Distribution of contaminants in the seasonally unsaturated zone of the Chalk aquifer

Distribution of contaminants in the seasonally unsaturated zone of the Chalk aquifer The Chalk is a dual-porosity aquifer in which most of the porosity is found within a fine-grained matrix. Groundwater in this matrix is virtually immobile and flow takes place almost entirely within fissures. The small volume of fissure porosity gives a low unconfined storage coefficient resulting in large changes in water table elevation in response to seasonal recharge. The zone over which the water table fluctuates, the seasonally unsaturated zone (SUZ), has a major influence on the behaviour of dissolved contaminants, initially slowing their movement, through diffusional exchange with the matrix, and at later stages acting as a reservoir of long-term contamination. Detailed profiles of contaminant distribution in matrix porewater through the SUZ have been constructed from cored boreholes at a study site in southern England. The physical and hydraulic environments of the SUZ have also been characterised. The field data have been used to construct a conceptual model of contaminant behaviour in the SUZ and from this a one-dimensional semi-analytical model has been developed to examine the role of matrix diffusion and fluctuating water tables in the distribution of contaminants in matrix porewater in the SUZ. The results of modelling have been compared to data gathered at the field site and similarities found which indicate a correct understanding of the processes involved. The model shows that contaminant distribution in the matrix evolves towards a situation where concentrations are greatest towards the top of the SUZ. The SUZ can thus form a secondary source of contamination, which may persist long after the original source has dissipated. A consequence of this distribution is that contaminant concentrations in groundwater show a characteristic seasonal high at high water stands. The work has examined a particular site but the processes involved are found over the Chalk outcrop, and in other hydrogeological environments characterised by dual porosity and a fluctuating water table.