Weathering rates for silicate minerals exert important controls on groundwater and surface water quality, nutrient availability, and the global carbon cycling and its impact on global climatic changes over geological time. Yet, weathering rates derived from watersheds and soil profiles are several others of magnitude slower than laboratory measurements. Understanding this large discrepancy remains a fundamental problem in modern geochemistry, hampering our ability to understand and predict surficial processes and environmental changes quantitatively.
To provide insights into discrepancy, we propose to study the in situ silicate dissolution and precipitation rates in large aquifer system with relatively simple and well-constrained chemistry and hydrology. The chosen field site is a sandstone aquifer, located in northern Arizona and mostly composed of the Navajo sandstone. Groundwater in this aquifer has an optimal residence time up to approximately 35,000 years. The kinetic rate constants derived from this aquifer first will be much better constrained than in watersheds and soil profiles because of the saturated hydrologic environment and unusually abundant data. A new transmission electron microscope at John Hopkins, with capabilities 0.12nm imaging and 1 nm compositional mapping resolution, will provide atomic scale information of microstructure and fine-scale chemical variations at weather feldspar.
