Health Concern

For the purposes of drinking water, there are two main water sources: surface water and groundwater. Surface water has been exposed to the atmosphere and is presumed to be contaminated with bacteria, viruses, and other pathogens. Groundwater is found below ground and therefore presumed to be naturally filtered, with no bacteria, viruses or pathogens.

However, in some cases subsurface drinking water sources (wells, springs, or infiltration galleries) may be close enough to surface water to be contaminated by that surface water. In this case the water needs to be treated to mitigate any risk to public health. The purpose of the groundwater under the direct influence of surface water (GWUDISW) rule is to identify those sources.

The health concern with GWUDISW is single-celled parasites such as Giardia lamblia and Cryptosporidium, which are not found in uncontaminated groundwater. Adequate distance between a surface water body and a public water source will provide some natural filtration. However, if a source is located close enough to nearby surface water, such as a river or lake, to receive immediate surface water recharge it may be directly influenced by surface water. Large diameter pathogens can then enter the source and be consumed by customers of the public water supply. These pathogens can cause extreme intestinal distress and may be difficult to treat. They are not inactivated by chlorine or chloramine disinfection and are therefore not controlled by normal groundwater treatment techniques.

Contaminant Source

If a public groundwater source is located close to a surface water body or is extremely shallow the source may be considered at risk of contamination. Sources most likely to be under the direct influence of surface water are:

  • Infiltration galleries and horizontal wells
  • Springs
  • Wells located close to surface water
  • Shallow wells

If a source is suspected of being GWUDISW, then it may be tested using microscopic particulate analysis (MPA) tests. This test identifies the number of primary surface water indicators trapped in a 1-micron filter after running water (1 gallon per minute) through the filter for about 18 hours, at not more than 10 pounds per square inch (about 1000 gallons).

Understanding Your Samples

If MPA samples are required for your source the interpretation of those results will be done by the lab and the GWUDISW rule manager at DEQ. The rule manager will help you understand your results.

Rather than looking for Giardia lamblia and Cryptosporidium directly the MPA test is designed to look for primary surface water indicators such as algae, diatoms, insect/larvae, plant debris, or rotifers. These particles would not be found in uncontaminated groundwater. They indicate that the water was exposed to sunlight within the last few weeks and that the pore spaces in the unsaturated zone and aquifer are allowing large particles to rapidly enter the drinking water source. Their presence indicates that the surrounding area does not provide adequate filtration of the water between the surface water and the public water source. This vulnerability requires that the water needs to be treated like surface water in order to be safely served to the public.


If two MPA results show that the source has a low risk of being under the direct influence of surface water (0-9 points), then that source can be determined to be groundwater. Any MPA score of 10 or higher (moderate or high risk) will result in the source being declared to be GWUDISW and subject to the surface water treatment rule (SWTR). The system will have 18 months to either modify the source or come into compliance with the SWTR and will work with DEQ to achieve that goal.

Score Risk Factor
0-9 Low
10-19 Moderate
20+ High

Protecting Your Water/Health

The GWUDISW rule requires that groundwater sources in Montana must be either confirmed to be groundwater or determined to be GWUDISW. See DEQ circular PWS-5, for the complete GWUDISW rule. A source of subsurface water (well, spring, horizontal well, or infiltration gallery) is presumed to be groundwater at the start of the process. Only if evidence indicates that the source is under the direct influence of surface water will the determination change to GWUDISW.

The first step in determining if a source is under direct influence of surface water is for DEQ staff to complete a preliminary assessment form. Based on the results of the preliminary assessment, the source will either be characterized as groundwater or be required to undergo further assessment.

Once the determination has been made for each source, a record of decision letter will be written to the public water system and a copy will be retained in DEQ permanent records.

Maintaining Compliance

Once a source is determined to be groundwater it will only be revisited if there is some indication of a change in the status of the source. This may be from flooding, the movement of a surface water body closer to the PWS source, installation of a man-made surface water body, a sudden increase in bacteriological contamination, deterioration of the source construction, or some other cause. A new preliminary assessment can be completed any time a field staff person visits the source for a sanitary survey or other reason and observes a change that justifies reassessment.


Katie Luther
GWUDISW Rule Manager
PO Box 200901
Helena, MT 59620
Phone: (406) 444-4633
DEQ: (406) 444-4400