GWUDISW
GWUDISW stands for Groundwater Under the Direct Influence of Surface Water.
In the GWUDISW process, all groundwater sources in Montana are to be assessed as either groundwater (GW) or GWUDISW.
GWUDISW is defined in federal regulations (40 CFR 141.2) as any water beneath the surface of the ground either:
- with significant occurrence of insects or other macro organisms, algae, or large-diameter pathogens such as Giardia lamblia or Cryptosporidium
- Or significant and relatively rapid shifts in water characteristics such as turbidity, temperature, conductivity, or pH which closely correlate to climatological or surface water conditions.
The process of determining whether or not a source is GWUDISW is regulated in the State of Montana Administrative Rules ARM 17.38.209, ARM 17.38.219, and in the DEQ Rules (Public Water Supply Circular PWS-5).
Why is it important that groundwater sources be evaluated through the GWUDISW process?
Compared to surface water, groundwater sources NOT under the influence of surface water (springs, wells, infiltration galleries) are typically much more likely to be free of pathogenic organisms (Giardia Lamblia, Cryptosporidium, bacteria, viruses) than surface water. Animals and humans are important reservoirs of these organisms. At the surface the higher temperatures and oxygen levels in the water allow these organisms to more easily reproduce.
If these organisms do enter groundwater, many of them can survive for extended periods of time and be transported to great distances from the point of contamination. Viruses are able to travel long distances in groundwater because of their small size and ability to survive a wide range of environmental conditions. Giardia and Cryptosporidium have two life stages: an actively feeding, growing and reproducing stage (trophzoite); a resistant dormant stage (cyst). Both organisms need a host animal in order for the trophzoite stage to survive. The cysts stages can survive for months in cold water above freezing.
Giardia and cryptosporidium are parasitic protozoas, relatively large in diameter compared to viruses and bacteria. Both organisms are typically found in the intestinal tracts of humans and animals and contaminate water through fecal pollution. Giardia and cryptosporidium are the most frequently identified protozoan parasites and have been responsible for many waterborne outbreaks of disease in the United States. Infection causes gastrointestinal distress including diarrhea and weight loss. People with immunodeficiency diseases are particularly susceptible.
What is the GWUDISW evaluation process?
Step 1: Preliminary Assessment
A source of subsurface water (well, spring, horizontal well or infiltration gallery) is presumed to be groundwater at the start of the process. It is only if evidence appears that there may be the influence of surface water in the source that the designation of the source could change to GWUDISW.
The first step is the completion of the Preliminary Assessment (PA) Form. The PA is completed by a professional engineer or water professional. For existing sources the PA is usually filled out by the person doing the periodic sanitary survey of the system. The PA of a source will be redone if there have been changes to the system that affect the source. During the stage of engineering plan development and review a professional engineer will complete the PA for the new source.
Using a numerical score, the person doing the PA assesses the type of structure (well, spring, horizontal well, or or infiltration gallery), distance between surface water and the source, historical bacteriological contamination over the last 3 years, and the historical pathogenic contamination. If the source is a horizontal well or spring, the source automatically fails the PA because these types of sources are often more susceptible to surface water influence.
If the source is a well, the source is also assessed with respect to well construction including the presence or absence of sanitary well caps, the depth to the top of the screen interval on the well, and static water level.
Step 2: Analyzing the results of the Preliminary Assessment
If the score on the PA is below 40, the source passes the PA. Unless there is other evidence suggesting surface water influence, DEQ will reconfirm the status of the source as groundwater.
If the score on the PA is 40 or over, the source fails the PA and either new information about the source, rehabilitation of the source or further assessment is required. A score of 40 or above does NOT mean that just based on the results of the PA that a source will be declared GWUDISW.
By providing new information about the source, the owners and operators of sources help DEQ make a more informed GWUDISW decision. Examples of information that can be provided are well log records. If a significant data gap is filled, then DEQ will reassess the source.
Source rehabilitation or repair can also change the PA score. An example of a repair is the replacement of a well cap with a sanitary well cap. A repair of a spring source would be the reconstruction of the spring box. An example of a rehabilitation of a spring source would be the reconstruction of the spring box.
Step 3: Further Assessment of the Source (if required)
DEQ will review and approve the methods proposed by the system for further evaluation. Generally the three methods are:
- Hydrogeologic Assessment (HA)
- Water Quality Assessment (WQA)
- Microscopic Particulate Analysis (MPA)
These methods are fully described in PWS-5. The HA analyzes geologic and hydrologic information to evaluate if a hydraulic pathway from surface water to ground water exists. The WQA also evaluates if a pathway exists by frequent and simultaneous water quality measurements at the source and nearby surface water. The MPA analyzes the source water for organisms associated with surface water. DEQ will decide on a case-by-case basis which methods will be used.
MPA testing can be a quick way to directly detect harmful surface water organisms in your water supply.
Based on the results of the further assessment, the source will either be characterized as ground water or GWUDISW.
If the source is classified as GWUDISW, then the system must install filtration and disinfection in accordance with the Surface Water Treatment Rule (40 CFR 141 Part H).
Additional Information about GWUDISW
EPA Power Point GWUDISW Slides 14-18 discuss the EPA Preliminary Assessment Process. Note that DEQ uses a different scheme for point scoring for the Preliminary Assessment.
State of Oregon Power Point GWUDISW. The general information is still good. There is also some information about the specific Oregon program especially on slides 11-16 that differs from the Montana GWUDISW rule.
State of Montana Department of Public Health and Human Services 2008 Communicable disease statistics. Look for "enteric" diseases in Table 2. Note: the statistics presented are for all occurrences of the disease, not specifically from drinking water from Public Water Supplies.
U.S. Waterborne Disease Statistics 1991-2000
Questions about GWUDISW?
Call Jake Kandelin at (406) 444-4633 or email him at jkandelin@mt.gov.
