Uneven E. coli levels found in local rivers

Submitted by the Souhegan Watershed Association

There is good news and bad news.

The good news: Most of the swimming spots along the Souhegan and Merrimack rivers tested OK for E. coli levels in the most recent testing. Only the swimming hole in Wilton called The Horseshoe had higher than acceptable levels for safe swimming. Unfortunately, this is the most popular spot on the Souhegan.

Watson Park in Merrimack, another popular spot, tested at 34, well below the 88 count for excellent conditions.

The bad news: Very high E. coli levels were found wherever the Souhegan passed through population centers. Beginning in Greenville and continuing through the centers of Wilton and Milford, E. coli levels grew until they reached dangerous levels.

Levels higher than 406 are dangerous for healthy swimming. The Swing Bridge in Milford reached 866, the highest we’ve seen on the river this year.

Luckily, these are scenic areas, not places where people are apt to come in contact with the water, and the levels began dropping and continued to drop to acceptable levels through Amherst and Merrimack.

The lack of rain, the low flow and higher temperatures all contributed to the higher pollution levels. Flow on the Souhegan measured in west Milford was only 12 cubic feet per second; historically, the level for this date is 76 cfs. As measured in Merrimack near Wildcat Falls, the flow was only 36 cfs, and historically, it should be at 103.

Flow on our stretch of the Merrimack River is determined by the release from the Amoskeag Dam in Manchester, not natural flow. It was still lower flow than it should be, but flow here is measured in the thousands. At the time of the testing, the flow was 1,580 cfs.

The larger flow is able to dilute small amounts of pollution coming in, and much of this river corridor has very little development between Manchester and Nashua.

All of the sites tested on the Merrimack between Manchester and Tyngsborough, Mass., had excellent E. coli levels. The highest level was a still excellent 64 at the boat ramp at Greeley Park in Nashua. This has been typical for the last several years; Merrimack River E. coli levels are good to excellent during normal weather.

Warmer temperatures and lower flows also have compromised the dissolved oxygen levels in both rivers. Dissolved oxygen levels give a good indication of the general health of the river and normally get lower as the summer progresses. This year, they seem to be getting lower sooner, especially through the slower moving shallow sections of the Souhegan. All of the data is available at www.souheganriver.org.

The rivers will be tested every two weeks during the summer. Thirty volunteers, many of whom have been with the program for many years, adopt a site and test every other Tuesday morning and carry the samples to the wastewater treatment plants in Greenville, Milford, Merrimack, Manchester and Nashua, where the samples are processed by licensed professionals.

There are still sites available, and new volunteers are always welcome; sites with "NA" are open right now.

More information is available on the SWA website.

The next test will be July 12.