Recovery from chronic and snowmelt acidification at Hubbard Brook: Long-term trends in stream and soil water chemistry

We investigated long-term chemistry trends in streamwater (1982-2011) and soil water (1984-2011) along an elevation gradient to evaluate the progress of recovery from the effects of acidic deposition at the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire, USA. Historic acidic deposition and the acid-sensitive nature of soils have led to chronic acidification of drainage waters in these headwater catchments. Additionally, episodic acidification associated with snowmelt makes spring the most acidic period of the annual cycle. Deposition of sulfate and nitrate has declined throughout the study period due to controls on emissions from electric utilities. The decreased input of acid anions has resulted in decreased leaching of base cations from the soil. Stream water pH has increased 0.01 units yr^-1 and the acid neutralizing capacity (ANC) has gained 0.69 µeq l^-1 yr^-1. While the changes in stream water chemistry broadly reflect changes in soil water chemistry, we found variation by landscape position in the magnitude and significance of changes in the chemistry of soil water draining the organic (Oa) and mineral (Bs) soil horizons. We also found the trend of recovery from acidification observed in the overall time series is very similar to that of the record representing the peak spring snowmelt of each year. The increase in the ANC of streamwater is very similar during the snowmelt periods as for the overall time series (0.69 and 0.78 µeq l^-1 yr^-1, respectively). Additionally, we found that for both the overall stream chemistry record and for the snowmelt period, the trends showed similar increases in pH, decreases in sulfate, and decreases in nitrate. The similarity between the trends in the overall time series and the snowmelt periods is an important finding that demonstrates the recovery acidification of drainage waters both during baseflow throughout the year and during the high flows of snowmelt. This finding indicates that episodic acidification associated with snowmelt is declining in severity.