[4] The water is fresh and acidic (pH 5.3), has a high temperature (26°C), is rich in organic compounds and oxygen, and has a good bathing history.

[5][6] The reddish-brown appearance of the waters is a consequence of the high concentration of salts such as iodine, in addition to the oxides, fulvic and humic acids, dissolved in the lagoon that are the results of the incomplete decomposition of organic material such as leaves and twigs.

The sand barrier is rebuilt by wave action and the lagoon returns to its normal water level within a few weeks.

[9][10][6] Located in the drainage basin of Rio das Ostras,[8] with a perimeter of 2.65 km, a length and width of 0.63 and 0.71 km[1] and an average depth of 1.5m,[11] the lagoon is positioned in the middle of the depression into which the rainwater and groundwater flow, forming the reflecting pool of the Iriry lagoon.

The relief of the lagoon area was shaped by the movements and oscillations of the sea level over the last 10,000 years, together with ocean currents, and more recently, by the action of winds.