Assessing the hazard from viruses in wastewater recharge of urban sandstone aquifers

Increasing water demand in urban areas is focusing attention on the possibilities of the re-use of urban wastewaters, waters that often contain human and animal (including avian) viruses. In urban red-bed sandstone aquifers in the UK, which are predominantly matrix flow systems, evidence from well and piezometer monitoring shows that viable human viruses can be transported to depths of at least 80 m. The aim of the studies described here is therefore to determine the processes controlling the virus transport as a basis for risk assessment. Laboratory column experiments show that virus breakthrough is severely attenuated in synthetic groundwater solutions, some viruses remaining effectively irreversibly attached to the rock: attenuation capacity is only slowly reduced as more viruses are eluted. However, addition of silica colloids (which when injected by themselves are also severely attenuated) to the virus solutions, results in breakthrough of the injected virus particles and release of previously attached virus particles. Forced-gradient tracer field experiments suggest that (severely attenuated) virus breakthrough occurs, but only through specific pathways. Current fieldwork is aimed at determining the location, and hence the hydraulic and geochemical characteristics of these pathways. It appears, therefore, that virus attenuation is reduced by the presence of other colloidal matter, low ionic strength, and continuous virus loading, and that conditions for transport occur only in specific pathways. Future laboratory work will be aimed at further quantifying these processes and relating them to the petrographic and geochemical properties of the various sandstone (hydro)lithofacies which the field experiments indicate are important. This will provide the understanding necessary for a process-based risk assessment procedure.