Applications and limitations of portable density meter measurements of Na-Ca-Mg-K-Cl-SO4 brines

Abstract

Knowledge of brine density and mineral saturation states under different temperatures contributes to understanding Earth systems, including dissolution and crystallization of chemical sediments, mixing water bodies with variable density, and mineral resource development and evolution. However, these interpretations often rely on geochemical measurements with low accuracies at high salinities. Here, we test the utility of density measurments to advance understanding of brine geochemistry. We demonstrate the accuracy and limits of determining a brine's density and halite saturation state at multiple temperatures with rapid and relatively inexpensive portable density meter measurements. Density measurements at different temperatures are used to create samples' equations of state, which are used to extrapolate measurements. This methodology presents multiple opportunities. We use brine samples from around the world to show that density and equation of state data provide semi-quantitative information about sample composition and temperature-sensitive mineral precipitation. Importantly, density measurements can be used to evaluate error-prone geochemical measurements. We illustrate applications with brine samples from the Bonneville Salt Flats, a landscape in Utah, USA, that is valued for mineral resources and recreation opportunities. We developed system-scale equations of state for these saline pan samples and their halite-saturated counterparts. We evaluated methodological limitations with duplication tests and by determining if diurnal fluctuations in salinity and halite saturation in a shallow pool's brine and shallow groundwater in contact with a halite crust were detectable. Average daily changes in halite saturation of shallow pool brine of approximately 3 kg m−3 were clearly detected, while diurnal halite saturation fluctuations in the shallow halite crust brine of 0.4 to 0.7 kg m−3 were near method limits. Because of their accuracy, low cost, and applications, portable density meter measurements of brines are invaluable in studying brine systems.