Coatepeque (on the E slope of Santa Ana Volcano), Ilopango (inside Ilopango Caldera) and Chanmico (associated with San Salvador Volcano) are volcanic lakes in El Salvador, showing a thermocline at a relatively shallow depth (from 30 to 40 m, from 20 to 40 m and from 5 to 15 m depth, respectively) and anoxic conditions below 33, 24 and 4 m depth, respectively. The Na +-Cl − composition of the Coatepeque and Ilopango lakes, displaying TDS values up to 1226 and 1216 mg/L, respectively, is likely due to hydrothermal fluids that feed these two lakes, as also confirmed by Cl − /Br − molar ratios ≤ 650, high As, B, Li and Si contents and Cl − /SO 4 2− ratio N 1. The Mg 2 +-HCO 3 − water composition of Lake Chanmico, whose TDS values were between 566 and 856 mg/L, suggests water-rock interaction processes with mafic/ultramafic rocks variably affected by serpentinization processes, which produced high Mg 2+ , Si and B concentrations. Waters at depth were characterized by the presence of CO 2 from an extra-lacustrine source, as suggested by δ 13 C-CO 2 values significantly less negative than those typically related to biogenic processes, albeit this gas was found in smaller quantities when compared to those recorded in other meromictic lakes hosted in quiescent volcanic systems (e.g. Lake Kivu in DRC, Monticchio, Albano and Averno lakes in Italy, Hule and Rio Cuarto lakes in Costa Rica, Lake Pavin in France). The occurrence of CH 4 , whose concentrations in Lake Chanmico were up to two orders of magnitude higher than those recorded in Coatepeque and Ilopango lakes, suggests bacterial methanogenesis.
