A Passive Flux Meter for Down-Hole Measurement of Water and Contaminant Fluxes in Rock Fractures

Abstract:

Fractured rock aquifers are ubiquitous and highly complex regarding their groundwater flow and transport behaviors. Current borehole logging techniques (e.g., optical, acoustical, thermal or borehole-dilution based) are capable of providing fundamental properties of fracture geometry and / or hydraulics. Moreover, groundwater sampling at discrete depths may assist in the characterization of contaminant plumes or transport. However, open borehole conditions (even over limited intervals) may significantly alter or, at best, smear out flow and transport conditions, thus complicating or invalidating a high resolution interpretation of results. Here, we present the Fractured Rock Passive Flux Meter (FRPFM) as a recently developed device to simultaneously measure cumulative water and contaminant mass fluxes in fractures intersecting a sealed borehole. The FRPFM consists of an inflatable packer lined on its outside with a sorbent felt containing invisible tracers and a dye. When installed over a desired borehole section, the packer presses the felt against the borehole wall, thus avoiding cross-fracture connections along the borehole, while forcing within-fracture flow circumferentially through the felt layer. Groundwater flow through the felt eludes tracers and dye, while the sorbent felt retains dissolved target contaminants intercepted. After a desired deployment period, the device is retrieved and dye marks on the felt allow visual identification of locations, orientations and incident flow directions of hydraulically active fractures (isolated “non-flowing” fractures are not detected). The amounts of dye lost (proportional to size of dye marks) or tracers lost (from laboratory analysis) from the felt are used to directly estimate undisturbed water fluxes in fractures. Masses of contaminants retained on the felt yield estimates of respective contaminant mass fluxes or flux-averaged concentrations in fractures. We present results of successful laboratory validation of the FRPFM using a bench-scale fracture flow simulator. We also present a method to upscale field measured fracture fluxes to estimate water and contaminant mass discharges with uncertainty across transects spanned by one or more observation boreholes.