ABSTRACT
In 1985, the United States Environmental Protection Agency (USEPA) proposed maximum contaminant level goals (MCLGs) for viruses and Giardia, a protozoan par asite (USEPA, 1985). These standards are in addition to the standard for the indicator microorganism, total coliform. Rather than require public water systems to monitor the water for the presence of these pathogenic microorganisms, the EPA proposed treatment requirements for groundwater with the goal that the level of pathogenic viruses and Giardia in the treated water would result in a risk of less than one infection per 10,000 persons per year (USEPA, 1992). Water utilities may avoid chemical dis infection of the source water if they meet one of the EPAs “natural disinfection” cri teria. The criteria include setback distance, depth to weD screen or thickness of unsaturated zone, groundwater travel time, and virus travel time. The numerical values for the criteria are based on an acceptable virus concentration of 2 viruses/10” liters at the wellhead. This concentration was calculated using a risk of less than one infection per 10,000 people per year (Regli et al., 1991).
