USING WATER QUALITY DATA AND PARTICULATE MATTER CHEMISTRY TO INVESTIGATE LOW DISSOLVED OXYGEN CONCENTRATIONS IN THE FOREST RIVER WATERSHED, SALEM MA

In 2020, it was discovered that sections of Forest River in Salem, MA contained very low dissolved oxygen concentrations. It was hypothesized that flow stagnation upstream of a hastily, human-made dam of sticks and branches, along with extreme drought conditions were the cause of the low oxygen concentrations. The dramatic slowing of flow, due to the dam, encouraged algae blooms that quickly deplete oxygen concentrations upon decay. This is particularly effective in the absence of rain or other significant recharge events. Dissolved oxygen concentrations rebound to normal levels downstream. This year, samples were collected to assess the integrity of the low oxygen zone during periods of high recharge to the watershed and gain a better understanding of the low oxygen zone.

Samples were collected upstream and downstream of the dam. A YSI-6600 multi-parameter geochemical probe was used to collect in-situ values of temperature, pH, specific conductance, and dissolved oxygen. Grab samples of water were taken for particulate matter to be analyzed for carbon, nitrogen and sulfur concentrations using an elemental analyzer and ¹³C and ¹⁵N isotopic values using a stable isotope mass spectrometer.

The results from this year are different from the 2020 study but not in the way it was expected. Instead of increased dissolved oxygen concentrations near the dam, the low oxygen environment had expanded upstream into Thomspon’s Meadow, which contained healthy dissolved oxygen concentrations during the previous year. Although the downstream locations indicate an increase in oxygen concentration as the water flows, it is lower than required for a healthy ecosystem. Carbon and nitrogen isotopic values and elemental concentrations indicate the particulate matter in the water is likely aquatic algae and may be impacted by the low oxygen conditions.