Reservoir ecosystems recover from atmospheric acidification: I. Trends of chemical reversal

In Saxony, about two million people are supplied with drinking-water from reservoirs, of which 16 suffered from atmospheric acidification. After the German reunification, the waterworks were modernized and adjusted on the treatment of acidic soft waters. But the emissions of oxidized sulfur (SO2) and nitrogen (NOx) compounds strongly declined within the 1990s. Whereas lakes of the Bohemian Mountains responded rapidly by chemical reversal from acidification (Kopáček et al., 2002), siliceous low mountain ranges in Germany showed a delayed or even no response (Alewell et al., 2001).
This paper presents major trends of chemical water composition monitored for 11 years in 7 acidified drinking water reservoirs and 22 tributaries of the Erzgebirge (Germany), and statistically evaluated by the Seasonal Kendall Test. About 85% of these surface waters showed significantly (p<0.05) declining concentrations of protons (-76%), nitrate ( 55%), sulfate (-26%), and reactive aluminum (Al, 104% on average). The strong decrease of toxic forms of Al provided a basis for recovery of sensitive aquatic organisms like fish (Meybohm and Ulrich, 2005).
Dependent on each initial acidification stage which differed, the study waters shifted one or two stages towards neutrality within a decade, thereby passing through a transitional stage of seasonally (episodically) fluctuating pH regime as illustrated by the Figure. The concentrations of dissolved organic carbon, ammonium, chloride, calcium, magnesium, manganese, and iron showed no trend in 60-75% of the investigated waters. But four reservoirs showed rising trends of iron, probably due to increased microbial activity in the sediment favoring anoxic release of ferrous iron.
The rapid chemical response of the study waters is attributed to the very high level and thus intense reduction of industrial SO2 and NOx emissions, which declined by 99% and 82% in southern Saxony between 1993 and 1999. Deposition rates of protons and sulfate decreased by a factor of ~3, but total N deposition remained almost constant. Soil protection liming, which was performed as a measure of forest remediation, contributed positively to the chemical reversal in about 20% of the surface waters, in particular of those where the cumulative dolomite supply on the total drainage area exceeded 7 t ha-1.

References
Alewell, C., Armbruster, M., Bittersohl, J., Evans, C.D., Meesenburg, H., Moritz, K. and Prechtel, A. (2001) Are there signs of acidification reversal in freshwaters of the low mountain ranges in Germany? Hydrol. Earth System Sci. 5(3), 367-378.
Kopáček, J., Stuchlík, E., Veselý, J., Schaumburg, J., Anderson, I., Fott, J., Hejzlar, J. and Vrba, J. (2002) Hysteresis in reversal of Central European mountain lakes from atmospheric acidification. Water Air Soil Pollut.: Focus 2, 91-114.
Meybohm, A. and Ulrich, K.-U. (2005) Reservoir ecosystems recover from atmospheric acidification: II. Signs of biological recovery. ACID RAIN 2005, Conference Proceedings, 000.