Groundwater Under Direct Influence of Surface Water (GWUDI)

What is GWUDI?

GWUDI is an anacronym for “ground water under direct influence” of surface water.  This means that the groundwater source (i.e. well, spring, mine, etc.) is located close enough to nearby surface water to receive direct surface water recharge. Since a portion of the groundwater sources recharge is from surface water, the groundwater source is considered at risk to certain contaminants which are not normally found in true ground waters, but are often found in surface waters. 

Water systems are required by law to sample for a GWUDI determination. If a well is determined to be GWUDI the system is subject to additional filtration and testing requirements.


Legislative and Regulatory Background

The Surface Water Treatment Rule (SWTR) was written by the EPA in 1989, as an amendment to the Safe Drinking Water Act. The SWTR requires all water systems using groundwater under the direct influence (GWUDI) of surface water to meet minimum standards for control of viruses, bacteria, and turbidity.  State regulations require all  GWUDI systems to adequately filter the water or a system might elect to develop another source.


For a GWUDI evaluation, groundwater sources are first separated into one of three groups according to how vulnerable they may be to surface water. Group 1 being low risk, Group 2 being moderate risk, and Group 3 being high risk for surface water influence. Based off which vulnerability group they fall under will determine the sampling methodology required for an evaluation. Factors that affect vulnerability is age/condition of the well, aquifer type, rock type, proximity to a surface water body, nearby caves, Ranney well sources and mine pools.
  • If bacteriological data is found to be inconclusive, other methods may be used. The agency should be contacted to determine needed analyses in cases where the bacteriological data is inconclusive.

Final Determinations

  • If an inspection, water quality tests, and follow up analyses show no evidence of an influence between surface water and the ground water source, then the source will be considered to be a NOT GWUDI source, and minimum treatment will be disinfection with an approved disinfectant contact time.
  • If water quality tests are positive for indicators of surface water such as the presence of coliform or changes in temperature, then the source will be classified as GWUDI and the public water system will be notified in writing by the agency. These systems are required to comply with the SWTR requirements for filtration, disinfection, and monitoring within 18 months following determination and notification by the agency.

Ground Water Under the Direct Influence (GWUDI) Determinations

Are > 1.0/100 mL E. Coli Bacteria Present?

Are < 1.0/100 mL E. Coli Bacteria Present?

Are > 100/100 mL Total Coliform Bacteria Present?

Are < 100/100 mL Total Coliform Bacteria Present?

Temperature varies more than 7⁰ F?





Temperature varies less than 7⁰ F?





What's a microscopic particulate analysis (MPA)?

If GWUDI testing is inconclusive systems may opt to do further testing in the form of a MPA.

MPA is a technique that filters source water and then analyzes the filter for various organisms. It evolved from the analysis of Giardia and filtration efficiency determinations. This method was developed through a collaborative effort of microbiologists and laboratories from around the country. 

The MPA is a quantitative approach tied to a relative risk factor places water in a high, medium, or low risk of surface water contamination. To conduct a MPA please follow the procedure from this document: Consensus Method for Determining Groundwaters Under the Direct Influence of Surface Water Using Microscopic Particulate Analysis (MPA) 

Requirements for GWUDI Sources

If a source is determined to be GWUDI, the public water supply has 18 months to comply with requirements of the SWTR. The following options may be considered, depending on the feasibility for each public water system:

  • Filter, disinfect, and monitor in accordance with the SWTR.
  • Abandon the source and develop a replacement source.
  • Rehabilitate the source to prevent surface water influence.