Determination of groundwater velocity in-situ by using passive samplers at sediment water interface

Contaminated bottom sediments are one of the major sources of contaminants in surface water. The contaminants may originated from human activities, enter the subsurface environment through waste disposal, spills, and land application of chemicals. These contaminants get transported through the porewater and may enter the surface water from the bottom sediments. Establishment of effective remediation systems and protection of public health rely on the ability to estimate the mass flux of the contaminants to surface water. The mass flux is a function of porewater velocity, retardation factor, and degradation coefficient. In this research, passive sampler (peepers) were used to determine the groundwater velocity insitu when the mass flux of the contaminants to surface water was low. This study suggested that the ability of the samplers to accurately capture produced tracer profiles depends on the velocity of the flow and equilibration time of the peeper. Low velocities allowed greater time for the sampler to equilibrate, approaching steady state with the pore water. The sampler was able to serve as a tracer source but was not able to provide a large enough flux of tracer to allow downstream locations to approach the source concentration. Regardless, steady state profiles developed rapidly, likely due to the rapid increase in area available for diffusional flux with distance from the source and the limited flux of source from the sampler cell. A sensitivity analysis indicated that, as expected, diffusional transport dominated at very low velocities and that the source dimensions were the critical determinant of the tracer concentration profile shape. Overall, these studies suggest that it may be possible to differentiate groundwater velocity, but that samplers with very low depths would be required (<0.5cm).