José A. Revilla

Flushing time as a descriptor for heavily modified water bodies classification and management: application to the Huelva Harbour.

Since the flushing time is a physical descriptor used to distinguish between different types of heavily modified water bodies (HMWB), the establishment of a methodology for its calculation becomes important. In order to achieve this task, a methodological procedure involving the tide mean value and variable river flow values is proposed. The hydrodynamics were assessed using a two-dimensional model which integrates the depth-averaged mass and momentum equations in the time and space domains and includes a wet-dry point treatment method. The hydrodynamic model calibration and validation were performed on the basis of tidal gauge and velocity current measurements. A reasonable agreement with the field measurements of water elevation and velocity were achieved. On the other hand, a two-dimensional mathematical model, which solves the depth-averaged advection-diffusion equation, was properly calibrated and used to evaluate the behaviour of a conservative tracer within a water body. The transport model calibration was developed according to the field survey data carried out during late spring when the rivers flows are low. This study allowed the flushing time estimation under four scenarios showing that only the estuarine mouth presents a high renewal rate because the current velocities are higher. For heavy rain periods, a flushing time decrease was observed as river flows modify the circulation in the main channel. Neglecting the river forcing was found to be valid for the dry period.

A model for predicting the temporal evolution of dissolved oxygen concentration in shallow estuaries

The design of sewage discharge systems in estuaries needs to consider the dissolved oxygen concentration among other water quality indicators. Due to the great number of factors affecting the dissolved oxygen, the prediction of the temporal evolution of this element requires the use of mathematical tools. In the case of shallow estuaries with extensive intertidal zones, the complexity of this task increases since the water domain varies continuously. This work describes a numerical model which solves the vertical integrated transport equation including the effect of extensive tidal flats. The model makes use of the tidal velocities computed by a previous run of a hydrodynamic model. This procedure allows the dissolved oxygen modelling to be performed using a larger time step than that used for the velocity field calculation. The numerical scheme developed for the model guarantees that even in the drying and wetting of intertidal areas the water column dissolved oxygen concentration is not modified by numerical errors. The representations of the processes included in the model (advection, dispersion and reaction) have been validated successfully in several theoretical cases. An application to the Urdaibai Estuary, a shallow estuary in the Basque Country, Northern Spain, is also presented.

The Prestige Oil Spill in Cantabria (Bay of Biscay). Part II. Environmental Assessment and Monitoring of Coastal Ecosystems

Abstract Assessment and monitoring activities constituted two main tasks of the emergency response system implemented by the regional government of Cantabria (north Spain) after the Prestige oil spill. The assessment covered four types of environmental units: estuaries, rocky shores, sand beaches, and subtidal areas, up to a 300-m depth. Monitoring procedures included the chemical quantification of total hydrocarbons and polycyclic aromatic hydrocarbons in water, sediments, and benthic organisms (clams and goose barnacles), and the analysis of structural changes in intertidal communities. Disturbance of oil cover was significantly more extensive in coastal areas, rather than in estuarine areas, where 50,000 m2 were directly covered by fuel. Otherwise, the presence of oil in subtidal areas was a rare event. Results from the bioeffects analyses were in agreement with the overall impact assessments, pointing to the coastal habitats as the areas where the bioavailability of toxic components from the Prestige spill presented a significant level of risk.

Baseline study of soft bottom benthic assemblages in the Bay of Santander (Gulf of Biscay)

The Bay of Santander constitutes one of the most important estuarine areas in northern Spain (Gulf of Biscay) which has been significantly stressed by sewage discharges. However, an improvement in the environmental quality of the bay is expected once the new sewer system comes into operation. The evaluation of such an evolution needs to be contrasted with the current situation, established in specific baseline studies. For this purpose, a field study was carried out on the soft bottom assemblages of the estuary in spring of 1998. At each station, the benthic fauna abundance, grain size, organic matter, carbon, nitrogen, phosphorus and heavy metals concentrations were analysed. The community structure and the physical and chemical characteristics of the sediment reflected the higher impacts both near the points of discharges and in the low water exchange zones, whereas those areas showing higher tidal flows presented an acceptable quality. High heavy metal concentrations for Cd, Hg, Pb and Zn indicated contamination of industrial sources in several areas of the Bay. This information allows to establish a reference situation in order to improve the design of a specific monitoring program for this area.

Assessment of susceptibility to pollution in littoral waters using the concept of recovery time

Susceptibility to pollution can be related to the flushing capacity of aquatic systems. Transport time scales constitute a useful tool for representing the water exchange and transport processes. A new transport time scale, recovery time, and a methodology to estimate it by means of numerical models is hereby developed. Recovery time, calculated in Gijon, Santander and Tarragona harbours, is significantly related to physical, chemical and biological water quality indicators. Susceptibility, assessed through recovery time values, provides spatial patterns of expected flushing capacity, being sensitive to physical and hydrodynamic characteristics. The developed method is appropriate to estimate recovery time and assess susceptibility against pollution in littoral waters having great potential to be applied to different disciplines. Recovery time could be used in littoral waters as a surrogate of water quality indicators, to establish efficient monitoring programs, to define and characterize modified water bodies or to improve the design of marine infrastructures.

Coastal outfalls, a sustainable alternative for improving water quality in north-east Atlantic estuaries.

The city of Santander ceased the discharge of sewage effluents into the bay of Santander in June, 2001 and began discharging at a site 2.4 km offshore in the nearby coastal area (Virgen del Mar, Bay of Biscay) at a water depth of about 40 m. The present study investigates the effects of the new outfall discharges on the water quality of the high-energy coastal area and the recovery of the perturbed temperate estuarine area now only affected by combined sewer overflows (CSOs). Nutrients, phytoplankton biomass and urban pollution indicators were analysed. No significant spatial or temporal change in water quality variables was found in the coastal area around the outfall. No signs of nutrification or increases in chlorophyll-a were observed throughout the study period, although a slight increase in phosphates, suspended solids and turbidity were observed two years after the relocation of the discharge. These changes were not attributed to outfall discharge but to a regional increase also observed at control stations and nearby coastal areas. Considerable reductions in indicators of urban discharges were observed in the estuary after the relocation of discharges, even at stations located around CSOs. Results from this study support the efficiency of ecological quality-driven designs of sanitation systems, which are used as management tools for sensitive and environmentally valuable coastal ecosystems in the north-east Atlantic.

ENVIRONMENTAL DESIGN AND MONITORING OF LARGE SUBMARINE OUTFALLS: AN INTEGRATED APPROACH FOR COASTAL PROTECTION

In order to address the environmental impact produced for decades to the water bodies, the authorities of the major coastal municipalities and counties in North Spain (Gulf of Biskay) initiated, during the 90‘s, the design and construction of new sanitation systems, according to the legal requirements imposed by the 91/271/EC Directive. Site planning, environmental risk assessment of discharges and objective-oriented monitoring were the three main tasks supporting the decision-making process related to the validation of the new sewerage projects. This process consists of four consecutive stages: 1) Design and selection of alternatives (strategic assessment), 2) Risk evaluation for different scenarios (impact assessment), 3) Verification of adverse effects (objective-oriented monitoring) and 4) Action plan for mitigation of registered impacts (management of pollution events).

Modeling heavy metal concentration distributions in estuarine waters: an application to Suances Estuary (Northern Spain)

Abstract Using numerical modeling as a tool to simplify management tasks is a less expensive complementary method to indirectly monitor estuarine zones. Nevertheless, information regarding the sedimentary dynamics and the pollutants present in the area, specifically in the sediment, must be known before a model can be used successfully. The two-dimensional transport model “SOLTOX” allowed the study of the fate and transport of heavy metals in estuarine waters, with particular application to the zinc evolution along the Suances Estuary (Northern Spain) which, historically, has suffered the effects of industrial pollution. As many pollutants can be adsorbed on suspended solids, advection–diffusion transport analyses were tackled to model not only the zinc but also the suspended solids concentration distributions. The importance of specific parameters related with the suspended solids transport modeling was also explored. Using the collected data during a whole tidal cycle for suspended solids and current velocities, a graphical method was applied to obtain the initial values for the calibration and validation of the model main parameters. The model was first calibrated against the field data for suspended solids. The obtained results exhibited the critical shear stress for deposition as the overriding parameter. The model was then used to investigate the zinc transport processes in the Suances Estuary. A reasonable agreement between modeled and measured data was achieved for the suspended solids and zinc. Moreover, the results demonstrated a dependence on the background concentration of the pollutant in the sediment. However, the background concentration in the water column had little effect on the model results.Graphical Abstract.