The use of O, H, B, Sr and S isotopes for tracing the origin of dissolved boron in groundwater in Central Macedonia, Greece

The groundwater B concentration in Mesozoic karst, Neogene and alluvial aquifers in the West part of Chalkidiki province in Central Macedonia, Greece reaches 6.45 mg L-1, which exceeds the limit of 1 mg L-(1,) set by the European Union for drinking water. The high B contents have been detected in this area, not only near the shoreline, where seawater intrusion occurs, but also in the inland part of the basin. Multi isotope (H-2, O-18, S-34, O-18((SO4)), B-11, Sr-87/Sr-86) data from borehole and thermal water springs allow identification of the possible B sources. The B dissolved in groundwater in the Chalkidiki area is mainly geogenic. The low delta B-11 values, 0-1 parts per thousand, similar to those of thermal fluids from continental geothermal fields, and the low Cl/B ratio compared to seawater both indicate a geothermal origin for B and reflect deep circulation and interaction with igneous rocks. The Sr-87/Sr-86 ratio also indicates that the deep-aquifer granodiorite is the predominant rock source of Sr, while the shallow limestone unit has negligible effects on the dissolved Sr budget in these thermal karst waters which O and H isotopes show to be of meteoric origin. The main source of high B in borehole water is mainly mixing with B-rich geothermal water. The mixing between geothermal water and water from the Neogene aquifer is also reflected by isotopic contents of SO4. (C) 2010 Elsevier Ltd. All rights reserved.