Sulfur, iron and carbon cycling following hydrological restoration of acidic freshwater wetlands

Abstract

Freshwater re-flooding is a relatively novel approach to remediate drained acid sulfate soil (ASS) wetlands. This study documents the geochemical consequences of restoring freshwater re-flooding for contemporary reduced inorganic sulfur (RIS) and iron species in two coastal floodplain ASS wetlands. Re-flooding has established predominantly reducing/suboxic conditions and encouraged organic carbon accumulation in surface sediments (~20-30%). The pH of former sulfuric horizons has increased by ~2-3 units, partly in response to alkalinity generation from anaerobic metabolism of organic carbon coupled with Fe(III) and SO42- reduction. Despite considerable sulfidisation, reactive Fe (FeR; sum of 1M HCl and citrate-dithionite extracts) and non-sulfidic Fe(II) remain abundant in both wetlands. High concentrations of Fe2+ (up to ~5mM) in wetland porewaters represent a considerable pool of labile net acidity and is partly a result of insufficient S(-II) to sequester excess Fe2+. Accumulation of iron sulfides appears to be constrained more by SO42- and carbon availability rather than FeR. Reformation and accumulation of RIS species is greatest in organic-rich surface horizons (~40-500μmolg-1), where time integrated RIS accumulation rates approximate 10-100nmolg-1 d-1. While pyrite is the dominant RIS species to have formed since re-flooding, there is anomalous accumulation of S(0) (up to 80μmolg-1), accounting for ~50% of the RIS pool in some samples. Greigite (Fe3S4) has formed in near-surface sediments and while AVS-S is a minor component of the RIS pool overall, at some locations maximum concentrations exceed 300μmolg-1. Contemporary near-surface pyrite is characterised by abundant small (200-300nm) crystals, in contrast to relic sedimentary pyrite of estuarine origin that is dominated by larger crystals with diverse habit. Although Fe and SO42- reduction are partly responsible for wetland-scale recovery from acute acidification, the resultant accumulation of diverse RIS species in surficial sediments indicates an oxidative component to the S-cycle and represents a hysteresis in S-cycling that contrasts markedly with the drained conditions existing before remediation. Analysis of seasonal climate fluctuations suggests that near-surface sediments containing contemporary RIS are vulnerable to oxidation and possible temporary re-acidification during future drought episodes. This study underscores the long-term legacy of ASS wetland drainage and highlights the need for both considered hydrological management of re-flooded wetlands and further study to quantify possible re-acidification risks associated with seasonal drought. © 2014 Elsevier B.V.