Mercury cycling in contaminated coastal environments: modeling the benthic-pelagic coupling and microbial resistance in the Venice Lagoon.

Anthropogenic activities have been releasing mercury for centuries, and despite global efforts to control emissions, concentrations in environmental media remain high. Coastal sediments can be a long-term repository for mercury, but also a secondary source, and competing processes in marine ecosystems can lead to the conversion of mercury into the toxic and bioaccumulative species methylmercury, which threatens ecosystem and human health. We investigate the fate and transport of three mercury species in a coastal lagoon affected by historical pollution using a novel high-resolution finite element model that integrates mercury biogeochemistry, sediment dynamics and hydrodynamics. The model resolves mercury dynamics in the seawater and the seabed taking into account partitioning, transport driven by water and sediment, and photochemical and microbial transformations. We simulated three years (early 2000s, 2019, and 2020) to assess the spatio-temporal distribution of mercury species concentrations and performed a sensitivity analysis to account for uncertainties. The modeled mercury species concentrations show high temporal and spatial variability, with water concentrations in some areas of the lagoon exceeding those of the open Mediterranean Sea by two orders of magnitude, consistent with available observations from the early 2000s. The results support conclusions about the importance of different processes in shaping the environmental gradients of mercury species. Due to the past accumulation of mercury in the lagoon sediments, inorganic mercury in the water is closely related to the resuspension of contaminated sediments, which is significantly reduced by the presence of benthic vegetation. The gradients of methylmercury depend on the combination of several factors, of which sediment resuspension and mercury methylation are the most relevant. The results add insights into mercury dynamics at coastal sites characterized by a combination of past pollution (i.e. sediment enrichment) and erosive processes, and suggest possible nature-based mitigation strategies such as the preservation of the integrity of benthic vegetation and morphology.

Modelling as decision support for the localisation of submarine urban wastewater outfall: Venice lagoon (Italy) as a case study.

Microbiological impact is critical in coastal areas where tourism is particularly important for both the local and regional economy. Submarine outfalls are commonly used to enhance the dispersion of treated sewage thus avoiding pollution along the coast. The Venice lagoon (North Italy) has a very sensitive ecosystem, due to the morphological and natural characteristics of the basin and the co-existence of human activities. To preserve the lagoon, the discharge from the treatment plant for urban wastewater collected from the Venezia-Mestre agglomeration, neighbouring areas and local industries (total of 400,000 population equivalent-PE) has been moved from the lagoon to the open Adriatic Sea since November 2013 by means of an approximately 20-km pipeline. Microbiological pollution inside the lagoon can affect shellfish breeding areas instead, along the coast it affects the quality of bathing waters. In this study, and for the first time, a 3D hydrodynamic SHYFEM model (shallow water finite element model) with high spatial resolution coupled with a microbiological module has been applied to the lagoon and to the Adriatic Sea, to evaluate the effectiveness of the location of the submarine outfall. Microbiological data have been produced by the control Authority according to official analytic methods and by the plant operator. The module of survival of free Escherichia coli follows a variable rate in dependence of UV radiation, temperature and salinity in the water. Two scenarios were modelled: final discharge into the lagoon before November 2013 and after into the open sea. In the latter case, two situations have been considered, one with "Bora" and the other with "Scirocco" winds. Our results indicate that the model correctly simulates microbiological decay and dispersion. The transferral of the final discharge point far from the shoreline improves pollution dispersion, thus preserving the lagoon without evidence of impacts on the bathing waters in all meteorological conditions.

Hydrodynamic properties of San Quintin Bay, Baja California: Merging models and observations.

We investigated the physical dynamics of San Quintin Bay, a coastal lagoon located on the Pacific coast of northern Baja California, Mexico. We implemented, validated and used a finite element 2-D hydrodynamic model to characterize the spatial and temporal variability of the hydrodynamic of the bay in response to variability in the tidal regime and in meteorological forcing patterns. Our analysis of general circulation, residual currents, residence times, and tidal propagation delays allowed us to characterize spatial variability in the hydrodynamic basin features. The eulerian water residence time is -on average and under reference conditions- approximately 7days, although this can change significantly by region and season and under different tidal and meteorological conditions. Ocean upwelling events that bring colder waters into the bay mouth affect hydrodynamic properties in all areas of the lagoon and may affect ecological dynamics. A return to pre-upwelling conditions would take approximately 10days.