The behaviour of contaminants in the water and sediments in river basins cannot be studied without taking into account the relevant processes in the basins and the boundaries with the upstream river system and the coastal region. The rivers that flow into these coastal areas take a considerable amount of contaminated sediments which are stored for longer or shorter periods in these estuaries. Retention of sediments will take place in the low-energy areas such as the smaller tributaries in the river basin.
Within this work package various empirical formulations and characteristics will be defined that typically determine the sediment retention (e.g. hydraulic load and specific runoff). The estuarine regions of a river basin represent a diverse and complex water system. The tidal motion and the density currents induced by the change from fresh to saltwater are of particular importance in describing the water quality of estuaries. In the estuary strong intrusion of saltwater landward and current reversal might occur.
The coastal area is characterised by the typical oscillations of the tidal movement and has a complicated current structure resulting from the horizontal intrusion of saline water and vertical stratification due to density differences. It is obvious that the estimation of the time and spatial behaviour of the exposure of contaminants in estuaries is complicated by the effects of tidal motion and chemical behaviour. In order to have an accurate description of the fate and distribution of contaminants in estuarine regions, a carefully analysis of model concepts and implementation is needed in this work package to assess the degree of complexity and valid merging of process formulations.
Bio-chemical fate processes
Besides transport processes compounds are subject to many distribution and transformation processes or reactions which determine the exposure of contaminants within a river basin. Physico-chemical processes such as sorption, partitioning and evaporation determine the distribution between the water, air and particulate phases. Most compounds are subjected to transformation or degradation reactions, such as hydrolysis, photo-degradation, redox reactions and degradation by micro-organisms. The significance of degradation processes may vary with depth.
For several compounds degradation is most prominent in the upper water layers, due to photo-degradation. Biodegradation rates in the lower water column are assumed to be lower. In anoxic sediments, biodegradation rates usually are much slower than in the water column. Many trace metals and persistent organic compounds are strongly bound to particulate phases or dissolved organic material or in the case of trace metals bound to inorganic and organic ligands. Usually only a limited fraction of a specific compound is present in a truly free dissolved state and available for uptake by aquatic organisms. Until 1995 the principle of linear equilibrium partitioning had been the main guiding principle in studying adsorption of hydrophobic pollutants onto sediments and soil. However, the literature of the nineties also showed that the partition coefficient increases at the progress of the ageing of sediment (Hatzinger and Alexander, 1995, Weber and Huang, 1996; Huang et al., 1997; Leboeuf and Weber, 1999; Kan et al., 1998; Jepsen and Lick, 1999; WL, 2003a).
The sediment and pollutant transport dynamics of the Elbe river upstream of the tidal range and the Llobregat river in Spain, is governed by the hydrological characteristics of the catchment and the subcatchments, respectively. The driving force for erosion, transport and sedimentation is the discharge and hence, the model input data must be chosen on a statistical basis in order to link the transport processes to the hydrological risk. This allows to investigate the ecological impact of representative discharge scenarios and to attribute a hydrological probability.
In addition, the spatial variability and uncertainty of the sediment data must be taken into account, e.g. by applying the Monte Carlo method to come up with statistical model results, i.e. expected values and variance for pollutant load and concentration, sedimentation rate, residence time, exposure duration etc. ( Li, 2004 ) which are useful and important results for the following ecological impact assessment in EXPO and EFFECT. Erosion and sedimentation, transport and mixing, residence time, sorption and 1-st order degradation can be described by the reactive model COSMOS (Kern, 1997 ) after calibration. The major hotspots in the Elbe river are located in the groyne fields which have a high trapping efficiency at low water level and become strong pollutant sources at higher erosive discharge. To account for the Elbe specific morphology a hierarchical model will be conceptualized consisting of a 2-dimensional model (TELEMAC- SUB2; Jacoub, Westrich, 2004)
which can be coupled or nested in a 1-dimensional model (COSMOS). Representative river portions with critical hotspots or high ecological vulnerability are selected and modelled by the 2-d model with high spatial and temporal resolution to cover the dynamic process whereas the 1-dimensional model is applied to capture the whole river Elbe with its most important hotspots located in the tributaries, i.e. Bilina in Czechia and Mulde, Saale in Germany, and to allow long term simulation for future exposure and environmental impact assessment very efficiently. A sensitivity analysis will be performed for model parameter ranking to come up with a limited number of key parameters. The intention is to validate the 2-d pollutant transport model by recent actual data collected by specific measuring campaigns in SITE 1 which provides a data set independent from the former calibration data set.
Within this work package available state-of-the-art knowledge and formulations of transport and bio-chemical processes will be compiled and linked with the sedimentation and erosion processes from work package 1. These formulations will be implemented in detailed sub-models to be applied for the Scheldt estuary and river Elbe. The models will be verified with field data compiled from the Sub-projects BASIN and SITE. A thorough analysis of the model performance will be conducted comprising a sensitivity analysis of the various bio-chemical parameters and environmental conditions. The results will be used to derive generalised transport and fate formulations that will be applied in the generic exposure assessment model of work package 4. At a later stage the models can be used to predict the spatial distribution of deposited key toxicant (KEYTOX) for various hydrological scenarios and to evaluate alternative hotspot remediation strategies resulting from DECIS.