The ultimate objective of mechanistic and analytical surface studies is to better characterize geochemical processes in the natural environment. Such application must also account for complexities not encountered in controlled experimental studies including spatial and chemical heterogeneity and time spans that can often not be simulated under laboratory conditions. Such complexities most often require that field studies involve significant and long-term financial and personnel commitments. These obstacles are principally responsible for the limited number of studies that have detailed surface geochemical processes under field conditions. Such information is however extremely important in validating experimental results and for calibrating hydrochemical transport models.
Studies on the effects of pH perturbations on sorption and dissolution of metals in stream waters have been conducted by McKnight and Bencala (1989) and Fuller and Davis (1989). In the first study, artificial acidification of a stream resulted in the dissolution of heterogeneous hydrous ferric oxides that where then reduced by solution phase photochemical reactions. In the latter study, natural pH perturbations, due to natural photosynthetic processes, were shown to control arsenate sorption on iron hydroxides and diurnal cycling in stream water. Extensive artificial tracer experiments in a shallow contaminated aquifer at Cape Cod demonstrated that chromium (VI) transport was controlled by reduction to Cr(III) and irreversible sorption on the sand and gravel substrate (Kent et al., 1994; Anderson et al., 1994). These studies demonstrate the importance of hydrologic and redox heterogeneity in controlling the spatial distributions of the tracer plumes. Heterogeneous redox processes were also shown to be the controlling factors in selenium contamination in groundwater. In the case of the Kesterson Reservoir in California, plumes of oxidizing surface water were shown to counteract the natural reducing groundwater environment permitting local mobility of Se(VI) (White et al., 1991). Elsewhere in California, Dubrovsky et al. (1990) found steady groundwater redox conditions, defined by a change in aquifer mineralogy, was the principal control of Se(VI) mobility. In both cases, the selenium was removed from the aqueous phase because of reduction to elemental selenium on solid substrates. All of the above examples demonstrate the importance of detailed geochemical and hydrologic characterization in order to define important surficial chemcial processes. Such information is critical in making informed and effective descisions concerning significant enviromental issues facing society.