In the 1920s, Xochimilco lost control of most of its water supply, taken by the city for its needs. The urban sprawl of Mexico City reached Xochimilco in the mid 20th century and it still affects the borough today. In the 1970s, the federal government began to replace the lost supply to the canals with treated water from the nearby Cerro de la Estrella. This is most of the water that now flows in the canals. The treated water is clear, but not potable due to bacteria and heavy metals. However, it is used to irrigate crops grown on the chinampas, even though the canals are further polluted by untreated sewage and garbage. The biggest threat to the canals and their ecosystem is uncontrolled sprawl, mostly due to illegal building on conservation land. These settlements are polluting canals with untreated garbage and waste, and filling in canals to make "new land." There are thirty-one illegal settlements in the historic center, with 2,700 constructions. Ten of these are in the chinampas of San Gregorio Atlapulco, San Luis Tlaxialtemalco, Santa Cruz Acalpixca, and Santa María Nativitas. The borough and UNESCO are at odds over what to do about the 450 hectares of illegal settlements. UNESCO demands their eviction, but the borough says this would be too difficult and better to legalize the settlements, putting efforts into preventing more.