Speciation of aluminum in circumneutral Japanese stream waters

Speciation of Al, including inorganic monomeric Al (Ali, the sum of aquo, hydroxy and inorganically complexed forms), organic monomeric Al (Alo, the organically complexed form), and colloidal mineral Al (Alc, the fine particulate form that passes through a 0.4-μm pore size membrane filter), was investigated in stream waters (pH 6–8) of 4 watersheds in Japan. Total dissolved Al (Alt, the sum of Alc, Ali and Alo) ranged from 0.03 to 3.31 μM, and Alc was a minor component (<22% of Alt) in most of the streams. Ali was dominant (71% of Alt) in stream waters with low concentrations of Alt (<0.25 μM), while the Alo fraction (37%) was almost as large as the Ali fraction (39%) in the highest class (Alt > 1 μM). In spite of the variation in the range of reactive Al (Alr, the sum of Ali and Alo) in the 4 watersheds (Miomote, 0.03–3.27 μM; Tsukuba, 0.06–0.71 μM; Dorokawa, 0.05–0.71 μM; Tama, 0.03–0.38 μM), the entire data set for Alr could be expressed as a function of the ratio of dissolved organic C (DOC) and Ca: [Alr (μM)] = 0.13 [DOC/Ca (mol/mol)] + 0.11 (r = 0.86, P < 0.001). Alr increased in proportion to the DOC/Ca ratio in Japanese stream waters. Although acidic deposition in Japan has already resulted in elevated concentrations of image in stream waters, a high level of Ca (instead of Al) is serving as a major counterion for image. However, an additional loading of acidic deposition may result in shortage of Ca and mobilization of Al as a counterion for image, and continuous observation of the speciation of Al in Japanese stream waters may reveal future change in the conditions for mobilization of Al from “low Ca and high DOC” to “low Ca and high image”.