Using halogens (Cl, Br, I) to understand the hydrogeochemical evolution of drought-derived saline porewater beneath a prairie wetland

Numerous closed-basin prairie wetlands throughout the Prairie Pothole Region (PPR) of North America maintain moderate surface pond salinities (total dissolved solids [TDS] from 1 to 10 g L^− 1) under semiarid climate by accumulation of gypsum and saline lenses of sulfate-rich porewater (TDS > 10 g L^− 1) in wetland sediments during droughts. In order to understand the hydrogeochemical origin and composition of these saline porewaters, we made a detailed geochemical survey of Cl⁻, SO₄^2 −, Br, and I in the porewater, pondwater, and upland groundwater of a typical closed-basin prairie wetland (P1 in the Cottonwood Lake study area, North Dakota). Concentrations of Cl⁻ ranged up to 5.9 mM in the saline porewaters, and was strongly correlated with SO₄^2 − and Br (Pearson's r > 0.7, p < 0.05; concentrations ranging up to 131 mM and 39 μM, respectively) due to the conservative effects of surface water evaporation. In contrast, total dissolved I was not significantly correlated with Cl⁻ (Pearson's r = 0.18, p = 0.273) and was concentrated in porewaters located above the saline lenses with a peak concentration of 4.1 μM beneath the center of the wetland— the highest value for dissolved I ever measured in a terrestrial aquatic system and an order of magnitude above that of seawater. We hypothesize that chromatographic separation between more mobile anions (Cl⁻, SO₄^2 −, Br⁻) and I occurs during droughts when wetland ponds dry and sedimentary iodide (I⁻) oxidizes to its less-mobile form, iodate (IO₃⁻). Understanding the origin and geochemical composition of porewater salinity that develops beneath prairie wetlands during drought can help to fingerprint sources of salinity to wetland ponds during wet climate and elucidate halogen systematics in saline and organic-rich subsurface environments associated with hydrocarbon generation.