The Ostian aqueduct was built during the early Empire, not later than the reign of Caligula. The aqueduct started in the Monti di S. Paolo di Acilia to the east of Ostia, or a bit further on, at the Malafede basin (the Malafede is one of the larger tributaries of the Tiber).
Various remains of the aqueducts that supplied Ostia, Portus and Laurentum (and perhaps private estates) have been found. In the 1990's the remains of two parallel aqueducts were found at Malafede-Infermeria. Remains of the Ostian aqueduct can furthermore be seen on a map of the environs of Ostia by Eufrosino della Volpaia, from 1547. Above the mediaeval village two rows of arches can be seen on the map. One belongs to the aqueduct, the other carried the road to Rome through the marshes that were present here. A stretch of the latter row was excavated in the 1990's near Acilia. Arches have been preserved in the mediaeval borgo of the modern village Ostia Antica. The aqueduct reached Ostia 120 metres to the south of the Porta Romana. Here remains of the arches were found, to the east of the first century BC city wall.
The aqueduct was at a height of 8 metres above sea-level. It will have taken at least 260 litres per second to Ostia (and perhaps 400). A large reservoir with four compartments, covered by barrel vaults, was set against the west side of the city wall. The total length of the reservoir is 57 meters, the width 6 and the height 5. The water reached a height of approximately 2 meters. The reservoir could contain 720.000 - 785.000 liters. It was supported by 19 small rooms to the west.
In the reservoir Domitianic brick stamps were found, but according to Gemma Jansen it could not supply the entire city after the raising of the level of the city under Domitian and Hadrian: the level is too low and only one lead pipe is connected to it. She maintains that it was not the main distribution reservoir, the castellum aquae. Perhaps the castellum was above the reservoir, and is now lost. The reservoir may have been filled with water to be used in times of shortage: a room at the south end contained a water-wheel, with which the water in the reservoir was taken to a higher level.
An inscription from 76/77 AD, found near the Porta Laurentina, suggests that during the reign of Vespasian the republican city wall became a branch of the aqueduct and supplied the south-west part of the city:
IMP CAESA[r Vesp]ASIANV[s Aug. pont. max. trib. pot.]
VIII IMP XV[I (vel II) pp cos VII (vel VIII)]
AQVAEDVCTVS IN COLONIA O[stiense ...]
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See the description of Nymphaeum III,VI,4 for a reconstruction drawing of the aqueduct on the city wall. Cistern IV,VIII,2 was also connected with the city wall.
