Legacy impacts of acid sulfate soil runoff on mangrove sediments: Reactive iron accumulation, altered sulfur cycling and trace metal enrichment

Abstract

The off-site impacts of iron-rich, acidic runoff from drained acid sulfate soils (ASS) on the geochemistry of estuarine sediments are relatively poorly constrained. In this study, we examine the geochemistry of intertidal zone mangrove sediments and porewaters from an estuarine tributary that received ~15–20years of acute ASS drainage, compared to adjacent control sites. Mixing of acidic, iron-rich ASS drainage waters with circumneutral pH estuarine waters drove precipitation of Fe(III) floc and has led to significant, localized accumulation of reactive-Fe species (FeR; Σ1M HCl and citrate-buffered dithionite extracts; up to 4200μmolg−1) as well as significantly elevated porewater Fe2+ (>1000μM) in ASS impacted sediments. X-ray diffraction indicates that goethite, lepidocrocite and ferrihydrite are important Fe(III) minerals in surficial sediments at ASS impacted sites. The abundant FeR is shifting the equilibrium of reduced inorganic sulfur (RIS) mineral-formation pathways towards accumulation of S(0) and acid volatile sulfide (AVS) species and is buffering porewater S(-II) to low concentrations. In contrast, RIS speciation at the control sites is dominated by pyrite (SCR) and porewater S(-II) is present. Some porewater metals (i.e. Al, Mn, Cu, Cr) are significantly elevated at sites proximal to the source of ASS drainage compared to control sites. Abundant FeR also provides a substrate for sorption and accumulation of metals and correspondingly there is significant relative enrichment of reactive metal fractions (1M HCl extractable) in sediments at the ASS impacted sites, with Al≈Mn≈U>Cu>Cr>Ni. This study demonstrates that ASS drainage, which causes accumulation of reactive Fe in mangrove environments, can substantially influence sediment biogeochemistry by altering reduced inorganic sulfur cycling and enhancing the availability of some trace metals.