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

Zhangye Basin (ZYB) is a typical inland arid intermountain basin-fill aquifer in northwest China which occurs extremely groundwater mixing. Scatter plots of 3 H and 14 C exhibits mixing of old and young water in the ZYB, however, the mixing process and diverse recharge resources remain insufficiently explored. Multiple environmental tracers ( 3 H, CFCs, SF 6 , 14 C, 4 He) 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. Comparisons of CFCs, 3 H and SF 6 suggest the short screens effectively eliminate the mixing effects. Plotting CFCs, 3 H, SF 6 against 14 C and 4 He 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 14 C 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 4 He 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. • Multiple tracers indicate extensive mixing of young and old groundwater • Multi-channel wells with short screens can eliminate the mixing effects in well • SF 6 enrichment due to excess air and geological process biases young age • Binary mixing model of PFM and EPM produces reasonable groundwater age distribution • Groundwater is conceptualized by mixture of local recharge and inter-basin source