Patos Lagoon: indicators of organic pollution

Abstract

Patos Lagoon is the largest receptor of fluvial waters used for agricultural, navigational and fishing activities in the state of Rio Grande do Sul. The dumping of industrial and domestic effluents into those waters is its worst polluting source. Aiming at assessing organic contamination, 16 points were sampled along the lagoon (winter and summer). Ammonium and BOD rates (chemoindicators of organic contamination) increased in summer in waters to the south of the lagoon adjacent to the cities of Rio Grande and São Lourenço and, more to the north, near Camaquã river. In the winter, the waters near the cities of São Lourenço and Tapes (located to the north of the lagoon) also presented increases in those parameters rates. Fecal coliform bacteria rates presented a similar pattern to that of chemical parameters, increasing in waters near the cities of Rio Grande and Pelotas (São Gonçalo Canal and Laranjal beach), both situated to the south of the lagoon. No contamination was verified in channel areas along the lagoon's axis, due to stronger hydrodynamics and the distance from urban contamination sources.An intrusion of saline water into the estuarine water at the south of the lagoon occurred in the winter. A marked stratification was defined in the water column, with oxygen subsaturation in bottom waters, due to resuspension of deposited sediment and release of semianoxic interstitial water. Although Patos lagoon is a national heritage of touristic, ecologic and economic importance, it has been degraded by effluents, urging their treatment and hydro-sanitary monitoring.