Field Performance of Point Velocity Probes at a Tidally Influenced Site



Conventional methods to estimate groundwater velocity that rely on Darcy's Law and average hydrogeologic parameter values are insensitive to local-scale heterogeneities and anisotropy that control advective flow velocity and direction. Furthermore, at sites that are tidally influenced or have extraction wells with variable pumping schedules, infrequent water-level measurements may not adequately characterize the range and significance of transient hydraulic conditions. The point velocity probe (PVP) is a recently developed instrument capable of directly measuring local-scale groundwater flow velocity and direction. In particular, PVPs may offer distinct advantages for sites with complex groundwater–surface water interactions and/or with spatially and temporally variable groundwater flow conditions. The PVP utilizes a small volume of saline tracer and inexpensive sensors to directly measure groundwater flow direction and velocity in situ at the centimeter-scale and discrete times. The probes are installed in conventional direct-push borings, rather than in wells, thus minimizing the changes and biases in the local flow field caused by well installation and construction. Six PVPs were installed at a tidally influenced site in North Carolina to evaluate their implementability, performance, and potential value as a new site characterization tool.

For this study, a new PVP prototype was developed using a rapid prototyping machine (i.e., a “three-dimensional printer'') and included both horizontally and vertically oriented tracer detectors. A site-specific testing protocol was developed to account for the spatially and temporally variable hydraulic conditions and groundwater salinity. The PVPs were tested multiple times, and the results were compared to the results of several different groundwater flux and velocity estimation tools and methods, including a heat-pulse flowmeter, passive flux meters, single-well tracer tests, and high-resolution hydraulic gradient analysis. Overall, the results confirmed that the PVP concept is valid and demonstrated that reliable estimates of groundwater velocity and direction can be obtained in simple settings. Also, PVPs can be successfully installed by conventional methods at sites where the formation consists primarily of noncohesive soils and the water table is relatively shallow.

Although some PVP tests yielded consistent and reliable results, several tests did not. This is likely due to the highly transient flow conditions and limitations associated with the PVP design and testing procedures. PVPs offer particular advantages over, and can effectively complement, other groundwater flow characterization techniques for certain conditions, and objectives may be useful for characterizing complex flow patterns under steady conditions; however, this study suggests that PVPs are best suited for conditions where the flow hydraulics are not highly transient. For sites where the hydraulic conditions are highly transient, the most reliable approach for understanding groundwater flow behavior and groundwater–surface water interactions would generally involve both a high-resolution hydraulic gradient analysis and another local-scale method, such as tracer testing. This study also highlighted some aspects of the current PVP design and testing protocol that can be improved upon, including a more robust connection between the PVP and injection line and further assessment of tracer solution density effects on vertical flow. © 2013 Wiley Periodicals, Inc.