Improving the characterization of Quaternary deposits for groundwater vulnerability assessments using maps of recharge and attenuation potential

Assessing aquifer vulnerability is difficult for bedrock aquifers concealed by highly variable superficial deposits such as glacial till. Many current groundwater vulnerability maps, and the geological maps on which they are based, do not adequately account for regional and vertical variations in the characteristics of superficial deposits. A new method for characterizing recharge potential and contaminant retardation potential of superficial deposits is presented here, which captures primary geological and hydrogeological expert knowledge in a systematic manner. The method modifies existing superficial geology maps using Quaternary domains and their descriptions, bedrock lithology and thickness of superficial deposits, and applies additional information on superficial geology and bedrock lithology. Central to the method is a matrix that allows local geological and hydrogeological knowledge to be incorporated in a systematic and traceable manner. The scale-independent method has been piloted at 1:625 000 scale to produce maps of recharge and attenuation potential for Great Britain. Preliminary verification against several indicators (HOST data, the Scottish vulnerability screening tool, and nitrate data) has been encouraging. The method is being used by the Environment Agency as part of its vulnerability assessments for the characterization of groundwater bodies as required by the Water Framework Directive.