High resolution modeling of aluminium transport in a fjord estuary with focus on mean circulation and irregular flow events.

Environmental impact assessments of trace metals and radionuclides in estuarine waters will benefit from numerical transport models that can provide detailed and accurate predictions of concentrations of harmful physico-chemical forms of contaminants at adequate spatial and temporal resolution. Aiming to study the potential of aluminium (Al) exposure to biota, a transport model (OpenDrift) including dynamic speciation and transformation processes was improved and applied, using three-dimensional hydrodynamic flow fields from a numerical ocean model (ROMS) at high horizontal resolution (32 m). Al transport and concentration was computed along the Sandnesfjorden Fjord, south-eastern Norway, from river outlet to open coastal waters. Validation of the circulation model with 29 hydrographic profiles from Sandnesfjorden showed substantial improvements compared to previous studies due to optimized model configuration (salinity overestimation decreased from >7 psu to <4 psu). Modeled Al data compared well with observed surface Al concentration from 12 locations and the along-fjord decreasing trend in Al-concentration was well reproduced (error ratios were <2 in Sandnesfjorden). Except in the channel area, both salinity and Al concentration estimates lie well within the expected variability. However, the transport modeling gave a more detailed site-specific picture of the Al concentration, suggesting more scattered and variable fields than indicated by observational data (variations of a factor 3-4 over short spatiotemporal scales). Reversed flow events (surface flow into the fjord) caused considerable mixing and redistribution of water masses, affecting both horizontal mixing of river discharges with coastal water as well as vertically as surface water mixed with deeper water masses. These blocking events strongly changed properties and distribution of the water masses giving rise to local and short-term high Al-exposure episodes (variations of a factor of 10 over a 12 h period) in the fjord that may pose risks to biota and therefore should be taken into account in impact and risk assessments.

Modeling key processes affecting Al speciation and transport in estuaries.

Assessments of the impacts of aluminium (Al) to aquatic organisms in estuarine waters have suffered from the lack of available models that can accurately predict the presence of toxic physico-chemical forms (species) of Al at adequate spatial and temporal resolution. In the present work, transport and distribution of river-discharged Al species through changing environmental conditions in the Sandnesfjorden estuary, South-Eastern Norway, was predicted using a numerical model system at relatively high spatial (32 m × 32 m in horizontal) and temporal (1 h) resolution. New model code was implemented, including dynamic, salinity-dependent speciation and transformation processes, based on in situ measurements from several Norwegian estuaries as well as experimental data. This is the first time such elemental speciation code including LMM, colloidal, particle and sediment species is utilized in an estuary case in combination with high resolution hydrodynamics and compared to an extensive observational dataset. Good agreement was obtained between modeled and observed total and fractionated Al concentration at several stations along the fjord transect. Without including background contribution of Al from the coastal water, the model predicted too low Al concentrations (by up to approximately a factor 4) near the fjord mouth. The surface Al concentrations were also underestimated due to overestimated near-surface vertical mixing in the hydrodynamic model. The observed correlation between salinity and total Al concentration was well reproduced by the model in situations with low upper layer volume flux, typical under low river flow conditions. In contrast, the predicted surface salinity and total Al concentration were less correlated under high-flux conditions. As the general trends of Al concentrations and speciation were well reproduced, this study demonstrated that by including carefully chosen transfer rates, the model can be used to predict spatio-temporal distribution of total contamination as well as concentration levels of the elemental species.

Coastal transport of river-discharged radionuclides: Impact of speciation and transformation processes in numerical model simulations.

Following a potential nuclear accident, river run-off may potentially become a significant source of radionuclide contamination to the coastal marine environment. In the present work, code for radionuclide speciation and dynamic transfer of radionuclides between the different species was implemented in a Lagrangian marine dispersion model. A case study was performed where the model system utilized ocean circulation fields at relatively high spatial (160 mâ€¯× 160 m in horizontal direction) and temporal resolution (1 hour), considering a hypothetical accident scenario including river discharges of 137Cs to the marine environment. Results from a number of simulations were compared to identify how factors associated with radionuclide speciation and transfer between the model compartments could affect the predicted radiocesium activity concentrations. The results showed that by including dynamic transfer of radionuclides between the model compartments, the total activity concentrations at far-field sites could vary with more than two orders of magnitude, demonstrating that this model configuration enables prediction of potential local hot-spots. However, the total activity concentration near the river outlets was less affected (< factor 10). The radionuclide speciation in the river discharges and the parameterization of 137Cs particle affinity greatly affected the specie distribution (> factor 103 increase in concentration of particle-associated 137Cs) as well as the settling of radionuclides towards the seabed (up to factor 102 increase in 137Cs sediment concentrations). These factors were therefore identified as important contributors to the overall uncertainty.

The impact of tidal and mesoscale eddy advection on the long term dispersion of 99Tc from Sellafield.

In the present work, numerical models are used to study the fate of the 99Tc discharges from Sellafield with a specific focus on the role of mesoscale eddy and tidal advection on the transport and dispersion of this radionuclide. Transport estimates are made with an ocean model that resolves a large part of the ocean mesoscale eddy field and also includes tides. Equivalent estimates are also computed with another model in which these processes are either absent or parametrized. Comparison with field observations shows that the coarse-resolution model can reproduce the general features of the observed time-space 99Tc distribution if the diffusivity in its eddy parametrization scheme is suitably chosen. However, the eddy-permitting simulations capture regional details better and show an overall higher prediction skill, with the model predictions agreeing with the observations within a factor of two to four. The importance of tidal advection is investigated by comparing transport in the eddy-permitting model when this is run either with tides included or with tides filtered out. The results point to systematic Lagrangian tidal drift in the Irish Sea and the North Sea that eventually impacts the 99Tc activity concentration levels also far downstream.