Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/27/2004
Publication Date: 2/5/2004
Citation: Darnault, C.G., Steenhuis, T.S., Garnier, P., Kim, Y.J., Jenkins, M., Ghiorse, W.C., Baveye, P.C., Parlange, J.Y. 2004. Preferential flow and transport of Cryptosporidium parvum oocysts through the vadose zone: Experiments and modeling. Vadose Zone Journal. 3:262-270.
Interpretive Summary: Cryptosporidium parvum (crypto for short) is a pathogenic microorganism associated with water, and has been the cause of several large-scale outbreaks of human gastrointestinal illness worldwide. In watersheds with animal agriculture such as cattle or dairies herds crypto is often detected in surface water. In particular new-borne calves can be a source of crypto. When shed onto pasture soil, the infective agents of crypto can be moved into surface waters by runoff from rain events. But subsurface movement of crypto as rainwater moves through the soil can also occur as has been documented by contaminated well water. Although subsurface movement of crypto is known to occur, the characteristics of its movement through the soil between the soil surface and groundwater needs to be elucidated. The objective of this research was to characterize the movement of crypto through columns of soil under a worst-case scenario, and develop a predictive mathematical model of the movement. This was achieved by placing a known quantity of calf feces containing a known quantity of crypto on the top of a column packed with sand, and a column containing an undisturbed soil core, and then simulating a steady rain fall. The simulated rainfall was sustained so that several volumes of water would pass through the columns. This water was collected at measured intervals and analyzed for the concentration of crypto and a chemical tracer, chloride. The movement of crypto was compared to the chemical tracer. In the packed sand column crypto moved at the same velocity as the chemical tracer and, thus, exhibited preferential flow. In the column with the intact soil core wherein flow was through large soil pores, movement of crypto was much slower than the chemical tracer. Although only a small fraction of the total number of crypto were recovered, the concentration was significantly greater than an infective dose. A mathematical model was developed that could predict movement of crypto for the sand columns; the model, however, failed to predict the movement of crypto through the large soil pores of the intact soil column. Information from this study is important to watershed hydrologists, and managers of municipal watersheds.
Technical Abstract: As a result of Cryptosporidium parvum in drinking water, several outbreaks of cryptosporidiosis have occurred in the last ten years. Although it is generally believed that movement of pathogens through the soil is minimal, recent research has shown that appreciable numbers of C. parvum oocysts may be transported via preferential or fingered flow to groundwater. The objective of the present research was to further investigate and model the transport of oocysts through preferential flow paths in the vadose zone under a 'worst-case' scenario. This was studied by adding calf feces containing C. parvum oocysts with a chloride (Cl) tracer to undisturbed silt loam columns and disturbed sand columns during a simulated steady state rain. The sand columns exhibited preferential flow in the form of fingers whereas macropore flow occurred in the undisturbed cores. In the columns with fingered flow, oocysts and Cl were transported rapidly with the same velocity through the columns. Although only 14 ' 86% of the amount applied, the number of oocysts transported across the columns was several orders of magnitude above an infective dose. The macropore columns had only a very limited breakthrough of oocyst, which appeared several pore volumes after the initial breakthrough of Cl. A simulation model for the transport of oocysts via preferential flow was developed on the basis of an existing preferential flow model for non-absorbing solutes, with addition of a first-order sink term for absorbance of C. parvum to the air/water/solid interfaces, and with velocity and dispersivity parameters derived from Cl transport. The breakthrough of C. parvum oocysts could be described realistically for the sand columns. However, the model could not describe oocyst transport in the columns with macropores.