Natural Protection of Spring and Well Drinking Water Against Surface Microbial Contamination: I. Hydrogeological Parameters

The Issue

One of the multiple barriers that can be claimed for human health protection is source water sequestration. In particular, some groundwaters may be so sequestered and not under the influence of surface water directly so as to preclude microbial intrusion. In order to verify the sequestered nature of these groundwaters, hydrogeological parameters were established.

Research Strategy

A practicing hydrogeologist (Mr. John Robertson, Hydro Geologic] and practicing medical microbiologist (Dr. Stephen Edberg, Yale University) collaborated to develop hydrogeological parameters that would ensure the integrity and sequestration of the source water.

Major Findings and Significance

The fate and transport of microbes in groundwater are controlled by physical/chemical 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 in groundwater migrate from a few hours to a few weeks. Examples of maximum reported migration distances of microbes in groundwater include: bacteria, 600 meters in a sandy aquifer; viruses, 1,000 – 1,600 meters in channeled limestones and 250-408 meters in glacial silt-sand aquifers; no reports were found which confirm the extent of migration distances for Cryptosporidium and Giardia. Investigations by the EPA have indicated that distances of 210 to 325 meters away from septic tanks are necessary to achieve a reduction in virus concentrations of an 11 order magnitude.

Robertson, J. B., and Edberg, S.C., Natural Protection of Spring and Well Drinking Water Against Surface Microbial Contamination: I. Hydrogeological Parameters, Critical Reviews in Microbiology, 23(2):143-178 (1997).