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