Spatial sensitivity of aquitard hydraulic parameters derived from pumping tests in multi-aquifer systems

Estimates of aquitard hydraulic parameters are typically derived from pumping test drawdowns in aquifers. Analytical solutions for leaky aquifers and semi-analytical solutions for multiple aquifer systems assume homogeneity of hydraulic parameters in both aquifers and aquitards. In settings where the hydraulic parameters cannot be assumed to be homogeneous, the parameters estimated with these methods are generally considered spatial averages. In this study, we investigate the spatial sensitivity of aquitard hydraulic properties at various observation times in both the pumped and overlying aquifers using a synthetic pumping test model. Results show that the area around both the observation and pumping wells exhibits the highest sensitivity in both the overlying and pumped aquifers. A parabolic shape in sensitivity is observed between the wells. Over time, the sensitive area shifts from an approximate line between the wells to an expanding ellipse. However, if the transmissivity of the overlying aquifer is lower than that of the pumped aquifer, the observation in the overlying aquifer becomes increasingly sensitive to the hydraulic conductivity near the observation well. Conversely, if the transmissivity of the pumped aquifer is lower than that of the overlying aquifer, the sensitivity shifts towards the area close to the extraction well. Understanding these sensitivity patterns is essential for translating pumping test results into parameters for regional groundwater flow models and improving pumping test design.