Solute transport and hydrodynamic characteristics in the Chalk aquifer at Tilmanstone, Kent
Research on the hydrodynamic characteristics of the Chalk aquifer is described in relation to the timescales and spatial extent of solute transport, focussing on the Tilmanstone - Eastry valley in east Kent. Groundwater contamination occurred there over a 70-year period as a result of coalfield brine infiltrating from surface lagoons. The resulting contamination is used as a large-scale, long-term conservative solute transport experiment, within which a series of geological and hydrogeological observations, and tracer tests, are used to investigate the transport properties of the Chalk. The objective is to consider appropriate methodologies for application to groundwater contamination investigations in the Chalk and characterisation of Chalk groundwater bodies in the context of the requirements of the EU Water Framework Directive. The geology and hydrogeology of the Tilmanstone area are reassessed in the light of recent work on Chalk lithostratigraphy, using available hydrological data and employing a regional groundwater flow model, coupled with a solute transport model utilising a first order mass transfer coefficient dual porosity approach. Field tests are analysed in terms of aquifer properties pertinent to solute transport at a scale of metres to tens of metres. Vertical profiles of hydraulic conductivity, groundwater velocity, fracture aperture and effective porosity are developed. A profile of chloride concentration in porewater is interpreted in the light of this work to develop a hydrostratigraphy for the area. A 1-D dual porosity model employing Fickian diffusive exchange is used to compute chloride concentration of fracture water and matrix porewater over time. This provides predictions for comparison with observations and the results of the mass transfer coefficient approach in the 3-D regional model. The combined results from the 1-D and 3-D models are used to direct development of a conceptual model of contaminant transport in the Chalk. Emphasis is placed on the effects of diffusive exchange between porewater and fracture water and the effects of the solute exchange approach adopted at different times in the plume history. Results are used to judge the applicability of methods for investigating contaminated groundwater and characterising groundwater bodies in the dual porosity Chalk aquifer.