Modelling flow and transport in the chalk unsaturated zone
The Chalk is an important aquifer representing 20% of all national water supplies in the United Kingdom (UK Groundwater Forum, 1998). Much of the aquifer is uncon¯ned and in places overlain by a deep unsaturated zone which may be 10's of metres thick. Understanding °ow and transport processes within the unsaturated zone is important as they dictate the fate and behaviour of groundwater contaminants and the quality of aquifer recharge. This thesis develops a uni¯ed physics-based modelling framework for unsaturated chalk that is consistent with observed (often apparently contradictory) phenomena at chalk sites. These include: a fast water table response to rainfall events (in days) and an absence of surface run-o® (Headworth, 1972); slow solute migration (in tens of years) with very little dispersion (Oakes et al., 1981); speci¯c yields that can vary by an order of magnitude during droughts (Price et al., 2000). Flow in unsaturated chalk is described using a modi¯ed Richards' equation for dual- permeability, whereby fast °ow occurs in fractures and much slower °ow occurs in the matrix. Hydraulic characterisation of the matrix is achieved using existing mercury intrusion data. The fractures are characterised by consideration of existing in situ hydraulic conductivity data. Hypothetical coupled transient °ow and solute transport simulations of the model are presented. A key ¯nding is that the system is highly non-linear causing di®erent temporal sampling rates of the precipitation time-series to yield very di®erent results. The simulations support the widely accepted hypothesis that peak preservation in observed solute pro¯les is an artefact of very little fracture °ow. The model further suggests that the Chalk unsaturated zone possesses signi¯cant storage properties (excluding the intergranular pores of the matrix) which attenuate in¯ltration to a rate which can be absorbed by the matrix and provide greater storage than that identi¯ed from conventional pumping test analyses.