Acid Sulfate Soils and Their Pathways of Impact: A Swedish Case Study.

Acid sulfate soils (ASS) are naturally occurring sediments rich in iron sulfides that, when exposed to oxygen, produce sulfuric acid, and mobilize toxic metals. Recent findings have identified acidic, metal-enriched layers in postglacial, organic-rich sediments in agricultural areas along the Swedish west coast. Although leaching from these soils into a nearby river system has been observed, the extent and downstream consequences remain poorly understood. This study investigates the environmental impact of ASS on a river-estuary system along the Swedish west coast, with a focus on metal mobility and ecological effects. To assess the environmental impact of ASS leachate, water quality parameters (pH and electrical conductivity) and metal concentrations were measured across a river-estuary gradient. Metal concentrations were also analyzed in sediment and biological tissues, and an ecological survey of aquatic flora, fauna, and microbial communities in the sediment was conducted. Additionally, a laboratory toxicity test was conducted on the freshwater amphipod using riverine water sampled near the ASS site. Results showed that although metal concentrations in water, sediment, and plant tissues remained elevated in the estuary, they declined with distance from the river mouth. Similarly, pH levels were notably lower at the river outflow compared to nearby reference rivers during both spring and autumn. In general, the estuary lacked macrobiodiversity, as low organism abundance of both plants and animals was encountered in the ecological survey. However, a rich microbial community dominated by different orders of cyanobacteria was identified through metabarcoding. Amphipods exposed to river water exhibited signs of developmental stress, including reduced head length, lower instar stages, and decreased dry weight compared to controls. The findings indicate that leaching from ASS leads to acidification and widespread metal contamination, with demonstrable ecological effects that may hinder the achievement of good water status under the European Union Water Framework Directive.