Preferential Transport of Cryptosporidium parvum Oocysts in Variably Saturated Subsurface Environments Christophe J. G. Darnault, Patricia Garnier, Young-Jin Kim, Kristina L. Oveson, Tammo S. Steenhuis, J.-Yves Parlange, Michael Jenkins, William C. Ghiorse, Philippe Baveye ABSTRACT: When oocysts of the protozoan Cryptosporidium parvum contaminate drinking water supplies, they 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 little attention to date; that is, because soils are often considered to be perfect filters, the transport of oocysts through the subsoil to groundwater is generally ignored. To evaluate the significance of this pathway, a series of laboratory experiments investigated subsurface transport of oocysts. Experiment 1 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 2 involved undisturbed, macroporous soil columns subjected to macropore flow. Experiment 3 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 potassium chloride to serve as a tracer, and rainfall was continued at the same rate. The breakthrough of oocysts and chloride, monitored in the effluent, demonstrate the importance of preferential flow on the transport of oocysts. Compared with chloride, peak oocyst concentrations were not appreciably delayed and, in some cases, occurred even before the chloride peak. Recovery rates for oocysts were low, ranging from 0.1 to 10.4% of the oocysts originally applied on the columns. However, the numbers of oocysts present in the effluents were still orders of magnitude higher than 10 oocysts, the infectious dose considered by the U.S. Food and Drug Administration, Center for Food Safety and Applied Nutrition, to be 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. Water Environ. Res., 75, 113 (2003). KEYWORDS: Cryptosporidium parvum, preferential flow, subsurface transport. Introduction Pathogenic bacteria, viruses, and protozoa in drinking water are a significant cause of animal and human death in many parts of the world. Chlorination is an efficient treatment against a wide range of pathogens. However, it has no effect on a number of micro- organisms, including the protozoan Cryptosporidium parvum, which causes Cryptosporidiosis, a common waterborne disease (Smith, 1992). Outside of the lower intestines of humans and domestic and wild animals, where C. parvum carries out the active part of its cycle, it is present in the environment in the form of 4- to 6-lm-long ovoid-shaped oocysts, with a double wall that is resistant to most oxidation processes (e.g., ozonation and chlorination) (Atwill et al., 1997; Current, 1986; Fayer and Ungar, 1986; Tzipori, 1983). During the past two decades, the presence of C. parvum in surface waters and groundwaters in the United States and Great Britain (Galbraith et al., 1987; LeChevallier et al., 1991; Rose et al., 1991) has been connected with several major outbreaks of Cryptosporidiosis (Hayes et al., 1989; MacKenzie et al., 1994; Smith et al., 1988). The presence of C. parvum oocysts in drinking water supplies results from a number of processes. For example, infected hosts such as cows or deer may defecate in streams and shallow ponds. As a result, feces may end up on the soil via direct release by infected animals or land spreading of manure. Land application of municipal or industrial wastewater sludge may also contribute significant numbers of oocysts at the soil surface. Here, runoff may carry oocysts to nearby waterbodies, or rain infiltration may transport oocysts in the subsoil to groundwater. Among the different pathways for transporting oocysts, rain infiltration was generally considered until recently to be of little significance, in line with the common assumption that soils are effective at filtering a wide range of pathogens (Tim et al., 1988). Several authors obtained results that seem to support this assumption. For example, Tan et al. (1994) found that movement of bacteria through disturbed, water-saturated soil matrices occurs to any significant extent only under extreme conditions of ionic strength, conditions that are unlikely to be found in agricultural fields. Under normal conditions in saturated and homogeneous media, Brush (1997) found C. parvum oocysts to be effectively strained by the soil matrix and to adsorb to soil particles; Harter et al. (2000) also found that the oocysts are subjected to velocity enhancement. Tate et al. (2000), who studied the transport of C. parvum oocysts on rangeland watersheds, hypothesized that oocysts can be transported from fecal deposits during rainfall events and are available for transport to waterbodies. During the last decade, laboratory and field experiments have shown that microorganisms can migrate far and fast along with other contaminants through unsaturated, undisturbed soil in both vertical and horizontal directions (Bitton and Harvey, 1992; Tan et al., 1992; van Eslas et al., 1991). This fast transport is due to preferential flow, which encompasses fingering, macropore flow, and funnel flow (Selker et al., 1996). Fingering-flow phenomena resulting from wetting front instabilities occurring at the interfaces of two fluids contributes to transport variability in sand–air–water systems. Hill and Parlange (1972) studied wetting front instability phenomenon and found that the vertical growth in the wetting front instability is driven by gravity, forming ‘‘fingers’’ that are stabilized horizontally by capillarity of the surrounding media. March/April 2003 113