The first two streams reach the Vallerano downstream of the hamlet il Castellaccio (after Via Cristoforo Colombo), a few hundred meters before it flows into the Tiber.

Its peculiarity derives from the presence, in the eastern part, of the Acqua Acetosa Ostiense archeological area and, at the south-eastern edge, of the San Paolo mineral water source.

The area shows a modest forest cover, modified by the intense agricultural use and by the surrounding urbanization, but with good chance of recovery.

In 1976 the village and the nearby necropolis were protected with the establishment of the archaeological area of Acqua Acetosa, which however is not part of the Nature Reserve.

This patch of land of a few hectares is actually a crucial knot to untie for a coherent development of the Reserve, since, in addition to Roman antiquities, it includes the 17th-century casale of San Sisto, the old bottling plant of Fonte San Paolo, the Judicial Deposit and an unofficial landfill.