Water quality in the Río Lerma, Mexico: An overview of the last quarter of the twentieth century

The Río Lerma basin is the most important watershed in the Central Plateau of Mexico. Major urban, industrial, agricultural and livestock regions are located in its catchment area. Regarded as a center of endemism for its fish fauna diversity, it is also the most polluted watercourse in Mexico. This study assesses spatial and long temporal variations in water quality over the last 25 years with two approaches: the use of a water quality index multiplicative and weighted (WQI) and a principal component analysis (PCA). The general rating scale for WQI range on a 0-100 with 100 indicating highest water quality. WQI scores ranging from 26.53 to 67.44 denote Rio Lerma water is not fit for drinking, requires treatment for most industrial and crop uses, and is suitable for coarse fish only. Navigation is impracticable and inexistent. PCA shows the monitoring stations arrayed along a set of environmental parameter gradients. Several endemic fish species have been lost: two silver side food fishes are extinct, two more species (one of them a food fish) are endangered, and another three are threatened.