Submitted to: American Water Works Association Annual Conference and Exposition
Publication Type: Proceedings
Publication Acceptance Date: 6/19/2001
Publication Date: 6/19/2001
Technical Abstract: Oocysts of Cryptosporidium parvum in the terrestrial environment pose a public health risk to drinking water. Soils are often considered filters that prevent transport of oocysts to groundwater. Recent experiments have shown that oocysts can move through soils in which preferential (finger or macropore) flow occurs. Within soils oocysts tend to accumulate in the upper part of the capillary fringe at gas-water interfaces. Our objective was to verify the influence of these interfaces in the region above the capillary fringe on the retention and transport of oocysts. Under laboratory conditions contaminated calf feces were applied to the surface of sand columns that were subjected to simulated rain. Results suggest a relationship between oocyst retention and the extent of gas-water interfaces; sharp increases in oocyst numbers were observed in regions of the sand where the water content had steep water gradients, and therefore maximal capillary menisci. These observations imply that oocyst transport in the vadose zone may be limited in the absence of preferential flow. Variability in oocyst recovery rates among experiments makes it uncertain, however, to what extent the accumulation of oocysts at gas-water interfaces is able to compensate for the efficiency of fingers to transport oocysts through the sand columns. Under all experimental conditions, oocyst numbers in the effluents were orders of magnitude higher than regulatory drinking water standards, confirming that the transport of oocysts in subsurface flow may create a significant risk of groundwater contamination.