To quantify chemical transformations in natural hydrologic environments the coupling of chemical transformations with flows needs to be understood. The objective of this proposal is to understand bimolecular chemical transformations in subsurface flows. For such chemical transformations, the reaction rate is proportional to the product of the concentrations of two reactants. Due to the spatial variability of flow, the concentrations of the different reactants undergoing transport can vary significantly around their average values. When the reactants are initially not overlapping, their concentrations can be significantly negatively correlated. This small-scale segregation of reactants can strongly influence the overall chemical transformation rate, and can cause it to be much smaller than that implied by the intrinsic chemical transformation rate constant and the average concentrations. The average chemical transformation rate depends on flow variability, small-scale mixing mechanisms, and the chemical transformation rate coefficient. The coupling of chemical transformations and transport processes will be examined in this work by conducting detailed numerical simulations and by performing laboratory experiments. In the numerical simulations the microstructure of the concentration and the spatially varying and tortuous flows will be resolved. In the laboratory experiments the concentrations of reactants undergoing bimolecular chemical transformations and transport processes will be monitored by a spectrophotometer. Addressing as it does the coupling of chemical transformations and hydrologic transport models, the expected outcome of this research is an improvement in the understanding of field-scale contamination. In addition, the results will also be applicable to field-scale geochemical modeling and understanding the composition of natural waters.
