[Diel variations of hydrochemistry and influencing factors in a surface stream in subtropical karst area, SW China].

In order to understand the diel variation and influencing factors of hydrochemistry in a surface creek fed by karst subterranean river in a subtropical area, where is located at Guancun Village, Daliang Township, Rong'an County of Guangxi Zhuang Autonomous Region, China, two monitoring sites were set simultaneously to launch Guancun subterranean river outlet (G1) and surface creek mouth (G2), respectively. Physical and hydrochemical parameters including pH, dissolved oxygen (DO), water temperature (T) and specific conductivity (Spc) were measured at 15-minute intervals and water samples for analyzing major ions such as Ca2+, HCO3- and NO3- as well as delta3C(DIC) were collected at 2-hour intervals. The results showed that: (1) G1 and G2 sites were both HCO3- Ca type water, however the two monitoring sites showed different diel variations of hydrogeochemical process; (2) The physical and hydrochemical parameters (T, DO, pH, Spc) and major ions (Ca2+, Mg2+, Na+, K+, HCO3-, SO4(2-), NO3-, Cl- in G1 site were basically stable, while the physical and hydrochemical parameters (T, DO, pH, Spc) and major ions (Ca2+, HCO3- and NO3-) in G2 site displayed regular diel variation during monitoring; (3) The dissolved inorganic carbon (DIC) and delta13C values in G2 monitoring site showed reverse characteristics in diurnal fluctuations, where DIC decreased in daylight and increased at night while the delta13C value increased in daylight and decreased at night, DIC also showed a negative correlation with the delta13C value (correlation coefficient is -0. 87, P < 0.01) in G2 site. These results indicated that photosynthesis and respiration of aquatic plants, water temperature and degassing jointly affected diurnal variation of hydrochemistry and controlled the cycling process of internal matter in this surface creek fed by karst subterranean river.