2D HYDROGEOLOGICAL MODELING OF POLLUTANTS (PATHOGENS) TRANSPORT IN AN ARID RIVERBANK FILTRATION SITE IN EL PASO TEXAS

Drawing of water through riverbank sediments may be an inexpensive method to filter drinking water for human consumption. Untreated water is an important component of the drinking water in many arid areas of the world. The hydrogeological factors and spatial changes in the water chemistry during the transport from the river to the aquifer have important implications on the quality of the produced water. A study was carried out at a bank filtration experiment site in the Rio Bosque Wetlands Park in El Paso, Texas, to model the hydrogeological aspects and chemical changes that occur as water passes through the sediment. The salinity of the water is slightly above the limit for drinking water, and the land was used as farm land with some contamination of organic compounds, especially pesticides.

A multiple pumping and tracer test was conducting using the cruciform array of the field site that consisted of a pumping well, 16 observation wells, and a stream sampling point. The average hydraulic conductivity of the geological media at the field site was about 2*10-3 m/s based on pumping test analysis. However, the type curve responses revealed significant heterogeneity of hydraulic conductivity throughout the field site. For the tracer test, bromide and microspheres were used as tracers. Microspheres were use to mimic the behavior of Giardia and Cryptosporidium. The tracers (bromide and microspheres of different sizes and colors) were injected in one observation well screened into the riverbank, one observation well screened into the geological medium at the field site, and into one piezometer pushed into the stream sediments within the stream. The bromide recovery in the pumping well and in the deeper observation wells showed an early and a late peak with a long tail indicating the possibility that the geological medium at the field site behaves like a double-porosity medium allowing the tracer to move relatively quickly through the higher conductivity units while being significantly retarded in the low hydraulic conductivity units. The analysis of the microspheres in the laboratory, which is not yet complete, will shed more light on the transport behavior of pathogens at the field site.