Density-driven free-convection model for isotopically fractionated geogenic nitrate in sabkha brine
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Abstract
Subsurface brines with high nitrate (NO3−) 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), NO3− in the coastal sabkhas of Abu Dhabi is enriched in 15N (mean δ15N ∼17‰), which is an enigma. A NO3− 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 Δ17O, δ15N, and δ18O data suggest approximately 80 to 90% of the NO3− 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 NO3− bearing water downward where it encounters reducing conditions and mixes with oxygen-free ascending geologic brines. In this environment, NO3− is partially reduced to nitrogen gas (N2), thus enriching the remaining NO3− in heavy isotopes. The isotopically fractionated NO3− 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 NO3− 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 NO3−.
Study Area
Publication type | Article |
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Publication Subtype | Journal Article |
Title | Density-driven free-convection model for isotopically fractionated geogenic nitrate in sabkha brine |
Series title | Groundwater |
DOI | 10.1111/gwat.12463 |
Volume | 55 |
Issue | 2 |
Year Published | 2017 |
Language | English |
Publisher | Wiley |
Contributing office(s) | National Research Program - Eastern Branch, Toxic Substances Hydrology Program |
Description | 9 p. |
First page | 199 |
Last page | 207 |
Country | United Arab Emirates |
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