Water Quality for Swimming and Paddling on the Souhegan and Merrimack

This post is about the Souhegan River Association's work to monitor the water quality of the Souhegan and Merrimack rivers. We do this every two weeks through the summer season. Our last session’s results show some outstandingly negative data regarding E. coli levels in the Merrimack River, although dissolved Oxygen and pH levels in all rivers were adequate for aquatic life.

The high E. coli levels were surprising. Usually, high E. coli bacteria levels are caused by significant rainfall on the preceding days. However, the water levels of the rivers did not fluctuate upward, which is an indicator that we didn't have a runoff event. Rainfall washes E. coli off the streets, lawns, and farms and into the rivers, which is usually the primary driver of higher E. coli levels. This process is called, in eco jargon, nonpoint source pollution or the acronym NSP. E. coli is a class of bacteria that has both harmless and dangerous types. E. E.coli-type bacteria are omnipotent in our environment. We track its levels broadly because it is an indicator of both the potential of bad bacteria as well as polluting nutrient inflow, oils, and other industrial chemistry such as PFAs, hydrocarbons (oils), and pesticides (from your lawn).

The lack of a rainfall driver leads us to think there may be some point source (dumping or septic) of E. coli and nutrients into the river. We at the SWA don’t have the resources to search for a point source, but we can recommend that folks don’t swim in the lower Merrimack these days. E. coli-type infections from exposure to contaminated water cause diarrhea, stomach pain, and fever, but they are usually not fatal.

The variations in E. coli levels between adjacent sites suggest to me that individual site data may have a wide tolerance. Hence, we should look at multi-site spanning trends.
Warm water can drive E. coli growth, but the preceding day's air and related water temperatures were not abnormally high. Again, this points us back to point source pollution sources.

I’ve added E.coli data for Wasserman Park and Horseshoe Pond. This gives us a comparison of the rivers to still pond bodies of water in our area. Both have impressively low E. coli numbers.

Again, I want to thank our monitor volunteers and the Merrimack and Manchester WWT Labs. We couldn’t do this work without a team.
(for river flow rates, precipitation, and temperature levels, see tab two within the linked results spreadsheet)

Reference Articles.
https://docs.google.com/.../1U43csj3XYdDs9Bvl4Idf.../edit...

https://www.epa.gov/.../basic-information-about-nonpoint....

https://www.epa.gov/pfas/pfas-...

https://news.climate.columbia....

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Eric Doberstein
Souhegan River Water Quality Monitoring Administrator
souhegan.river.monitoring@gmail.com