Long-Term Temporal Trends in Surface Water Chemistry

Abstract

This chapter (1) synthesizes the data presented in the foregoing 11 case study chapters regarding past changes in surface water chemistry in response to acidic deposition, (2) quantifies the acidification that has occurred, where possible, and (3) evaluates likely future responses of surface waters in the study regions to projected changes in acidic deposition. Based on spatial evaluation of surface water chemical ion ratios within the regions under investigation, waters that are currently acidic largely as a consequence of high SO₄²⁻ relative to base cation concentrations are most prevalent in the southwestern Adirondack Mountains, the Catskill Mountains, Virginia, Florida, Maine (small aquifer and high-elevation lakes), and northern Michigan and Wisconsin. The SO₄²⁻ concentrations in most of the low ANC (≤ 100 µeq L⁻¹) waters in these regions can generally be ascribed to evapoconcentration of atmospheric inputs.

Empirical distributions of lakewater chemistry across a depositional gradient in the Adirondack and Maine regions suggest that concentrations of ANC, base cations, inorganic Al, and possibly organic acid anions have changed in response to acidic deposition in drainage lakes with current ANC ≤ 50 µeq L⁻¹.

Historical reconstructions of lakewater chemistry based on diatom remains in sediment are presented for 50 lakes in the eastern, upper mid western, and western United States, with the greatest attention to the Adirondack region. Estimated ANC change from preindustrial times to the present for Adirondack lakes with ANC ≤ 50 µeq L⁻¹ ranged from a loss of 35 µeq L⁻¹ to a gain of 3 µeq L⁻¹, with a median value of −13 µeq L⁻¹. Assuming that all inorganic monomeric Al (Al_i,) in these lakes is attributable to increased mineral acidity, biologically relevant change in chemistry is given by (ΔANC — ΔA1 _i ,), and shows a median value of -24 µeq L^-1, assuming an average Al valence of +2. Only two lakes in the Adirondack paleoecological data set had current ANC > 50 µeq L^-1, and both showed no indication of historical acidification.

Paleoecological data for lakes in Maine, the Upper Midwest, and Florida suggested that some lakes had decreased in pH since preindustrial times, but the data were not as consistent as data for the Adirondacks in suggesting widespread acidification.