This proposal seeks renewal of a NSF-sponsored integrated research and education program in field, microscopic, and modeling studies of silicate reaction kinetics in a groundwater aquifer the Navajo sandstone aquifer at Black Mesa, Arizona. One of the fundamental problems in modern hydrogeology and geochemistry is the orders of magnitude discrepancy between silicate dissolution rates derived from watersheds and soil profiles versus those derived from laboratory measurements. This large discrepancy indicates our lack of basic understanding of the fundamental physical and chemical processes controlling silicate dissolution kinetics in nature. The proposed study will test two hypotheses to explain this discrepancy: (1) A large part of the discrepancy between in situ and laboratory rates results from different saturation states under which feldspars dissolve in natural systems and in most laboratory experiments. While most laboratory experiments attempt to measure the rate of a single congruent dissolution reaction far from equilibrium, field studies measure in situ rates amid a complex web of reaction networks and at a condition very close to equilibrium with respect to feldspars. Among these reactions is the precipitation of clay, which removes solutes from groundwater and hence promotes continued feldspar dissolution. However, contrary to the prevailing assumption that clays are at equilibrium with groundwater, we believe that clay precipitation is much slower than feldspar dissolution, and hence groundwater chemistry is constrained to be close to equilibrium with feldspars. Over time, a steady state of groundwater chemistry is reached with near constant rates of feldspar dissolution and clay precipitation; and (2) A leached layer forms on weathered feldspar surfaces, which is an important part of how silicate dissolution occurs in nature and must be considered in rate laws that properly describe reaction kinetics. The proposed study will test the first hypothesis by analyzing dissolved Al3+ concentrations in the aquifer, evaluating saturation indices and their variations along a flow path, and numerical geochemical modeling to elucidate the complex network of reactions associated with feldspar dissolution in aquifers. We aim to establish a theoretical framework for interpreting laboratory experiments and field data. To test the second hypothesis, we will characterize microstructures and detailed chemistry at the feldspar-clay interfaces using an atomic scale Field Emission Gun Transmission Electron Microscope. The overall objectives are to advance our understanding of the two key possibilities for discrepancies between laboratory and field rates: the diminishing thermodynamic drive and the characteristics of weathered feldspar surfaces. Broader Impacts The PIs will integrate the research activities into undergraduate and graduate-level geology courses. A summer mentorship program will sponsor undergraduate students, particularly from underrepresented groups, to conduct research in the PI's laboratories. Partnerships with the National Energy Technology Laboratory and Los Alamos National Laboratory will be developed to facilitate dissemination and applications of the research results to the carbon sequestration and environmental restoration programs. Groundwater is a key source of drinking water and is essential to life on Earth. Groundwater also represents 98% of fresh water readily available to humans. Therefore, reaction rates in aquifers are critical to water resource and water quality management, waste disposal, and global warming mitigation strategies. The findings from this project will therefore contribute to our understanding of the environment, and help to build a scientific basis for environmental policies and strategies.
