Isotopically Enriched Geogenic Delta Br-81 And Delta Cl-37: Primary Evidence For The Ascending Brine Model

Mass balance calculations and hydrodynamics of groundwater flow suggest that the solutes in brines of the coastal sabkha aquifer from the Emirate of Abu Dhabi are derived largely from ascending geologic brines into the sabkha from the underlying formations. Solute interpretation for the ascending brine model (ABM) was based on two independent but secondary lines of evidence (solute ratios and solute fluxes). In the current study, direct primary evidence for this ABM was provided through analyses of delta Br-81, delta Cl-37, and Sr-87/Sr-86. Different solute histories of geologic brine and sea water provide an "isotopic fingerprint" that can uniquely distinguish between the two possible sources. Samples from the coastal sabkha aquifer of Abu Dhabi were determined to have a mean delta Br-81 of 1.17 parts per thousand that is statistically equal, at the 95% confidence level, to the mean of 1.11 parts per thousand observed in the underlying geologic brine and statistically different than sea water. Similarly, the delta Cl-37 in sabkha brine has a mean of 0.25 parts per thousand and is statistically equal to a mean of 0.21 parts per thousand in the underlying geologic brines at the 95% confidence level and statistically different from sea water. Also, dissolved strontium isotope data are consistent with the ABM and even with the complex set of processes in the sabkha, the variance in strontium isotope results is similar to the geologic brine. These observations provide primary direct evidence consistent that the major source of these solutes (and presumably others in the aquifer) is from discharging geologic brines, not from adjacent sea water.