A TALE OF TWO RATES: IMPLICATIONS OF THE DISCREPANCY BETWEEN LABORATORY AND FIELD FELDSPAR DISSOLUTION RATES ON GEOLOGICAL CARBON SEQUESTRATION

Because of the global warming trend, mitigation strategies for greenhouse gas emissions are essential, particularly for CO₂, which comprises more than half of man-made greenhouse gas emissions. One of the best approaches for reducing CO₂ emissions is its disposal in deep geological formations. To ensure long-term storage, one of the most desirable sequestration mechanisms is mineral trapping through reactions such as,

Anorthite + CO₂ + 2H₂O=calcite + kaolinite

Prediction of the effectiveness and storage capacity of the deep aquifers then requires the knowledge of feldspar dissolution rates. However, the large discrepancy between lab and field-based weathering rates of feldspars remains one of the fundamental problems in modern geochemistry. Field-based rates are typically several orders of magnitude slower than lab rates under similar pH conditions. The causes of this discrepancy and realistic in-situ dissolution rates are fundamental to assessing the success of geological carbon sequestration.

We have studied the in-situ feldspar dissolution rates in the Navajo sandstone at Black Mesa, Arizona. The Navajo sandstone is an aeolian quartz sandstone with about 2% feldspars and forms a part of a confined regional aquifer. Although the Navajo sandstone itself is not a suitable candidate for CO₂ sequestration, the numerous hydrogeological and geochemical data available (cf. Zhu et al., Geology, 1998, v.26, 127-130; Zhu, Water Resources Research, 2000, v.36, 2607-2620) make it one of the best aquifers to study in-situ dissolution kinetics. The saturated aquifer essentially provides a natural flow-through experiment up to the groundwater residence time of 35 ky. Inverse mass balance modeling and 14C dating show feldspar dissolution rates of log k+=-16.0 mol/m²s. This compares to the experimentally determined rate of log k+=-9.9 mol/m²s at 25^o C and pH 8. The observed in-situ rates in Navajo sandstone are about 10⁶ times slower than the experimental rates. This discrepancy is even larger than that found in soil and watershed studies, where natural rates are a factor of 10² to 10⁴ slower than those measured in the lab. Feldspar grains in the sandstones are rimmed with authigenic clay coatings of kaolinite and illite, which indicates dissolution conditions in the field that are drastically different from lab experiments.