EFFECTS OF BEAVER PONDS ON THE CHEMISTRY OF AVERY BROOK, WEST WHATELY, MA

Avery Brook is a second order stream within a 756 hectare forested watershed having 230m of relief located in the Conway State Forest in West Whately, Massachusetts. It is the principle tributary feeding the main drinking water reservoir for the City of Northampton. Numerous beaver ponds occur in the upper reaches of the stream and these exert a strong influence on stream chemistry during low flow conditions.

Water samples were collected biweekly from streams, groundwater seeps, and beaver ponds between May and September 2010. In addition, sediment cores were collected from several of the ponds. Water samples were analyzed for base cations (Ca²⁺, Mg²⁺, Na⁺, K⁺, SiO₂) and trace metals (Pb, Cr, Ba, Fe, Mn, Zn, Cu, Al) by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). Anions (Cl^-, F^-, SO₄^2-, NO₃^-) were determined by Ion Chromatography (IC), and acid neutralizing capacity (ANC) was measured by Gran titration. Sediment core samples underwent a mild organic decomposition procedure using commercial grade sodium hypochlorite (Clorox Bleach) and the supernate was analyzed for dissolved metals by ICP-OES.

During the warmest part of the summer the alkalinity of the beaver ponds was higher than the inlet streams or groundwater seeps. Much of this increase was due to sulfate reduction, as the average of streams above the beaver dam complex was >100 µeq/L while the average in the ponds was <25 µeq/L. Mn in pond waters reached concentrations greater than 2000 µg/L while Fe concentrations remained low. The source of the Mn is unknown but is likely associated with remobilization of organically concentrated Mn under reducing conditions. Analysis of the cores dissolved organic matter revealed Fe concentrations 3 times greater than Mn. Although there was some downstream dilution, the beaver ponds had a significant influence on the chemistry of water entering the reservoir during low flow conditions.