ASSESSING A TOOL TO MEASURE GROUNDWATER VELOCITIES AT THE CENTIMETER SCALE

Accurate groundwater velocity measurements are necessary for determining the fate and transport of solutes in the subsurface and the selection of appropriate remediation schemes. The point velocity probe (PVP) is a tool recently developed for directly measuring groundwater velocity at the centimeter scale. The PVP functions without a well, requires no calibration, and provides velocity estimates without resorting to Darcy's Law calculations. Avoiding the use of Darcy's Law is advantageous because there is no need for hydraulic gradients which are only measurable at scales on the order of meters. Groundwater velocity measurements are based on the injection and detection of a small saline tracer that migrates around the circumference of the probe (a cylinder) with the ambient groundwater. Tracer breakthrough curves are recorded as electrical conductivity signals at discrete locations along the probe surface, which is in direct contact with the aquifer. The measured tracer breakthrough curves provide apparent velocities that are later converted to average linear groundwater velocity direction and magnitude through a geometric correction. Previous PVP models successfully applied in field and laboratory experiments measured apparent velocities with a pair of electrical conductivity detectors. The magnitudes of these velocities compared well with expected values, but the flow directions varied considerably. Such variation may be expected with centimeter scale measurements, but it could also be indicative of instrument error. To address this issue, a Super-PVP was constructed with ten detectors around one side of the probe perimeter. This device permitted the apparent velocities near the probe surface to be measured in detail for the assessment of flow direction accuracy. A series of bench-top experiments were performed to compare observed trends in apparent velocity to mathematically predicted values. The probe has been shown to function well in relatively homogeneous porous media. Further testing is underway to demonstrate the utility of the tool in controlled heterogeneous media.