A general review of the chemical changes which have occurred in the reservoir of Ahuachapan geothermal field is given with emphasis on the main
processes resulting from intensive mass extraction in twenty years of commercial exploitation. New data and several new geochemical interpretations have been incorporated into the chemical conceptual
model. The main discovery is that two different recharge fluids are moving into the well field. The main hot fluid recharge comes from the South near
the Laguna Verde volcanic complex and moves through the field mixing with cold water inflow from above mainly at the SE part. Due to the fault system,
the main recharge from the upflow zone reaches the SW part as undiluted fluid, as indicated by the salinity and geothermometer temperatures that are
almost the same as original values (near 9,000 mg/kg and 240oC respectively). In the central and NE parts of the reservoir the fluid is strongly affected by the above recharge water. The gas results suggests a deep
fluid contribution with maximum temperatures around 300oC. The increase in gas contents developed over the years suggests an open boundary
at the bottom of the reservoir. This means that magmatic fluid can ascend through fractures or the matrix of the basement rock. The reservoir processes
governing fluid chemistry seems to be dilution from above or lateral inflow of relatively cold water, boiling and a high temperature fluid recharge.

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