7 Gas Losses

In addition to mineral precipitation altering the solute chemistry, there is a loss of CO₂ gas from precipitation of CaCO₃ (calcite). As groundwater which has 10 to 70 times greater partial pressure of CO₂ reaches the surface it degasses CO₂ and precipitates CaCO₃ (calcite). Nitrogen (N₂) gas is also lost from shallow surface environments (Wood and Bohlke, 2017). Mass balance of the Abu Dhabi sabkha solutes suggests that the dominant source of nitrogen is atmospheric precipitation. However, the nitrogen isotopes of the sabkha suggest an isotopically heavier deep basin source. This apparent paradox is resolved when it is recognized that density-driven convection within the sabkha aquifer transports nitrate (NO₃^–1) with isotopically light atmospheric nitrogen from the oxygenated surface to a reducing environment at the base of the aquifer. Here, some nitrate is reduced to nitrogen gas N₂, which is isotopically lighter. The light N₂ is then carried back to the surface by the convective flow where it escapes to the atmosphere, leaving isotopically heavier nitrogen behind (Figure 21). This process is repeated each time density-driven free convection occurs and gradually changes the remaining nitrogen to an isotopically heavier form (Wood and Böhlke, 2017).

Figure 21 – Schematic depiction of a density-driven free-convection model that carries isotopically light atmospheric nitrogen from the oxygenated surface down to a reducing environment at the base of the aquifer where some nitrate is reduced to nitrogen gas N², which is isotopically lighter. Light N² then escapes to the atmosphere, leaving isotopically heavier nitrogen results in reduction of NO₃^-1 to N² which is carried back to the surface and lost to the atmosphere as a gas (modified from Wood and Böhlke, 2017).

In addition to nitrogen gas loss, bromide is lost as a gas from both saline lakes and salt flats including the Dead Sea (Hebestreit et al., 1999; Matveev et al., 2001), Great Salt Lake (Stutz et al., 2002), and Salar de Uyuni in Bolivia (Honninger et al., 2004). This is measured using differential optical absorption spectroscopy (DOAS). Utilizing a mass balance approach on the sabkha from Abu Dhabi suggests an annual bromide flux loss of 85 kg/km² (Wood and Sanford, 2007).