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1. 贵州师范大学喀斯特研究院/地理与环境科学学院,贵阳 550001
2. 贵州省喀斯特山地生态环境国家重点实验室培育基地,贵阳 550001
3. 国家喀斯特石漠化防治工程技术研究中心,贵阳 550001

Variation of CO2 and its response to the drip hydrogeochemistry in caves under the short-time high-strength tourism activities
ZHANG Jie1,2,, ZHOU Zhongfa1,2,, WANG Yanlin1,3, PAN Yanxi1,3, XUE Bingqing1,3, ZHANG Haotian1,2, TIAN Zhonghui1,2
1. School of Karst Science/College of Geography and Environmental Sciences, Guizhou Normal University, Guiyang 550001, China
2. The State Key Laboratory Incubation Base for Karst Mountain Ecology Environment of Guizhou Province, Guiyang 550001, China
3. State Engineering Technology Center of Karst Rock Desertification Rehabilitation, Guiyang 550001, China
Abstract

The presence of CO2 in the caves affected by intense tourism activities has a significant impact on the drip hydrogeochemistry and sedimentation. In this investigation, a continuous monitoring on the indexes such as CO2, temperature, relative humidity, tourist number and drip hydrochemistry was conducted in Guizhou Suiyang Dafeng Cave from 30 September 2017 to 9 October 2017. Following the collection of data, different methods were applied systematically to analyze a number of elements comprehensively. The observed results show that, under the influence of factors such as tourist number and ventilation effect of cave, the partial pressure of CO2 in the cave (PCO2 (A)) presented obvious diurnal and interdiurnal variations in the time sequence, and showed a higher value in daytime whereas a lower value at nighttime, and also a higher value on days with many tourists and a lower value on days with few tourists. In space variation, due to different ventilation degrees and cavity volumes, the PCO2(A) of different monitoring points had obvious differences, from the deep cave to the entrance of cave as 3# (Magical Spring and Dewdrop) >1# (Time Tunnel)>2# (Legendary Luminous Pearl). Through the comparison of PCO2(A) and drip temperature, the former had more significant influence on the solubility of CO2 than the latter did. Moreover, the drip temperature and drip partial pressure of CO2 (PCO2(W)) of cave generally had the same trend of variation with that of PCO2(A), and also presented obvious diurnal and interdiurnal variations. The pH, SIc, and HCO3- generally had a trend of variation in the opposite way to that of PCO2 (A), EC and Ca2+ had no obvious diurnal variation but certain interdiurnal variation. With an increase in the strength of tourism activities, the variation amplitude of drip hydrochemistry gradually increased. Furthermore, the differences in the factors such as cavity structure, size and closeness caused differences in the diffusion speed of PCO2(A) and cave ventilation degree, and further influenced the hydrochemistry of the constituents of cave drip and sedimentation conditions of cave. Overall, this study will have a significant impact on the research on protection and management of cave environment as well as its karst cave carbon cycle.

Keyword: anthropogenic CO2; short-time scale; high intensity tourism activity; cave drip water; hydrogeochemistry; Dafeng Cave;
1 引言

2 研究区概况

 Figure Option 图1 研究区概况图 Fig. 1 The sketch map of research area

3 数据来源与研究方法

$T V = T × ( 1 + 0.6079 r V - 0.3419 r C )$ （1）

 Figure Option 图2 大风洞监测点分布示意图 Fig. 2 Schematic diagram of the distribution of the monitoring points of Dafeng Cave

$PC O 2 ( W ) = [ H + ] [ HC O 3 - ] K 1 K h$ （2）

$SIc = log IAP K$ （3）

4 结果
4.1 洞穴空气CO浓度的时空变化特征

PCO2（A）变化的主要影响因素包括气候变化导致的植被和土壤生物活动产生的CO2浓度变化、通风效应和游客数量及其在洞内滞留的时间长短[45]。而洞穴短时间尺度CO2浓度变化主要与通风效应和游客数量及其滞留的时间长短等因素有关。图3所示,对3个监测点PCO2（A）进行10 d的连续自动监测,发现监测期间PCO2（A）浓度总体上呈现明显的昼夜变化,表现为白天CO2浓度出现明显的峰值,而夜间出现明显的低值。各监测点PCO2（A）最高值均出现在当天游客数量最多的14:00-14:30时间段。经过夜间的自净,CO2浓度逐渐降低与限制性通风时期（夏秋季）的背景值相近（约1000 mg/L）。日际变化上PCO2（A）的变化与进入洞穴的游客数量大体一致,即当进入洞穴的游客数量迅速增加时,PCO2（A）也出现显著地增加,增加幅度在不同监测点存在差异,分别为2.1 mg/L、1.2 mg/L和4.0 mg/L,这与其他旅游洞穴基本一致。至游客量达到最大值时,PCO2（A）出现高而尖的峰值,如10月3日1#、2#和3#分别为1481 mg/L、1455 mg/L和2351 mg/L（图4a、4c）;而当游客数量较少时,CO2波动较小,出现宽缓的峰值,如10月9日1#、2#和3#分别约为1108 mg/L、1138 mg/L和1192 mg/L（图4b、4d）。此外,10月5日-10月6日CO2浓度出现异常低值,但1#（528 mg/L）和3#（948 mg/L）点低值均在8:30-9:00之间,而2#点最低值（874 mg/L）则在10:00左右,稍有一定的滞后。

 Figure Option 图3 大风洞不同监测点CO2浓度短时间尺度变化 Fig. 3 Short-time scale variation in the concentration of CO2 at different monitoring points of Dafeng Cave

 Figure Option 图4 游客在洞内滞留和游客出洞后CO2的变化趋势 Fig. 4 The trend of variation of CO2 when the tourists linger in the cave and exit the cave

4.2 洞穴滴水水文地球化学特征分析

 Figure Option 图5 2#和3#滴水点水文地球化学特征 Fig. 5 The characteristics of hydrology geochemistry of 2# and 3# drip points

5 讨论
5.1 短时间尺度PCO变化的驱动因素

5.1.1 游客量 游客数量及其在洞穴内滞留的时间是导致洞穴瞬时CO2浓度变化幅度较大的最直接原因[31]。其他条件不变的情况下,当一定数量的游客到达监测点时,PCO2（A）开始逐渐上升,随着滞留时间的增加和游客数量的增加,PCO2（A）开始处于快速上升阶段,当游客离开监测点后的一段时间内,由于CO2的累积效应,监测点的PCO2（A）继续缓慢升高至最大值,而后由于洞穴通风效应的影响,使洞内CO2不断扩散,稀释净化,最终回到自然背景值（Cn）,由此日复一日地使PCO2（A）呈现出明显的昼夜变化（图6）。如图4a所示,在游客数量最多的10月3日,8:30游客还未进入洞穴,各监测点CO2浓度基本接近背景值,随着游客进入洞穴的数量增加,PCO2（A）逐渐增加,但由于受洞穴洞腔大小和洞穴通风效应的影响,不同监测点CO2浓度增加的幅度存在明显差异,分别为3#>1#>2#。由于CO2的累积效应,至16:00时洞穴PCO2（A）逐渐达到最大值,随后由于游客逐渐减少,CO2浓度梯度达到最大值,同时由于洞内外温差逐渐缩小,洞内外气流交换增强,CO2开始迅速扩散,至午夜或次日第一批游客来临之前达到自然背景值。而游客较少的10月9日与10月3日相比,CO2变化的模式相似,但是CO2在上升段和下降段的变化幅度明显较小。

 Figure Option 图6 游客进入洞穴期间空气CO2浓度的演变过程[5] Fig. 6 The variation in the concentration of CO2 in the air during the period when the tourists enter the cave

5.1.2 洞穴通风效应 短时间尺度的洞穴通风过程主要受洞内外温度差[47,48]、洞穴形态[49]、气压和风[50,51]等因素的控制。目前对洞穴通风的估计忽略了高浓度二氧化碳和相对湿度的作用,因而一种更精确的浮力测量方法是虚拟温度（Tv）,虚拟温度综合计算了洞穴气温、相对湿度和空气CO2,其包括每个气团的主要分子组成的变化对其浮力的影响[52]。因此通过计算洞内外虚拟温度差（△Tv =TvCave-TvOutside）来估算浮力差的变化,能够进一步准确估计洞穴通风情况。即当△Tv>0时,空气浮力差为正值,表明更多的暖而轻的洞穴空气离开洞穴并被洞外的冷重气流所取代;当△Tv<0时,空气浮力差为负值,则意味着洞穴空气将滞留在洞内,洞内外气流交换受到抑制,洞穴通风程度较弱。如图8所示,监测的绝大部分时间△Tv<0,空气浮力差为负值,说明在监测的大部分时间里,洞内外气流交换主要处于限制性通风状态,即洞内外气流交换受到限制,导致洞穴空气滞留于洞内。尤其是晴朗天气的午后,这种限制性通风状态尤为明显（图7a）,而至夜间有所缓和,洞内外气流具有一定的交换,但此时洞内外气流交换可能主要受洞内外空气密度差控制（图7b）,也就是当洞内外温差相近时,洞内由于旅游活动,PCO2（A）浓度增高,使洞穴空气密度增加,而洞外空气密度相对恒定,从而导致洞内外空气密度差产生,洞内气流缓慢流向洞外,最终经过夜间的自净,至第二天游客进入洞穴之前,达到自然背景值。但当旅游活动过于强烈,可能使PCO2（A）产生大量累积,导致洞穴空气无法在短时间内自净,进而可能抑制洞穴沉积物生长。

 Figure Option 图7 监测期间大风洞洞穴通风模式图 Fig. 7 The mode chart for cave ventilation of Dafeng Cave during the monitoring period

 Figure Option 图8 洞内外虚拟温度差(△Tv) Fig. 8 The virtual temperature difference between the inside and outside of cave

5.2 PCO对洞穴碳酸钙沉积过程的驱动作用

H2O+CO2=H2CO3 （4）

CaCO3+H2CO3=Ca2++2HCO3- （5）

Ca2++2HCO3-=CaCO3↓+CO2↑+H2O （6）

CO2（S）=Cab×PCO2×1.963 （7）

5.3 洞穴空气CO与洞穴滴水水化学

 Figure Option 图9 PCO2（A）与洞穴滴水水文地球化学指标的关系 Fig. 9 Relationship between PCO2（A） and hydrogeochemical indicators of the cave dripping water

 Figure Option 图10 洞穴CO2对洞内环境影响的概念模型 Fig. 10 Conceptual model of the influences of CO2 on the internal cave environment

6 结论

（1）监测期间,PCO2（A）在时间变化上呈现明显的昼夜变化和日际变化,主要受游客数量和短时间尺度的洞穴通风效应等因素的影响。在空间变化上,由于洞腔体积和通风程度不同,不同监测点PCO2（A）浓度存在明显差异,由洞内深处至洞口分别为3#>1#>2#。

（2）通过对PCO2（A）和洞穴滴水水温对溶解CO2的影响的比较,发现大风洞PCO2（A）变化幅度为1500 mg/L,洞穴水温变化幅度基本在0.5 ℃以内,表明PCO2（A）浓度变化对CO2溶解度影响比温度变化更为显著,也证明了PCO2（A）是洞穴沉积物沉积过程的最重要驱动因素之一。

（3）洞穴滴水水温和PCO2（w）变化趋势基本相似,表现为白昼高而夜间低,pH、SIc和HCO3-变化大体上呈现出白昼低而夜间高的特征,EC和Ca2+则无明显的昼夜变化。随着旅游活动强度的增大,PCO2（A）迅速增加,滴水水化学变化幅度也逐渐增大,但当CO2浓度达到一定阈值时,水化学变化幅度逐渐变小。PCO2（A）的扩散和通风程度的差异,会进一步影响洞穴滴水水化学的变化,表现为洞穴通风程度好,洞穴滴水水化学变化相对平稳;但洞穴通风程度差,使洞穴积聚大量CO2,对洞穴滴水水化学变化影响显著,也可能在一定程度上抑制洞穴沉积物的沉积。

（4）短时间尺度洞穴人为CO2变化对洞穴水化学影响的研究,有利于洞穴环境管理和保护,为旅游洞穴碳循环的深入研究提供了一定的理论依据。但由于洞穴是一个极其复杂的系统,现有的监测数据并不能完全揭示相关规律。因此洞穴环境监测研究需要分辨率更高的数据,才可能更加逼近实际的规律。

The authors have declared that no competing interests exist.

Reference
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A spurious relationship was alternatively considered,in which external temperature was the universal predictorof cave CO2 levels.      [本文引用:1] [13] Faimon J, Ličbinská M, Zajíček P, et al.Partial pressures of CO2 in epikarstic zone deduced from hydrogeochemistry of permanent drips, the Moravian Karst Czech Republic. Acta Carsologica, 2012, 41(1): 47-57. [本文引用:1] [14] Holland H D, Kirsipu T V, Huebner J S, et al.On some aspects of the chemical evolution of cave water. Journal of Geology, 1964, 72(1): 36-67. The evolution of cave waters can be divided naturally into three stages: a stage of carbonation in the soil zone, a stage of solution of calcite and/or dolomite, and a stage of equilibration with cave air. Much of the necessary physical-chemical data for the interpretation of the chemistry of cave waters is available: the solubility of $CO_{2}$ in water is well known, as is the solubility of calcite and aragonite in pure water and in solutions containing dissolved $CO_{2}$. Only the solubility of dolomite is still in doubt. Water samples from Indian Echo Cave and Carpenter Cave in Pennsylvania and from the Luray Caverns in Virginia were analyzed, and their composition was used to interpret their chemical evolution. The waters in each cave must have absorbed a large amount of $CO_{2}$ on their passage through the soil zone. Even so, the calcium and magnesium content of some of the water at Luray is so large that impossibly high $CO_{2}$ pressures would have had to prevail in the soil zone if the solubility data of Yanat''eva (1954) and of Garrels, Thompson, and Siever (1960) for dolomite were correct. We suggest that the solubility product of dolomite is probably near $10^{17}$, about two orders of magnitude higher than the value proposed by Garrels, Thompson, and Siever (1960). During the precipitation of $O_{3}$ from cave waters, the activity product $a_{Ca^{+2}}\cdot a_{CO_{3}^{-}}$ has been found to exceed the solubility product of calcite by as much as a factor of seven. In bodies of standing water from which no $CaCO_{3}$ is precipitating the activity product is very nearly equal to the accepted value of the solubility product of calcite. Precipitation of $O_{3}$ takes place essentially entirely by escape of $CO_{2}$ from cave water to the cave air. This is demonstrated convincingly by the observed constancy of the magnesium concentration in these waters during $O_{3}$ precipitation. At Luray nearly all of the water is supersaturated with respect to dolomite. However, no dolomite has been observed to precipitate in the cave even where the activity product $a_{Ca^{+2}}\cdot a_{Mg^{+2}}\cdot a_{CO_{3}^{-}}^{2}$ is in excess of $2 \times 10^{-15}$. The strontium content of $O_{3}$ precipitated from cave water depends on the mineralogy of the precipitate and on the ratio of the concentration of strontium to the concentration of calcium in the parent water. The strontium content of our Luray aragonites is always greater than the strontium content of the Luray calcites, and the absolute value of the strontium concentration can be related satisfactorily to the strontium-calcium ratio in the parent water via the distribution coefficients $k_{Sr}^{C}$ and $k_{Sr}^{A}$ determined in the laboratory. The strontium content of the cave waters at Luray is determined by the amount of dolomite dissolved by the carbonated rain water, by the strontium content of the dissolved dolomite, and by the amount and mineralogy of the $O_{3}$ deposited during equilibration with cave air. DOI:10.1086/626964      [本文引用:1] [15] Lang M, Faimon J, Ek C.A case study of anthropogenic impact on the CO2 levels in low-volume proﬁle of the Balcarka Cave (Moravian Karst, Czech Republic). Acta Carsologica, 2015, 44(1): 71-80. [本文引用:1] [16] Faimon J, Štelcl J, Sas D.Anthropogenic CO2-flux into cave atmosphere and its environmental impact: A case study in the Císarská Cave (Moravian Karst, Czech Republic). Science of the Total Environment, 2006, 369(1-3): 231-245.      [本文引用:2] [17] Lang M, Faimon J, Godissart J, et al.Carbon dioxide seasonality indynamic caves: The roles of ventilation modes and advective fluxes. Theoretical and Applied Climatology, 2017, 129(3/4): 1355-1372. The seasonality in cave COlevels was studied based on (1) a new data set from the dynamically ventilated Comblain-au-Pont Cave (Dinant Karst Basin, Belgium), (2) archive data from Moravian Karst caves, and (3) published data from caves worldwide. A simplified dynamic model was proposed for testing the effect of all conceivable COfluxes on cave COlevels. Considering generally accepted fluxes, i.e., the direct diffusive flux from soils/epikarst, the indirect flux derived from dripwater degassing, and the input/output fluxes linked to cave ventilation, gives the cave COlevel maxima of 1.9 10mol m(i.e., 440 ppmv), which only slightly exceed external values. This indicates that an additional input COflux is necessary for reaching usual cave COlevel maxima. The modeling indicates that the additional flux could be a convective advective COflux from soil/epikarst driven by airflow (cave ventilation) and enhanced soil/epikarstic COconcentrations. Such flux reaching up to 170 mol sis capable of providing the cave COlevel maxima up to 3 10mol m(70,000 ppmv). This value corresponds to the maxima known from caves worldwide. Based on cave geometry, three types of dynamic caves were distinguished: (1) the caves with the advective COflux from soil/epikarst at downward airflow ventilation mode, (2) the caves with the advective soil/epikarstic flux at upward airflow ventilation mode, and (3) the caves without any soil/epikarstic advective flux. In addition to COseasonality, the model explains both the short-term and seasonal variations in C in cave air CO.      [本文引用:1] [18] Fernández-Cortes A, Sanchez-Moral S, Cuezva S, et al.Annual and transient signatures of gas exchange and transport in the Castañar de Ibor Cave (Spain). International Journal of Speleology, 2009, 38(2): 153-162.      [本文引用:1] [19] Spötl C, Fairchild I J, Tooth A F.Cave air control on dripwater geochemistry, Obir Caves (Austria): Implications for speleothem deposition in dynamically ventilated caves. Geochimica et Cosmochimica Acta, 2005, 69(10): 2451-2468.      [本文引用:3] [20] Faimon J, Troppová D, Baldík V, et al.Air circulation andits impact on microclimatic variables in the Císařská Cave (Moravian Karst, Czech Republic). International Journal of Climatology, 2012, 32(4): 599-623.      [本文引用:1] [21] Ban Fengmei, Cai Binggui.Research on seasonal variations of the air's main environmental factors in the Shihua Cave, Beijing. Carsologica Sinica, 2011, 30(2): 132-137. [本文引用:1] [班凤梅, 蔡炳贵. 北京石花洞空气环境主要因子季节性变化特征研究. 中国岩溶, 2011, 30(2): 132-137.] 洞穴大气CO2浓度不仅是影响洞穴沉积物沉积(或者溶蚀)的重要 因素之一,而且在旅游洞穴,它关系到沉积物景观的稳定性以及旅游环境的舒适性.本文通过对石花洞洞穴大气温度、湿度及CO2浓度近4个水文年的观测,结果 表明:(1)洞穴温度在15℃上下波动,夏季约高1℃,主要与洞内外温差的季节性交化和旅游活动有关;(2)洞穴CO,浓度随着大气温度上升而缓慢升高, 至每年的7月上甸雨季来临时,气温、降水及土壤中CO,大幅提高,降水溶解大量的土壤CO2并渗入洞穴中,导致洞穴CO2浓度迅速上升,8月观测到的最高 浓度可达到4 334ppm,在雨季结束后,随着大气温度降低,CO2浓度缓慢下降,2月份平均值达到最低,为360～458 ppm.另外,在5月份和10月份的旅游黄金周,旅游人数的增加,洞穴CO2浓度异常增高.在进行洞穴管理与规划时,应综合考虑自然和人为因素对洞穴的影 响. [22] Cai Binggui, Shen Linmei, Zheng Wei, et al.Spatial distribution and diurnal variationin CO2 concentration, temperature and relative humidity of the cave air: A case study from Water Cave, Benxi, Liaoning, China. Carsologica Sinica, 2009, 28(4): 348-354. [本文引用:0] [蔡炳贵, 沈凛梅, 郑伟, 等. 本溪水洞洞穴空气CO2浓度与温、湿度的空间分布和昼夜变化特征. 中国岩溶, 2009, 28(4): 348-354.] [23] Zhang Ping, Yang Yan, Sun Zhe, et al.Comparisons between seasonal and diurnal patterns of cave air CO2 and control factors in Jiguan Cave, Henan Province, China. Environmental Science, 2017, 38(1): 60-69. [本文引用:0] [张萍, 杨琰, 孙喆, 等. 河南鸡冠洞CO2季节和昼夜变化特征及影响因子比较. 环境科学, 2017, 38(1): 60-69.] 岩溶洞穴空气CO_2变化影响次生沉积物沉积和溶蚀,它关系到洞穴旅游景观的稳定性及洞穴环境的舒适性,是岩溶作用发生的关键因素,进行洞穴空气CO_2变化的机制研究对于理解岩溶作用发生规律和现代洞穴合理保护具有重要意义.本文基于对我国南北地理分界区域河南西部鸡冠洞2011年12月至2016年5月近5年连续洞穴CO_2、水文地球化学指标、洞内外温度及湿度、大气降水和游客量等数据监测,并结合2016年5月19~20日洞穴CO_2等指标的昼夜的系统监测,分析了鸡冠洞洞穴空气CO_2时空变化特征和昼夜变化特征及其影响因素,结果表明:1在空间尺度上,越靠近洞口通风效应越强,洞穴空气p CO_2越低,越接近大气的p CO_2;洞穴结构及外界环境变化尤其是气候变化导致的土壤中p CO_2变化也会对鸡冠洞空气p CO_2变化产生影响.2在长时间尺度,鸡冠洞洞穴空气p CO_2夏季明显高于冬季,对比分析发现旅游活动和岩溶作用是其主要的影响因子.3在短时间尺度上(昼夜变化),鸡冠洞洞穴空气p CO_2变化主要受旅游活动的影响,建议景区在进行旅游开发的时候要考虑高峰期游客人数对CO_2的影响及岩溶景观的合理保护. [24] He Haibo, Tang Jing, Liu Shuhua, et al.Spatial and temporal variation of environments and influencing factors in Loufang Cave, northeast of Sichuan Province. Tropical Geography, 2014, 34(5): 696-703. [本文引用:0] [贺海波, 汤静, 刘淑华, 等. 川东北楼房洞洞穴环境时空变化与影响因素. 热带地理, 2014, 34(5): 696-703.]