Diagnosis of water quality in a cascade system of dam reservoirs in the San Juan river, Querétaro, Mexico.

Abstract

In recent years, the number of man-made dam reservoirs has increased dramatically due to the urgent need to supply water, food and energy to the population. When in a fluvial system several reservoirs are located in the same basin, a cascade effect is produced due to the water release downstream, producing changes in water quality. This paper contributes information to identify water quality variations in a cascade system where three dam reservoirs regulate the quantity of water and, subsequently, its quality. Eight monitoring sites where located along 86 km of the main river channel, three to represent the conditions in the reservoirs and five those in the river segment before the Zimapán dam. At each site, measurements were made for 11 physiochemical parameters, five determined in-situ using multi-parametric equipment and six ex-situ using spectrophotometric methods. Bimonthly water collection samples were conducted from October 2004 to June 2006. Statistical analyses ( ANOVA, MANOVA, principal components and cluster) were carried out to determine temporal and spatial variations in the water quality of the system. The ANOVA analysis resulted in a significant positive correlation (p < 0.05) among conductivity, total dissolved solids, sulphates and phosphates. The MANOVA analysis determined that no significant differences existed between water quality in the river among dams and that in the reservoir sites. Five of the 11 parameters determined 73% of the variation. According to water quality criteria, its use is restricted to agricultural irrigation. The system presented enrichment identified as euthrophic and in the process of sedimentation, not only due to the entrance of nutrients but also because of the lifespan for which the dam reservoirs in the San Juan river, Querétero were built