Oocysts of the protozoan Cryptosporidium parvum, when they contaminate drinking water supplies, can cause outbreaks of Cryptosporidiosis, a common waterborne disease. Of 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 through the subsoil to groundwater is generally ignored. To evaluate its significance, three series of laboratory experiments investigated subsurface transport of oocysts. Experiment I was carried out
in a vertical 18 cm-long column filled either with glass beads or silica sand, under conditions known to foster fingered flow. Experiment II involved undisturbed, macroporous soil column subjected to macropores flow. Experiment III aimed to study the lateral flow on an undisturbed soil block.
The columns and soil samples were subjected to artificial rainfall and were allowed to reach steady-state. At that point, feces of contaminated calves were applied at the surface, along with a known amount of KCl to serve as tracer, and rainfall was continued at the same rate. The breakthrough
of oocysts and Cl−, monitored in the effluent, demonstrate the importance of preferential flow on the transport of oocysts. Peak oocyst concentrations were not appreciably delayed, compared to Cl−, and in some cases, occurred even before the Cl−
peak. Recovery rates for oocysts were low, ranging from 0.1 to 10.4 percent of the oocysts originally applied on the columns. However, the numbers of oocysts present in the effluents were still orders of magnitude higher than the 5 to 10 oocysts per liter that are considered sufficient to
cause Cryptosporidiosis in healthy adults. These results suggest that the transport of oocysts in the subsurface via preferential flow may create a significant risk of groundwater contamination in some situations.
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