Evidences from Helium Isotope for Groundwater Supplement,Circul ation and Mixing—Taking the Shiyanghe and Heihe river basins as an example

Key words: 
• Shiyanghe river basin /
• groundwater /
• helium isotope /
• mixing

Abstract: Shiyanghe and Heihe rivers,situated in the east part of Hexi Corridor,Gansu province,are two typical rivers in the arid and semi-arid area. According to the groundwateic helium isotope composition,most samples collected from these two river basins are of meteoric characteristics. Theirδ³He values range from -93.8%-181.4%, but most of them are close to the atmosphere helium with δ³He ranging between-20-+200%³He/²⁰Ne distribute in the range of 4.65×10^-7-7.43×10^-7, which is slightly higher than that of the air, indicating that their origin is mainly from the meteoric water. In the northwest of Wuwei basin,Shiyanghe river the He³/He⁴ values of groundwater is lower than atmosphere helium in 1-2 magnitude of orders,implying that the mixing of radiogenic helium from adjacent granite bodies in which abundant minerals containing U and Th are observed. The decay of U and Th can produce a large amount of ⁴He resulting in the excess of helium in the groundwater. The eminent accumulation of radiogenic helium shows that the circulation period of groundwater in this area could be 1000 years. Along the hidden fault in the front of the Qilian Mountains can observe the much higher ³He/⁴He values with δ³He ranging from 70%-180 %, indicating that the eminent contribution of mantle helium which implies that the Fault is still active.