Cook, S.J.; (2010) The hydrogeology of bromate contamination in the Hertfordshire Chalk: incorporating karst in predictive models. Doctoral thesis , UCL (University College London).

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Abstract

Point source contamination of groundwater by bromate (BrO3^-) in the Hertfordshire Chalk Aquifer forced the closure in 2000, of a major public supply ground- water abstraction and presents a continuing threat to regional water resources. Solute transport in the Chalk is dominantly dual porosity in character but in the bromate-aected region of Hertfordshire, karst development along the feather edge of overlying Palaeocene sands and gravels complicates groundwater ow and transport. Tracer tests conducted in the early 20th century indicated rapid (1-3km/day) flow paths of 8-15km in length but a lack of quantitative data has meant that the karst system has not been adequately incorporated into existing models of bromate transport. A new suite of quantitative tracer tests was conducted to investigate the spatial relationship between karst flows and distribution of the bromate contamination, and to establish hydrodynamic transport parameters. Bacteriophage were introduced into the aquifer at 3 locations and their appearance was monitored downgradient. Results indicate that karstication is more widespread than previously indicated. Multiple flow pathways are indicated and evidence of karst penetrating into regions not previously considered karstic. The interpretation has contributed to the development of a new conceptual understanding of the function, geometry and evolution of the Hertfordshire Chalk karst and its influence upon the bromate contamination. Tracer breakthrough analysis suggests a `dual porosity' exchange between karst conduits and the Chalk matrix, and/or micro-fractures. The analysis enabled determination of transport parameters for incorporation within a regional groundwater flow and transport model.The new conceptual understanding has been incorporated into a spatially distributed groundwater ow and transport model which has been able to reproduce the main features of the karst and transport of the bacteriophage tracers. The model has been applied to simulate bromate transport towards points of interest in the catchment incorporating possible bromate source histories and stresses.

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