The main source of water in the region was wells drilled into the local alluvial aquifer, which was fed by infiltration from the river bed, return flows from irrigation, and losses from the municipal water supply and sewerage systems.

The aquifer potential fell short of demand, as a result of which overpumpage took place, causing intrusion of saline water into the aquifer and pollution of the groundwater.

River flows were used principally for irrigation (65- 70%). The supplementary volume required for supply of potable water for the target year of 2020 was about 80 million cu.m, with a mean discharge of 2.8 cu.m/sec.

During some months of the year there was no water in the river and irrigated agriculture cannot be sustained.

Two alternatives were examined for utilization of storm-water flows:

• Construction of a reservoir with a capacity of 120 million cu.m to receive surplus flows from Río Chillón between January and April, which would otherwise reach the sea unutilized.

• Drilling of a series of wells for artificial recharge and production along the Río Chillón channel in the vicinity of the San Antonio reservoir.

In parallel, suitable sites were located for construction of river diversion works and a treatment plant. In view of the apparent shortage of water in the Río Chillón basin, the possibility was examined of utilizing neighbouring basins such as those in the Alcay and La Viuda mountain ranges. A preliminary analysis indicated that a total average discharge of about 2.5 cu.m/sec could be obtained from eight small basins by means of small canals and tunnels.

Work included design of 70 km of main transmission pipelines, as well as siting and design of a new hydrometric station. Eight rain gauging stations and two meteorological stations would be constructed and furnished with modern equipment.