On cessation of the 2001 outbreak of Foot and Mouth Disease, the Environment Agency in the North West Region wished to review the overall significance of the authorised farm burials of pyre ash and carcasses to the groundwater resources of the Region. ESI was commisioned to develop a standard methodology and software tool that might be used for the following tasks:
Derivation of cordons sanitaire around ash and carcass disposal sites, within which development of groundwater resources ought to be excluded, or only proceed based on a site specific risk assessment.
Identification of disposal sites at which water quality monitoring might be required.
Identification of disposal sites for which there is unlikely to be concern in the short term, to aid prioritisation of site-specific risk assessment and/or investigation efforts.
ESI undertook the initial assessment of all the on-farm ash and carcass disposal sites in the Region. With the new software tool ESI were able to predict contaminant breakthrough concentrations in nearby watercourses and using these, predict concentrations at key surface water supply points in the rivers downstream.
It was recognised that one of the the key factors in attenuation of contaminants was the travel time through the unsaturated zone. UK groundwater risk assessments have tended to model this in a simplistic and conservative manner. Since the Agency had specifically targetted areas with thick unsaturated zones, this additional attenuation had to be accounted for in the transport modelling. ESI therefore developed a stand-alone software model, based on RAM, that accounts for attenuation in unsaturated low permeability drift, aquitard units and/or the aquifer unit using semi-analytical one-dimensional variably saturated flow modelling techniques. A custom Newton-Raphson solver was used to solve the flow equations while the RAM Laplace Transform solver was used for contaminant transport. Sites posing most risk to groundwater were thus readily identified. Other features of the software tool were the database of contaminant attenuation parameters, compiled specially for the aquifers of the North West Region, and automated model generation routines that allowed rapid translation of the conceptual model of the geology to the numerical model.