons Learned: The bench-scale tests provided gas production rates for sediments that are currently buried and therefore could not be screened for ebullition potential in the field, but could have implications for contaminant transport and cap integrity following remedy implementation. Gas production was indistinguishable from negative controls at 13 °Celsius but quantifiable at 28 °Celsius. At the higher temperature, gas production was, on average, approximately five times greater in vessels with surficial fill compared to the underlying alluvial deposits. Following dredging of surficial fill, ebullition will likely represent a minor mechanism of contaminant transport.