Haloform formation in coastal wetlands along a salinity gradient at South Carolina, United States

Abstract

Environmental contextNatural haloform emissions contribute to stratospheric ozone depletion but there are major unknown or underestimated sources of these gases. This study demonstrates that soil and water at tidal wetlands are important haloform sources, and emissions peak at the forest–marsh transition zone. The low-lying forested wetlands of the south-eastern United States that are facing sea-level rise and seawater intrusion may become hotspots for haloform emission. AbstractSoil haloform emissions are sources of reactive halogens that catalytically deplete ozone in the stratosphere but there are still unknown or underestimated haloform sources. The >200000ha of low-lying tidal freshwater swamps (forests and marshes) in the south-eastern United States could be haloform (CHX3, X=Cl or Br) sources because sea-level rise and saltwater intrusion bring halides inland where they mix with terrestrial humic substances. To evaluate the spatial variation along the common forest–marsh salinity gradient (freshwater wetland, oligohaline wetland and mesohaline saltmarsh), we measured chloroform emissions from in situ chambers and from laboratory incubations of soil and water samples collected from Winyah Bay, South Carolina. The in situ and soil-core haloform emissions were both highest in the oligohaline wetland, whereas the aqueous production was highest in mesohaline saltmarsh. The predominant source shifted from sediment emission to water emission from freshwater wetland to mesohaline saltmarsh. Spreading out soil samples increased soil haloform emission, suggesting that soil pores can trap high amounts of CHCl3. Soil sterilisation did not suppress CHCl3 emission, indicating the important contribution of abiotic soil CHCl3 formation. Surface wetland water samples from eight locations along a salinity gradient with different management practices (natural v. managed) were subjected to radical-based halogenation by Fenton-like reagents. Halide availability, organic matter source, temperature and light irradiation were all found to affect the radical-based abiotic haloform formation from surface water. This study clearly indicates that soil and water from the studied coastal wetlands are both haloform sources, which however appear to have different formation mechanisms.