title: Density-Driven Free-Convection Model for Isotopically Fractionated Geogenic Nitrate in Sabkha Brine.
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 in15N (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-.
authors.affiliation: Department of Geologic Sciences, Michigan State University, 288 Farm Lane, East Lansing, MI 48824.
authors.forename: Warren W
authors.initials: WW
authors.lastname: Wood
authors.affiliation: U.S. Geological Survey, 431 National Center, Reston, VA 20192.
authors.forename: J K
authors.initials: JK
authors.lastname: Böhlke
mesh_terms: D018481:Convection; D004784:Environmental Monitoring; D060587:Groundwater; D009566:Nitrates; D009584:Nitrogen; D009587:Nitrogen Isotopes; D012492:Salts; D014874:Water Pollutants, Chemical
chemical_list: D009566:Nitrates; D009587:Nitrogen Isotopes; D012492:Salts; D014874:Water Pollutants, Chemical; C017082:brine; D009584:Nitrogen