DIRECT GROUNDWATER VELOCITY AND SPECIFIC DISCHARGE MEASUREMENTS IN AQUIFERS, SURFACE WATER SEDIMENTS AND FRACTURED ROCK USING CENTIMETER-SCALE TRACER TESTS (Invited Presentation)

The practice of contaminant hydrogeology to investigate and remediate polluted sites justifiably places great emphasis on finding and delineating source zones and plumes, i.e., the emphasis is on the ‘contaminant’ half of the work. The hydrogeology half of the work is commonly addressed by measuring water levels, estimating hydraulic gradients, and employing Darcy’s Law to determine flow magnitudes and directions. This approach has served the industry well for approximately fifty years of modern contaminant hydrogeological investigations. However, estimating contaminant transport on the basis of water level measurements is subject to limitations that may be overcome with novel site investigation tools, such as the point velocity probes (PVPs). For example, the goal of water level measurements in contaminant work is to enable accurate predictions of groundwater seepage velocity, or flux. To achieve this desired end, not only must the water levels be measured, but so must hydraulic conductivity and effective porosity. These latter two parameters are notoriously difficult to measure with high confidence, particularly hydraulic conductivity. The direct measurements of velocity or flux reduces the sources of uncertainty to those arising from the direct measurements themselves. It might also be noted that water level measurements can cause misleading velocity predictions if the wells used are in poor hydraulic connection, have different screen lengths, or are completed at different depths. To facilitate the direct measurement of groundwater velocity and flux, a family of point velocity probes have been developed at the University of Waterloo and the University of Kansas to characterize porous media and fractured rock aquifers. The first of the PVPs was introduced in 2007 and underwent ten years of development before the introduction of the Streambed PVP (SBPVP) and the In-Well PVP (IWPVP) instruments were created. The PVP instruments have been used to address such practical problems as transient flow in bioremediation zones, mass discharge estimation into a stream, hydrogeological characterization of a lake with pronounced sediment stratification, geostatistical characterization of a sandy aquifer, and rapid velocity determinations using existing infrastructure in granular and fractured rock aquifers.