Applications and implications of direct groundwater velocity measurement at the centimetre scale.

Three projects involving point velocity probes (PVPs) illustrate the advantages of direct groundwater velocity measurements. In the first, a glacial till and outwash aquifer was characterized using conventional methods and multilevel PVPs for designing a bioremediation program. The PVPs revealed a highly conductive zone that dominated the transport of injected substances. These findings were later confirmed with a natural gradient tracer test. In the second, PVPs were used to map a groundwater velocity field around a dipole recirculation well. The PVPs showed higher than expected velocities near the well, assuming homogeneity in the aquifer, leading to improved representations of the aquifer heterogeneity in a 3D flow model, and an improved match between the modelled and experimental tracer breakthrough curves. In the third study, PVPs detected subtle changes in aquifer permeability downgradient of a biostimulation experiment. The changes were apparently reversible once the oxygen source was depleted, but in locations where the oxygen source lingered, velocities remained low. PVPs can be a useful addition to the hydrogeologist's toolbox, because they can be constructed inexpensively, they provide data in support of models, and they can provide information on flow in unprecedented detail.