DIRECT GROUNDWATER VELOCITY ESTIMATES FROM PVP AND GPR MEASUREMENTS

Groundwater velocity is an important parameter for modeling applications, assessing the fate and transport of contaminants, and for in situ remediation. Groundwater velocity estimates are usually based on field techniques relying on hydraulic conductivity and Darcy's Law, e.g., slug testing, coring, pumping tests. These techniques lead to velocities averaged over scales often larger than the area being characterized. Direct velocity measurement can overcome this limitation. The point velocity probe (PVP) was developed to address the above need. The probe method is based on the movement of a saline tracer around a PVC casing. As the tracer flows around the casing, electrical conductivity breakthrough curves are detected by electrodes placed at known angles around the casing. Breakthrough curves are fitted with a solution of the advection dispersion equation from which apparent velocity is calculated. Geometrical corrections convert apparent velocities to actual velocities.

A gasoline plume located in the Borden aquifer has been examined with an array of 20 PVPs down gradient from the source. The array comprises five standpipes with four PVPs each, forming a fence across the plume. A high-resolution borehole GPR survey was employed around the array to serve as an independent method in aquifer characterization and plume monitoring. The two independent methods provided complimentary and consistent data that defined small-scale heterogeneities in the aquifer. Zones of highest BTEX attenuation, following the application of ORC, saw decreases in groundwater velocity corresponding to increases in radar wave velocity suggesting the trends were real. Ongoing work is aimed at establishing the specific causes of the trends.