FIELD DEGASSING AS A NEW SAMPLING METHOD FOR 14C ANALYSES IN OLD GROUNDWATER

Radiocarbon (C-14) activity in groundwater can be used to determine subsurface residence time up to similar to 40 kyr. providing crucial information on dynamic properties of groundwater and on paleoclimate. However, commonly applied sampling methods for dissolved inorganic carbon (DIC-C-14) are prone to low level of modern atmospheric contamination, resulting in underestimation of groundwater ages that cluster around 30-40 kyr. We extract CO2 gas from groundwater using a device originally developed for studies of noble gas radionuclides. Carbon is collected in the gas phase, eliminating the possibility of fostering microbial activities and aqueous chemical reactions during sample storage. This method collects CO3-C-14 and radiokrypton (Kr-81 and Kr-85) samples simultaneously. The presence of any shorter-lived Kr-85 is used to evaluate the degree of atmospheric contamination during sampling or mixing of young groundwater. Most groundwater samples showed lower CO2-C-14 activities than those of DIC-C-14, presumably dire to the absence of atmospheric contamination. Samples with Kr-81 age exceeding 150 kyr have no detectable CO2-C-14 except where mixing sources of young groundwater is suspected. These field data serve as confirmations for the reliability of the newly presented sample collection and CO3-C-14 method, and for the outstanding roles of radiokrypton isotopes in characterizing old groundwater.