Low flows on local rivers means higher E. coli levels
One of the longtime volunteer water quality monitors noted that the Souhegan River at his site was "as low as I’ve ever seen it," and most of the monitors at the other 30 sites echoed this by noting the level as "very low."
At the time of the testing on Aug. 9, flow measured at the gauge in west Milford was 4 cubic feet per second, just 20 percent of the expected historic flow there. Measured in Merrimack closer to the mouth of the river, the flow was just half of its expected flow.
The level of the Merrimack River in the Nashua area is controlled by the dam in Lowell, Mass., which backs the ponding back to Merrimack and so is less noticeable. But the water level in Manchester was below the top of the Amoskeag Dam, and flow just downstream was down by one-third of its expected flow.
Low flows mean less dilution for any pollution present. Just downstream of the Amoskeag Dam, at Arms Park in Manchester, the E. coli level was higher than the 88 level the program uses to measure clean water and the level that the state uses for swimming at recognized beaches.
"The Merrimack typically tests as clean for E. coli, so that is unusual this week," said George May, coordinator of the testing program.
The Merrimack cleaned up enough to drop below the 88 level after leaving Manchester and stayed clean to the last site tested in Tyngsborough, Mass., where the level was only 2.
The Souhegan River is a much smaller river and so is much more influenced by pollution at low flows. E. coli measured in downtown Wilton reached 770, a level dangerous for human contact. The river improved as it flowed downstream, but never reached levels safe for swimming. The Swing Bridge in Milford just south of the Oval was 133.
The Souhegan becomes a shallow, meandering river through Amherst, an area conducive to higher E. coli counts. The E. coli count at the Amherst Canoeport on Boston Post Road was 172, too high for safe swimming. E. coli levels improved to acceptable levels through Merrimack to its confluence with the Merrimack.
Other swimming holes tested OK for swimming. The Horseshoe in Wilton was 43, very clean. Indian Ledges in Merrimack was 45. Watson Park near the mouth of the river where it joins the Merrimack was 31.
The SWA no longer posts the E. coli levels at Watson Park in Merrimack because of a disagreement with the town, which will take over posting. Swimmers can still find the information at the Souhegan Watershed Association Facebook page each Wednesday through Labor Day.
E. coli monitoring is for human use of the rivers only, but the SWA also monitors for dissolved oxygen in both rivers. DO levels give the best general indication of the health of the river. Monitors found generally good levels of DO again last week, which has been true all season. The rivers are clean for fish and other river creatures and for the environment.
More information about DO and the actual readings for each site are listed at www.souhegan river.org.
Thirty volunteers, many of whom have been with the program for many years, spend about 20 minutes at the river taking samples and then bring the samples to one of the local wastewater treatment plants in Greenville, Milford, Merrimack, Manchester and Nashua, where the samples are processed by licensed professionals.
The next test, on Tuesday, Aug. 23, will be the final test for this season.
For more information, visit www.souheganriver.org.
– Submitted by Souhegan Watershed Association