Evaluation of Analytical Methods to Study Aquifer Properties with Pumping Tests in Coastal Aquifers with Numerical Modelling (Motril-Salobreña Aquifer)

Two pumping tests were performed in the unconfined Motril-Salobreña detrital aquifer in a 250 m-deep well 300 m from the coastline containing both freshwater and saltwater. It is an artesian well as it is in the discharge zone of this coastal aquifer. The two observation wells where the drawdowns are measured record the influence of tidal fluctuations, and the well lithological columns reveal high vertical heterogeneity in the aquifer. The Theis and Cooper-Jacob approaches give average transmissivity (T) and storage coefficient (S) values of 1460 m2 /d and 0.027, respectively. Other analytical solutions, modified to be more accurate in the boundary conditions found in coastal aquifers, provide similar T values to those found with the Theis and Cooper-Jacob methods, but give very different S values or could not estimate them. Numerical modelling in a synthetic model was applied to analyse the sensitivity of the Theis and Cooper-Jacob approaches to the usual boundary conditions in coastal aquifers. The T and S values calculated from the numerical modelling drawdowns indicate that the regional flow, variable pumping flows, and tidal effect produce an error of under 10 % compared to results obtained with classic methods. Fluids of different density (freshwater and saltwater) cause an error of 20 % in estimating T and of over 100 % in calculating S. The factor most affecting T and S results in the pumping test interpretation is vertical heterogeneity in sediments, which can produce errors of over 100 % in both parameters.This research has been financed by Project CGL2012-32892 (Ministerio de Economía y Competitividad of Spain) and by the Research Group Sedimentary Geology and Groundwater (RNM-369) of the Junta de Andalucía

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