J.K. Bohlke Warren W. Wood 2017 <p><span>Subsurface brines with high nitrate (NO</span>₃⁻<span>) concentration are common in desert environments as atmospheric nitrogen is concentrated by the evaporation of precipitation and little nitrogen uptake. However, in addition to having an elevated mean concentration of ∼525 mg/L (as N), NO</span>₃⁻<span> in the coastal sabkhas of Abu Dhabi is enriched in </span>¹⁵<span>N (mean δ</span>¹⁵<span>N ∼17‰), which is an enigma. A NO</span>₃⁻<span> solute mass balance analysis of the sabkha aquifer system suggests that more than 90% of the nitrogen is from local atmospheric deposition and the remainder from ascending brine. In contrast, isotopic mass balances based on Δ</span>¹⁷<span>O, δ</span>¹⁵<span>N, and δ</span>¹⁸<span>O data suggest approximately 80 to 90% of the NO</span>₃⁻<span> could be from ascending brine. As the sabkha has essentially no soil, no vegetation, and no anthropogenic land or water use, we propose to resolve this apparent contradiction with a density-driven free-convection transport model. In this conceptual model, the density of rain is increased by solution of surface salts, transporting near-surface oxygenated NO</span>₃⁻<span> bearing water downward where it encounters reducing conditions and mixes with oxygen-free ascending geologic brines. In this environment, NO</span>₃⁻<span> is partially reduced to nitrogen gas (N</span>₂<span>), thus enriching the remaining NO</span>₃⁻<span> in heavy isotopes. The isotopically fractionated NO</span>₃⁻<span> and nitrogen gas return to the near-surface oxidizing environment on the upward displacement leg of the free-convection cycle, where the nitrogen gas is released to the atmosphere and new NO</span>₃⁻<span> is added to the system from atmospheric deposition. This recharge/recycling process has operated over many cycles in the 8000-year history of the shallow aquifer, progressively concentrating and isotopically fractionating the NO</span>₃⁻<span>.</span></p> application/pdf 10.1111/gwat.12463 en Wiley Density-driven free-convection model for isotopically fractionated geogenic nitrate in sabkha brine article