Modern and Paleolimnological Evidence for Accelerated Leaching and Metal Accumulation in Soils in New England, Caused by Atmospheric Deposition

Abstract

Empirical field evidence for changing chemical processes in soils caused by atmospheric deposition of pollutants consists of: (1) Long-term water quality data including total dissolved solids, concentrations of specific metals (e.g. Ca), and conductivity; (2) Cation exchange capacity and base saturation values for soils located on precipitation pH gradients; (3) Lysimeter studies; and (4) Chemical analysis of organic soils on precipitation pH and metal gradients. For well-drained organic soils, as precipitation pH decreases, metals are differentially leached at an accelerated rate (Mn > Ca > Mg ≥ Zn > Cd and Na > Al). Experimental field and laboratory lysimeter studies on soil columns yield similar results, with increases in leaching rates for soil solutions with pH = 3 up to 100 × values for soil solutions with pH = 5. Nearly 100% of the Pb from precipitation is accumulating in the organic soil layer or sediments. Zn is accumulating in soils and sediments where the pH’s of precipitation, soil solutions, and surface waters are generally above 5 to 5.5. At lower pH values Zn and other chemically similar elements are desorbed/leached (net) at an accelerated rate.

Chemical analyses of dated sediment cores from high and low altitude lakes, with drainage basins relatively undisturbed for the last 200 + yr, reveal that increased deposition of metals on a regional scale started in the northeastern United States as early as 1880, consistent with increased fossil fuel consumption. This suggests acidified precipitation as early as 1880. Cores from historically acidified lakes (pH < ~5.3 to 5.5) indicate that, as acidification of surface waters occurs (caused by acidic deposition), concentrations of Zn, Mn, and Ca decrease in the sediment. Apparently the metals are leached from the detritus prior to sedimentation. This conclusion results from data from experimental acidification of sediment cores and the general observation that precipitation pH is generally ≥ 0.5 pH units lower than lake water pH. Accelerated leaching of soil in New England dates to earlier than 1900.