An integrated geophysical investigation of the hydrogeology of an anisotropic unconfined aquifer

The predictive capability of groundwater flow models is frequently restricted by insufficient characterisation of a typically heterogeneous and anisotropic subsurface. Trace levels of volatile organic compounds have been detected at municipal water supply wells in Gray, Maine. Groundwater flow modelling based on available hydrogeologic data defines a dominant W–E transport vector that is inconsistent with the apparent N40E transport of a plume emanating from a road salt storage facility, as mapped with a terrain conductivity instrument. A local-scale geophysical study at an undisturbed site in the glacial-marine delta deposit aquifer was conducted to investigate the possible influence of anisotropy on flow through these unconsolidated sediments. Ground penetrating radar and terrain conductivity measurements reveal evidence for structural features that are likely to promote preferential transport paths orientated in a general NE–SW direction. Two conductive tracers, one deep and one surficial, were injected and monitored for direct evaluation of the groundwater flow vector using resistivity and self potential methods. Although interpretation was limited by an incomplete resistivity dataset, the results and modelling of both methods supports a general N30E–N40E flow vector. Furthermore, consistent flow velocity estimates (∼0.15m/day) are obtained from the two methods. Analysis of this integrated dataset suggests that anisotropy exerts a significant control on flow in this unconsolidated aquifer. Predictions of sources of groundwater contamination at municipal wells based on flow models not accounting for this anisotropy will likely be in error. This study illustrates the potential value of an integrated geophysical study, which will aid the development of accurate flow models for unconsolidated aquifers.