'Acid rain', dissolved aluminum and chemical weathering at the Hubbard Brook Experimental Forest, New Hampshire

Contemporary 'acid rain' in the Hubbard Brook ecosystem has induced a series of geochemical responses. Neutralization is accomplished in essentially a 2-step process. Initially, hydrogen ion acidity is neutralized by the dissolution of reactive alumina primarilly found in the soil zone. In the Hubbard Brook area this reactive alumina has solution properties much like natural gibbsite. Aluminum-rich surface waters with a pH of 4.7 5.2 are typical of this neutralization stage. In a second step, both hydrogen ion acidity and aluminum acidity are neutralized by the chemical weathering of primary silicate minerals, i.e. by the alkali and alkaline earths contained in the bedrock and glacial till of the watershed. The chemical weathering reaction is much slower than the alumina dissolution reaction, so that the aluminum acidity stage (pH 4.7 5.2) may persist for substantial periods. Typically, however, in the Hubbard Brook area the aluminum acidity has been neutralized and a pH > 5.2 is obtained before surface waters reach a third-over stream channel. Because of the relatively low pH's throughout the soil zone and in the streamwater, carbonic acid reactions are essentially absent at the present time in the Hubbard Brook system. Water pathlength (or residence time) in the soil zone is the crucial factor in the state of acid rain neutralization, aluminum chemistry and chemical weathering. As measured by the losses of alkali and alkaline earths from the ecosystem, chemical weathering rate in the Hubhard Brook area at the present time is not especially high relative to other areas.