Trends and patterns in lake acidification in the State of Vermont: Evidence from the Long-Term Monitoring Project

Abstract

Twenty-four low acid neutralizing capacity (ANC) lakes in Vermont have been monitored since 1980 to characterize their chemical variability, and to determine if they exhibit temporal trends in acid/base chemistry. Many of the lakes exhibit significant decreasing trends in SO₄ ²⁻ and base cation (C_B) concentrations, but few exhibit significant changes in pH or ANC. An examination of all trend results (significant and insignificant) suggests a tendency for ANC and pH values in these lakes to be increasing, but either the changes are too small, or the number of observations too small, for these trends to be significant. Data from these lakes suggest that the primary responses of surface waters in this region to declining rates of SO₄ ²⁻ deposition are decreases in SO₄ ²⁻ concentrations and rates of cation leaching from watershed soils. Decreasing rates of c_b deposition may combine with lower rates of cation leaching to produce declines in c_B that are very similar to measured declines in SO₄ ²⁻ concentration. Vermont lakes exhibit their lowest ANC values in spring, attributable, for the most part, to dilution of c_B concentrations during spring snow melt. Concentrations of SO₄ ²⁻ are also more dilute in the spring, but c_B decreases are greater, and the net effect is a lowering of ANC. One quarter of the Vermont lakes monitored exhibit strong seasonality in NO₃ ⁻ concentrations, with peak concentrations near 70 Μeq L⁻¹. In these lakes, spring increases in NO₃ ⁻ concentrations are more important than C_B dilution in producing minimal spring ANC values.