The hydrogeology of bromate contamination in the Hertfordshire Chalk : double-porosity effects on catchment-scale evolution

• Main Author: Fitzpatrick, C. M.
• Published: University College London (University of London) 2011
• Subjects: 550
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.594303

Bromate contamination over an area of more than 40 km2 in the Hertfordshire Chalk aquifer was first detected in 2000 and is the largest case of point-source groundwater contamination in the UK. Bromate is a possible human carcinogen, and a regulatory limit for drinking water of 10 μg l-1 had been implemented in the U.K. since 2003. Background concentrations of bromate in groundwater are believed to be effectively zero. In the affected area, bromate at concentrations of several 100 μg l-1 have forced the closure of a large public water supply source and restricted the use of seven other public supply boreholes up to 20 km from the contamination source. The source has been identified as a former industrial site which operated between 1955 and 1983. Residual contamination at the site provides a continuing source of bromate to groundwater. A range of conceptual scenarios for bromate mobilisation and release to groundwater have been developed and quantified based on interpretation of the available data, and constrained by interpolation of the observed concentrations. Analysis and interpretation of all available monitoring and investigation data throughout the catchment has revealed the influence of recharge, water level, and groundwater abstractions on bromate concentrations. These relationships, integrated with observations of the geology and hydrogeology of the area, support a conceptualisation of transport of bromate by dominantly double-porosity processes within the Vale of St. Albans area, which maintains a highly attenuated, stable contaminant distribution west of Hatfield. An extensive karst network related to the position of the Palaeogene overlap of the Chalk influences bromate transport to the east of Hatfield, dispersing bromate rapidly over large distances toward the Lea Valley. The revised conceptual understanding has enabled the development of a new interpretation of bromate transport within the catchment between 2000 and 2008. A new analytical network modelling approach has been developed to predict the long-term, largescale transport of bromate. The model simulates Fickian double-porosity diffusive exchange along interconnecting flow-lines, linked to rapid karst flow. The model is parameterised on the basis of single borehole dilution testing, catchment-scale natural gradient tracer testing, and literature derived values. The network model, combined with quantified bromate source terms, simulates bromate and bromide concentrations of the order of magnitude of those observed at locations within the Vale of St. Albans, and predicts bromate concentrations to remain above regulatory limits for around 200 years. This highlights the importance of double-porosity diffusion for the long-term evolution of contaminants at catchmentscale in the Chalk aquifer.