Inferring Young Groundwater from Deep Geothermal Water using CFCs and Isotope Data: Implication for Circulation of Groundwater in the Xi'an Geothermal Field, Shaanxi Province, China

Abstract

Chlorofluorocarbons (CFCs) and isotopic properties of water from wells in the Xian geothermal field, Xian, Shaanxi Province, China, were measured to identify young groundwater from the geothermal system, and to refer circulation of geothermal groundwater. Microbial degradation of CFCs in deep geothermal groundwater (temperature: 35? to 95?) has not been observed, and this may be interpreted as a temperature effect that has depressed microbial activities. Seven of twenty samples from well in depth of 300 to 3000 meters were determined as CFC-bearing, and the others were CFC-free. Water samples near the Qinling Mountain have the highest fraction of young water, and their stable isotope data fall on the local meteoric water line. The fraction of young water indicates mixing between young and old groundwater. CFCs input to the geothermal water aquifers may relate to surficial or shallow recharge. This may imply that recent infiltration could be responsible for young groundwater into deep geothermal water aquifers. However, the result is inconsistent with tritium concentration of less than 0.32 TU for all of waters. This may be explained by the different transport mechanism for gaseous CFCs and aqueous tritium in a deep groundwater system. Two samples (GZH-9, GZH-20) near the Mountain front have relatively high 14C activities of 50 to 20 pmc, indicating the presence of young recharge. Two groups of waters were identified based on CFCs, stable isotopes and 14C: (1) a regional geothermal water, that is CFC-free, enriched 18O and low 14C; and (2) young groundwater from the Qinling Mt., shallow aquifers and/or the Wei River, that is CFC-bearing, high 14C, and featured by isotopic composition close to precipitation. The recharge of geothermal water is dominated from the Qinling Mountain area and has been taken deep circulation toward inner basin.