Development of a Point Velocity Probe for in-Well Use

With the development of in-situ remediation technologies, the need for reliable estimates of groundwater velocities at local scales has increased.  The traditional approach of Darcy-based calculations is subject to uncertainties arising from scale limitations and estimations of hydraulic conductivity. This study presents a novel device designed to make groundwater velocity measurements in wells and to compliment velocity estimations based on Darcy calculations.  The device is a modification of the point velocity probe (PVP), which functions in a dedicated borehole without a well, and is called the in-well PVP (IWPVP).  The IWPVP is placed within the screened interval of groundwater monitoring wells.  Velocities are determined from travel times of tracer pulses travelling through channels inside the device. Preliminary laboratory experiments utilized a hand-cut well-screen installed in a homogeneous sand aquifer simulator to evaluate the viability of IWPVP designs. For initial experiments, the well-screen openings were aligned in the direction of groundwater flow, promoting flow directly through the probe. Preliminary results showed a linear relationship between calculated and measured velocities. The slope of the calibration line, describing the known vs. measured velocities, indicated that groundwater traveled through the IWPVP at a speed  about 15 times faster than ambient speeds (10–600 cm/day), with an R² value of 0.9926. A two-dimensional model was constructed using COMSOL Multi-Physics to verify these initial results. The model estimated a velocity magnification factor (calibration slope) of about 11, in reasonable agreement with experimental results. Further work aims at improving model-experiment agreement through modifications to the experimental method and moving to three-dimensional modeling.  Future experiments will also be conducted using commercial well screens. On the basis of the early results, the IWPVP may become an inexpensive tool to use in conjunction with more traditional methods of assessing groundwater velocity, to gain insights concerning local flow rates in aquifers.