420Aluminum is the most abundant metal in the earth’s crust, occurring in a host of primary aluminosilicate minerals. “Normal” chemical weathering of these minerals by carbonic acid or various organic acids (i.e., abundantly occurring weak acids) generally causes only a short-lived mobilization of the aluminum. The high surface affinity of dissolved aluminum species (see Chapter 6) and the relative insolubility of both primary aluminosilicate minerals and secondary aluminum oxide minerals (see Chapter 3) ensure that the quantity of aluminum released to natural waters is always very small; most of it is retained within the soil B horizon. However, since the pool of aluminum within the soil environment is so large, even a small perturbation of the normal weathering (i.e., soil acidification) may cause a large increase in aluminum mobility and profoundly increase the concentration of this element observed in some natural waters, particularly those waters spatially close to the weathering site. Acidic precipitation is one such perturbation.1,2 Numerous publications have detailed the occurrence of elevated aluminum concentrations in natural waters as the effect of acidic deposition (for example, see reviews3–5). Environmental concern for this increased aluminum mobility rests with its ultimate toxic effect on both terrestrial6 and aquatic biota.7–11