The Possible Importance of Forest Soil Processes in Defining Surface Water pH Depressions

Abstract

It has been hypothesized that 1) during the winter, acidic deposition from the atmosphere is collected by the snowpack and released during spring runoff periods and 2) during the fall, heavy storms cause flushing events that result in acidic materials on the soil surface (derived from dry atmospheric deposition) being carried to surface waters. Some researchers have ascribed episodic increases In surface water ion concentrations to atmospheric deposition. Other researchers have noted that precipitation chemistry is significantly altered by contact with vegetation and soil and have concluded that acidity of surface waters may be largely independent of atmospheric deposition. An assessment of natural biogeochemical processes clearly reveals that H⁺ ions can be produced in temperate forest ecosystems. The potential also exists for these H⁺ ions to be transported by soil water flow to surface waters where their input may significantly influence the H⁺ ion concentration, particularly during storm or snowmelt events. Research data evaluated In this paper suggest that the observed increase in H⁺ ion concentration in some forest watershed surface waters during storm events and snowmelt runoff may be more a function of key climatic and biogeochemical processes rather than precipitation chemistry. Given the fact that natural processes do acidify soils and that these processes appear to contribute to the acidity and aluminum concentrations observed in many surface waters, it is important to determine the relative contribution of the anthropogenic sources of acidity compared to the acidity generated through natural processes. Until further data are provided that separate anthropogenically-caused surface water pH depressions from those resulting from natural soil processes, caution is suggested in linking acidic deposition with pH declines in streams, rivers, and lakes.