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SCREENING MODELS TO PREDICT PROBABILITY OF CONTAMINATION BY PATHOGENIC VIRUSES IN DRINKING WATER AQUIFERS
Citation:
Faulkner*, B P., W. G. Lyon, G. P. Breidenbach, J. Cruz, S. Chattopadhyay, AND F A. Khan*. SCREENING MODELS TO PREDICT PROBABILITY OF CONTAMINATION BY PATHOGENIC VIRUSES IN DRINKING WATER AQUIFERS. Presented at EPA Science Forum 2002, Washington, DC, April 30, 2002.
Impact/Purpose:
To inform the public.
Description:
The Safe Drinking Water Act's 1996 Amendments broadened the definition of public water systems (PWS) to include systems which serve drinking water to as few as 25 individuals. Implementation of the proposed Ground Water Rule for Pathogens will place an increased burden on utilities to evaluate compliance. Many of the utilities consist of small PWS with few resources available for determining compliance. Due to their small size and resistance to inactivation, viruses pose a risk when septic systems or other sources exist above shallow PWS source aquifers. In most cases the risk is very small. However, since as few as one or two virus particles may cause illness if consumed by humans, the risk must be considered. Outbreaks of illness due to viruses in drinking water wells have been documented.
The purpose of this research is to develop mathematical models to predict the fate and transport of viruses during percolation in soils. Modeling is subject to considerable uncertainty, due to the many factors that affect virus survival during transport. We have developed a physical model of virus transport and a database of probability distribution functions for the parameters of the model. Monte Carlo methods were used to predict the probability of failure of a soil to prevent leaching of viruses to aquifers at depth. We have applied this model with several viruses which are listed on EPA's candidate contaminant list. The most significant factors appear to be the hydraulic conductivity of the soil and the inactivation rate of viruses suspended in soil water. The screening model "VirMod" will soon be available for downloading at http://www.epa.gov/ada/ the Web site for the Ada, Oklahoma Division of the EPA-ORD National Risk Management Research Laboratory.
Application of the model indicated the following probabilities (p) of leaching of viruses below a 0.5-meter column of soil for Hepatitis A virus: sand (p = 0.0007), loam (p = 0.0023), and clay (p = 0.0045). The Monte Carlo methods that produced these results considered volumetric soil water content to be unknown, hence varying between the residual water content and the saturated water content. In this case finer grained soils produced increased risk for a relatively thin layer of soil, and coarser-grained soils reduced leaching by adsorption to the air-water interface. This result highlights the importance of water content in evaluating leaching of viruses.