In situ feldspar dissolution rates in an aquifer

In situ silicate dissolution rates within the saturated Navajo sandstone, at Black Mesa, Arizona were determined from elemental fluxes in the aquifer. The mass transfer between groundwater and mineral matrix along flow paths was calculated from inverse mass balance modeling. The reaction time is bound by 14C-based travel time. BET surface areas were measured with N2 gas adsorption. Dissolution rates for K-feldspar and plagioclase are 10−19 and 10−16 mol (feldspar) m−2 s−1, respectively, which are ∼105 times slower than laboratory experiment-derived rates under similar pH and temperature but at far from equilibrium conditions. The rates obtained in this study are consistent with the slower field rates found in numerous watershed and soil profile studies. However, these rates are from saturated aquifers, overcoming some concerns on estimated rates from unsaturated systems. The Navajo sandstone is a quartz-sandstone with a relatively simple and well-studied hydrogeology, groundwater geochemistry, and lithology, a large number of groundwater analyses and 14C groundwater ages, groundwater residence times up to ∼37 ky, groundwater pH from ∼8 to 10, and temperature from ∼15 to 35°C.