Calcium isotope fractionation in groundwater: Molecular scale processes influencing field scale behavior

It is the purpose of this study to demonstrate that the molecular scale reaction mechanisms describing calcite precipitation and calcium isotope fractionations under highly controlled laboratory conditions also reproduce field scale measurements of delta Ca-44 in groundwater systems. We present data collected from an aquifer during active carbonate mineral precipitation and develop a reactive transport model capturing the observed chemical and isotopic variations. Carbonate mineral precipitation and associated fluid delta Ca-44 data were measured in multiple clogged well bores during organic carbon amended biogenic reduction of a uranium contaminated aquifer in western Colorado, USA. Secondary mineral formation induced by carbonate alkalinity generated during the biostimulation process lead to substantial permeability reduction in multiple electron-donor injection wells at the field site. These conditions resulted in removal of aqueous calcium from a background concentration of 6 mM to <1 mM while delta Ca-44 enrichment ranged from 1 parts per thousand to greater than 2.5 parts per thousand. The relationship between aqueous calcium removal and isotopic enrichment did not conform to Rayleigh model behavior. Explicit treatment of the individual isotopes of calcium within the CrunchFlow reactive transport code demonstrates that the system did not achieve isotopic reequilibration over the time scale of sample collection. Measured fluid delta Ca-44 values are accurately reproduced by a linear rate law when the Ca2+:CO32- activity ratio remains substantially greater than unity. Variation in the measured delta Ca-44 between wells is shown to originate from a difference in carbonate alkalinity generated in each well bore. The influence of fluid Ca2+:CO32- ratio on the precipitation rate and delta Ca-44 is modeled by coupling the CrunchFlow reactive transport code to an ion by ion growth model. This study presents the first coupled ion-by-ion and reactive transport model for isotopic enrichment and demonstrates that reproducing field-scale delta Ca-44 enrichment in groundwater where Ca2+:CO32- approaches unity is only accomplished by utilizing such a coupled modeling approach. (C) 2013 Elsevier Ltd. All rights reserved.