NATURAL PROTECTION OF SPRING AND WELL DRINKING-WATER AGAINST SURFACE MICROBIAL-CONTAMINATION .1. HYDROGEOLOGICAL PARAMETERS

The fate and transport of microbes in groundwater are controlled by ph ysicochemical characteristics of the microbe and of the groundwater/aq uifer media. Key characteristics of the microbe include size, inactiva tion (die-off) rate, and surface electrostatic properties. Key propert ies of the groundwater/aquifer system include flow velocity, aquifer g rain (or pore) size, porosity, solid organic carbon content, temperatu re, pH, and other chemical characteristics of water and mineral compos ition. Because of size and surface electrical properties, viruses are much more mobile in groundwater than Cryptosporidium and Giardia (whic h are about 100 times or more larger than viruses). The inactivation o r die-off rate is usually the most important factor governing how far microbes can migrate in significant numbers in groundwater. Typical ha lf-lives of microbes in groundwater range from a few hours to a few we eks. Examples of maximum reported migration distances of microbes in g roundwater include: bacteria, 600 m in a sandy aquifer; viruses, 1000 to 1600 m in channeled limestones and 250 to 408 m in glacial silt-san d aquifers; Cryptosporidium and Giardia, no confirmed reports found of significant migration distances. Investigations by the EPA have indic ated that distances of 210 to 325 m away from septic tanks are necessa ry to achieve with high confidence an 11 order of magnitude reduction in virus concentrations.