Tidally driven water column hydro-geochemistry in a remediating acidic wetland

Managed tidal inundation is a newly evolved technique for remediating coastal acid sulphate soil (CASS) wetlands. However, there remains considerable uncertainty regarding the hydro-geochemical pathways and spatiotemporal dynamics of residual H⁺ and metal(loid) mobilisation into the tidal fringe surface waters of these uniquely iron-rich landscapes. Here, we examine the hydrology and water column chemistry across the intertidal slope of a remediating CASS wetland during several tide cycles. There was extreme spatial and temporal dynamism in water column chemistry, with pH fluctuating by ∼3 units (∼3.5–6.5) during a single tide cycle. Acute acidity was spatially confined to the upper intertidal slope, reflecting surface sediment properties, and tidal overtopping is an important pathway for mobilisation of residual H⁺ and Al³⁺ to the water column. Marine derived HCO₃⁻ was depleted from surface waters migrating across the intertidal slope and a strong gradient in HCO₃⁻ was observed from the tidal fringe to the adjacent tributary channel and nearby estuary. Tidal forcing generated oscillating hydraulic gradients in the shallow fringing aquifer, favouring ebb-tide seepage and driving rapid, heterogeneous advection of groundwater on the lower intertidal slope via surface connected macropores. A combination of diffusive and advective flux across the sediment–water interface led to persistent, elevated surface water Fe²⁺ (∼10–1000 μM). The geochemical processes associated with Fe²⁺ mobilisation displayed distinct spatial zonation, with low pH, proton-promoted desorption occurring on the upper intertidal slope, whilst circum-neutral pH, Fe(III)-reducing processes dominated the lower intertidal slope. Arsenic was also mobilised into surface waters on the lower intertidal slope under moderate pH (∼6.0) conditions and was strongly positively correlated with Fe²⁺. Saturation index values for aragonite were substantially depressed (−1 to −5) and significantly negatively correlated with elevation, thereby presenting a barrier to re-colonisation of the upper intertidal slope by calcifying benthic organisms. These findings highlight the spatially complex hydrological and geochemical controls on surface water quality that can occur in tidally inundated acid sulphate soil environments.