Measuring the flux at the interface of coal-tar impacted sediment and river water near a former MGP site.

Determining water movement through contaminated sediment is critical for characterizing transport of chemicals from the sediment to the overlying water. Field studies to characterize the water flow across the sediment-water interface within a river adjacent to a former manufactured gas plant site were conducted. For this purpose, a new design of an interfacial flow meter was developed and tested. The in situ components of the system consisted of: a cylinder with an interfacial area of 2342 cm2; a dome attached to the cylinder; and a flow tube that allows water to flow from inside the dome to the river at the rate equal to the specific discharge across the sediment-water boundary. A 'heat-pulse' method was used to measure flow by heating the center of the flow tube for a brief time period and measuring the temperature profile within the tube over time. The system was calibrated to measure volumetric flux in the range 1.5-4.0 cm d(-1), however using a flow-addition method, the measurement of lower velocities also was accomplished, and calibration at higher fluxes is possible. From the groundwater flow at the interface of the coal-tar impacted sediment and information on the sediment pore water concentrations of several PAHs (poly-cyclic aromatic hydrocarbons), the mass flux of these PAHs to the river were estimated. Information on PAH mass flux at the sediment-water interface is useful for site assessment, including the evaluation of remediation alternatives and longer term site characterization.