Using multi-environmental tracers to investigate groundwater residence times, age distribution and recharge processes in an arid intermountain basin-fill aquifer

Study region: Zhangye Basin (ZYB) is a typical inland arid intermountain basin-fill aquifer in northwest China which occurs extremely groundwater mixing. Study focus: Scatter plots of 3H and 14C exhibits mixing of old and young water in the ZYB, however, the mixing process and diverse recharge resources remain insufficiently explored. Multiple environmental tracers (3H, CFCs, SF6, 14C, 4He) were sampled from multi-channel monitoring wells with 31cm-length screens and long-screen wells along a transect crossing the piedmont to the central basin. New hydrological insights for the region: Comparisons of CFCs, 3H and SF6 suggest the short screens effectively eliminate the mixing effects. Plotting CFCs, 3H, SF6 against 14C and 4He reveal mixing of groundwater with diverse time scales. Groundwater age distributions were determined using a binary mixing model (BMM) composed of a PFM (Piston Flow Model) and an EPM (Exponential Piston Flow Model). The mean age of young and old groundwater components derived by the BMM using 14C range from 11 to 45 yrs, 1.2–28.3 kyrs, respectively. The fraction of young groundwater component varies from 7 % to 70 % exhibiting a general decreasing trend with depth. The mean residence time of old groundwater component derived from 4He reaches up to 500 kyrs. Considerable old groundwater fraction in the discharge zone suggests existence of deep regional flow in the ZYB originating from mountain block recharge.