INFLUENCES OF LAND USE ON WATER QUALITY, MILL RIVER WATERSHED, HATFIELD, MASSACHUSETTS

The Mill River Watershed (MRW), a 125 km² catchment of the Connecticut River, possesses heterogeneous human settlement patterns suitable to distinguish natural sources of chemical loading to rivers from anthropogenic sources. The MRW is divided into catchments by drainage patterns of dominant tributaries, which are further classified into land-use zones defined by intensity of human activity. Water chemistry in Zone I areas, where human activity is minimal to absent, serves as a baseline for assessing human impacts on water quality from within the watershed. Zone II areas are affected by water removal from drinking water reservoirs on two tributaries (~9500 m³ per day, combined). Zone III regions receive runoff from agricultural, residential, and transportation areas. Since 1997, water samples collected from 13 sites in MRW have been analyzed for specific conductance, temperature, pH, ANC, base cations (Ca, Mg, Na, K, NH₄), anions (Cl, SO₄, NO₃, PO₄), and dissolved silica. GIS software was used to calculate percent area of different land uses that drain to each sample site.

Average concentrations of both NO₃ and SO₄ show a positive correlation with percent catchment area altered by human land uses (R²=0.68), and concentrations of Cl increase with road density (R²=0.82). However, water removal from municipal reservoirs increases the downstream concentration of NO₃ and SO₄ over that predicted by land use, showing that removal of high quality upstream water concentrates pollutants downstream. Streams impacted by road salt show a strong correlation between Na and Cl (R²=0.86 to 0.95); yet Cl exceeds Na by 10-15% due to cation exchange reactions that bind Na to soil. The net effect of nonpoint source pollution is to elevate ANC in the most developed areas, which impacts the natural acidity of a large swamp. ANC of the stream draining the swamp shows high variability, ranging from –90 to 600 µeq/L. The sum of base cations (C_B) exceeds ANC for all tributaries, due to addition of salts that add C_B, addition of acids that remove ANC, or both. Plotting C_B against ANC and subtracting Cl quantifies the impact of road salt from the impact of strong acids. For all Zone III sites, salt impacts are greater than local sources of acid. An ANC loss of 100 µeq/L by local acids is observed downstream of a municipal reservoir, accounting for 32% of excess C_B.