RAIN-EVENT HYDROLOGY AND THE BEHAVIOR OF TURBIDITY AND DISSOLVED ORGANIC CARBON IN A FORESTED WATERSHED, WESTERN MASSACHUSETTS

The Avery Brook (AB) subcatchment of the Mill River Watershed is the primary water source flowing to the Northampton, MA reservoir. The stream flows through undeveloped temperate forest situated in glacial till. Large wetlands and swamp forests created by beavers are present at the headwaters. The Northampton Dept. of Public Works has recently observed high turbidity levels in the reservoir. This project will attempt to determine the source of turbidity and how it relates to rain events.

Avery Brook, its major tributaries, and a groundwater well were sampled for major chemistry (Ca²⁺, Mg²⁺, Na⁺, K⁺, NH₄⁺, SO₄^2-, NO₃^-, Cl^-, F^-, SiO₂, pH, and ANC), as well as dissolved organic carbon (DOC), suspended sediment load, and d¹⁸O and dD concentrations. Rain events were sampled using an ISCO autosampler or by grab samples. A Druck pressure transducer coupled to a Campbell Scientific 10X datalogger was compared to a temporary Campbell Scientific 21X datalogger installed upstream to compare time lags in rain-event and base flow hydrographs. Organic acids (indicated by DOC) and suspended sediment load can create turbidity. While the suspended sediment load in AB did not show a significant increase during a rain event, the concentrations of DOC increased greatly from 4.7 to 10.2 mg/L, showing that DOC is the primary source of elevated turbidity. Mineral weathering produces ANC and base cations in equal proportions. However, in AB there is less ANC present than would be expected from cation concentrations. The presence of strong acids from precipitation and organic acids flushed out of the wetlands both contribute to the loss of ANC.

The majority of water contributing to Avery Brook during rain events comes from overland flow. The hydrograph peaks are steep and “flashy” during rain events. A peak in the concentration of base cations occurs in AB, two hours before the peak in the hydrograph. A peak in DOC occurs more simultaneously with the hydrograph, only 15 minutes before. Because DOC follows the hydrograph more closely, the main source of DOC must from the headwater wetlands, not local overland flow.

Further work using a stable isotope technique will be investigated to determine the relative contributions of rainwater and groundwater to Avery Brook.