Field Comparison: Point Velocity Probes (PVPs), Heat-Pulse Flowmeter, Passive Flux Meter, and Single-Well Tracer Testing

Large-scale Darcy’s Law-based average estimates of groundwater velocity are largely insensitive to local-scale heterogeneities that control advective flow rate and direction.  Several methods have been developed over the years for measuring local-scale groundwater flow velocity in situ.  For examples, Devlin et al. (2009) recently developed the point velocity probe (PVP) which utilizes a small volume of saline tracer and inexpensive sensors to directly measure groundwater flow direction and velocity in situ at discrete times.  The saline tracer can be tailored to provide sufficient electrical conductivity contrast from the groundwater.  As the injected tracer is redistributed around the PVP by ambient groundwater flow, the change in electrical conductivity is detected by the sensors, yielding tracer breakthrough curves.  The probes are installed in conventional borings, rather than in wells, thus minimizing the changes and biases in the local flowfield caused by well construction.  Other available tools and techniques for measuring local-scale groundwater flux and/or velocity include heat-pulse flow meters, passive flux meters, and single-well tracer testing, and this study was designed to compare the performance of PVPs to these other techniques.  For this study, five PVPs were installed at a tidally-influenced site in North Carolina that included both horizontally- and vertically-oriented detectors.  A site-specific testing protocol was developed to account for the spatially- and temporally-variable hydraulic conditions and groundwater salinity.  Overall, the PVPs were easy to install and operate, and they provided in situ measurements of groundwater flow velocity that were generally consistent with the results obtained from the other techniques. However, some PVP tests produced ambiguous and uncertain results, likely caused by the highly transient flow conditions at the site.  This presentation summarizes and compares the results from the various methods, and highlights the applicability, strengths, and limitations of each technique.