Ground Water Quality Straining , Attachment , and Detachment of Cryptosporidium Oocysts in Saturated Porous Media

of cryptosporidiosis have been reported in industrialized countries (Craun, 1990; Craun et al., 1998). A single Accurate knowledge of the transport and deposition behavior for outbreak of cryptosporidiosis in 1993 caused illness in pathogenic Cryptosporidium parvum oocysts is needed to assess con370 000 individuals from Milwaukee, Wisconsin. Ingestamination and protect water resources. Experimental and modeling studies were undertaken to examine the roles of attachment, detachtion of contaminated water containing as few as 10 ooment, and straining on oocyst transport and retention. Saturated colcysts can lead to infection (Olson et al., 1999). Hence, umn studies were conducted using Ottawa aquifer sands (U.S. Silica, contamination of drinking water by oocysts is a serious Ottawa, IL) with median grain sizes of 710, 360, and 150 m. Decreasconcern for public health. ing the median sand size tended to produce lower effluent concentraFew commercial disinfectants are effective against tions, greater oocyst retention in the sand near the column inlet, and oocysts in drinking water (Sobsey, 1989; Peeters et al., breakthrough of oocysts at later times. Oocyst transport data also 1989), including chlorination at normal drinking water exhibited concentration tailing. Mathematical modeling of the oocyst treatment levels. Riverbank filtration, infiltration basins transport data using fitted first-order attachment and detachment and trenches, and sand filters are sometimes used to coefficients provided a satisfactory description of the observed effluent concentration curves, but a poor characterization of the oocyst remove oocysts from surface water or effluent from spatial distribution. Modeling of these data using an irreversible strainsewage treatment. The efficacy of soil treatments deing term that is depth dependent provided a better description of the pends on the retention and survival of oocysts in porous oocyst spatial distribution, but could not account for the observed medium. Several published studies have examined transeffluent concentration tailing or late breakthrough times. A more physiport and retention behavior of Cryptosporidium oocysts cally realistic description of the data was obtained by modeling attach(Mawdsley et al., 1996a, 1996b; Brush et al., 1999; Harter ment, detachment, and straining. The percentage of total oocysts et al., 2000; Logan et al., 2001). Mawdsley et al. (1996a, retained by straining was estimated from effluent mass balance consid1996b) found significant subsurface transport of oocysts erations to be 68% for 710m sand, 79% for 360m sand, and 87% in tilted soil boxes and undisturbed soil columns. These for 150m sand. Straining coefficients were then selected to achieve these percentages of total oocyst retention, and attachment and deauthors concluded that rapid flow of water through mactachment coefficients were fitted to the effluent concentration curves. ropores largely bypasses the filtering and adsorptive efDramatic differences in the predicted oocyst breakthrough curves fects of soil and increases the risk of oocyst transport were observed at greater transport distances for the various model forto ground water. Brush et al. (1999) measured effluent mulations (inclusion or exclusion of straining). Justification for oocyst concentration curves in several sands, and analyzed their straining was provided by trends in the transport data, simulation data using an analytical solution to the transport equaresults, pore size distribution information, and published literature. tion that accounts for advection, dispersion, linear equilibrium sorption, and first-order decay. Harter et al. (2000) measured oocyst effluent concentration curves C ryptosporidium parvum is a protozoan parasite and spatial distributions in various sands at several that infects the intestines of a variety of wild and transport velocities. The collected data were characterdomesticated animals, as well as humans (Current, 1986). ized using traditional clean-bed (first-order) attachment The infectious stage of this parasite is biologically dortheory. Logan et al. (2001) also conducted oocyst transmant oocysts that are shed in high numbers within the port experiments that examined the influence of water feces of infected animals. Cryptosporidium oocysts are quality, sand size, and flow rate, and observed that the ubiquitous in surface water (Rose, 1988; Rose et al., 1991; vast majority of oocyst retention occurred in the sand LeChevallier et al., 1991; Hoogenboezem et al., 2001). adjacent to the column inlet. Brush et al. (1999), Harter Field surveys also indicate that low levels of oocyst conet al. (2000), and Logan et al. (2001) all observed trends tamination in ground water are rather frequent (Lisle of increasing oocyst retention with decreasing sand size. and Rose, 1995; Hancock et al., 1997). Many outbreaks The retention of colloids (such as oocysts) in porous media is typically ascribed to attachment (Ryan and S.A. Bradford, USDA-ARS, George E. Brown Jr. Salinity LaboraElimelech, 1996; Kretzschmar et al., 1999). Effluent tory, 450 W. Big Springs Road, Riverside, CA 92507-4617. M. Bettaconcentration curves and spatial distributions of rehar, Parsons, 100 West Walnut Street, Pasadena, CA 91124. Mention tained colloids in porous media, however, have not alof trade names and company names in this manuscript does not imply ways been in agreement with colloid attachment theory any endorsement or preferential treatment by the USDA. Received 2 June 2004. *Corresponding author (sbradford@ussl.ars.usda.gov). (Camesano and Logan, 1998; Bolster et al., 1999; Redman et al., 2001; Bradford et al., 2002; Tufenkji et al., Published in J. Environ. Qual. 34:469–478 (2005). 2003). Some of these discrepancies have been attributed © ASA, CSSA, SSSA 677 S. Segoe Rd., Madison, WI 53711 USA to soil surface roughness (Kretzschmar et al., 1997; Red-