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When they contaminate drinking water supplies, oocysts of the protozoan Cryptosporidium parvum can cause outbreaks of cryptosporidiosis, a common waterborne disease. Among the different pathways by which oocysts can wind up in drinking water, one has received very little attention to date; because soils are often considered to be perfect filters, the transport of oocysts via the subsoil to groundwater is generally ignored. Recent experiments have shown that appreciable numbers of oocysts can migrate in soils in which preferential (finger or macropore) flow occurs, and that, within soils, oocysts seem to have a tendency to accumulate in the upper part of the capillary fringe, presumably because of adsorption at gas-water interfaces. The objective of the present research was to verify the influence of these interfaces in the region above the capillary fringe on the retention and transport of oocysts. Laboratory experiments were carried out with sand columns subjected to simulated rainfall and at the surface of which the feces of contaminated calves were applied. The approach adopted toinvestigate the effect of gas-water interfaces was to modify the hydrodynamical conditions in the sand columns by varying the rainfall intensity or by making the sand water-repellent. In a third series of experiments, water-repellent barriers were artificially created at two different depths. Experimental results suggest a close relationship between oocyst retention and the extent of gas-water interfaces; sharp increases in oocyst numbers are consistently observed in regions of the sand where the water content has steep gradients, and therefore where one expects capillary meniscii to have maximal extent. Theseobservations imply that oocyst transport in the vadose zone is likely to be very limited in the absence of preferential flow. Variability in oocyst recovery rates among the different 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 used in the experiments. In all cases, regardless of experimental conditions, oocyst numbers in the effluents were orders of magnitude higher than regulatory drinking water standards, confirming earlier indications that the transport of oocysts in the subsurface via preferential flow may create a significant risk of groundwater contamination in some situations. Includes 24 references, table, figures.

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Edition: Vol. - No. Published: 06/01/2001 Number of Pages: 16File Size: 1 file , 430 KB