Evaluation of sediment contamination and trophic state in the La Purisima water reservoir (central Mexico)

Abstract. La Purisima is a water reservoir integrating one of the largest hydrological systems in central Mexico. It provides water to irrigate a vast agricultural region, recharges the aquifers and allows fishing activities. Since the last decades it has been strongly impacted by mining, urbanization, and agriculture. The aim of this study is to assess the level of contamination and to determine the trophic state of the lake. Bottom sediment samples were collected to quantify total concentrations of As, Cu, Pb, and Zn. Texture, organic matter (OM) and total nitrogen (TN) were also determined; carbon/nitrogen ratio was calculated. Samples of water were taken to characterize the hydrochemistry. Trophic state indexes (TSI) were calculated from the Secchi disc depth and total phosphorous concentration. Geoaccumulation indexes (I_geo) with respect to the metal content of the upper continental crust were used to discriminate between natural and anthropogenic sources of provenance. OM and TN contents and C/N ratios indicate that phytoplankton is the dominant source of OM. The Geo-accumulation indexes of the analyzed elements reflect an anthropic accumulation of As in the sediments. Water of La Purisima lake are diluted and the dominant composition is bicarbonate-calcium type, probably as a result of silicate weathering. The TSI calculated from DS and TP indicates that the lake is mesotrophic/eutrophic/hypereutrophic, which is a sign of an important contribution of nutrients from the hydrological basin. This study provides important information for understanding the potential risk of accumulation of toxic elements and nutrients in the lake.