Group B Public Water Systems - GWI

Using Groundwater Sources Under the Direct Influence of Surface Water


Groundwater sources under the direct influence of surface water (GWI) are vulnerable to microbiological contamination.

"Under the direct influence of surface water" means the groundwater source is located close enough to nearby surface water, such as a river or lake, to receive direct surface water recharge. Since a portion of the groundwater source's recharge is from surface water, the groundwater source is considered at risk of contamination from pathogens such as Giardia lamblia and viruses, which are not normally found in true groundwater.

Potential GWI sources include:

  • Wells less than 50 feet deep that are located within 200 feet of lakes, streams, rivers, or wetlands.
  • Infiltration galleries and Ranney wells located near surface waters.
  • Springs.

The evaluation of a potential GWI must be completed prior to submitting the design for a new or expanding Group B system to the reviewing authority.

We won't approve a new or expanding Group B system with a GWI source.

Process for determining if sources are GWI

Determination of hydraulic connection. A new or expanding Group B system must conduct a hydrogeologic investigation to determine whether the potential GWI source is hydraulically connected to nearby surface water if the source is a potential GWI. The hydrogeologic investigation requires a licensed geologist. If the hydrogeologic investigation indicates a hydraulic connection, the source is designated as a groundwater in hydraulic connection with surface water, requiring a microscopic particulate analysis.

Microscopic particulate analysis. The owner of the new or expanding Group B system with a source in hydraulic connection with surface water must collect a sample of source water and send the sample to a laboratory for a microscopic particulate analysis. If certain numbers or types of surface water organisms are found in the samples, the source is designated to be GWI.