Studies on the geochemical controls of surface and groundwater compositions have characteristically dealt with geologic settings that are mineralogically simple and relatively uniform, allowing quantitative estimates of mineral weathering rates in natural environments (Drever, 1997; White and Brantley, 1995). However, clastic sediments comprising multiple mineral (polyphase) assemblages, exhibiting a wide range of chemical compositions, are more typical for many watersheds or aquifers. Laboratory dissolution studies of individual silicate and carbonate phases permit comparison of the relative weathering rates of specific mineral phases (Velbel, 1993), but application to estimating rates for fieldbased, multiple-phase environments, that are typical of most weathering situations, is difficult and frought with uncertainties. It is generally recognized that mineral weathering rates are determined in no small part by the specific detailed chemical composition of the phases, especially the plagioclase feldspars and the principal mafic silicates, such as biotite, amphibole, and pyroxene. Complex mixtures of these phases in clastic systems reflecting multiple source rocks unevenly distributed over the terrain (e.g. glacial deposits) are common and cannot be ignored in attempting to understand mass transfer in specific watersheds or groundwater aquifers. Often, significant attention has been paid to the mineralogy and chemical composition of the reactant (source) and product (sink) phases, but to highly varying degrees in different studies. Despite the equal importance of products of silicate weathering processes as the reactants, the sufficient definition of mineralogic character and chemical composition has been commonly lacking, largely due to the difficulty of identifying phases and chemical compositions of specific clay minerals and oxyhydroxide products in surficial enviroments, as well as the problem of distinguishing between autochthanous weathering products and allochthanous clays from sediment source areas. A shallow, carbonate free, silicate aquifer in Department of Geology and Geophysics, University of Wisconsin, Madison, WI, USA