On the Initial 14C Activity of Karst Aquifers with Short Mean Residence Time

The 14C activity of total dissolved inorganic carbon (DIC) as well as tritium activity and stable isotope content (13C, 2H, 18O) of spring water were measured for 3 consecutive years at 3 karst springs that feed Plitvice Lakes, NW Yugoslavia. The constant chemical and stable isotope composition and temperature indicated well mixed aquifers. A very short mean residence time of underground water (2–5 yr) was determined by tritium activity measurements. Measured C activities of 3 karst springs varied from 60.0% to 82.5%. The measured C activities were compared with calculated 14C activities based on various models using measured values of C activity of soil at different depths and 13C content of limestone, plants, and soils from the recharge area. Our measurements indicate that recharge systems in karst aquifers are open to the atmosphere. The atmospheric CO2 contributes to the 13C content and the C activity of ground water to an estimated extent which varies between 10 and 40% of DIC.