What is GWUDI?
GWUDI is the acronym for Groundwater Under the Direct Influence of Surface Water. This term specifically refers to groundwater sources where conditions are such that pathogens such as giardia lamblia are proven or likely to travel from nearby surface water into the groundwater source. The Surface Water Treatment Rule includes both surface water and GWUDI sources. EPA left it up to the states to develop programs to make the determination of whether or not a source is GWUDI. The state of Oregon has been working with the other states in Region 10 (Alaska, Idaho, and Washington) to create protocol for making these determinations.
For a groundwater source (wells, springs, and infiltration galleries) to be potentially GWUDI, the source must be:
1) within 500 feet to a surface water source,
and EITHER
2a) have shown a source-related presence of coliform bacteria,
OR
2b) have an inadequate grout seal or no impervious aquifer barrier.
Once identified, the water system has the option of conducting a Hydrogeologic Assessment (performed by a Registered Geologist), or a Water Quality Assessment, which can be done by a water system operator. Information gathered during this step is then analyzed to determine if there is a "hydraulic connection" between the groundwater and surface water sources.
If a hydraulic connection is established, meaning if the water is shared between the two sources, a series of samples are taken to determine the likelihood of surface water indicator organisms or giardia cysts actually reaching the groundwater source. This analysis is called an MPA, or Microscopic Particulate Analysis. The results of an MPA are in the form of a Risk Score. After a minimum of two MPAs are conducted, the Oregon Health Services determines whether or not the groundwater source will be considered to be under the direct influence of surface water and therefore subject to the Surface Water Treatment rule.
A more detailed explanation of this program is included in documents listed on the Groundwater page. |
