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From winter 2009 to spring 2010, southern China suffered from an extensive and lengthy drought. Subsequently, southern China experienced torrential rains a dozen times during the flood season of April–June. The prolonged drought and subsequent intensive rainfall dramatically altered the hydrodynamic conditions of the karst areas, causing hundreds of widely distributed karst collapses, with extensive damage to houses and fields and necessitating the evacuation of many people. Understanding the causes of these collapses will provide a scientific basis for the prediction or prevention of the related risk. Thus, a geologic survey of karst collapses was performed that systematically allowed to analyze the basic geological conditions, groundwater hydrodynamic conditions, overlying strata properties, and environmental and climatic conditions in southern China. The results showed that environmental and climatic changes, especially the drastic changes in rainfall, gave rise to striking changes in karst hydrodynamic conditions that accelerated infiltration, suffosion, dissolution, and transportation rates and also changed the physical and mechanical properties of the strata overlying karst caves. These stressors triggered or facilitated both cave formation and collapse.
The karst geological system in north China is different from that in the south. Due to differences in basic geological conditions and environment, the karst evolutional pattern and karst water resources, among other things, are also different in the two regions. This paper, based on on-site fieldwork and analysis of an extensive database of karst in China, presents a systematic and contrastive analysis of karst geological environment and karst water resources in north versus south China, highlights the differences between the two regions in basic karst geological conditions, groundwater dynamics and karst water resources, and concludes with the economic, environmental and engineering implications of these differences. These findings will be helpful for the strategic planning and decision-making processes associated with exploitation of karst geological resources and with prevention and control of karst geological hazards.
The Diana Cave in SW Romania develops along a fault line and hosts a spring of hot (Tavg = 51 °C), sulfate-rich, sodium-calcium-chloride bearing water of near-neutral pH. Abundant steam and H2S rises from the thermal water to condensate on the walls and ceiling of the cave. The sulfuric acid produced by H2S oxidation/hydrolysis causes a strong acid-sulfate weathering of the cave bedrock generating a sulfate-dominated mineral assemblage that includes rapidcreekite, Ca2(SO4)(CO3)•4H2O closely associated with gypsum and halotrichite group minerals. Rapidcreekite forms bundles of colorless tabular orthorhombic crystals elongated along [001] and reaching up to 1.5 mm in length. For verifying the hydrogen bond scheme and obtaining crystal-chemical details of the carbonate group a single-crystal structure refinement of rapidcreekite was performed. Its unit-cell parameters are: a = 15.524(2), b = 19.218(3), c = 6.161(1) Å; V = 1838.1(5) Å3, Z = 8, space group Pcnb. Chemi¬cal composition (wt%): CaO 35.65, SO3 24.97, CO2 13.7, H2O 23.9, Na2O 0.291, MgO 0.173, Al2O3 0.07, total 98.75%. The empirical formula, based on 7 non-water O atoms pfu, is: Ca1.98Na0.029Mg0.013 Al0.004(S0.971 O4)(C0.97O3)•4.13H2O. The d34S and d18O values of rapidcreekite and other cave sulfates range from 18 to 19.5‰ CDT and from –9.7 to 7.8‰ SMOW, respectively, indicating that the source of sulfur is a marine evaporite and that during hydration of the minerals it has been an abundant 18O exchange with percolating water but almost no oxygen is derived from O2(aq). This is the first descrip¬tion of rapidcreekite from a cave environment and one of the very few natural occurrences worldwide. We also report on the mineral stability and solubility, parameters considered critical to understand the co-precipitation of carbonates and sulfates, a process that has wide applications in cement industry and scaling prevention.