Grain size and organic carbon controls polyaromatic hydrocarbons (PAH), mercury (Hg) and toxicity of surface sediments in the River Conwy Estuary, Wales, UK

The Conwy estuary was evaluated for sediment quality. Microtox bioassay revealed 38 of 39 sites were non-toxic. Hg ranged from 0.001 to 0.153 μg kg−1, mean 0.026 mg kg−1, Σ16 PAH from 18 to 1578 μg kg−1, mean 269 μg kg−1, Σ22 PAH, 18 to 1871 μg kg−1 mean to 312 μg kg−1, two sites had high perylene relative to ΣPAH. Σ22PAH correlated positively with TOC, clay and silt (R2 0.89, 0.92, 0.90) and negatively with sand. Multivariate statistics, delineated four spatial (site) and five variable (measurements) clusters. Spatial clustering relates to sediment grain size, in response to hydrodynamic processes in estuary; fine (clay to silt) sized sediments exhibit the highest Hg and PAH content, because these components partitioned into the fine fraction. Comparison to national and international environmental standards suggests Hg and PAH content of Conwy sediments are unlikely to harm ecology or transfer up into the human food chain.