Using 81Kr and isotopic tracers to characterise old groundwater in the Bangkok metropolitan and vicinity areas.

Krypton-81 was applied to investigate the age of groundwater in the aquifer system in the Bangkok metropolitan and vicinity areas. Stable (²H, ¹⁸O and ¹³C) and radioactive (³H, ⁸⁵Kr and ¹⁴C) isotopes and noble gases were applied in parallel. Low levels of ¹⁴C and significant radiogenic ⁴He confirm that groundwater in the deep aquifers is older than 30 ka. ⁸¹Kr analysis identified groundwater with ages ranging from 17 to 300 ka. At some sites, large age discrepancies between ⁸¹Kr and ¹⁴C indicated that inter-aquifer mixing is likely occurring. The interpretation of the noble gases suggests that groundwaters in the deeper aquifers, with apparent ages of 300 to 10 ka, have recharged in slightly colder and wetter climates than those found in the upper aquifers with apparent ages < 10 ka. Degradation of water quality from seawater intrusion was identified in the upper four aquifers. This was also evidenced by higher δ¹⁸O and δ²H values, typical of seawater. The four deeper aquifers contain high quality water characterised by less enriched ¹⁸O and ²H. This work presents new findings of very old groundwater in the Bangkok aquifer system.