We detected arsenic near the Tolimique Dam, an archaeological ecotourism area in Aguascalientes, México. Fortunately, the concentration range of this metalloid (0.001 to 0.003 mg L−1) did not exceed the total arsenic in water allowed by NOM 127 SSA1 1994 (0.025 mg L−1). However, because arsenic is a high risk priority pollutant at low concentrations in aquatic ecosystems, we studied the acute toxicity, bioaccumulation, and endocrine disruption of arsenic using native species of Rotifera (Brachionus calyciflorus Pallas, 1766, Brachionus rubens Ehrenberg, 1838, and Lecane cornuta Müller, 1786), Oligochaeta (Aeolosoma hemprichi Ehrenberg, 1838), and fish (Oreochromis nilotica Linnaeus, 1758). We found 20 species in the Tolimique Dam, including 13 rotifer species, 4 cladocerans species, and one of each of the following species: copepods, oligochaetes, and fish. Environmental Risk Assessment (ERA) was performed using the Predicted No Effect Concentration (PNEC) and the Measured Environmental Concentration (MEC) of arsenic. In conclusion, arsenic produced celular necrosis in A. hemprichi at a concentration of 5.0 mg L−1 and endocrine disruption in B. calyciflorus at a concentration of 0.05 mg L−1. The detection of arsenic at an average of 2.05 ± 0.30 mg Kg−1 in O. niloticus fish suggests a probable biomagnification of arsenic.