It was discovered last fall that the water of city well No. 4, located near city hall, was contaminated with 16 parts per billion (ppb) of benzene due to leaks over the years at the former gas station now known as Midtown Auto. Well No. 4 was then shut down.
After doing so, well No. 3, located on Railroad Street, became contaminated because it was then the closest well to the benzene plume.
Jim McArthur, hydrologist with the Minnesota Pollution Control Agency (MPCA), said that typically gas leaks are restricted to 15-20 feet below ground surface. This benzene contamination is closer to 70+ feet under the surface.
ďItís an unusual situation,Ē he said.
The MPCA recommended that in order to keep city wells No. 5 and 6 from being contaminated, well No. 3 can continue to be pumped for drinking water at a reduced rate without fear of contamination, and well No. 4 should be pumped and the water discharged into the sanitary sewer system.
The problem with that solution, according to Public Works Director Ron Mergen, is that the sewage ponds are not built to handle that extra volume of water from well No. 4.
However, pumping the water has brought contamination levels down. Benzene is now at three ppb at well No. 4.
McArthur emphasized that there is presently no detectable benzene in the city drinking water.
The plan suggested by the MPCA includes installing a new well which will serve the sole purpose of pumping contaminated water into the storm sewer. The well will potentially be located either on the property of Caseyís or MNDoT property by Highway 23.
The city has applied for a permit with the MPCA to discharge the con-taminated water from well No. 4 and the new well into the storm sewer, which flows into the North Fork Crow River. The EPAís standard of contamination for dumping into surface water is 114 ppb, and the contamination level of well No. 4 is three ppb.
The permit is for a term of approximately five years. Approval or denial of the permit request will be made after July 15.
A draft of the permit is available for review at the MPCA central office in St. Paul from Deborah Schumann, Point Source Compliance Section, Water Quality Division, MPCA, 520 Lafayette Road North, St. Paul, 55155-4194, or by calling 218-828-2492. A copy of the permit will be mailed after the MPCA receives either a written or oral request.
After the background of the problem was reviewed by McArthur, the council had no questions.
Darlene Thyen asked if any other options would be possible and how big the area of the leak is.
McArthur answered that there is little to do at the site of the contamination. According to McArthur, 1500 cubic yards have been excavated, and crews are considering using the soil vacuum extraction system, which is putting wells in the contaminated soil and vaporizing the gasoline.
ďUnfortunately, the soil in question has a lot of clay, so the extraction system may not be effective,Ē McArthur said.
McArthur said that the known problem of benzene is that it is a carcinogen, or cancer-causing agent.
McArthur added that current levels of benzene, both in the treated water and the proposed water to be discharged, are well below the safety standards established by the EPA.
A calculation was made by Koronis Lake Association President Peter Jacobson that pumping the water from well No. 4, contaminated at 3 ppb, into the river at 100 gallons a minute for five years would introduce less than one gallon of benzene into the river.
McArthur said that far more contaminants are introduced into public waters by oil leaks from cars, street drain runoff, personal watercraft and boat usage.
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