The use of pumping for hydraulic characterisation of dykes

Abstract Faults and dykes can control groundwater flow, acting as barriers, conduits, or composite conduit–barrier systems. This study investigates dyke hydraulic behaviour using numerical modelling of field-scale pumping tests at two sites in Western Australia, supported by a uniquely detailed hydrogeological dataset. At each site, pumping tests were conducted across dykes that impeded groundwater flow, as evidenced by asymmetric drawdown cones and delayed responses in observation wells situated across the dyke from the pumping well. High-resolution three-dimensional geological frameworks were developed, and transient groundwater flow models were calibrated using automated inversion with and without spatially variable parameterisation (pilot points) to represent heterogeneity. In all calibrated models, the horizontal hydraulic conductivity ( K h ) of dykes was estimated to be one to eight orders of magnitude lower than that of the surrounding formations. Representation of dykes in the model produced the largest improvement in model fit at both sites, reducing the parameterisation objective function by one to two orders of magnitude relative to model parameterisations without dykes. Further improvements in model fit were achieved by introducing heterogeneity into formations, dykes or both. Although including formation heterogeneity reduced posterior uncertainty in dyke K h estimates, posterior variance patterns revealed that, in some locations, calibration preferentially adjusted K h in formations adjacent to dykes rather than within the dykes. Dyke K h was tightly constrained in areas with available field data and observable drawdown. These findings demonstrate the importance of representing structural features when interpreting pumping-test responses in structurally complex groundwater systems.