旅游活动输入岩溶地下河系统的水化学指纹记录

地下河系统是岩溶地质景观的重要组成部分，利用特殊岩溶地形地貌建立的国家重大科技基础设施成为科研和旅游的胜地。为了解旅游活动对岩溶地下河系统水化学的影响，文章分析了大小井地下河系统入口和出口在不同旅游时段的水化学特征。结果表明：受旅游人数影响，污水水化学变化较大；大小井水化学类型为Ca-HCO3型，水岩相互作用产生的Ca2+、Mg2+和${rm{HCO}}_3^{-}$从入口到出口逐渐增加，而人类活动输入的K+、Na+、Cl−、${rm{NO}}_3^{-}$和${rm{SO}}_4^{2-}$则表现出差异性特征。小井地下河系统因受沿途城镇生活排放污水和农业活动影响，水化学波动较大，出口K+、Na+、Cl−和${rm{NO}}_3^{-}$升高；大井地下河虽受旅游活动输入影响，但K+、Na+、Cl−、${rm{NO}}_3^{-}$和${rm{SO}}_4^{2-}$随着地下径流长度增加而降低。HFE−D和Gibbs模型以及离子比例系数分析显示，地下河系统受到了城镇生活排放、旅游和农业等人类活动输入的影响，这一现象应引起足够的重视。

Effects of tourism activities on hydrochemical fingerprints in the karst underground river system

The underground river system is the main carrier of water resources and an important source of urban and rural water supply in karst areas, and is also an essential element of karst geological wonders. Nowadays, people are showing the increasing preference for the cultural tourism integrated with science and technology. Meanwhile, the Five-hundred-meter Aperture Spherical Radio Telescope (FAST), a significant national science and technology infrastructure built at the special topography of karst, has become an aspiring destination for astronomy and tourism enthusiasts. However, pollutants from tourism activities will easily affect the water quality of underground river through the surface runoff via sinkholes, shafts and cracks. Using hydrochemical facies diagram (HFE-D), Gibbs models and coefficients of ion ratio, we have analyzed the characteristics and changes in water chemistry of wastewater from the tourist attraction and from the Dajing and Xiaojing underground river systems during different tourism periods in order to understand the periodic, concentrated and sudden effects of tourism activities on the water chemistry of underground river systems. The results show that: (1) The pH and conductivity values of the sewage from the tourist attraction varied little during the May Day holiday, ranging from 7.57 to 7.84 and 540.00 to 761.00 mS·cm−1, respectively. The sewage pH values during the National Day holiday ranged from 7.66 to 7.95 with an average value of 7.78; however, the conductivity varied greatly from 488.00 to 934.50 mS·cm−1. The water chemistry type of sewage is Ca−HCO3, and the values of K+, Na+, Cl− and ${rm{NO}}_3^{-}$ subject to tourism activities fluctuated greatly. Their maximum values appeared on May 2 to May 3 and October 2 to October 4, which is consistent with the increasing number of tourists in the peak period of tourism. Compared with Dajing and Xiaojing underground river systems, increases of Na+, K+ and Cl− in the Durov diagram are responsible for domestic discharge and input from disinfection of Cl2 or NaClO in water treatment plants. (2) During the May Day holiday, the pH values of water from the inlet and outlet of the Dajing and Xiaojing underground river systems varied from 7.72 to 8.42 and 7.36 to 8.39, respectively. The water conductivity in the inlet ranged from 256.00 to 338.00 mS·cm−1 with an average of 281.79 mS·cm−1. The water conductivity in the outlet increased slightly, changing from 297.00 to 413.00 mS·cm−1 with an average of 313.59 mS·cm−1. Compared with the May Day holiday, the pH values of water from inlet to outlet of the Dajing underground river system increased slightly from 8.03 to 8.27 during the National Day holiday, while its conductivity values decreased slightly due to the dilution of rainfall, averaging between 317.51 mS·cm−1 and 265.11 mS·cm−1. Differently, the Xiaojing underground river system showed an increase in pH and conductivity vaules during both May Day holiday and National Day holiday, with the average of pH increasing from 7.66 to 8.02 and from 8.03 to 8.17, respectively. Average values of conductivity increased from 295.67 mS·cm−1 to 342.97 mS·cm−1 during the May Day holiday and from 307.63 mS·cm−1 to 359.72 mS·cm−1 during the National Day holiday, both of which were influenced by the urban or rural domestic discharges and the input from agricultural activities. (3) The water chemistry type of Dajing and Xiaojing underground river systems is Ca−HCO3, which is mainly controlled by karstification. The TDS increased from the inlet to the outlet as a result of sufficient water-rock interaction and effects of human activities. There are some differences in spatial variation of water chemistry between Dajing and Xiaojing underground river systems. Ca2+, Mg2+ and ${rm{HCO}}_3^{-}$ produced by water-rock interaction in the Dajing underground river system gradually increased from inlet to outlet, while K+, Na+, Cl−, ${rm{NO}}_3^{-}$ and ${rm{SO}}_4^{2-}$ caused by human activities declined along groundwater. K+, Na+, Cl− and ${rm{NO}}_3^{-}$ in Xiaojing underground river system increased from inlet to outlet. The value of ${rm{NO}}_3^{-}$ fluctuated the most with a variation coefficient of 747.97. More fluctuations in water chemistry showed in Xiaojing underground river system due to the effect of urban or rural domestic discharges and input from agricultural activities. (4) Influenced by tourism activities, the HFE-D showed that continuous 5-day responses happened after the 24-days effect of tourism activities on the water chemistry in outlet of the Dajing underground river system. The response time lengthened 2 days compared with 3-day peak tourism in the May Day holiday. But the response duration prolonged more than 13 days during the National Day holiday. It can be contributed to a concentrated and explosive effect of tourism activities and needs an extended recovery time for the water chemistry values of the underground river system. The water chemistry at the inlet of Xiaojing underground river system had been affected before the peak of tourism no matter whether in the May Day holiday or National Day holiday, indicating that the water chemistry of the underground river is influenced not only by the sewage from tourist attraction, but also by the urban or rural domestics discharge and input from agricultural activities. The HFE-D indicates a 2-day response in water chemistry at the outlet of the Xiaojing underground river system 27 days after the May Day holiday. The poor continuity and regularity of the water chemistry response observed at the outlet of the Xiaojing underground river system during the National Day holiday may be related to multiple effects along the underground river. Both Gibbs model and analysis of ion ratio showed that the underground river system was influenced by the input of human activities such as urban domestic discharge, tourism and agriculture activities. Therefore, the human effect on water chemistry of the underground river system should be given adequate attention.