How safe is the Merrimack for Swimming?

Pathogen Trend Analysis:

Both public health and recreational enjoyment of the river are impacted by high bacterial levels in the river. For Massachusetts, the limit for E. coli bacteria at fresh water beaches is 235 cfu/100 mL in a single sample; the limit for Enterococcus bacteria at marine beaches is 104 cfu/100 mL in a single sample. For New Hampshire, the limit for E. coli bacteria at fresh water beaches is 88 cfu/100 mL in a single sample. At levels above these, the water is not safe for swimming.

E. Coli is common bacteria from the digestive systems of humans and animals and only a few strains can cause illness. In our testing E. Coli is used as an indicator for the possible presence of more harmful bacteria. Sources of this fecal contamination can include agricultural runoff, wildlife, manure runoff, and waste water treatment plants. Heavy precipitation causes runoff and increased levels of E. Coli.

Below are trend graphs for E. coli and Enterococcus bacteria in the Merrimack River.  These measurements were taken along the mainstem of the river from 2008 to 2012.  An average and standard deviation were calculated for 8 regions:  Nashua, Tyngsboro, Lowell, Andover, Lawrence, and Haverhill (for E. coli); West Newbury and Newburyport (for Enterococcus).

Graphs include the state maximum limits for bacteria in a single sample. The points show the total rainfall (inches) during the week preceding the sampling date.  Pathogens can increase in the water in regions with combined sewer overflows (CSOs). These include: the cities of Manchester and Nashua (NH),  and the cities of Lowell, the Greater Lawrence Sanitary District, and Haverhill (MA).

The good news is that several regions have shown some improvement in the average counts of E. coli bacteria over the last 4 years (Nashua, Tyngsboro and Lowell regions). However, other regions have shown no change.


E. Coli:

Cities Showing improvement:                         No improvement:

Nashua                                                                                    Andover

Nashua_Ecoli_lines           Andover_Ecoli_lines

Tyngsborough                                                                             Lawrence

Tyngsboro_Ecoli_lines          Lawrence_Ecoli_lines

Lowell                                                                                      Haverhill

Lowell_Ecoli_lines          Haverhill_Ecoli_lines



West Newbury





Below is a link to a .pdf containing all of the above graphs:

Average Monthly Bacteria by Region, 2008-2012


For 2013, we also have data provided by the Gulf of Maine Institute on Enterococci in Newburyport

and have partnered with Ground Work Lawrence to collect data on E. Coli in the Spicket River

and the Souhegan River Watershed Association to collect data on E. Coli in the Souhegan River