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Analysis of variations in major ion chemistry in the Mill River watershed reveals the importance of anthropogenic activities in controlling streamwater chemistry. Average concentrations of NO3- and SO4(2-) show a positive correlation with percent catchment area altered by human land uses, and concentrations of Cl- increase with road density. Water removal from municipal reservoirs increases the downstream concentration of NO3- and SO4(2-) over that predicted by land use changes, showing that removal of high quality upstream water concentrates pollutants downstream. In salt-impacted streams, 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. The sum of base cations (C(B)) exceeds ANC for all samples. Plotting C(B) against ANC and subtracting Cl- quantifies the impact of road salt from the impact of the addition of strong acids.

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