The kinetics of water-rock interactions in natural systems: Isotopic methods for quantifying rates and a multicomponent reactive transport model for interpreting them

Isotopic reactive transport formulations are used to quantify fluid flow and reaction rates in natural systems. At the Hanford Site, Washington, Sr isotopes in pore fluids give the ratio of infiltration flux to mineral dissolution rate. Using dissolution rates based on independent soil studies, the infiltration flux for the 70 meter column is estimated to be 7 +/- 3 mm/yr. This infiltration flux corresponds to a total transport time of water through the vadose zone to the water table of 600--1,600 years. The infiltration flux has important implications for the transport of radionuclides at the site. The 234U/238U in pore water at the Hanford Site also constrains the infiltration/dissolution rate ratio, but depends on the alpha-recoil loss fraction from solid grains. Geometric and chemical techniques were employed to estimate the alpha-recoil loss fraction. Geometric predictions of the alpha-recoil loss fraction based on grain size agree well with estimates from strong acid leaches and the finer grain size fractions. When coupled, the Sr and U isotopes can be used to uniquely constrain the infiltration flux and silicate dissolution rate in natural systems. At Hanford, these two methods converge on an infiltration flux of 3--5 mm/yr and bulk dissolution rates between 10-6.25 and 10 -7.0 yr-1.; Weathering rates are also calculated for silicate-rich deep-sea sediments using the 234U/238U approach. The rates calculated for both the vadose zone and deep-sea sediments are similar, and 3--5 orders of magnitude slower than expected based on laboratory experiments. A multi-component reactive transport model is used to assess the possible factors contributing to this discrepancy. The slow natural reaction rates may be explained by a reduction in the number of reactive sites per unit of mineral surface area. Plagioclase dissolution rates exhibit a strong dependence on the rate of secondary clay precipitation. Slow clay precipitation, by controlling the reaction affinity (proximity to chemical equilibrium) of the plagioclase, allows dissolution to occur closer to equilibrium and hence more slowly.