The response of soil and stream chemistry to decreases in acid deposition in the Catskill Mountains, New York, USA

The Catskill Mountains have been adversely impacted by decades of acid deposition, however, since the early 1990s, levels have decreased sharply as a result of decreases in emissions of sulfur dioxide and nitrogen oxides. This study examines trends in acid deposition, stream-water chemistry, and soil chemistry in the southeastern Catskill Mountains. We measured significant reductions in acid deposition and improvement in stream-water quality in 5 streams included in this study from 1992 to 2014. The largest, most significant trends were for sulfate (SO₄²⁻) concentrations (mean trend of −2.5 μeq L⁻¹ yr⁻¹); hydrogen ion (H⁺) and inorganic monomeric aluminum (Al_im) also decreased significantly (mean trends of −0.3 μeq L⁻¹ yr⁻¹ for H⁺ and −0.1 μeq L⁻¹ yr⁻¹ for Al_im for the 3 most acidic sites). Acid neutralizing capacity (ANC) increased by a mean of 0.65 μeq L⁻¹ yr⁻¹ for all 5 sites, which was 4 fold less than the decrease in SO₄²⁻concentrations. These upward trends in ANC were limited by coincident decreases in base cations (−1.3 μeq L⁻¹ yr⁻¹ for calcium + magnesium). No significant trends were detected in stream-water nitrate (NO₃⁻) concentrations despite significant decreasing trends in NO₃⁻ wet deposition. We measured no recovery in soil chemistry which we attributed to an initially low soil buffering capacity that has been further depleted by decades of acid deposition. Tightly coupled decreasing trends in stream-water silicon (Si) (−0.2 μeq L⁻¹ yr⁻¹) and base cations suggest a decrease in the soil mineral weathering rate. We hypothesize that a decrease in the ionic strength of soil water and shallow groundwater may be the principal driver of this apparent decrease in the weathering rate. A decreasing weathering rate would help to explain the slow recovery of stream pH and ANC as well as that of soil base cations.