Abstract
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.
Highlights
The input of trace elements and radionuclides to the marine environment is largely attributed to river transport via estuaries to the coastal zone
Particles are defined as entities with nominal diameter greater than 0.45 lm, colloidal species are within the range from 10 kDa to 0.45 lm and low molecular mass species (LMM) species are less than 10 kDa (Salbu, 2009)
It was assumed that the Al concentration in River Storelva depends on the flow rate to that of River Lona (Teien et al, 2006b), and the number of trajectories released in particle compartment (Np) increased exponentially with increasing river flow (Q); Np = (Q fp)kx where k is a constant and the scale factor x ensures that the total number of trajectories released in the simulation is reasonable
Summary
The input of trace elements and radionuclides to the marine environment is largely attributed to river transport via estuaries to the coastal zone. Along the estuarine transport pathways, river-discharged trace elements such as aluminium (Al) can appear in a series of physico-chemical forms (species), ranging from single ions and low molecular mass species (LMM) to larger colloids and particles (Salbu, 2009). It was assumed that the Al concentration in River Storelva depends on the flow rate to that of River Lona (Teien et al, 2006b), and the number of trajectories released in particle compartment (Np) increased exponentially with increasing river flow (Q); Np = (Q fp)kx (2). As an approximation of the real discharges, the input concentrations of the LMM and colloidal species were assumed to be constant, i.e., the number of released numerical trajectories in these model compartments (Nlmm and Nc, respectively) was linearly proportional to the daily river flow, described by the following equations: Nc = Qfcx (3). 251, 874 trajectories were released with 4-hour time intervals
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