The Edremit geothermal field, with 42-62 degrees C discharge temperatures, is utilized for space heating. Alternation of permeable and impermeable units created two superimposed aquifers in the area: an upper unconfined and a lower confined. Water samples from 21 (hot-cold) wells were taken in this study. 8 of these wells penetrate the deeper confined aquifer, while 13 penetrate the shallower unconfined aquifer. Geochemical analyses revealed that Na + K - SO4 (>40 degrees C), Ca - HCO3 (<30 degrees C) and Ca - SO4 (30-40 degrees C) waters occur. delta O-18-delta D compositions point to a meteoric origin for all waters, while C-14 analyses suggest longer subsurface residence times for the hot waters, compared to the cold/warm waters. Chemical and isotopic compositions indicate that mixing and water-rock interaction are the possible subsurface processes. When silica and cation geothermometers are evaluated together with fluid-mineral equilibria calculations, a reservoir temperature range of 92-150 degrees C is evaluated. Saturation indices do not indicate a serious potential of scaling in the field. The hydrogeology of the study area is highly affected by faults. Infiltrated meteoric water percolates (down to 4.5 km depth) via deep seated step faults, becomes heated and ascends to the surface at the low lands, especially through intersection of buried, mid-graben faults. During its ascent, geothermal water invades the two superimposed aquifers where mixing between the hot and cold waters takes place. (C) 2013 Elsevier B.V. All rights reserved.