IN SITU WEATHERING RATES IN AQUIFERS AND THE SURFACES OF NATURALLY WEATHERED FELDSPARS

Despite numerous kinetic studies in soils and watersheds, the studies of reaction kinetics in groundwater systems are relatively scarce. Yet, aquifers are sources of drinking water, major sites for water-rock interactions, and a significant segment of the hydrological cycle and global elemental cycling. In this study, in situ silicate dissolution rates in a number of sandy aquifers were derived and compiled from inverse mass balance modeling of solute fluxes along flow paths. These rates are two to five orders of magnitude slower than those from laboratory experiments at comparable temperature and pH but at far from equilibrium conditions, and also much slower than those derived from studies of soils and watersheds.

Atomic scale electron microscopy studies of the naturally weathered K-feldspars in the Navajo sandstone, Arizona (Zhu et al., 2006, GCA) show that there is a nano-meter thick amorphous layer on the dissolving feldspar surfaces. Furthermore, K-feldspars from the Navajo sandstone show tightly adhered kaolinite coating and a 3-5 micron thick smectite rind. We believe the amorphous layer and secondary coating layers bear significances on the feldspar dissolution kinetics. We advance two hypotheses for explaining the field-lab discrepancy. First, the secondary minerals rinding on feldspars are not at local equilibrium with groundwater, as traditionally assumed, but their slower precipitation rates can raise the aqueous solution saturation state with respect to feldspars to very close to equilibrium, and hence retard feldspar dissolution. Feldspar dissolution reaction in natural systems is within a complex web of reactions, in which secondary clay precipitation plays a key role. Second, the presence of the amorphous layer on weathered feldspars requires re-consideration of the details of surface reaction controlled mechanism.