Characterization of Flow in Laboratory and Rock Fractures Using an In-Well Point Velocity Probe

Abstract

The In-Well Point Velocity Probe (IWPVP) is a tool deployed in the screened interval of a well that utilizes an internal small-scale tracer test to directly measure groundwater velocity on the centimeter scale. The ability of the tool to function within a well-developed screened portion of a porous media well led to the hypothesis that the IWPVP could also be utilized in fractured rock. A fracture flow apparatus (FFA) was built to conduct preliminary work in the laboratory and allow design adaptations to ensure the IWPVP would properly measure flow magnitude and direction. Calibration factors dependent on well diameter and aperture size were determined experimentally and agreed well with predictions based on a flux balance. Additionally, the IWPVP could identify flow direction accurately to within about ± 15° under idealized laboratory testing conditions. Following successful laboratory work, field testing of adapted IWPVPs (3” and 6” diameter) was carried out at a contaminated fractured rock aquifer at Edwards Air Force Base in California. The IWPVPs were able to identify zones of both relatively low and high flow, in agreement with a variety of other technologies used on site, including passive flux meters, oxidation-reduction sensors, and FLUTe liners. Fluxes internal to the probes were observed to range between 300 – 5,300 cm/d. Additional data on discrete fractures from acoustic borehole televiewers allowed determination of calibration factors from which water fluxes in the fractures were estimated to range from 370 cm/d – 2,239 cm/d. Also, flow directions determined by the IWPVP compared favorably to expected regional flow direction and showed the drastic impact a rain event had on flow directions in the fractures. Overall, this work has supported the IWPVP’s use in fractured rock in addition to porous media. With minor adaptations, the probe was able to successfully characterize fracture flow on the centimeter-scale, in near real-time, and at a low fabrication cost.