Hydrogeochemical characterization and conceptual modeling of the Edremit geothermal field (NW Turkey)

Abstract

The Edremit geothermal field, with 42–62°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°C), Ca−HCO3 (<30°C) and Ca−SO4 (30–40°C) waters occur. δ18O-δD compositions point to a meteoric origin for all waters, while 14C 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°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 4.5km 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.