Development and Testing of an In-well Point Velocity Probe for Preliminary Site Characterization

Abstract

The in-well point velocity probe (IWPVP) is a novel device for the centimeter-scale measurement of groundwater velocity within the screened interval of a monitoring well. IWPVP measurements are based on the detection of a tracer pulse that is injected into a central mixing chamber and carried over a detector as groundwater passes through the well and the probe. The viability of the IWPVP design was confirmed by numerical modeling followed by a series of laboratory tank experiments. Initial laboratory tank tests showed the IWPVP design to be viable, measuring groundwater velocity within ± 6%, on average, regardless of the orientation of the probe within the well screen, when velocities in the tank ranged between 48 cm/day and 400 cm/day. Following the initial laboratory tank experiments, a new IWPVP was designed with a packing system that permitted easy deployment in the field. Through a second round of laboratory testing, the new IWPVP design was shown to perform nearly identical to the original IWPVP design. Initial field testing of the IWPVP was carried out in an alluvial sand and gravel aquifer at the O’Rourke Bridge Site in Pawnee County, Kansas. Field results showed the IWPVP was able to estimate the flow direction correctly, within the limits of the device, and the groundwater velocity within a factor of three compared to a site-averaged Darcy estimate of groundwater velocity. The latter comparison was hampered by the condition of the well in which the tests were performed. Overall, the IWPVP shows promise as a cost-effective technology for the measurement of groundwater velocity within the screened interval of groundwater monitoring wells.