Origin of halogens and their use as environmental tracers in aquifers of Thriassion Plain, Attica, Greece

The processes that control halide concentrations in coastal aquifers of the Thriassion Plain in Greece include both natural and anthropogenic factors as follows: evapotranspiration, dissolution of evaporites, domestic sewage as well as dissolution of minerals containing fluoride. Groundwater monitoring data indicate that the Cl⁻/Br⁻ mass ratio ranges between 43 and 1510. Ratios ranging round the value 292 are attributed to sea intrusion; 300–600 to domestic sewage; and 900–1510 to dissolution of evaporites. The evaporation of irrigated water plays an important role as well. Fluoride concentrations ranging from 0.1 up to 9.94 mg/l possibly originate from Holocene as well as Plio–Pleistocene sediments which contain organic material. It is also likely that fluoride originates from fluorite dissolution or comes from Palaeozoic-Middle Triassic volcano-sedimentary rocks which occur around the wider area and constitute the basement of the study area. In those rocks, the alteration of biotite to chlorite takes place under the influence of hydrothermal fluids, as the Thriassion Plain is close to South Aegean volcanic arc where volcanic activity exists.