A basin-scale groundwater flow model in the Columbia Plateau (Pacific Northwest, USA); insights for management of fractured aquifer-types

Mechanical discontinuities control groundwater flow in fractured aquifers. Bedding plane and sub-vertical discontinuities create fracture networks geometrically organized both horizontally and vertically in areas un-affected by compressional tectonic forces. In this structural setting, we use the Columbia River Basalt aquifer in the Palouse to show how the combination of previous acquired stable isotope data and geological, groundwater, and particle tracking modeling better describes groundwater flow in three dimensions. We present a steady-state flow model simulating backward particle traces from abstraction wells to the recharge boundaries. Backwards particle analysis coupled with the ¹⁴C isotope vertical concentration distribution shows how the aquifer system is characterized by two separate zones. A shallow (<120 mBGL) zone of freshwater circulation is characterized by higher ¹⁴C concentrations and low particle travel times with respect to the deeper (>120 mBGL) aquifer zone. Here, penetration of particles is partially impeded by the low vertical hydraulic conductivity of the volcano-sedimentary layers and recharge preferentially occurs in correspondence of discontinuities related to a geological unconformity. Hence, the outputs of a particle tracking analysis fits stable isotope data either validating a 3D groundwater flow model or aiding detail to conceptualization of a fractured aquifer.

The Columbia River Basalt aquifer is also horizontally anisotropic due to sub-vertical tectonic fractures which are related to gentle folding and faulting. This horizontal anisotropy significantly influences particle tracking analysis in the basin up to 120 mBGL. Well-head protection areas are defined globally by backward particle tracking analyses at shallow depths. Thus, as a consequence of this research we envisage introduction of horizontal anisotropies in groundwater flow models for definition of well capture zones.