On the use of virus transport modeling for determining regulatory compliance

The U.S. Environmental Protection Agency (USEPA) has developed a draft Ground Water Disinfection Rule (GWDR) proposing that all public water systems using groundwater must disinfect the source water from each of its wells unless the wells meet one of the natural disinfection criteria they have developed or the system qualifies for a variance. A water utility that wished to avoid chemical or physical disinfection of its well water and did not meet the prescribed horizontal or vertical setback distances would have the option of using a virus transport model to determine whether there was adequate natural disinfection in their system. We think that this procedure is unwise, because there is enormous uncertainty in the estimates of the parameters required by the transport model. This uncertainty is amplified nonlinearly into the model prediction of maximum virus concentration the USEPA wishes to use as the basis for regulatory decisions. For typical levels of uncertainty in the input parameters, we predict that the concentration uncertainty will span many orders of magnitude, rendering model estimates of natural disinfection useless for regulatory purposes.