Dr. J.F. Devlin
The University of Kansas
J.F. Devlin currently works in the Department of Geology, University of Kansas where he conducts research in Hydrogeology and Geochemistry. Recent projects include work on the performance of zero valent iron in reactive barriers, in situ denitrification, and the development of tools for the measurement of groundwater velocity. The latter work has led to devices able to quantify groundwater flow with or without wells, in fractured rock, and flow across the groundwater-surface water interface.
Case studies showing the use and value of point velocity measurements
Most hydrogeological site investigations begin with the installation of monitoring wells, that also serve as piezometers, and the gathering of water level data. Years of experience have led the hydrogeological community to trust this basic dataset to provide sufficient information for the determination of groundwater flow direction and magnitude. However, this trust can be misplaced in commonly encountered cases. For example, the calculation of velocity depends on accurate knowledge of hydraulic conductivity, a parameter known to be scale dependent and difficult to measure with accuracy and precision. The hydraulic gradient is also required in the calculation and is subject to many sources of error including 1) poor measurability in closely spaced wells and 2) highly permeable sediments, 3) accuracy problems arising when the well field is installed with differing screen lengths that may intersect geologic units in poor hydraulic connection, 4) or in similarly constructed wells that are not hydraulically connected to each other due to poorly developed well screens or poorly connected strata, 5) errors arising when waters of different density are present in the wells being sounded, as might occur in deep groundwater systems or near coastlines where seawater intrudes into aquifers. In response to all these problems, the last decade and a half as seen a growing literature emerge concerned with developing alternative, complimentary methods of measuring groundwater velocity. This presentation relates several case studies that relied on the Point Velocity Probe (PVP) family of tools for determining groundwater velocity at the centimeter scale. This family includes the In-Well PVP (IWPVP), the PVP, and the Streambed PVP (SBPVP). In many of these cases new information was obtained from the small scale measurements that proved to be of great value to the assessment of contaminant fate and transport at the sites. For example, PVPs provided the first evidence of a highly permeable stratum conducting groundwater at 10X the average rate on a rural site in Ontario, Canada, that was under consideration for remediation of nitrate pollution. PVPs and SBPVPs were used together to achieve a contaminant mass discharge balance across the stream’s bank and bed at a site in Denmark. The SBPVP recently revealed a preferred flow conduit at the base of a small lake in Minnesota with important implications for the transport of oil spill metabolites in the lake and underlying aquifer. Most recently, PVPs have been employed in transects to enable water flux and contaminant flux estimates across control planes with uncertainties that can be set at the design stage of a project. At one time small scale measurements of flow were dismissed as irrelevant for the scales of typical hydrogeological investigations. The experiences mentioned above, and others to be presented, give cause to rethink that mindset.