REEVALUATING PALOUSE GROUNDWATER BASIN COMPARTMENTALIZATION

Ever since the first investigators started evaluating well hydraulic connections among Grande Ronde wells, they realized that variable degrees of hydraulic continuity exist spatially within the Palouse Groundwater Basin (PGB). Wells were found to respond differently to short-term (days to weeks) hydraulic stresses on a spatial basis, but similarly to long-term (annual) hydraulic stresses. Analysis of water level data and drawdown data for the PGB clearly indicate that the groundwater resource system known locally as the Grande Ronde Aquifer is compartmentalized on both short-term and long-term time frames. Aquifer compartmentalization has been identified within the PGB during this investigation (Lateral Compartmentalization). Lateral Compartmentalization is defined in this investigation as the division of the Grande Ronde Formation into irregularly shaped blocks (i.e., compartments) that are hydraulically separated laterally by sub-vertical, low hydraulic conductivity zones. Hydrostructures may exist sub-regionally and locally that consist of geologic folds and fault/fracture zones that play a role in defining the limits of groundwater compartments inside the PGB. Well hydraulic connections, groundwater trends, groundwater temperature, and groundwater age dates identified in previous investigation and during this investigation were used for designating compartment sizes within the PGB. With the evidence for compartmentalization from this work, five individual compartments in the PGB are hypothesized. Recent research by Conrey (2014) using well log stratigraphy and drilling chips has located subsurface faults in the vicinity of proposed compartment boundaries.