Virus Survival and Transport in Ground Water

Abstract

Viruses are a significant cause of waterborne disease in the United States; it has been estimated that they may be responsible for as much as 50% of the reported outbreaks. This fact has led the U.S. Environmental Protection Agency to propose a maximum contaminant level goal (MCLG) for viruses in drinking water. Septic tanks, which contribute over one trillion gallons of waste to the subsurface every year, are a major source of viruses in soils and ground water. The purpose of this research was to develop a model which could be used to estimate safe distances between septic tanks, or other sources of contamination, and drinking-water wells. The model was based on ground-water flow characteristics and the length of time that viruses remain infective in the subsurface environment. Water samples were collected from 71 continuously pumping municipal drinking-water wells. Viruses were inoculated into the water samples, and the rate at which the viruses were inactivated was calculated for each sample. The inactivation rates were determined to be spatially correlated by calculating a semivariogram. Kriging, a geostatistical technique, was used to estimate virus inactivation rates at unsampled locations using the measured values at nearby locations. The measured and kriged virus inactivation rates were used in conjunction with the regional ground-water flow characteristics to calculate septic tank setback distances over a city-wide area. The setback distance was defined as the distance required for a 7-log reduction in virus number in the time that the water traveled from the source of contamination to a drinking-water well. The model has been extended to account for alterations in the flow field caused by the presence of pumping wells. Setback distances of less than 15 m to greater than 300 m have been calculated using these models. The results of this research may be useful for community planning purposes, because areas with higher potentials for viral contamination of ground water may be identified based on the maps generated by the model. In addition, the models may be useful in granting variances from the mandatory ground-water disinfection requirement under consideration by the Environmental Protection Agency.