Evolution Of Noble Gas And Water Isotopes Along The Regional Groundwater Flow Path Of The Konya Closed Basin, Turkey

Abstract

Noble gas and water isotope compositions of regional groundwater were investigated along two transects in the Konya Closed Basin (KCB) of central Turkey. According to the He-3/He-4 versus Ne/He plot of samples, crust (up to 86%) and mantle (up to 26%) appear to be the primary and secondary sources of dissolved He in groundwater, respectively. After the beginning of both transects where the flow domain is confined, both He-3 and He-4 accumulate steadily in groundwater. Thereafter, the intermediate recharge from the surface in the unconfined part of regional flow system disrupts the steady accumulation trend of He-3 and He-4. Effect of intermediate recharge on regional groundwater is also indicated by the spatial variations in specific conductance, temperature, water isotopes, and tritium signals. At the last part of the flow domain where the system becomes confined again He-3 and He-4 start to accumulate in the eastern transect whereas they continue to decline in the western transect probably because of degassing to atmosphere through thinner confining unit. The 4 He accumulation rates based on radiocarbon ages and He-4 concentrations along both transects between the mountain flank and Obruk Plateau are in agreement with literature values but differ from each other. The accumulation rate along the eastern and western transects are 4.93 x 10(-11) cm(3) STP/g.year and 1.99 x 10(-10) cm(3) STP/g.year, respectively. These values reveal that the He-4 accumulation rates may differ at least four times at different parts of a single aquifer, which are located 30 to 60 km apart. It is understood that any groundwater age-dating attempt by He-4 requires a firm pre-assessment of spatial He-4 accumulation rates. He isotope signal in the groundwater of KCB proves the mantle and/or crustal gas input into the carbonate aquifer in which an on-going hypogenic karst development has been suspected.