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International audience; A mathematical model describing the steady state flows in a forced gradient tracer test between an injection and pumping borehole in a multilayered sandstone aquifer has been developed that includes the effect of vertically variable background heads. A second model describing the recovery of tracer from a layer in which there are discharges due to vertical flow in the injection borehole is also presented. Application of the models to field tracer test data indicates that the observed recoveries, which are not proportional to the abstraction rate in each layer, are consistent with the hydraulic behavior of the aquifer when natural vertical head gradients are taken into account. Investigation with the models illustrates that the vertical distribution of tracer recovery depends strongly upon the background heads and that tracer tests conducted in the same aquifer, but at different times, may interrogate different aquifer layers. It is also shown generally that for a given abstraction rate the vertical distribution of tracer recovery in small-scale tracer tests is controlled largely by the transmissivity distribution but that as the spatial scale of the test increases, the distribution of recovery becomes proportional to the discharges from the injection borehole because of vertical flows within it, which may be natural or induced by pumping in the monitoring borehole. Uncertainties inherent in the design of forced gradient tracer tests in multilayered aquifers and the problems of applying the results of such tests to natural gradient contaminant migration are discussed.

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.