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The association between data obtained from ground penetrating radar (GPR) and from boreholes at the Itaipuaçú coastal plain (Maricá-RJ) revealed information regarding the evolution of this coast. It has been possible to recognize a paleobarrier-lagoon environment and its associated inlets. The results indicated that the sedimentary sequence above basement is about 3 to 4 meters thick on the old sandy barrier and it is 10 meters thick towards the younger sandy barrier. The sedimentary sequence contains several horizontal to subhorizontal reflectors in addition to seaward dipping layers 15 to 40º, similar to the present-day Itaipuaçú beach storm scarps. A bore hole trough the younger sandy barrier showed that the main sediment is coarse sand; however, at about 7,5 meters down the section an organic-rich lagoonal mud is present and 14 C dating indicated the age of 5.900-6.400 years before present for such lagoonal environment. The water table is just 1 meter below surface on the plain between the two barriers deepening to 4 to 5 meters towards the younger sandy barrier. Water collected from this reservoir presents good quality for human consumption.

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