Short-term seawater inundation induces metal mobilisation in freshwater and acid sulfate soil environments

Climate change is leading to global sea level rise. Storm surges and higher tides will generate short-term 'pulses' of seawater into freshwater systems, often for the first time in over 3000 years. The effect of increased seawater inundation upon soil geochemistry is poorly understood. We identified 12 sites in South Australia which are predicted to be inundated by seawater storm surges in the next 20 years. Within these 12 sites are three distinct environments; fresh water streams and lakes, hypersaline saltmarsh and mangroves, and acid sulfate soils. Soils were inundated with seawater under laboratory conditions to replicate a short-term (two weeks) inundation by a storm surge. Lowering of redox potential and dissolution of high concentrations of reactive Mn and Fe in freshwater environments lead to the release of dissolved Fe and Mn in the soils from freshwater environments. Soils also released As, Cu, Ni, Cd and Co, while Zn and Pb were less mobilised. Concentrations of metals released exceeded water quality guidelines to protect freshwater aquatic ecosystems in most cases. By comparison, hypersaline soils only released minor amounts of Mn, Fe, Cd and Ni, and only in some of the soils. The moderately acidic acid sulfate soil (pH 5.41) reductively dissolved Mn and Fe releasing significant amount of Fe and Mn as well as As, Cu, Ni, Cd and Co, whereas almost all metal species decreased in the porewaters of the strongly acidic acid sulfate soil (pH 2.77). The response to short-term seawater inundation in acid sulfate soils was dependent upon the baseline soil acidification status. This study highlights the need for further research on seawater inundation of coastal soils as sea levels rise and storm surges penetrate further inland.