|Automated Long-Term Remote Monitoring of Sediment-Water Interfacial Flux (132 pp, 2.27 MB) (EPA/600/R-10/110) October 2010
Advective flux across the sediment-water interface is temporally and spatially heterogeneous in nature. For contaminated sediment sites, monitoring spatial as well as temporal variation of advective flux is of importance to proper risk management. This project was conducted to develop a ruggedized advective flux meter capable of unattended long-term remote monitoring of the sediment-water interfacial flux.
Based on the heat-pulse technique, the flux meter is capable of quantitatively measuring bidirectional seepage flux from 0 to 8 centimeters per day with the current configuration. The system has automatic data acquisition, real-time data display, and signal display/analysis capability. A remotely controlled monitoring module was included to permit system monitoring and modification over the internet.
The instrument has undergone several calibrations to establish relations between the flow rate and the heat-pulse travel time. For very low flow rates, an alternate approach was adopted based on the ratio of temperature rises. Using this approach, it is possible to accurately interpret flow rates down to zero-flow conditions. Depending on the magnitude, flow rate can be derived as a function of peak temperature arrival time or as a ratio of rise in temperature on both sides of the heater. Using these two methods, reliable measurements can be made over a wide bi-directional range of flows.
In the field operation, the flow across the sediment-water interface is isolated by a dome chamber and funneled through a flow sensor where the flow rate is measured. The advective flux through the sediment-water interface is calculated in terms of the vertical Darcy velocity by dividing the flow rate through the flow sensor by the cross-section area of the dome.
The flux meter underwent a continuous long-term field test from June 2008 to April 2009 in the Red Cove area of Plow Shop Pond, Fort Devens, Massachusetts. Two flux meters were placed side by side; the unattended operation ran automatically with a self-sufficient power supply system. The measurement frequency was changeable through the wireless network. Seepage flux data was available for download after each measurement cycle. All of the data suggest very low flow and the majority of the data suggest an upward flow direction from the groundwater to the surface water.
In terms of Darcy velocity, the average advective flux for flux meter unit SN300 and SN303 was 0.44 ± 0.34 millimeters per day (mm/day) and 0.76 ± 0.33 mm/day, respectively. Calculated seepage flux based on hydraulic conductivity and gradient data suggests the average flux was 1.12 ± 0.71 mm/day. During the winter months when the surface water was frozen, the data superficially suggest there was significant rapid oscillatory pumping of the ice-covered water during this period of time.
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