Anecdotal evidence from boat owners suggested anodes corroded quicker on the Hamble compared to other estuaries, with a number of theories proposed. Anode corrosion led to higher total, labile and free zinc concentrations to be present within the Hamble estuary. A survey to boat owners on the Hamble along with anode experiments confirmed anodes do not corrode quicker on the Hamble. It did, however, indicate there was a general lack of knowledge and awareness regarding anode use, which partially derives from conflicting advice available for boat owners. The anode experiments and survey suggested salinity and stray currents influence anode corrosion. Anode release rates calculated from the survey and anode experiments, predicted a zinc load of up to 6.95t/yr to the estuary from anodes. Anodes are by far the largest zinc source to the Hamble Estuary, the EQS (European Quality Standard) subsequently was exceeded at times on the Hamble. Zinc samples were analysed using cathodic stripping voltammetry (CSV), higher concentrations were observed in areas of greater boat density, with total dissolved zinc levels reaching 20-25μg/l. An estuary average concentration of 8.07μg/l was recorded across the sampling period, which exceeded the EQS of 7.9μg/l for zinc. Areas of lower boat densities or from outside the estuary achieved the EQS. Complexation with Dissolved Organic Carbon (DOC) meant labile zinc was generally a quarter to two thirds the total dissolved zinc concentration, with free zinc a quarter or below of the labile. The BLM model (Biotic Ligand Model) site specific EQS predictions for the Hamble freshwater sites were all higher than the observed zinc concentrations. Zinc concentrations within sediments on the Hamble were comparably low, the zinc levels within SPM (Suspended Particulate Matter) however tended to be high, suggesting SPM acts as a vector for zinc released from anodes. The GEMCO (Generic Estuary Model for Contaminates) and MAMPEC (Marine Antifoulant Model to Predict Environmental Concentrations) models confirmed anodes as a major zinc source to the Hamble estuary, predicted concentrations compared well with observed values. The MAMPEC model generally under predicted zinc in areas of high boat density. The GEMCO model predicted that 200-800 (out of 3000) vessels would need to use aluminium anodes for the EQS to be met, leading to environmental benefits for the Hamble estuarine environment. Vessels in brackish waters could benefit from using aluminium anodes.
|Date of Award||Jan 2019|