Minnesota’s Assessment Source Water Monitoring Study

The Minnesota Department of Health (MDH) conducted a pilot study to evaluate the approach it might use for conducting assessment source water monitoring under the federal Ground Water Rule. A scoring routine was developed to identify public water supply wells that may be at risk to pathogen contamination using information about 1) well construction, 2) hydrogeological setting, 3) previous monitoring history, and 4) nearby potential pathogen sources.  Then, a subgrouping of potential high risk candidate wells was sent to MDH field staff for their review and comment prior to contacting public water supply systems to participate.  The sampling set consisted of 52 community and 43 noncommunity wells. Wells were sampled on a monthly basis for one year and each raw water sample was tested for bacteria (total coliform with e. coli follow-up) and general chemistry indicators (ammonia, bromide, chloride, nitrate, and sulfate).  All water samples were submitted to the MDH laboratory.

A groundwater classification scheme that reflects the pathogen risk potential to a well was developed and is based on the analytical results obtained from the bacteriological and chemical parameters used for this study.  This initial scheme has been enhanced by adding analytes such as dissolved oxygen, tritium, and stable isotopes of oxygen and hydrogen that are used to refine groundwater classification. Other factors such as the impacts of surrounding land uses within the likely capture area for a well and the variability in analyte concentrations on a seasonal basis are also considered. This additional information along with each monthly sample set was used to build an overall assessment of the likelihood of whether a well is at risk to pathogen sources.  

The study shows that MDH should include assessment monitoring as part of Ground Water Rule implementation but it will likely be used on a case-by-case basis instead of for all wells.  The scoring routine appears to be a good first step to identify wells that may be at greatest risk and follow-up assessment work would be triggered by a total coliform bacteria positive from a source water sample.  Also, the approach used for assessment monitoring may prove valuable for determining whether a well should be designated as being under the direct influence of surface water.