Regional analysis of lake acidification trends for the northeastern U.S., 1982-1994

Acidic deposition is a regional phenomenon, but its effects have traditionally been studied using site-specific, intensive monitoring. We present trends information for 36 lakes of high-to-moderate acid sensitivity (defined as acid neutralizing capacity [ANC] < 100 μeq L^-1), and 15 deposition monitoring stations, in the northeastern U.S. for the period 1982-1994. Trends at each site were assessed through use of the Seasonal Kendall tau test; the resulting statistics were combined, through a technique analogous to analysis of variance, to produce quasi-regional estimates of change for key chemical variables. Rates of sulfate deposition declined significantly across all of the northeastern region during this time period, while rates of nitrate and ammonium deposition were unchanged. All lakes exhibited strong decreases in sulfate concentrations (ΔSO₄²- = -1.7 μeq L^-1yr^-1, p < 0.001) in response to declining sulfate deposition, but there was a strong contrast in the response of acid/base status between lakes in New England and lakes in the Adirondacks. As a group, the New England lakes exhibited recovery (ΔANC = +0.8 μeq L^-1yr^-1 p < 0.001), while the Adirondack lakes exhibited either no trend or further acidification (as a group, ΔANC = -0.5 μeq L^-1yr^-1, p < 0.01). This contrast can be attributed to changes in base cation concentrations: New England lakes exhibited base cations declines that were smaller in magnitude than declines in sulfate, producing the observed recovery in ANC; Adirondack lakes showed base cation declines that were very similar to those of sulfate, and no recovery was evident.