Methane sink of subterranean space in an integrated atmosphere-soil-cave system.

CH4 serves as an important greenhouse gas, yet limited knowledge is available in global and regional CH4 cycling, particularly in widely distributed karst terrain. In this study, we investigated an upland in Puding Karst Ecosystem Research Station, and explored CH4 concentration and/or flux in atmosphere, soil and cave using a closed static chamber method and an eddy covariance system. Meanwhile, we monitored atmospheric temperature, precipitation, temperature and wind velocity in the cave entrance. The results demonstrated that atmospheric CH4 and actual soil CH4 fluxes in the source area of eddy covariance system were -0.19 ± 8.64 nmols-1m-2 and -0.16 nmols-1m-2 respectively. The CH4 concentrations in Shawan Cave exhibited 10 âˆ¼ 100-fold lower than that of the external atmosphere. CH4 oxidation rate dominated by methane-oxidizing bacteria was 1.98 nmols-1m-2 in Shawan Cave when it combined with temperature difference between cave and external atmosphere. Therefore, CH4 sink in global karst subterranean spaces was estimated at 106.2 Tg CH4 yr-1. We supplemented an understanding of CH4 cycling paths and fluxes in karst terrain, as well as CH4 sinks in karst subterranean space. Further works require to establish a karst ecosystem observation network to conduct long-term integrated studies on CH4 fluxes regarding atmosphere, soils, plants and caves.

The effect of cave ventilation on carbon and oxygen isotopic fractionation between calcite and drip water.

Rapid CO2 degassing and calcite precipitation driven by cave ventilation influence the speleothem δ18O and δ13C. However, the drivers of cave ventilation are not completely understood due to the lack of monitoring of multiple environmental factors. Furthermore, the understanding of isotope fractionation caused by the dissolution of speleothem in undersaturated drip water is limited during the cave air stagnation. In this study, we displayed four years of cave microenvironment monitoring in Shawan Cave, Southwestern China, and analyzed the δ13CDIC and δ18O of drip water, and calcite precipitation δ18O and δ13C. The results show that the ventilation process is attributed to buoyancy airflow between external atmosphere, fissure air, and cave air. This causes that the higher (lower) cave air pCO2 in the summer (winter) is associated with upward airflow mode (downward airflow mode). Furthermore, cave ventilation could control the isotopic fractionation. Specifically, when cave air pCO2 is lower, the carbon isotopic disequilibrium between calcite and dissolved inorganic carbon (DIC) is controlled by the degassing of CO2 associated with calcite precipitation. The disequilibrium fractionation in carbon isotopes is less pronounced at slower drip-rate sites. The oxygen isotope fractionation between calcite and the drip water is found to be close to equilibrium. However, the high cave air pCO2 (exceeding 10,000 ppm) may result in drip water undersaturation to drive the dissolution of speleothem calcite. The δ18O values of drip water are pulled away from their original values to disequilibrate to the calcite because the exchange time of oxygen in the dissolved carbonates with the oxygen in the water is sufficiently long. Hence, the dissolution of speleothems may be a new mechanism to explain the oxygen isotopic disequilibrium between the calcite and drip water during the cave air stagnation. The carbon isotope fractionation between calcite and drip water is close to equilibrium.

Response of drip water Mg/Ca and Sr/Ca variations in ventilated caves to hydroclimate.

Mg/Ca and Sr/Ca in speleothems which record valuable information regarding past variations of precipitation and cave air pCO2 are promising proxies because the degrees of water-rock interaction (WRI) and prior calcite precipitation (PCP) are directly and indirectly related to these changes. However, the controls on Mg/Ca and Sr/Ca can be complex, and most studies ignored the combined effects of rainfall and cave air pCO2. Moreover, knowledge of the influence of seasonal rainfall and cave air pCO2 on seasonal fluctuations in drip water Mg/Ca and Sr/Ca are limited for caves with different regions and ventilation types. Drip water Mg/Ca and Sr/Ca were monitored for five years at Shawan Cave. The results indicate that the irregular seasonal oscillation in drip water Mg/Ca and Sr/Ca is controlled by inverse-phase seasonal changes between rainfall and cave air pCO2. The rainfall amount may be the primary controlling factor of the interannual variation in drip water Mg/Ca, whereas the interannual variation in drip water Sr/Ca is most likely controlled by cave air pCO2. Furthermore, we compared drip water Mg/Ca and Sr/Ca of caves in different regions to fully understand how drip water Mg/Ca and Sr/Ca respond to hydroclimate changes. The drip water element/Ca, for seasonal ventilation caves with a fairly narrow range of cave air pCO2 respond well to the local hydroclimate associated with rainfall variation. If the range of cave air pCO2 is considerably large, the element/Ca in seasonal ventilation caves of subtropical humid regions may not reflect hydroclimate and that of Mediterranean and semi-arid regions may be primarily controlled by cave air pCO2. The element/Ca in the low year-round pCO2 caves could reflect the hydroclimate associated with surface temperature. Therefore, observations of drip water monitoring and comparative analysis can provide a reference for the explanation of speleothems element/Ca ratios from seasonally ventilated caves worldwide.

Predicting the leachate generation from wet phosphogypsum stack using a water-balance-analysis based model.

Leachate from wet phosphogypsum (PG) stack should be properly managed to mitigate the negative environmental impact of phosphoric industry. Accurate prediction of leachate amount is the prerequisite for efficient leachate management. In this study, a model using water balance analysis to predict leachate production from wet PG stack is established. The extruded water, which is related to PG deformation, is innovatively introduced as a variable in the model to account for the porewater's contribution. Model simulation suggested that at the early stage, fresh water need to be added to PG to facilitate the transfer or PG slurries; however, as the leachate accumulates in the tailings pond, a net discharge of PG is required starting at the fourth year for the studied PG stack. Model simulation also indicated that the leachate generation increased gradually over time and that the leachate generation in each month could deviate from the average leachate generation during the life cycle of the stack. The model output matches with measured values reasonably well, which confirmed the model's accuracy. Sensitivity analysis indicated that average precipitation and evaporation are the two most important factors that determine leachate generation rate. Monthly leachate generation rates vary significantly within the year, as the precipitation and evaporation vary in different seasons. The highest leachate generation rates were reached in rainy seasons and the lowest rates were reached in wintery months. This study could be used to optimize the PG leachate managements and to mitigate the PG related pollution to the environment.

High 222Rn concentrations and dynamics in Shawan Cave, southwest China.

Cave 222Rn has been a major health issue and subject of scientific debate for decades. While the basics of natural ventilation physics are well understood, it is difficult to make blind predictions of 222Rn concentrations in a given cave due to the complexity of cave systems. In-situ continuous observation is necessary to improve our ability to quantify radiation dose exposure and reduce radiation hazard to cave users, and trace the air exchange patterns occurring in caves. In this study, continuous monitoring using a RAD7 radon detector revealed high 222Rn concentrations and large fluctuations in 222Rn concentration in a small karst cave in southwest China, Shawan Cave. From August 2016 to July 2017, the average annual concentration was 47,419 Bqm-3 and ranged between 3720 and 123,000 Bqm-3, with lower values during summer than other seasons. Taking Shawan Cave as a case study, we suggest a framework to evaluate the potential dose exposure, allowing cave users to minimize risk of exposure to hazardous levels of 222Rn. Furthermore, we comparing results from this study with other studies in 35 caves worldwide, and conclude that there are three patterns of seasonal 222Rn variation. They were classified into five types of ventilation mode based on diversity of cave locations, geometry and connectivity of bed rock fracture networks, together with temperature differences between outside atmosphere and cave air.