Natural protection of spring and well drinking water against surface microbial contamination. I. Hydrogeological parameters.

The fate and transport of microbes in groundwater are controlled by physicochemical characteristics of the microbe and of the groundwater/aquifer media. Key characteristics of the microbe include size, inactivation (die-off) rate, and surface electrostatic properties. Key properties of the groundwater/aquifer system include flow velocity, aquifer grain (or pore) size, porosity, solid organic carbon content, temperature, pH, and other chemical characteristics of water and mineral composition. Because of size and surface electrical properties, viruses are much more mobile in groundwater than Cryptosporidium and Giardia (which are about 100 times or more larger than viruses). The inactivation or die-off rate is usually the most important factor governing how far microbes can migrate in significant numbers in groundwater. Typical half-lives of microbes in groundwater range from a few hours to a few weeks. Examples of maximum reported migration distances of microbes in groundwater include: bacteria, 600 m in a sandy aquifer: viruses, 1000 to 1600 m in channeled limestones and 250 to 408 m in glacial silt-sand aquifers; Cryptosporidium and Giardia, no confirmed reports found of significant migration distances. Investigations by the EPA have indicated that distances of 210 to 325 m away from septic tanks are necessary to achieve with high confidence an 11 order of magnitude reduction in virus concentrations.