Lake Cajititlán is an endorheic tropical lake located in the state of Jalisco, Mexico, which has suffered the consequences of high levels of eutrophication. This study assessed the presence of heavy metals and metalloids in Lake Cajititlán to ascertain possible risks to its aquatic biota and the environment. Eleven monitoring sites were selected throughout the lake; from each site, one sample was taken from the sediments and another from the sediment-water interface by using an Ekman dredger and a Van Dorn bottle, respectively. The measured metals in each sample were As, Cd, Cr, Hg, Pb, Zn, Cu, Ni, Fe, Mn, and Al. The results showed the following sequence of heavy metal concentrations Al > Fe > Mn > Zn > Cu > Cr, wherein Al had the highest average concentration and Cr had the lowest. As, Cd, Hg, and Pb were practically undetectable. Because the predominant rock in the lake basin is volcanic tuff and the soil is vertisol, the high quantities of Al and Fe suggest the weathering of the basin's minerals. The analyses of the sediment-water samples contained small amounts of dissolved Al, Fe, and Mn. According to the Håkanson equations with Hg, Cd, As, Cu, Pb, Cr, and Zn, the calculations of the contamination degree and ecological risk revealed that the presence of metals and metalloids does not present a potential risk to the aquatic biota; nonetheless, the water is not suitable for local human consumption due to an unrelated factor associated with nutrient and bacteriological contamination. The results show that heavy metals in the sediments were dispersed throughout the lake mostly because of the weathering of minerals from the local basin and not because of the punctual discharges of the pollutants from the municipal wastewater treatment plants (WWTPs). However, to ensure that the small amounts of dissolved metals (Al, Fe, and Mn) do not affect the aquatic biota, the fish species and phytoplankton need to be internally analyzed.
Keywords: Ecological risk assessment; Endorheic basins; Shallow lakes; Subtropical lakes; Wastewater pollution.