Estuaries constitute the parts of the ocean where the greatest salinity gradients are found. Changes in salinity cause a sharp variation in the ionic strength within a relatively few kilometres, which produces a strong level of reactivity for many chemicals transported by rivers. In this way, estuaries act as a chemical barrier for many chemicals, regulating their residence time in the ocean. So, many investigation projects about estuaries are being developed actually. Without denying the need to work on site (at field) for this kind of studies, it is possible to significantly reduce the amount of time and money spent on field samplings by employing laboratory simulations.
A dynamic laboratory simulator of estuaries has been designed and developed to reproduce the process of continuous mixing of fresh and seawater in a vertically and laterally homogeneous estuary. Residence times similar those of real estuaries can be obtained by choosing conveniently operation conditions. So, the device is able to generate a large number of longitudinal salinity gradients at stationary state as well as to simulate the tidal effect on the estuary. With this laboratory dynamic simulator it is possible to establish the behaviour of synthetic chemicals before their entry to real estuaries as well as it supports environmental management testing alternative solutions to assess their efficiency in the presence of pollution episodes (real or hypothetical). So, the laboratory simulator appears like a powerful tool to the environmental risk assessment in the estuarine systems.
A set of environmental applications of the simulator has been developed to show its possibilities. So, the behaviour of inorganic carbon along different salinity gradients has been studied. The quantified CO2 fluxes water-atmosphere ranged between 95-80 mmol m-2 d-1 in the fluvial zone of the estuary and 10-25 mmol m-2 d-1 at the end of the estuary. These values are similar to the same found in natural estuaries. Tidal variations at CO2 fluxes also were registered. By other hand, nutrient reactivity has been characterised, putting special attention in the laboratory verification of apatite formation from dissolved inorganic phosphorus. In other set of laboratory assays, effects of a episode of acute pollution by heavy metal on a population of estuarine clams have been assessed. Behaviour of linear alquilbebzene sulphonate (LAS), an anionic synthetic tensioactive, has been studied during its transit along different salinity gradients appearing, in all cases, biodegradation of LAS and sulphophenylcarboxylic acids (SPC) formation. All these obtained results show the usefulness of the built simulator.