Controls on preferential recharge to Chalk aquifers

There is evidence that, under certain conditions, rapid preferential recharge via the fracture network can occur in Chalk aquifers. This has potentially important implications for contaminant migration through the Chalk unsaturated zone, CUZ, and for groundwater flooding in Chalk catchments. In the case of groundwater flooding, deficiencies in modelling aquifer response have been attributed to inadequate representation of flow processes in the CUZ. In this paper we consider two complementary approaches for assessing controls on preferential recharge to Chalk aquifers: an empirical approach and a physically-based modelling approach. We show that the main controls on preferential recharge to Chalk aquifers are the characteristics of rainfall events, in terms of duration and intensity, the physical properties of the near surface, and the antecedent soil moisture in the near surface. We demonstrate a number of deficiencies when past models of the CUZ are applied to the problem of simulating preferential recharge, notably that the assumption of instantaneous equilibrium between fractures and matrix is not valid, particularly during extreme recharge events. In order to simulate preferential recharge, fractures and matrix must be modelled as separate but interacting domains. This was achieved using a dual continua model. The model was computationally demanding, but was able to reproduce observed behaviour, including apparently hysteretic soil moisture characteristic relationships in the near surface, and rapid preferential recharge fluxes in response to high intensity rainfall events.