Detection and Treatment of GWUDI

By Steve Winkley

Introduction In the Winter 2001/2002 Aquafacts issue, Jim Hyde of the New York State Department of Health (NYSDOH) explained how you would determine if your source of ground water is under the direct influence of surface water. In this article, I wanted to elaborate on two aspects of ground water under the direct influence of surface water (GWUDI): detection of GWUDI using detailed temperature monitoring and possible treatment technologies for GWUDI.To refresh your memory, Part 5, Subpart 5-1 defines GWUDI as "any water beneath the surface of the ground which exhibits significant and rapid shifts in water characteristics such as turbidity, temperature, conductivity, or pH which closely correlates to climatological or surface water conditions and/or which contains macroorganisms, algae, large diameter pathogens or insect parts of a surface water origin." For regulatory purposes, a source that is GWUDI is similar to a surface water source and must be treated through filtration. There are over 1,100 groundwater sources that the NYSDOH has identified may be GWUDI. If your groundwater source is less than 200 feet from a surface water body and/or has less than 50 feet of casing, NYSDOH believes that you could be subject to GWUDI.

Detection of GWUDI

Systems with groundwater sources that are suspected of GWUDI will have to determine if GWUDI actually exists. The most common way of doing this will be to conduct a water quality assessment that is designed to detect the rapid movement of surface water to a groundwater source. During this assessment, daily temperature and conductivity measurements would be taken at the source(s) and at the nearby surface water source (if there is one). These readings would be taken for an extended period of time (up to one year) and plotted to see if there is strong correlation between surface water fluctuations and groundwater fluctuations. Ideally, these measurements could be taken by a logging device that would take frequent measurements and store these highly accurate readings. This would save manpower and improve the quality and consistency of the data. A relatively inexpensive temperature-logging device is available on the market for approximately $115 to $200. One unit, the Water Temp Pro from Onset Computer Corporation can take measurements with an accuracy of 0.2 ^oC at intervals ranging between one second to 9 hours. The data can be periodically downloaded to a laptop computer. I recently purchased one of these units and would be glad to demonstrate it to you. Unfortunately, there is no comparable conductivity-logging device. Conductivity logging devices will cost at least $ 2,000 each. Fortunately, conductivity is not a sensitive parameter and daily measurements could be taken with a relatively inexpensive ($50) meter.If the temperature and conductivity measurements indicate that a surface water influence may be possible, a Microscopic Particulate Analysis (MPA) would be conducted during the period of most susceptibility to GWUDI. The purpose of the MPA is to determine if pathogens and other indicator organisms such as algae are present, indicating inadequate natural filtration.

Treatment Technology

If a MPA indicates that the source is GWUDI, the water system may do one of four things: (1) fix structural defects that allow surface water into the source; (2) develop a new source that is not GWUDI; (3) obtain filtration avoidance status; or (4) install filtration. Historically, option 4 above, was thought to be prohibitively expensive for many small water systems. However, relatively new bag and cartridge filtration is making filtration cost-effective for many smaller systems. A bag filter is a polypropylene fabric filter that is shaped into a bag-like shape. Since bag filters are relatively inexpensive to replace, the typical setup would be to first have a 5-micron bag filter to remove debris and larger waterborne biological contaminants. The second stage of filtration would be a 1-micron absolute cartridge filter that is similar to a bag filter, but is wrapped around a rigid supporting structure. These cartridge filters are much more expensive than the bag filter to replace, but would not typically need to be replaced for months depending upon the turbidity of the water, etc. Bag and cartridge filters are replaced based upon the pressures observed before and after filtration. Prechlorination is sometimes installed in order to control algae growth. UV light disinfection can also be installed in situations where contact time is in question.Based upon a 2001 study by the Pennsylvania Department of Environmental Protection entitled Drinking Water Treatment Technologies For Groundwater Systems Under the Direct Influence of Surface Water, the capital costs for a slow sand filtration plant range from $ 0.80/gpd to $1.20/gpd, and the capital costs for a conventional filtration system could range from $1.20/gpd to as much as $1.70/gpd. In contrast, the capital costs for a bag and cartridge filtration system would typically range from $0.30/gpd to $ 0.80/gpd.