ULTRA-VIOLET ABSORPTION AS A TRACING TECHNIQUE FOR SEWAGE DISPERSION AND WATER MASSES IN 2 RED-SEA COASTAL LAGOONS IN JEDDAH

The u.v. absorbance is measured in water samples, collected on six successive cruises made within 1 year, from two Saudi sewage-affected fjord-like lagoons in Jeddah (Arbaeen and Reayat Al-Shabab). The hydrochemical elements, salinity, phosphate and ammonia are also measured. The horizontal and vertical distribution pattern of u.v. absorbance and that for the other hydrochemical elements are found to coincide with each other to a great extent. The direction of the discharged wastewater spread at the surface can be easily identified. Three distinct water layers are observed. The surface layer is diluted by sewage effluent containing high values of u.v. absorbance, phosphate and ammonia. The intermediary layer has almost the same properties as the water outside the lagoon mouth at the same level where it is less diluted (i.e. less polluted). The third layer at the bottom (old water) where the water is dammed by the sills is more saline. It has exceptionally high values out of the four elements studied. This situation prevails almost throughout the year except during the intermittent flushing periods in winter and early spring, when the stratification is disturbed. The advantage of using u.v. absorbance measurement is that it can be read directly in the water samples without any chemical pre-treatment and so no special training is needed.