Median THg concentrations from the three untreated wastewater discharges to Lake Zapotlán were high (265.4 ng/L, range: 17.5–942.3 ng/L, n = 13) and largely associated with particles (89%; Fig. 2). Median unfiltered THg concentrations in the water flows from the C. Guzman wastewater treatment plant (363.5 ng/L, range: 133.8–942.3, n = 9) were an order of magnitude higher than those MPC 6827 from the less urbanized municipality of San Sebastián del Sur (27.7 ng/L, range: 17.5–64.2, n = 4). A one-time sampling of inflow and treated outflow water from the primary body largest C. Guzmán treatment plant demonstrated high THg removal efficiencies for both particle-bound (97–98%) and dissolved THg (82%) with effluent THg concentrations < 10.0 ng Hg/L. This difference certainly suggested that, on the whole, bypassing untreated wastewater dominated the volume of the channel conducting flows from the C. Guzman wastewater treatment plant.