Using soils as filters to prevent 'crypto' from moving to the groundwater

02/27/04

A study published in the Vadose Zone Journal examines how different soil types affect Cryptosporidium parvum's transport.



MADISON, WI, FEBRUARY 27, 2004 Groundwater is generally considered a safe source of drinking water because pathogens are presumably filtered out during their transport through unsaturated soils. Nevertheless, pathogen-contaminated groundwater has been the cause of many disease outbreaks in the last 10 years including cryptosporidiosis caused by the protozoan pathogen Cryptosporidium parvum. Standard chlorine disinfection is not effective at killing the pathogen.

Although several "crypto" outbreaks have occurred in the U.S., the most severe problem is in Africa where both the sanitary conditions are poor and a significant portion of the population has HIV. It is especially dangerous for people with a compromised immune system because it can lead to severe diarrhea and lethal dehydration. the most notable being a cholera epidemic in London in 1854.

A study published in the February 2004 issue of Vadose Zone Journal, which was funded by the USDA-National Research Initiative program and performed at Cornell University Department of Biological and Environmental Engineering, examines how C. parvum oocysts may be transported via preferential flow to groundwater.

A simulation model was developed on the basis of an existing preferential flow model. Column experiments with sandy and undisturbed structured soils were performed. Calf manure with cryptosporidiosis was applied to the columns soil surface, followed by rain. Chloride was also added as a tracer.

"Traditionally, people assume that soils filter out pathogens. This is based upon the premise that water and pathogens move slowly through all of the soil. We found that for sandy soil, where water moved rapidly through, there was indeed very little filtering initially. Surprisingly, the undisturbed, structured columns with large macropores had only a very limited breakthrough," said Tammo Steenhuis, Cornell University, who was one of the researchers of the study.

Although this study shows that the pathogen could enter the groundwater easily at certain levels, the experiments were carried out by designing transport-enhancing, worst-case conditions consisting of infected fresh manure spread shortly before a major rainstorm on a coarse sandy soil with a short distance to ground water. Contamination is much less likely to occur when conditions are not as extreme, for example when manure is dry before the rainfall starts or when the groundwater is not as shallow.

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