Discharge-Targeted Hydraulic Tomography to Quantify and Locate Aquifer Discharge

Quantifying and localizing groundwater discharge is inherently difficult. It requires knowledge about hydraulic conductivity and the hydraulic gradient on the scale of interest. Conventional hydraulic testing, such as pumping tests, may fail in the presence of heterogeneity and complex structural boundaries. While advanced 2D and 3D hydraulic tomography may resolve small-scale heterogeneity, it is typically limited to small spatial scales and requires costly field installations. We propose a simplified tomographic approach using a limited number of pumping and observation wells spatially distributed over a well profile in the order of 100 m transverse to the direction of ambient flow. To infer the spatially variable hydraulic-conductivity field from drawdown data with its uncertainty, we apply an iterative ensemble smoother. Subsequently, the posterior ensemble of hydraulic-conductivity fields is used to calculate total and specific discharge based on the observed ambient hydraulic heads in the same wells. We test our approach in a synthetic scenario mimicking a channel-like aquifer such as the quaternary fill in a small river valley. The results demonstrate that multiple spatially distributed pumping tests are suitable to quantify total discharge and its associated uncertainty. The approach is more reliable than a conventional one that estimates effective transmissivity from fitting analytical solutions to pumping-test data. The tomographic analysis additionally allows locating spatial patterns of specific discharge at a resolution similar to the spacing of the wells, which may be important when assessing and remediating contaminant plumes.