The Ostian aqueduct was built during the early Empire, not later than the reign of Caligula. It was rebuilt in the early second century AD. The visible remains belong to the Severan period. 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). It must have been some 14 kilometres long, starting at a height of some 13-14 metres above sea-level, and going to Ostia with a decline of c. 0.40 m. per km.
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 of the aqueduct have been preserved in the mediaeval borgo of the modern village Ostia Antica. It 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 and a hexagonal tower.
The aqueduct arrived at Ostia at a height of approximately 8 metres above sea-level. It will have taken at least 260 litres per second to the city, and perhaps as much as 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, as can be deduced from the height of waterproof opus signinum. The reservoir could contain 720.000 - 785.000 liters. It was supported by 19 small rooms to the west. The connection between the reservoir and the aqueduct has not been preserved.
In the reservoir Domitianic brick stamps were found, indicating that it was built in the late first century AD. According to 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 to feed many reservoirs, and only one outgoing lead pipe was connected to it. She maintains that it was not the main distribution reservoir, the castellum aquae. The reservoir may have been filled with water to be used in times of shortage and regulated the pressure. Perhaps the castellum was above the reservoir or to the east of the city wall. A small basin on top of the city wall, at the point where the aqueduct arrived, might be related.
An inscription from 76/77 AD, found in the unexcavated south-east part of Ostia, near the city wall, mentions work on the aqueduct:
IMP(erator) CAESA[r Vesp]ASIANV[s Aug(ustus) pont(ifex) max(imus) tribibunic(ia) potestat(e)]
VIII IMP(erator) XVI[II p(ater) p(atriae) co(n)s(ul) VII design(atus)]
AQVAE DVCTVS IN COLONIA Os[t(iensi) (e.g. specum novum sua impensa adiecit)]
From the inscription has been deduced 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. The inscription may have been attached to a secondary gate. 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. Near Porta Marina the gutter (specus) was at a height of c. 7.50 m. A curious stretch of aqueduct is found to the east of the Terme del Foro.