The artificial lake of Itaboraí, Rio de Janeiro State, Brazil.

The artificial lake of Itaboraí, Rio de Janeiro State, Brazil. Benedicto H. Rodrigues-Francisco National Museum-UFRJ ************************************** In Rio de Janeiro, as result of its lithodiversity, it is very important to monitor the quality of water and soil composition. Certain problems have already been detected, such as a high fluorine content in certain areas. The geological map of the state enables risk areas to be previously selected as a result of the diversity of the lithology. In this way, areas of alkaline rocks constitute potential sites for fluorine concentration, which has been proven by the preliminary geochemical survey of the state of Rio de Janeiro. The Geo-medical Project of the Stratigraphic and Environmental Geology Sector of the Geology Department of the National Museum - UFRJ is aimed at establishing preventive control programs for toxic elements, by selecting areas with the highest risk potential, working together with the authorities in the health area to monitor possible occurrences of sicknesses related with anomalies in the soil and/or river, lake and underground water. As a preliminary pilot area, the region of Itaboraí was selected, where there is a high rate of infant mortality. The lake of Itaboraí (São José) was formed when the Mauá cement industry left the quarry that it had been operating for more than 50 years, in 1983. Since then, groundwater has slowly occupied the open space left by the quarry of Tertiary limestone of the Itaboraí Formation. Currently, about one-third of the artificial pit opened by the mining has now filled up with water, forming an artificial lake. In order to try and solve the chronic water supply problem in the region, the City Hall allowed water from the lake to be utilized for distribution to houses in the surrounding region, since 1993. At the time, the dissolved calcium carbonate content of the water was not known, but it could have been foreseen that it would be reasonably high, considering that in addition to the sedimentary limestone, there is a marble lens located a few meters below the bottom of the lake between the gneisses comprising the bedrock. After a few years, an increase in the carbonate content can be expected, bearing in mind the solubility of carbonate rocks, principally in tropical climates. In addition, clays contained in the marls of the Tertiary sedimentary sequence may release chemical elements that are potentially toxic to humans. We should add to this situation the fact that fishing is allowed in the artificial lake, and that the fish are consumed by the population without knowing whether they are contaminated by any chemical elements harmful to humans. In order to find out the real extent of the problem, an immediate study of the chemical composition of the water distributed to the population is required, plus that of the fish consumed. The analyses carried out have already proven contamination by fecal coliformes in the lake water in quantities above the limits allowed by Brazilian and international standards.