Human viruses as tracers of wastewater pathways into deep municipal wells

Enteric viruses, because of their small size, have a high potential to move deeply through the subsurface environment, penetrate aquitards, and reach confined aquifers. Until recently, few water utilities or researchers were aware of possible virus presence in deep aquifers and wells. Over the past several years, repeated detection of viruses in water from deep wells in Madison, Wisconsin, shows that viruses can be significant groundwater contaminants and potential threats to human health.

During 2008 and 2009 we collected a time series of 26 approximately monthly virus samples from six deep municipal water-supply wells in Madison. Sampling for viruses requires a time series approach because virus concentrations, and virus species, vary with time in individual wells. The wells range in depth from approximately 700 to 900 feet and draw water from a series of Cambrian sandstones. Three of these wells are reportedly cased and grouted through a regional aquitard thought to protect the wells from surface contamination, and three have shallow casings. We also sampled local lakes and untreated sewage as potential virus sources.

Viruses were detected up to 61 percent of the time in each well sampled, and many groundwater samples were positive for virus infectivity. Lake samples contained viruses over 75 percent of the time. Sewage samples were extremely high 1n viruses, with all samples positive. Virus concentrations varied significantly with time, and there was apparent temporal correlation between virus detections in sewage, lakes, and groundwater.

Correlation between viral serotypes found in sewage, lakes, and groundwater suggests very rapid transport, on the order of weeks, from the source(s) to wells. Water isotope analyses indicated surface water to be an unlikely source of viruses; the most likely source of the viruses in the wells is leakage of untreated sewage from the Madison sewer system, which contains a large number of clay pipes installed before 1950.

Potential pathways for virus transport from the surface to the wells include porous-media flow, rapid transport through fractures, transport down failed well casings, and flow through cross-connecting wells. Human enteric viruses might be excellent tracers of recently recharged groundwater in urban settings when virus sources exist.