MULTI-MEDIA FLUX METERS TO EVALUATE CONTAMINANT MIGRATION THROUGH GROUNDWATER-SURFACE WATER INTERFACE

Multi-media flux meters were designed and installed to determine the contaminant flux through the groundwater-surface water interface to facilitate design of a sand cap placed to isolate polynuclear aromatic hydrocarbon (PAH)-impacted sediment at the St. Louis River/Interlake/Duluth Tar (SLRIDT) site in Duluth, MN. The remedial design included a sand cap to isolate PAH-impacted sediment up to 12 feet thick in an 11-acre part of a 40-acre shallow, sheltered bay near the intersection of a native estuary shoreline and a peninsula constructed of waste material from site operations. Sheen-causing gas bubbles were observed in the area to be capped, requiring the flux meters to measure groundwater flux, as well as the flux of gas, sediment, and NAPL transported through the interface to the surface water by ebullition. The site is on the seiche-influenced St. Louis River estuary approximately 4 miles from Lake Superior, with the dominant 7-hour seiche cycle causing a water level fluctuation of up to 2 feet.

The flux meters were designed to collect the data necessary to limit the potential for ebullition, groundwater or expressed pore water from the underlying sediment to impact the cap due to the potential contamination migration pathway through the groundwater-surface water interface. Five flux meters were installed in Stryker Bay; three in locations within the cap area where higher groundwater flow was expected based on sediment temperature profiles, and two outside the cap area to complete aerial coverage of the bay and provide data from locations with typical sediment temperature profiles. Vibrating wire piezometers installed at depths of 5 and 20 feet below the mudline at each flux meter location were synchronized with the site water level meter to evaluate temporal and spatial variations in groundwater gradient, flow direction, and head distribution. Each flux meter captured all gas and water entering the base of the 3.14-square-foot flux meter. A dome was welded to the inside wall of the flux meter to capture gas and water in a flexible bag, and glass wool mesh was suspended just below the dome to capture sediment or NAPL. Temperature probes were installed in the sediment and the water column to allow for gas volume standardization.

Data, results and conclusions from deployment of these multi-media flux meters will be presented.