Dušan Žagar

Mercury in contaminated coastal environments; a case study : the Gulf of Trieste

Some general facts, uncertainties and gaps in current knowledge of Hg cycling in coastal and oceanic environments are given. As a case study the Gulf of Trieste is chosen. The Gulf is subject to substantial Hg pollution, originating from the Soca river, that drains the cinnabar deposits of the worlds second largest Hg mining area, Idrija, Slovenia. The Gulf belongs to one of the most polluted areas in the Mediterranean. Apart from Hg problems, the Gulf is also a subject to industrial and sewage pollution. Due to deteriorating water quality in the Gulf there is a great concern that Hg can be remobilized from sediments to the water column as well as enhance methylation rates which may consequently increase already elevated Hg levels in aquatic organisms. The paper presents data from a recent study which aims to assess the extent of contamination of the Gulf of Trieste after the closure of the Hg mine. Mercury and methylmercury were measured in various environmental compartments (estuarine and marine waters, sediments, and organisms) during the period 1995-1997. Data obtained show that even 10 years after closure of the Hg mine, Hg concentrations in river sediments and water are still very high and did not show the expected decrease of Hg in the Gulf of Trieste. A provisional annual mercury mass balance was established for the Gulf of Trieste showing that the major source of inorganic mercury is still the River Soca (Isonzo) while the major source of methylmercury is the bottom sediment of the Gulf.

Mercury speciation driven by seasonal changes in a contaminated estuarine environment

In this study, seasonal changes of mercury (Hg) species in the highly variable estuary of Soča/Isonzo River (northern Adriatic Sea) were investigated. Samplings were performed on a seasonal basis (September 2009, May, August and October 2010) and Hg species (total Hg, methylmercury (MeHg), dissolved gaseous Hg (DGM)) in waters, sediments and pore waters were determined. In addition, a range of ancillary parameters were measured (salinity, nutrients, organic carbon (OC), nitrogen species). Hg values were interpreted using these parameters and hydrological conditions (river flow, wave height) around the time of sampling. There were no significant changes in Hg load from river to the gulf, compared to previous studies. The load was temporarily higher in May 2010 due to higher river flow. Wave height, through changing hydrostatic pressure, was most likely to cause resuspension of already deposited Hg from the bottom (August 2010). The estuary is a net source of DGM to the atmosphere as suggested by DGM profiles, with salinity, redox potential and organic matter as the most probable controls over its production. MeHg is produced in situ in sediment or in water column, rather than transported by river, as indicated by its correlation with OC of the marine origin. Calculated fluxes for THg and MeHg showed sediment as a source for both the water column. In pore waters, OC in part affects partitioning of both THg and MeHg; however other factors (e.g. sulphide and/or oxyhydroxides precipitation and dissolution) are also probably important.

Mercury speciation in the Adriatic Sea

Mercury and its speciation were studied in surface and deep waters of the Adriatic Sea. Several mercury species (i.e. DGM – dissolved gaseous Hg, RHg – reactive Hg, THg – total Hg, MeHg – monomethyl Hg and DMeHg – dimethylmercury) together with other water parameters were measured in coastal and open sea deep water profiles. THg concentrations in the water column, as well as in sediments and pore waters, were the highest in the northern, most polluted part of the Adriatic Sea as the consequence of Hg mining in Idrija and the heavy industry of northern Italy. Certain profiles in the South Adriatic Pit exhibit an increase of DGM just over the bottom due to its diffusion from sediment as a consequence of microbial and/or tectonic activity. Furthermore, a Hg mass balance for the Adriatic Sea was calculated based on measurements and literature data.

High-resolution pollutant dispersion modelling in contaminated coastal sites.

The recent developments in pollutant measurement methods and techniques necessitate improvements in modelling approaches. The models used so far have been based on seasonally averaged data, which is insufficient for making short-term predictions. We have improved the existing modelling tools for pollutant transport and dispersion on three levels. We significantly refined the numerical grid; we used temporally and spatially non-uniform meteorological parameters for predicting pollutant dispersion and transformation processes; we used grid nesting in order to improve the open boundary condition. We worked on a typical contaminated site (The Gulf of Trieste), where mercury poses a significant environmental threat and where an oil-spill is a realistic possibility. By calculating evasion we improved the mass balance of mercury in the Gulf. We demonstrated that the spreading of river plumes under typical wind conditions is different than has so far been indicated by model simulations. We also simulated an oil-spill in real time. The improved modelling approaches and the upgraded models are now suitable for use with the state-of-the-art measurements technology and can represent an important contribution to the decision-making process.

Large-scale oil spill simulation using the lattice Boltzmann method, validation on the Lebanon oil spill case.

This paper tests the adequacy of using the lattice Boltzmann method in large-scale oil spill modelling, such as the Lebanon oil spill. Several numerical experiments were performed in order to select the most appropriate lattice and to decide between the single- and two-relaxation time models. Large-scale oil spills require simulations with short computational times. In order to speed up the computation and preserve adequate accuracy of the model, five different flux limiting interpolation techniques were compared and evaluated. The model was validated on the Lebanon oil spill with regard to the oil-slick position and concentrations in the sea, and the beaching area on the coast. Good agreement with satellite images of the slick and field data on beaching was achieved. The main advantages of the applied method are the capability of simulating very low oil concentrations and computational times that are by an order of magnitude shorter compared to similar models.

MODELING MERCURY FATE AND TRANSPORT IN AQUATIC SYSTEMS

Mercury in the aquatic environment is a neurotoxin with several known adverse effects on the natural ecosystem and the human health. Mathematical modeling is a costeffective way for assessing the risk associated with mercury to aquatic organisms and for developing management plans for the reduction of mercury exposure in such systems. However, the analysis of mercury fate and transport in the aquatic environment requires multiple disciplines of science ranging from sediment transport and hydraulics, to geochemistry and microbiology. Also, it involves the knowledge of some less understood processes such as the microbial and diagenetic processes affecting the chemical speciation of mercury and various mechanisms involved in the mass-exchange of mercury species between the benthic sediments and the overlying water. Due to these complexities, there are many challenges involved in developing an integrated mercury fate and transport model in aquatic systems. This paper identifies the various processes that are potentially important in mercury fate and transport as well as the knowns and unknowns about these processes. Also, an integrated multi-component reactive transport modeling approach is suggested to capture several of those processes. This integrated modeling framework includes the coupled advective-dispersive transport of mercury species in the water body, both in dissolved phase and as associated to mobile suspended sediments. The flux of mercury in the benthic sediments as a result of diffusive mass exchange, bio-dispersion, and hyporheic flow, and the flow generated due to consolidation of newly deposited sediments is also addressed. The model considers in addition the deposition and resuspension of sediments and their effect on the mass exchange of mercury species between the top water and the benthic sediments. As for the biogeochemical processes, the effect of redox stratification and activities of sulfate and iron-reducing bacteria on the methylation of mercury is discussed, and the modeling approach is described. Some results for the application of the model to the Colusa Basin Drain in California are presented. At the end of the paper, the shortcomings of our current knowledge in predicting the fate of mercury in water-sediment systems, the potential improvements, and additional complexities required to make the model more realistic, are discussed.

Different ways of defining wall shear in smoothed particle hydrodynamics simulations of a dam-break wave

ABSTRACT The paper describes two different ways of defining the terrain roughness in smoothed particle hydrodynamics (SPH) simulations performed with the Tis Isat model, developed at the University of Ljubljana. The model introduces into the SPH method a non-discrete boundary condition with friction. Two basic definitions of terrain roughness are used: (a) as a hydraulically smooth wall, where roughness was controlled by the wall–particle eddy viscosity coefficient; and (b) as a hydraulically rough terrain by elevating the mesh-nodes. The undertaken SPH simulations relate to a dam break at the upper storage reservoir of the pumped-storage hydro power plant Kolarjev vrh in Slovenia. For the first time, such study was performed on a real topography. Water depths at the gauges along the valley were compared with measurements on a physical model and to results obtained using a finite volume (FV) model. The comparison showed satisfactory agreement with the measurements, which are comparable with the FV model simulations.

Numerical modelling of mercury evasion in a two-layered Adriatic Sea using a coupled atmosphere-ocean model

A new mercury (Hg) evasion model for the Adriatic Sea was developed accounting for the ocean mixed layer depth in order to decrease Hg depletion at the surface. Previously modelled airborne Hg species and measured Hg in the ocean were used. Simulations were run using one- and two-way coupled atmosphere-ocean models. Discrepancies in evasion between the applied coupling schemes were shown to be insignificant. The model was evaluated by applying various wind parameterisations and diffusive coefficient formulae. Relatively high discrepancies among the applied methods were observed. The results of a shorter simulation were extrapolated over a one-year period by applying a measurement-based adaptation. We obtained good agreement with previously published data on Hg evasion in the entire Mediterranean area, thus confirming the suitability of the new model for Hg evasion simulations. Model computations performed for the Adriatic Sea resulted in levels of evasion approximately two times lower than previously estimated.