GEOCHEMICAL CHARACTERIZATION OF GROUNDWATER OF DISTINCTIVELY HIGH ALKALINITY AND CO2 PRESSURE FROM AN UNCONFINED AQUIFER IN A VOLCANIC TERRAIN: PUEBLA VALLEY, MEXICO

We report on a regional hydrogeochemical study of an area located in the Puebla Valley in the central part of the Trans Mexican Volcanic Belt. At least 8 families of groundwater can be distinguished. Among these is a distinctive subgroup identified by its high alkalinity, associated with high CO₂ content. Puebla Valley is delimited by 3 large volcanos: Mt Popocatepetl (newly active since 1994), Mt. Iztaccihuatl (no longer active) and Malinche (dormant). A major populated area (Puebla City with 1.5 million people) is located within the Valley. The area is a semi-arid region with an average precipitation of 900 mm/y, and an average elevation of 2100 masl.

The reported samples come from a pumping well system drilled within an unconfined aquifer formed mainly of Quaternary volcanic and alluvial deposits from surrounding volcanoes. Among the 8 families of groundwater one is characterized by the presence of Ca-Mg-Na-HCO₃ as predominant major ions. Furthermore, based on major ions, trace elements and stable isotopes, we can clearly distinguish a subgroup (Atlixcayotl-Cholula subgroup) within this family that is notable for its high alkalinity, high CO₂ pressure, relatively high B, Li, Sr and relatively good correlation between Mg and HCO₃^- (r²=0.92). Three possible sources for CO₂ are considered: Anthropogenic contamination, ancient lacustrine deposits (organic matter decaying) and CO₂ released from a magmatic body. The former possibility is excluded by the low NO₃^- concentration in these waters; also, wells are deep enough (around l80 m) to make it an unlikely possibility. However, the depth of the wells make the second alternative possible, since the wells may be extracting water from deposits rich in decaying organic matter, but there is not strong evidence of reducing conditions. Trace elements (mentioned above) associated with igneous rocks and minerals may suggest a magmatic origin. d¹⁸O and deuterium analyses suggest that the Atlixcayotl-Cholula subgroup is recharged from a slightly lower elevation than the other samples within its family group. Analyses are in progress to determine the amount and isotopic composition of the following gases dissolved in these waters: CO₂, He, Ar, and Ne. These determinations will help identify the source(s) of the gas being released and will also help to confirm the recharge area.