The Biogeochemical Response of Eastern U.S. Watersheds to Declines in Acid Deposition

Over recent decades, reductions in acid deposition have led to the recovery of watersheds across the eastern United States. Examining the acid-base status of surface waters is important for understanding the changes in chronic and episodic acidification of surface waters. This thesis investigates the biogeochemical response of forested watersheds to major declines in acidic inputs.

The soils of the southeast U.S. have a high sulfate adsorption capacity compared to the northeast soils, leading to a delay in the recovery of surface waters. To identify regional differences in the recovery of watersheds, a mass balance analysis of sulfur for 16 watersheds (8 in the northeast, 8 in the southeast) was completed. The northeast sites (with the exception of one) have been net exporters since the 1990s to the early 2000s, while the southeast sites have only recently converted from a net retention within the last decade. There is evidence that sulfate adsorption is an irreversible process based on the whole-watershed scale. Additionally, results from the temporal analysis of the flow-normalized concentrations of sulfate reveal that while the majority of the northeast sites have declined to around the same concentration, there is more variation between the southeast sites. This may be due to differences in bedrock, which influences surface water response.

While the watersheds in the southeast have shown a net export of sulfur recently, and thus a trajectory towards recovery from years of excess acid deposition, the main drivers for changes in the stream chemistry during episodic acidification remained unclear, and thus were investigated for three watersheds in Shenandoah National Park. Internal biogeochemical processes were more dominant than deposition in affecting changes in the acid-base chemistry of surface waters. The largest declines in base cation supply were evident at the most base-poor site as the soil buffering capacity at these watersheds is influenced by the underlying bedrock.

This work shows the importance of long-term monitoring of surface waters to evaluate the current acid-base status of watersheds in response to decreases in acid deposition. This study may lead to further research into the effects of levels of acid deposition on soils, as well as a further investigation of the sulfate isotherm.