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TECHNICAL REPORTS

Vadose Zone Processes and Chemical Transport

Developing Risk Models of Cryptosporidium Transport in Soils from Vegetated, Tilted Soilbox Experiments

^a Dep. of Land, Air, and Water, Univ. of California—Davis, Davis, California 95616-8628
^b Veterinary Medicine Extension, School of Veterinary Medicine, Univ. of California—Davis, Davis, California 95616
^c Veterinary Medicine Teaching and Research Center, School of Veterinary Medicine, Univ. of California—Davis, Tulare, California 93274
^d Dep. of Plant Sciences, Univ. of California—Davis, Davis, California 95616

^* Corresponding Author (ThHarter{at}ucdavis.edu).

Received for publication July 19, 2006. Transport of Cryptosporidium parvum through macroporous soils is poorly understood yet critical for assessing the risk of groundwater contamination. We developed a conceptual model of the physics of flow and transport in packed, tilted, and vegetated soilboxes during and immediately after a simulated rainfall event and applied it to 54 experiments implemented with different soils, slopes, and rainfall rates. Using a parsimonious inverse modeling procedure, we show that a significant amount of subsurface outflow from the soilboxes is due to macropore flow. The effective hydraulic properties of the macropore space were obtained by calibration of a simple two-domain flow and transport model that accounts for coupled flow in the matrix and in the macropores of the soils. Using linear mixed-effects analysis, macropore hydraulic properties and oocyst attenuation were shown to be associated with soil bulk density and rainfall rate. Macropore flow was shown to be responsible for bromide and C. parvum transport through the soil into the underlying pore space observed during the 4-h experiments. We confirmed this finding by conducting a pair of saturated soil column studies under homogeneously repacked conditions with no macropores in which no C. parvum transport was observed in the effluent. The linear mixed-effects and logistic regression models developed from the soilbox experiments provide a basis for estimating macropore hydraulic properties and the risk of C. parvum transport through shallow soils from bulk density, precipitation, and total shallow subsurface flow rate. The risk assessment is consistent with the reported occurrence of oocysts in springs or groundwater from fractured or karstic rocks protected only by shallow overlying soils.

Abbreviations: BTC, breakthrough curves • LME, linear mixed effect • SSDI, sterile degassed deionized