Evaluating water-quality conditions in the mainstem and tidal reaches of the Merrimack River in Massachusetts, June to September 2020

In summer and early fall (June to September) 2020, water-quality data were collected at 13 stations along the mainstem of the Merrimack River and into the Merrimack River estuary. The data are allocated among three different datasets: discrete water sample data, discrete vertical profile data, and continuous data. The collective purpose of these datasets is to enable assessment of the overall water-quality conditions in the Merrimack River and estuary and to identify areas for potentially more targeted water-quality monitoring in the future.

The highest concentrations of nutrients—nitrogen and phosphorus—were found at the stations downstream from wastewater treatment plants in Lowell, Lawrence, and Haverhill. Nutrient concentrations measured in the Merrimack River estuary were not as high as those measured in the Merrimack River, indicating that other processes are affecting nutrient concentrations in the system. These data were collected coincident with a severe flash drought in New England. Analysis of the vertical profile and continuous data indicated that, for intermittent periods up to 5 days, water quality in some sections of the Merrimack River may not support designated uses for the waterbody as established in the Massachusetts surface water quality standards.