Hydrogeochemistry of prairie pothole region wetlands: Role of long-term critical zone processes

This study addresses the geologic and hydrogeochemical processes operating at a range of scales within the prairie pothole region (PPR). The PPR is a 750,000 km²portion of north central North America that hosts millions of small wetlands known to be critical habitat for waterfowl and other wildlife. At a local scale, we characterized the geochemical evolution of the 92-ha Cottonwood Lake study area (CWLSA), located in North Dakota, USA. Critical zone processes are the long-term determinant of wetland water and groundwater geochemistry via the interaction of oxygenated groundwater with pyrite in the underlying glacial till. Pyrite oxidation produced a brown, iron oxide-bearing surface layer locally over 13 m thick and an estimated minimum of 1.3 × 10¹⁰ g sulfate (SO₄^2 −) at CWLSA. We show that the majority of this SO₄²⁻ now resides in solid-phase gypsum (CaSO₄•2H₂O) and gypsum-saturated groundwater.

Results from the CWLSA were scaled up to a 9700 km² area surrounding CWLSA using ~ 1800 drill logs and literature data on wetland water chemistry for 178 wetlands within this larger area. The oxidized brown zone depth and wetland water compositional trends are very similar to the CWLSA. Additionally, surface water data from 176 southern Canadian pothole wetlands that conform to the same wetland water geochemical trends as those recorded in the CWLSA further corroborate that SO₄^2 − accumulation driven by pyrite oxidation is a nearly ubiquitous process in the prairie pothole region and distinguishes PPR wetlands from other wetlands worldwide that have a similar overall hydrology.