Investigating aquitard heterogeneity by inverse groundwater modelling of drinking water extraction sites

Aquitards are important hydrogeological features and their properties play an important role in e.g. water resources management, subsidence, aquifer thermal energy storage and contamination transport. The hydraulic conductivity of aquitards is typically parameterized by analytical interpretation of pumping tests or model calibration. However, these methods may not be very accurate for aquitards and usually do not account for spatial variability and uncertainty. Alternatively, core-scale measurements of hydraulic conductivity are used in geostatistical upscaling methods, for which their correlation lengths are needed. However, this information is extremely difficult to obtain. In this study we investigate whether readily available data from three drinking water extraction sites in The Netherlands can be used to obtain the geostatistical parameters of aquitard hydraulic conductivity needed for upscaling and to provide information about spatial variability of the hydraulic conductivity. We generated conditional random realizations from core scale data with varying correlation lengths and lithology distributions, upscaled these to model block scale and inserted these into a groundwater flow model that simulates the impacts of drinking water extraction and natural variability in precipitation and evapotranspiration over an extensive time period. We select the realizations that best fit observed groundwater heads to extract information about aquitard correlation lengths and lithology distributions and derive upscaled spatially varying aquitard conductivities.