This chapter discusses the occurrence and variability of aluminum concentrations and species in surface and subsurface waters at the catchment scale, placing particular emphasis on discerning the biogeochemical factors that control the variability. Aluminum 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 generally causes only a short-lived mobilization of the aluminum. The flowpath by which water reaches a stream channel has a critical impact on stream-water quality. Stormflow describes the situation in which water from a precipitation or snowmelt event, or water already in the soil, is rapidly transmitted through the soil matrix, yielding a sharp rise in the stream hydrograph.