 Untreated wastewater is also disposed of in surrounding rivers that run to the sea, but this runoff water polluted with untreated waste water is also used for irrigation as farmers have found that the high concentration of nitrogen and phosphorus nutrients effectively fertilize their crops and increase crop yields.Though there is a national norm regulating pollution limits in untreated waste water, CONAGUA reported that the Valley of Mexico basin had 50% heavily contaminated, 25% contaminated, 20.8% acceptable, and only 4.2% excellent water quality based on Biochemical Oxygen Demand (BOD) levels sampled at selected sites.Leaks in the distribution system are a major cause of concern for drinking water quality.