Development of a passive sensor for measuring water and contaminant mass flux in lake sediments and streambeds.

摘要

Contaminated aquatic sediment sites are difficult to characterize and manage because of the tendency for solutes to be retained within the sediments for long periods of time due to the hydrophobic nature of some key compounds. Currently, technologies can identify groundwater discharge zones and infer estimates for contaminant mass flux based on contaminant concentration in bulk sediment, though it is generally accepted that freely dissolved concentration in pore water is a better measure of potential exposure. The purpose of this research project is to demonstrate a new tool to provide more accurate characterization of vertical water and contaminant fluxes within saturated sediments through direct in-situ measurement. The sediment bed passive flux meter (SBPFM) is designed to passively provide direct in-situ measurements of cumulative water flux and contaminant mass flux vertically through the upper surface sediment layer and into the overlying water column.;The SBPFM consists of an internal permeable sorbent which is impregnated with one or more water soluble tracers and is contained in a dedicated drive-point with upper and lower screened openings, allowing vertical flow through the device given a preexisting hydraulic gradient. The tracers are displaced from the sorbent at rates proportional to the average specific discharge. Similarly, the mass of sorbed contaminants provides a direct measurement of the vertical contaminant flux during deployment.;Axisymmetric potential flow theory was used to determine the flow convergence to relate measured flux to the ambient flux in the surrounding sediments. Bench scale tests were conducted that demonstrated the theoretical equations were valid for estimating water and contaminant mass flux under conditions of unidirectional flow. To permit use within tidally influenced sediment beds, a method was developed using spatial moment analysis of resident tracer concentrations to determine the vertically upwards and downwards components of water flux. The SBPFM was then modified to accommodate the new approach and the method was assessed in the laboratory to some success. Subsequent analysis of the results through comparison to simulated tests showed the extent of the capabilities and limitations of the modified design and several alternative methods were described.