Calibration of spatial aquitard distribution using hydraulic head changes and regularisation

Irregularly expressed aquitards have a significant impact on hydraulics and redox conditions. The method expands the scope and applicability of characterising spatial structures close to well fields. The test site is an aquifer system with two aquifers divided by an aquitard (glacial till), located at a well field for drinking water abstraction. During operation, the wells are frequently switched and hydraulic head data are recorded at 10 wells in both aquifers. The data contain information about the impact of each abstraction well on each observation well. We develop an inverse modelling procedure to calibrate spatial aquifer parameters from this data. Instead of heads we calibrate to selected head differences to keep the model concise to short term fluctuations and to reduce data. The calibrated model parameters are storativity of the upper aquifer, hydraulic conductivity of both aquifers and leakage of the aquitard. The calibration is carried out spatially by pilot points with the software PEST using regularisation. Different cross validations show that the leakage can be calibrated in a physically meaningful way, but not the hydraulic conductivities and not the storativity. Geostatistic assumptions are not required. The calibrated spatial aquitard distribution coincides with bore profiles and explains anomalies of redox conditions.