Hydrogen and oxygen isotope characteristics controlled by thermal stratification effect in a subtropical reservoir, SW China

To examine the relationship between thermal stratification and spatial distribution of hydrogen and oxygen isotope in Wulixia Reservoir, Guangxi, spatial characteristics and factors influencing hydrogen and oxygen isotopes were studied in during the summer of 2014. Results showed that ① Wulixia Reservoir showed obvious thermal stratification in 2014 summer which can be divided into three layers, epilimnion (from surface water to a depth of 5 m), thermocline (depths of from 5 m to 20 m) and hypolimnion (below 20 m); ② All the δ¹⁸O and δD values of Wulixia Reservoir (line equation:δD=4.66δ¹⁸O-10.85, R²=0.76) scattered roughly along Global Meteorological Water Line (GMWL), which showed the reservoir water to originate from modern meteorological precipitation. However, water δ¹⁸O and δD of Wulixia Reservoir, located in the right lower area of the Guilin Local Meteorological Water Line (CLMWL) and the slope of their line equation were obviously different from the slope of GMWL and CLMWL line, which indicate that evaporation and water-rock interaction could occur before water entered the reservoir; ③ δ¹⁸O and δD values of water generally decreased as depth increased, in the following order epilimnion > inflow water > thermocline > hypolimnion outflow water (here ">" means "more positive"). Results indicate that seasonal variation of inflow water δ¹⁸O and δD and water evaporation decreased as water depth increased. These are the two primary factors influencing and controlling the vertical distribution of δ¹⁸O and δD in Wulixia Reservoir during the thermal stratification that takes place during summer.