Chloride and bromide sources in water: Quantitative model use and uncertainty

Dissolved chloride is a commonly used geochemical tracer in hydrological studies. Assumptions underlying many chloride-based tracer methods do not hold where processes such as halide-bearing mineral dissolution, fluid mixing, or diffusion modify dissolved Cl⁻ concentrations. Failure to identify, quantify, or correct such processes can introduce significant uncertainty to chloride-based tracer calculations. Mass balance or isotopic techniques offer a means to address this uncertainty, however, concurrent evaporation or transpiration can complicate corrections. In this study Cl/Br ratios are used to derive equations that can be used to correct a solution's total dissolved Cl⁻ and Br⁻ concentration for inputs from mineral dissolution and/or binary mixing. We demonstrate the equations’ applicability to waters modified by evapotranspiration. The equations can be used to quickly determine the maximum proportion of dissolved Cl⁻ and Br⁻ from each end-member, providing no halide-bearing minerals have precipitated and the Cl/Br ratio of each end member is known. This allows rapid evaluation of halite dissolution or binary mixing contributions to total dissolved Cl⁻ and Br⁻. Equation sensitivity to heterogeneity and analytical uncertainty is demonstrated through bench-top experiments simulating halite dissolution and variable degrees of evapotranspiration, as commonly occur in arid environments. The predictions agree with the experimental results to within 6% and typically much less, with the sensitivity of the predicted results varying as a function of end-member compositions and analytical uncertainty. Finally, we present a case-study illustrating how the equations presented here can be used to quantify Cl⁻ and Br⁻ sources and sinks in surface water and groundwater and how the equations can be applied to constrain uncertainty in chloride-based tracer calculations.