| Karst region abounds in groundwater, providing a large amount of4.77billion m3/a water resources for local people. Due to the fragile hydrogeologic structure of karst, it leads to intrinsic vulnerability of environment with a lack of protective soil which can prevent contaminants derived from surface activities entering the karst aquifers and subterranean stream. Especially, in a case of urbanization, the increasing waste gas and waste water discharged from the residential areas are pouring into the groundwater system to contaminate the system. In this study, a case from the Laolongdong Subterranean stream system in Chonging, China, was chose, targeting the effects of groundwater from natural and anthropogenic factors based on hydrochemistry and C、S、Sr isotopes in the process of urbanization.The Laolongdong subterranean system responds quickly to outside environmental changes. The variations of hydrological processes showed characteristics of karst conduits that both the recharge and discharge were strongly dynamic. Precipitation is the main recharge of karst groundwater. The rainfall in study area always appears in April to September. In this period of time, the discharge of groundwater is much larger than the opposing seasons and concentrations of Ca2+、Sr2+, SO42-、PO43-also showed the similar trends to the discharge.The hydrochemistry of precipitation is Ca-SO4-HCO3, two springs showed a type of Ca-HCO3-SO4and Ca-Mg-HCO3-SO4respectively, whereas the subterranean stream presented a type of Ca-HCO3-SO4, which reflects the dominance of carbonate rocks. The hydrochemistry of Laolongdong subterranean stream stays in the midst of three end-members, namely, epikarst springs, rainwater and sewage water, which suggest contributions of multiple recharges. Thus, the formation of karst groundwater is affected by geological processes, precipitation and changes of land use. Domestic water showed a type of Ca-Na-HCO3. Four principal components which amount to86%of accumulated contribution were extracted to represent geological or lithologic backgrounds, domestic sewage water, soil erosion and nitrification, separately. The first component, related to Ec, Ca2+, Mg2+, HCO3-, illustrates contributions from the dissolution of carbonate rocks. The second component, related to K+and PO43-, showed contributions from discharge of domestic sewage water. The third component is highly connected to total Fe, total Mn, Al3+and discharge, which demonstrates the soil erosion attacked by precipitation. The fourth one was only correlated to NO3-, showing contribution from nitrification process.Attributed to the dilution processes of raining, DIC concentrations of groundwater were lower in high-flow season than in low-flow season, specificly, domestic sewage water (467.8mg/L)> Zhaojiayuanzi Spring (346.6mg/L)> Laolongdong subterranean stream (306.5mg/L)> Guihuawan Spring (292.5mg/L). The highest carbon isotopes took place in the spring and summer, which was different from previous reports that carbon isotopes tend to be light in summers because of the influence of the active biological processes in soils. There was no significant relationship between δ13CDiC and DIC, indicating they were originated from multiple sources and controlled by different factors.It was proved that nitric acid and sulfuric acid took part in dissolving carbonate rocks, which leads to heavy carbon isotopes. The range of carbon isotopes in subterraneam stream was from-13.3‰to-4.8‰with an average value of-9.8‰. While in the two springs, the average values of carbon isotopes were-12.2‰and-12.4‰, respectively, much lighter than the subterranean stream.Sr is mainly originated from the dissolution of carbonate rocks based on analysis of Sr2+concentration and87Sr/86Sr ratios. However, anthropogenic-originated Sr had similar rang of87Sr/86Sr ratios, which may elevate87Sr/86Sr ratios in groundwater. Even though we observed that elevated87Sr/86Sr ratios could be attributed to some anthropogenic activities, some further evidences should be introduced to support this.Features of δ34S showed that there were many sources to contribute to concentrations of SO42-Especially, local precipitation was acid with high concentration of SO42-and a sulphur-bearing seam of coal as well as anthropogenic activities, such as fertilizing, could make contributions to SO42-of karst groundwater. |
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