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The Montevideo coastal zone is located in the middle Rııo de la Plata basin (Ayup, 1986), on the east coast of South America. It covers an area of about 38,800 km2 and drains a 3,170,000 km2 basin, the second largest in South America. The Rııo de la Plata may be defined as a funnel-type coastal plain tidal river with a semi-enclosed shelf sea at the mouth. The mean annual river flow is 25,000 m3 sÀ1, with a maximum in June and a minimum in January (Nagy et al., 1997). Tides are semidiurnal with an amplitude of about 40 cm on the Uruguayan coast. Features such as salinity, depth of the halocline and vertical mixing vary with astronomic tidal oscillation on an hourly basis, while axial winds influence water height and salinity variations on a daily basis. The river flow governs monthly to interannual variations. Fine grained sediments (silty clays, clayey silts and silts) are confined to the upper and middle Rııo de la Plata, while sand covers almost the entire outer Rııo de la Plata and adjacent continental shelf (Lóopez-Laborde, 1997). Within the Montevideo coastal zone, Montevideo Bay, with an approximate area of 10 km2 and mean depth of 5 m, is an important region because it harbours the ANCAP refinery (petroleum Uruguayan refinery Company), the Batlle stream water plant (UTE, Electrician Uruguayan Company) and the Port of Montevideo. Three streams flow into Montevideo Bay, the Miguelete Stream, the Pantanoso Stream and the Seco Stream, the latter flowing through apipe (Fig. 1). These streams carry wastes from many different industries and urban centers, as well as from a great number of sewage sources. The entrance to the …