Identification of the mixing processes in brine discharges carried out in Barranco del Toro Beach, south of Gran Canaria

The main environmental problem of the industries dedicated to the desalination of seawater is the brine discharge coming from the processes of desalination of seawater by means of reverse osmosis. Ionics Ibérica S. A. develops its industrial activity around the desalination of sea and brackish water to obtain potable water and the brine effluents produced by this activity are discharged into the sea. The necessity to carry out these discharges in a way that respects the environment led to in the month of August 2.000 that the Department of Quality, Industrial Security and Environment of Ionics Ibérica, S.A., designed a campaign with the objective of determining what are the effects of the brine discharge in the marine environment. That is to say: pursuit of the brine volume once it is discharged into the coastal environment; quantification of the mixing processes of these brine discharges; identification of the effects produced by the discharge on the fauna and flora of the area, with special attention to the presence of seagrass prairies on the sandy seabeds, “cebadales” (generally associations of two species: Cymodocea nodosa and Caulerpa prolifera) of great biological importance on the sandy seabeds of the coasts of the Canary Islands.

The study of the brine discharge produced by the desalination plant Maspalomas II has been carried out on the basis of taking of samples according to three radials starting from the discharge area with the purpose of determining the dilution of the brine in distant areas, and on the other hand samples were taken in front of one of the outlets of the discharge to determine the dilution of the brine in areas nearby. Once represented, the values of salinity of the samples, the following conclusions have been reached: The dilution in nearby areas is high, taking place a dilution of the brine discharge from 75 psu to 38 psu at some 20 m away from the discharge outfalls outlet. The dilution in more distant areas takes place in such a way that the salinity in a radius of a mile stays practically in the normal values for seawater. The evolution of the mass of brine water in breakup, due to the dynamics of the discharge area, means the dilution takes place in a northeast direction, starting from the discharge area. This is mainly due to the bathymetry of the area and the presence of two outfalls of treated water that are to the southeast of the brine discharge area. The day the samples were taken was characterized by the extreme calm of the sea, such as tides, wind and surf, which allowed a correct sampling and the correct determina-tion of the position, but it is very reasonable to think that the dilution of the brine was much less that the one that would take place in normal conditions of more wind and surf. In a discharge like the one that we are studying that takes place in depths of around 8 m, the marine dynamics are very important for its dilution, that is why we believe that the dilution has been underestimated. The study of the distribution of the reflective turbidity clearly reflects the entrance of fresh water (less dense than the seawater) in the discharge. The dilution of this mass of water takes place on the surface and it starts to interact with the brine mass producing lower values of salinity in the area southwest of the brine discharge mixture. This interaction totally agrees with the high nitrogen values that were reflected in the analytics of the feed water, which we had supposed were of an origin of treated wastewaters discharged in the area. The values of salinity and pH of the samples taken in the bathing areas adjacent to the discharge are in the ranges settled down by legislation and normal for seawater. The values of salinity in the whole study area are very near what we could consider as the normal range of salinity for the seawater of the coasts of Gran Canaria. Therefore the current effects on the marine biological communities are nonexistent. We believe that the oceanographic conditions at the time of taking the samples allows us a very high margin of trust due mainly to the fact that we have taken dilution values below the real ones and inappropriate of the normal situation of the marine dynamics of the area. This way we are contemplating the most unfavorable case (from the point of view of the marine dynamics) that could take place in the future and in which the salinity of the area would be higher. We can be assured that the dilution of the brine in the future under normal conditions and of extreme calm would be sufficiently enough so as not to negatively affect the marine life and the quality of the bathing waters of the area. The flora and fauna of this area is seen to be affected by the discharge of treated wastewater as demonstrated in the underwater pictures. Paradoxically the patches of “cebadales” that we have found in perfect condition and in more extension were in the northeast area, where salinity values have been shown to be a little higher. (Although not at all dangerous. The range of salinity that these types of communities can endure is considerably wider as studies of the distribution of marine grasses in the Mar Menor, done by the Department of Ecology and Environment of the University of Murcia have shown.) In the area south of the discharge, due to the storms, the flooding from the Barranco del Toro and the treated wastewater discharge, the state of conservation of the seabeds is less, with reduced patches of “cebadales” which are less dense. In the sandy seabeds where there are prairies of C. nodosa and C. prolifera present, we find a vast quantity of young fish of different species and life is abundant. In general, in the rocky seabeds near the reef line the state of conservation is good.