Assessing the Vulnerability of Public Drinking Water Wells to Microbial Contamination

David Jorgenson
International Consultants, Incorporated (ICI), Dayton, OH

Mike Wireman
USEPA Region 8, Denver, CO

Brent Huntsman
Terran Corporation, Beavercreek, OH

Daniel Olson
Office of Ground Water and Drinking Water, USEPA, Washington, DC

The U.S. Environmental Protection Agency (EPA) is currently promulgating a Ground Water Disinfection Rule (GWDR) in response to requirements of the Safe Drinking Water Act.   As part of this process, EPA has been working with stakeholders to develop a means to assess the vulnerability of public ground water supply wells to contamination by infective microorganisms resulting from fecal wastes.  This paper describes one of the vulnerability assessment strategies being considered by EPA for use in implementation of the GWDR.

The vulnerability assessment strategy involves reviewing existing data to evaluate the combined effects of contaminant source controls, ground water sensitivity, and well construction criteria to assess a well's vulnerability to microbial contamination.   The vulnerability assessment methodology provides qualitative assessments of public drinking water wells, resulting in three possible vulnerability classifications:

  1. High vulnerability (likely to be contaminated now or in the future);
  2. Low vulnerability (not likely to be contaminated); and
  3. Unknown vulnerability because of a lack, or uncertainty, of data.

The assessment process uses flow diagrams which provide a series of steps to be followed while making decisions based on the data available for the well and ground water source, guiding the user to a determination of a vulnerability ranking for a particular well.  The vulnerability of a well incorporates the results of four categories of data evaluation:

  1. Results of Past Microbe Monitoring:  If microbes indicative of fecal contamination have been found in the past, the well has high vulnerability since the well has been, or is, contaminated.
  2. Contaminant Source Risk:  A ranking of low vulnerability will be given a well if there are no nearby contaminant sources that might affect the well.
  3. Ground Water Sensitivity:  Ground water sensitivity is dependent on the ability of the natural hydrogeologic system to prevent the migration of viable pathogenic microorganisms to the public water supply well.  In the presence of a source of contamination, a well in a high sensitivity aquifer would have high vulnerability.
  4. Well Construction and Condition:  A poorly constructed well, or a well in a deteriorated condition, may provide a direct conduit for contaminants to reach the aquifer.  Therefore, in the presence of a contaminant source, an unacceptable well will result in a condition of high vulnerability.

The results of the vulnerability assessment may be used to make a determination of whether or not disinfection would be required.

This paper was presented by Brent Huntsman at "Gambling with Groundwater, Physical, Chemical, and Biological Aspects of Aquifer-Stream Relations", International Association of Hydrogeologists, XXVIII Congress & Annual Meeting of the American Institute of Hydrology, Las Vegas, Nevada, USA, September 27-October 2, 1998.