Half of Nairobi’s population lives in slums, but they consume only 34,500 m3/day, corresponding to less than 20 litre per capita per day. The heavily polluted and relatively small Nairobi River that flows through the city is not used for drinking water supply. The Nairobi water utility relies almost exclusively on surface water to satisfy the growing city's water needs. Surface water supply for Nairobi stood at 484,500 m3/day in 2010. Reservoirs have sufficient storage to supply the city during a normal dry season, but during extended droughts water supply can be lower than average. The above figure is for 99% supply reliability. At 90% reliability available water supply is higher at a total of 577,000 m3/day. However, storage capacity in reservoirs is gradually reduced through sedimentation. The Nairobi utility receives 94% of its water from the Tana River basin north of the city through three reservoirs: the Sasumua Dam on the Chania River, the Thika Dam and the Chania-B Dam. The Thika Dam is the largest, supplying 225,000 m3/day. Water from the reservoirs is treated in two treatment plants, including the largest one in Ngethu. The remaining 6% comes from local sources: the Kikuyu Springs and the Ruiru Dam, both located in the Athi River Basin and whose water is treated in two smaller treatment plants. The spillway of the Sasumua Dam has been badly damaged during the 2003 El Nino floods. The spillway was rehabilitated between 2009 and 2011 with financing from the French Development Agency at a cost of 65m Euro. Groundwater supplies an additional 85.000 m3 per day or more from an estimated 3000 boreholes, up from an estimated 2250 boreholes in 2001. The groundwater table has declined; in one well it declined by 40 meters between 1958 and 1996. The average depth of new wells in 2001 was 238 meters. In that year 97 new wells were drilled because of a drought. Most wells are operated by industrial enterprises, hotels, farms for flower production in greenhouses, and private houses in parts of the city that receive only intermittent supply (e.g. Langata and Karen). Groundwater is also used to irrigate gardens and to supply tankers that resell the water. Many private well owners are also connected to the mains water supply network and use groundwater as a back-up supply. Natural groundwater quality is good.