BIOGEOCHEMICAL CONTROLS ON THE CHEMISTRY OF AVERY BROOK, WEST WHATELY, MA

Avery Brook is a small, second order stream, flowing within a 756 hectare watershed located in the Conway State Forest near West Whately, Massachusetts. It is the primary source of water entering the City of Northampton’s main water supply reservoir. The watershed is mostly underlain by Devonian Williamsburg granodiorite that contains abundant xenoliths of pelitic and mafic schist. The bedrock is overlain by glacial till of variable thickness with some isolated areas of stratified drift. Stream chemistry is controlled by a combination of weathering reactions within the soil and till and biologically mediated redox reactions occurring in riparian wetlands and beaver ponds.

Water samples were collected from the beaver ponds and their associated inlet and outlet streams in the upper part of the watershed. Samples were analyzed for major cations, anions, and silica. Alkalinity was measured by Gran titration and Dissolved Oxygen (DO) was determined, in the field, using an optical sensing DO meter. Alkalinity was found to be tightly controlled by Ca²⁺ and SO₄^2- concentrations. Dissolved Ca²⁺ appears to be primarily derived from mineral weathering while SO₄² is derived from precipitation but its concentration is mediated by redox reactions involving sulfur-reducing bacteria in the beaver ponds. During summer, dissolved oxygen concentrations are lowered by organic decomposition and SO₄^2- is reduced to sulfide. At these times the inlet streams have much higher concentrations of SO₄^2- than the beaver ponds. Conversely, in the fall, high stream flows and cooler temperatures re-oxidize the reduced sulfur causing SO₄^2- concentrations in the ponds to rise significantly higher than the inlet streams. Biochemical reactions within the ponds are also responsible for significant decreases in both SiO_2aq and Cl^-. Aqueous silica is a weathering product that is removed by the growth of aquatic diatoms during the summer. Chloride is derived from precipitation but is also removed from the ponds in the summer at a similar rate as SiO_2aq. During summer baseflow conditions the chemistry of Avery Brook is strongly influenced by biochemical reactions occurring in the beaver ponds and riparian wetlands while at other times the chemistry is determined by other processes such as variable mixing of surface water and groundwater.