Density‐Driven Free‐Convection Model for Isotopically Fractionated Geogenic Nitrate in Sabkha Brine. Issue 2 (28th November 2016)

• Record Type: Journal Article
• Authors: Wood, Warren W.; Böhlke, J.K.
• Is Part Of: Ground water. Volume 55:Issue 2(2017)
• Page Start: 199
• Page End: 207
• Publication Date: 2016-11-28
• Subjects: Groundwater -- Periodicals; Wells -- Periodicals; Eau souterraine -- Périodiques; Puits -- Périodiques; Grondwater; Eau souterraine; Puits; Electronic journals; Périodique électronique (Descripteur de forme); Ressource Internet (Descripteur de forme); 551.49
• DOI: 10.1111/gwat.12463
• Languages: English
• ISSNs: 0017-467X

Abstract

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 15 N (mean δ 15 N ∼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 Δ 17 O, δ 15 N, and δ 18 O 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 − . Abstract : Article impact statement : Density‐driven free convection provides a mechanism for accumulating isotopically fractionated reactive solutes in brines.