FACTORS CONTROLLING SURFACE WATER CHEMISTRY IN A SMALL CATCHMENT BASIN, SOUTH COASTAL WATERSHED, SE MASSACHUSETTS

The Wildcat Brook / Third Herring Brook catchment subbasin is a small drainage system (approximately 10 mi²) within the North and South River Watershed which is a subdivision of the south coastal drainage system. Both streams, as well as their confluence, flow through Old Pond Meadows aquifer, the main municipal water supply field for the towns of Norwell and Hanover. Hydrochemical characteristics of this aquifer show an existence of two diverse groundwater systems each associated and delineated along the individual streams. For this study we selected 15 sampling stations uniformly distributed between the headwaters and the downstreams, monitored and sampled monthly since August 2000. Five standard water quality parameters along with the nitrates are measured on site by Hydrolab® multiprobe and samples of water from each site are analyzed by Inductively Coupled Plasma (ICP) spectroscopy. The nitrates range from 1.5 to 44 mg/l-N, pH - from 5.0 to 7.5, and specific conductance is between 90 to 500 mS/cm. Ca varies between 1.4 and 11 ppm, Mg has a narrower range between 0.8 to 3.5 ppm, and Fe varies from 0.4 to 2.5 ppm. Na levels show a large range from a maximum of 72 ppm to a minimum of 8.8 ppm. Some of the controlling variables are from chemical weathering of the surrounding granites and the newly identified gabbroic body recognized by its signature of a nearly constant Ca/Mg ratio. The higher than expected Na levels suggest a role of ocean spray as well as the application of road salt during the winter season. Other identified controlling factors include anthropogenic effects due to discharge of waste water through septic systems and use of fertilizers as seen by the higher NO₃^- concentrations. A minor role comes from the nutrient cycling within the biosphere.