Context: Municipal drinking water wells are often drilled deep into the ground to reach aquifers lying under relatively impermeable layers called aquitards. Aquifers bounded above and below by aquitards are called contained aquifers. The aquitards are thought to protect the aquifers from surface contamination.
Shallow groundwater near waste water injection well sites can contain bacteria and viruses. Injecting waste water from treatment plants into the ground to recharge wells continues even though it's been known for 30 years that the recharged well water can be contaminated with pathogens (Vaughn et al. 1978). Bacteria found in human intestines have been measured as well in aquifers in the United Kingdom (Powell et al. 2003).
Viruses have a greater likelihood of reaching aquifers to contaminate drinking water than bacteria, protozoa and other waterborne, disease-causing organisms because their small size may allow them to pass through aquitards to reach aquifer waters. Until now, this possibility has not been tested explicitly. In a survey of over 400 sites across the United States, groundwater samples tested positive for at least one virus type (Abbaszadegan et al. 2003). This study, however, did not identify which of the aquifers were contained aquifers. |
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What did they do? Borchardt et al. looked for for human viruses in water from a contained aquifer in Wisconsin to see if viruses can move through the overlying protective aquitard to contaminate underground drinking water sources. They collected samples from the Mount Simon sandstone aquifer, which supplies drinking water to Madison, Wisconsin via 19 separate wells.
Samples were collected from wells, 5, 7 and 24 every month for ten months. Wells 7 and 24 are located in the center of the city, approximately one half mile from Lake Mendota. Both are cased through the aquitard and so draw water only from the Mount Simon aquifer.
In contrast, well 5 is adjacent to a wastewater treatment plant on the outskirts of Madison. Importantly, the casing for well 5 does not reach the aquifer, so water pumped from this well is likely a mixture of deep and shallow groundwater.
Water samples were analyzed for relative age, presence of viruses and infectivity of viruses. Relative age of the aquifer water was determined using stable isotopes and chloride ion analyses. Results were compared to surface water characteristics. Viruses were measured using RT-PCR (reverse transcription polymerase chain reaction) and primers specific to the virus type.
Lab cultured cells were exposed to virus-containing water samples to determine if the viruses were infectious.
What did they find? Taken together, the stable isotopes mix and the chloride concentrations both indicated that the water was a younger age that probably did not come from nearby lakes. The researchers estimated it takes between one and 40 years for water to percolate through the aquitard and recharge the aquifer.
Of the 30 samples, seven taken from wells 7 and 24 contained human intestinal viruses, including three subtypes of coxsackievirus, two subtypes of echovirus, and poliovirus sabin 1. No viruses were detected from well 5 located adjacent to the wastewater treatment plant. Only one of the seven human intestinal viruses, echovirus 18, was infectious. |
