Recovery from chronic and snowmelt acidification: Long‐term trends in stream and soil water chemistry at the Hubbard Brook Experimental Forest, New Hampshire, USA

Abstract

Atmospheric acid deposition of sulfate and nitrate has declined markedly in the northeastern United States due to emissions controls. We investigated long-term trends in soil water (1984-2011) and stream water (1982-2011) chemistry along an elevation gradient of a forested watershed to evaluate the progress of recovery of drainage waters from acidic deposition at the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire, USA. We found slowed losses of base cations from soil and decreased mobilization of dissolved inorganic aluminum. Stream water pH at the watershed outlet increased at a rate of 0.01 units yr À1 , and the acid neutralizing capacity (ANC) gained 0.88 μeq L À1 yr À1 . Dissolved organic carbon generally decreased in stream water and soil solutions, contrary to trends observed at many North American and European sites. We compared whole-year hydrochemical trends with those during snowmelt, which is the highest-flow and lowest ANC period of the year, indicative of episodic acidification. Stream water during snowmelt had long-term trends of increasing ANC and pH at a rate very similar to the whole-year record, with closely related steady decreases in sulfate. A more rapid decline in stream water nitrate during snowmelt compared with the whole-year trend may be due, in part, to the marked decrease in atmospheric nitrate deposition during the last decade. The similarity between the whole-year trends and those of the snowmelt period is an important finding that demonstrates a consistency between recovery from chronic acidification during base flow and abatement of snowmelt acidification. FUSS ET AL.

Thirty‐year trends in acid deposition and neutralization in two headwater catchments, northwestern Massachusetts, USA

“…The significant upward trends in Ca 2+ and ANC highlight the recovery in Birch Brook's buffering capacity and distinguish its chemistry from that reported for most poorly buffered catchments in the NE USA, where neither median Ca 2+ nor ANC has increased significantly (cf. Fuss et al, ). Median SO 4 2− concentration in Birch Brook decreased from 170 ± 10 to 110 ± 6 μeq L −1 , a long‐term decrease of ~2 μeq L −1 year −1 since 1983.…”

“…Increasing Ca 2+ and ANC concentrations in Birch Brook and decreasing export of hydronium and SO 4 2− since about 2005 record stream recovery coincident with long‐term decreases in acid SO 4 2− deposition. Long‐term trends suggest that effects of acid deposition also are decreasing in some streams and lakes in poorly buffered northern Appalachians catchments (Fuss et al, ; McHale et al, ; Stoddard et al, ; Strock et al, ). Local biogeochemical buffering and flow pathways likely control the differential response in stream chemistry and timing in different NE USA catchments.…”

“…At the Birch Brook sampling site, Ca 2+ , ANC, and pH (not shown) have increased significantly since about 2004 (Figure ), whereas at HB, stream ANC has increased slightly, the frequency of snowmelt acidification episodes has decreased, and net export of Ca 2+ and changes in soil exchange Ca 2+ both have stabilized since about 1984 (Fuss et al, ). The sharp increases measured in Birch Brook are consistent with increases in available Ca 2+ in HMF soils following a period when acid sulfate deposition had decreased base saturation.…”

“…Controls on emissions of acid rain precursors such as SO 2 and NO x , mandated by amendments to the Clean Air Act, have increased precipitation pH and decreased SO 4 2− deposition in the NE USA since about 1990 (Galloway, Norton, & Robbins Church, ; McHale, Burns, Siemion, & Antidormi, ). Evidence from upland streams and lakes demonstrates increases of pH and ANC and decreases of SO 4 2− in some ecosystems in the United States, eastern Canada, and northern Europe (Burns, Riva‐Murray, Bode & Passy, ; Campbell et al, ; Fuss, Driscoll, & Campbell, ; Murdoch & Shanley, ; Stoddard et al, ; Strock et al, ) over the same period. In other areas, minimal or delayed recovery from acidification may result from low soil buffering capacity, long‐term soil calcium depletion and slow replacement of exchangeable Ca 2+ in the soil, mediated by slow rates of silicate weathering, or the influence of increased dissolved organic carbon (DOC) concentrations (Burns, Lawrence, & Murdoch, ; Likens et al, ; McHale et al, ; Mitchell et al, ).…”

Chronic and episodic acidification of streams along the Appalachian Trail corridor, eastern United States

“…With widespread decreases in atmospheric S and N deposition across eastern North America in recent decades, recovery of surface waters from chronic acidification is occurring in the north‐eastern United States (Driscoll, Driscoll, Fakhraei, & Civerolo, ; Fuss et al, ; McHale, Burns, Siemion, & Antidormi, ), and the initiation of recovery in south‐eastern United States surface waters is likely imminent (Rice, Scanlon, et al, ). However, episodic acidification remains a concern and is likely to persist further into the future than chronic acidification because streams with low‐flow ANC values as high as 100 μeq/L can become sufficiently acidic during high‐flow to reach toxic levels of Al i (Baldigo et al, ; Eshleman et al, ).…”