Coupled Silicate Reaction Kinetics in an Aquifer

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0509755ZhuThis proposal seeks renewal of a NSF-sponsored integrated research and education program infield, microscopic, and modeling studies of silicate reaction kinetics in a groundwater aquifer theNavajo sandstone aquifer at Black Mesa, Arizona. One of the fundamental problems in modernhydrogeology and geochemistry is the orders of magnitude discrepancy between silicate dissolutionrates 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 andchemical processes controlling silicate dissolution kinetics in nature.The proposed study will test two hypotheses to explain this discrepancy: (1) A large part of thediscrepancy between in situ and laboratory rates results from different saturation states under whichfeldspars dissolve in natural systems and in most laboratory experiments. While most laboratoryexperiments 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 veryclose to equilibrium with respect to feldspars. Among these reactions is the precipitation of clay, whichremoves solutes from groundwater and hence promotes continued feldspar dissolution. However,contrary to the prevailing assumption that clays are at equilibrium with groundwater, we believe thatclay precipitation is much slower than feldspar dissolution, and hence groundwater chemistry isconstrained to be close to equilibrium with feldspars. Over time, a steady state of groundwaterchemistry is reached with near constant rates of feldspar dissolution and clay precipitation; and (2) Aleached layer forms on weathered feldspar surfaces, which is an important part of how silicatedissolution occurs in nature and must be considered in rate laws that properly describe reactionkinetics.The proposed study will test the first hypothesis by analyzing dissolved Al3+ concentrations inthe aquifer, evaluating saturation indices and their variations along a flow path, and numericalgeochemical modeling to elucidate the complex network of reactions associated with feldspardissolution in aquifers. We aim to establish a theoretical framework for interpreting laboratoryexperiments and field data. To test the second hypothesis, we will characterize microstructures anddetailed chemistry at the feldspar-clay interfaces using an atomic scale Field Emission GunTransmission Electron Microscope. The overall objectives are to advance our understanding of the twokey possibilities for discrepancies between laboratory and field rates: the diminishing thermodynamicdrive and the characteristics of weathered feldspar surfaces.Broader ImpactsThe PIs will integrate the research activities into undergraduate and graduate-level geologycourses. A summer mentorship program will sponsor undergraduate students, particularly fromunderrepresented groups, to conduct research in the PI's laboratories. Partnerships with the NationalEnergy Technology Laboratory and Los Alamos National Laboratory will be developed to facilitatedissemination and applications of the research results to the carbon sequestration and environmentalrestoration programs.Groundwater is a key source of drinking water and is essential to life on Earth. Groundwateralso represents 98% of fresh water readily available to humans. Therefore, reaction rates in aquifers arecritical to water resource and water quality management, waste disposal, and global warmingmitigation strategies. The findings from this project will therefore contribute to our understanding ofthe environment, and help to build a scientific basis for environmental policies and strategies.