IMPACT OF TROPICAL STORM IRENE ON THE HYDROLOGY AND CHEMISTRY OF AVERY BROOK, WEST WHATELY, MASSACHUSETTS

Runoff from rains associated with Tropical Storm Irene caused major changes in water quality in Avery Brook, a small second order stream lying within a 756 hectare watershed in West Whately, Massachusetts. Avery Brook, is also the principle tributary to the major water supply reservoir for the nearby City of Northampton. During the storm over 155 mm of rain fell causing the discharge of the brook to increase from a baseflow value of less than 0.05 cms to a peak flow of over 24 cms within a 6 hour period. An ISCO water sampler installed at the gage station collected hourly water samples starting 6 hours before the start of the storm. Samples were filtered through a 0.45 µm filter and weighed to determine suspended sediment concentrations. Concentrations ranged from less than 5 mg/L immediately prior to the event to over 1,500 mg/L at peak flow. Suspended sediment concentrations then quickly decreased to pre-event values even though stream discharge remained near peak values for several hours. The total sediment flux delivered to the reservoir was calculated to be over 280 metric tons and the resulting high suspended sediment concentrations caused the temporary shutdown of Northampton’s water treatment plant.

The filtered water samples were analyzed for major anions, cations, alkalinity, silica, and Dissolved Organic Carbon (DOC) as well as delta¹⁸O and delta²H. Concentrations of delta¹⁸O, delta²H and DOC showed significant peaks during the rising limb of the hydrograph that occurred almost 3 hours before peak stream flow. All other chemical constituents showed a 2 step dilution with an initial rapid dilution ending at the time of the peak in isotope and DOC concentrations and a second slower dilution ending about 2 hours after the peak in stream discharge. These changes in concentration reflect the different source areas for water entering the stream with the peak in isotopes and DOC representing water entering from flooded riparian wetlands and overflowing beaver ponds. The rapid decline in suspended sediment concentrations reflects a limited sediment source that was exhausted as soon as the stream reached peak stage.