Water flux profiling in fractured rock boreholes with an In-Well Point Velocity Probe (IWPVP)

Abstract

• First field measurements with the fracture adapted IWPVP reached 38 m below ground surface. • Depth profile of water fluxes compares well with PFM, ORP, and FLUTe liner profiles. • Demonstrated calibration of probe measurements with acoustic borehole televiewer data. • IWPVP shown to be a simple, cost-effective tool for measuring flow in fractures. Flow in a fractured rock aquifer beneath the Edwards Air Force Base in California was characterized by depth profiling two wells with In-Well Point Velocity Probes (IWPVPs). The probes, which were originally designed for use in porous media wells, were optimized for use in fractured rock wells and to meet several challenges including: sampling depths up to 38 m; high background site-water salinity; and variable well construction (screened vs open borehole, and well diameters of 7.62 cm and 15.8 cm). The IWPVP measures water flux inside the probe (internal flux), which is converted to fluxes in the aquifer through calibration. At this site, the internal fluxes ranged from 3 to 53 m/d. The channeled internal design of the probe inherently provides information about general flow directions and allows further interpretation of specific flow direction within ±15°, which at this site was generally consistent with the expected regional flow direction. Notably, a significant shift in the flow system was observed following a rain event (up to 180° shift in the flow direction). The IWPVP identified highly transmissive zones in the fractured rock, which were independently confirmed by passive flux meters and oxidation-reduction potential sensors. Transmissivity profiles determined during a FLUTe (Flexible Liner Underground Technologies™) liner deployment also showed similarities in the depths of potentially high flow rates, particularly in the shallower portion of the well (<20 m). With additional information on fracture apertures provided by an acoustic televiewer, the internal fluxes were converted to water fluxes in the observed fractures, indicating seepage velocities in the rock aquifer between 3.7 m/d and 22.4 m/d.