Vulnerability of unconfined aquifers to virus contamination.

An empirical formula was developed for determining the vulnerability of unconfined sandy aquifers to virus contamination, expressed as a dimensionless setback distance r(s)(*). The formula can be used to calculate the setback distance required for the protection of drinking water production wells against virus contamination. This empirical formula takes into account the intrinsic properties of the virus and the unconfined sandy aquifer. Virus removal is described by a rate coefficient that accounts for virus inactivation and attachment to sand grains. The formula also includes pumping rate, saturated thickness of the aquifer, depth of the screen of the pumping well, and anisotropy of the aquifer. This means that it accounts also for dilution effects as well as horizontal and vertical virus transport. Because the empirical model includes virus source concentration it can be used as an integral part of a quantitative viral risk assessment.