A framework for rapidly assessing the pollutant retardation capacity of aquifers and sediments

Pollutants are subject to a variety of processes that attenuate their transport within aquifers and sediments. Natural attenuation processes are commonly considered during the assessment and management of risks associated with contaminated land and groundwater. A framework has been developed that allows rapid assessment of the potential for aquifers or river-bed sediments to retard dissolved pollutants. Retardation by sorption to organic carbon, cation exchange and acid buffering are considered for a range of common and relevant groundwater pollutants. Based on simulated pollutant behaviour, thresholds for high, moderate and low attenuation capacity have been generated. The methods are described so that users can adjust the boundaries (of the attenuation classifications) to best suit the needs of other assessment decisions. The framework relies solely on measurements of the fraction of organic carbon, cation exchange capacity and total inorganic carbon content in an aquifer or sediment. It can be undertaken during the initial evaluation of a hydrogeological risk assessment or monitored natural attenuation evaluation, and will help assessors to prioritize subsequent site-specific investigations on pollutant attenuation. The framework has been tested against field data and is shown to usefully differentiate British strata, based on the geochemical properties that control pollutant retardation.