Daoxian Yuan

Natural and anthropogenic factors affecting the groundwater quality in the Nandong karst underground river system in Yunan, China.

The Nandong Underground River System (NURS) is located in a typical karst agriculture dominated area in the southeast Yunnan Province, China. Groundwater plays an important role for social and economical development in the area. However, with the rapid increase in population and expansion of farm land, groundwater quality has degraded. 42 groundwater samples collected from springs in the NURS showed great variation of chemical compositions across the study basin. With increased anthropogenic contamination in the area, the groundwater chemistry has changed from the typical Ca-HCO(3) or Ca (Mg)-HCO(3) type in karst groundwater to the Ca-Cl (+NO(3)) or Ca (Mg)-Cl (+NO(3)), and Ca-Cl (+NO(3)+SO(4)) or Ca (Mg)-Cl (+NO(3)+SO(4)) type, indicating increases in NO(3)(-), Cl(-) and SO(4)(2-) concentrations that were caused most likely by human activities in the region. This study implemented the R-mode factor analysis to investigate the chemical characteristics of groundwater and to distinguish the natural and anthropogenic processes affecting groundwater quality in the system. The R-mode factor analysis together with geology and land uses revealed that: (a) contamination from human activities such as sewage effluents and agricultural fertilizers; (b) water-rock interaction in the limestone-dominated system; and (c) water-rock interaction in the dolomite-dominated system were the three major factors contributing to groundwater quality. Natural dissolution of carbonate rock (water-rock interaction) was the primary source of Ca(2+) and HCO(3)(-) in groundwater, water-rock interaction in dolomite-dominated system resulted in higher Mg(2+) in the groundwater, and human activities were likely others sources. Sewage effluents and fertilizers could be the main contributor of Cl(-), NO(3)(-), SO(4)(2-), Na(+) and K(+) to the groundwater system in the area. This study suggested that both natural and anthropogenic processes contributed to chemical composition of groundwater in the NURS, human activities played the most important role, however.

Human impacts on karst groundwater contamination deduced by coupled nitrogen with strontium isotopes in the Nandong Underground River System in Yunan, China.

With the rapid increase in population and economy, groundwater quality has degraded in the Nandong Underground River System (NURS), a typical karst underground river developed in carbonate rocks (limestone and dolomite), which is located in an agriculture-dominated area in the southeast Yunnan Province, China. Determining sources of contamination in the groundwater is an important first step toward us improving its quality by emission control. It is with this aim that we reviewed here the benefit of using a coupled isotopic approach (delta15N and 87Sr/86Sr) to trace the origin of contamination in groundwater. Thirty-six representative groundwater samples, which were collected at different aquifers and land use types, showed significant disparities for major element concentrations and Sr and N isotopic composition in the NURS. Nitrate, along with Cl- and SO4(2-) and some Na+ and K+, pollution of groundwater is a significant problem in the NURS. The joint use of nitrogen and strontium isotope systematics in each context deciphered the origin of contamination in groundwater in the NURS as agricultural fertilizers and sewage effluents. Therefore, an increase in knowledge of groundwater geochemistry by means of hydrochemical and isotopic data will be helpful for understanding water-rock interactions and the influence of human activities on the hydrogeochemical environment of karst groundwater and provide a scientific basis for protection and rational utilization of groundwater resources in karst regions.

Hydrochemical indications of human impact on karst groundwater in a subtropical karst area, Chongqing, China

The application of combined isotopic and hydrochemical compositions may be useful for evaluating water quality problems in karst aquifers in which it is difficult to distinguish the sources of solutes from the natural background of those due to human activities. Multiple isotopes (δ13C–DIC, δ34S–SO42−) and chemical parameters were measured in rainwater, groundwater and sewage in order to elucidate the solute sources and impacts from human activities and natural background in the Laolongdong karst catchment in Chongqing Municipality, SW China. Overall, the dissolution of carbonate rock controls Ca2+, Mg2+ and HCO3− content in rainwater and karst groundwater. SO42− originated mainly from gypsum dissolution in karst groundwater. Carbonate rocks in the studied site could be dissolved jointly by H2CO3 from the natural CO2–H2O reaction and other acids (organic acids and HNO3) from sewage and soils. Sewage discharge from urban areas and agriculture activities lead to the increase of NO3−, PO43− and Cl− in karst groundwater. To protect and sustainably utilize the karst aquifer, sewage originating from urban areas must be controlled and treated and the use of fertilizer should be limited.

Assessment of water resource carrying capacity in karst area of Southwest China

Abstract Water resources are essential for sustainable economic and social development, especially in the karst regions of Southwest China. Guizhou Province was selected as a representative area to explore water resource carrying capacity (WRCC) in karst regions. The concepts of carrying capacity of utilization amount of water resource (CCUWR) and gross amount of water resources (CCGWR) were proposed to evaluate the scales of population, agriculture and economy that local water resources can support. Results show that actual values of population and cultivated land were much larger than the CCUWR, indicating that both population and agriculture were overburdened from 1999 to 2012. However, actual values of GDP were smaller than those of CCGWR and CCUWR, suggesting that water resources were in surplus relative to economic development. In comparison between actual values and those of CCGWR, the WRCC potential for the scales of population and cultivated land is low, but the potential for the economy is high. The discrepancy between the water resources and cultivated land was most pronounced in Guizhou. This paper indicates that it is urgent to enforce rational policies to ensure utilization efficiency of water resources in karst regions.

Arsenic in a Speleothem from Central China: Stadial-Interstadial Variations and Implications

In a pilot study, arsenic in a stalagmite (SJ3) collected from Central China was measured, and its association with past climate and environment was explored. Most of the SJ3 arsenic concentrations ranged from 120 to 320 ppb with the highest concentrations associated with relatively warm and humid climatic phases and lowest concentrations with cold and dry phases. The SJ3 arsenic record was very similar to the manganese record of SJ3. Variations of arsenic in SJ3 might be controlled by metal oxides of iron, manganese, and aluminum in karst groundwater at the study site, which in turn were closely related with changes in past climate and environment. A considerable proportion of arsenic was in excess over manganese in SJ3, which might be related with incorporation of arsenic into the calcite lattice during the formation of SJ3. It was speculated that more arsenic was released due to stronger weathering of the surface soils and sequestrated by metal oxides in karst groundwater under warm-humid climatic phases than under cold-dry phases. This suggested that climate shift might alter arsenic balance in sedimentary areas and aquifer systems and potentially exert significant influence on global arsenic contamination.

Stable isotope and aquatic geochemistry of a typical subtropical karst subterranean stream in southwest China

A hydrogeochemical and isotopic study was conducted on the subterranean karst stream, namely the Guancun subterranean stream (GSS). The hydrogeochemical processes of the GSS were controlled through calcite dissolution and precipitation and were driven by the concentration of CO2, which controlled changes in the pH and of PCO2 in the water. The δ18O and δD values of the GSS were within the global meteoric water line and the local meteoric water line, thereby indicating that the water of the GSS comes from precipitation. Certain abnormal δ18O and δD values suggest the effect of evaporation on the GSS given its use in a particular irrigation system, wherein the GSS in transformed into a surface stream and flows for a relatively long time on the surface during the wet season. The δ13CDIC values of the GSS range from −13.5 to −11.3‰ in the dry season and from −13.9 to −9.5‰ in the wet season, thereby indicating that the GSS belongs to a semi-open system. The δ13CDIC values in the GSS were formed by the δ13CDIC values of the soil CO2 and carbonate dissolution at different proportions. According to the simplified mass balance formula, the contributions of carbonate dissolution to the dissolved inorganic carbon (DIC) of the GSS were calculated to be 50.2–58.3% and 48.7–64.7% in the dry and wet seasons, respectively, thereby indicating a less than 50% carbonate dissolution contribution during the formation of DIC in karst groundwater. Moreover, sulfuric acid and nitric acid were observed to participate in karst processes.

Hydrochemical Variation and Its Influencing Factors in Typical Karst Subterranean River in SW China

Groundwater was monitored to investigate its hydrochemical variation and possible influencing factors from April, 2007 to May, 2009 at the inlet (D1) and outlet (S2) of Qingmuguan subterranean river in Chongqing, China. The response of the subterranean river discharge to rainfall events was very fast and dramatic, as the water level rising suddenly and dropping sharply with rainfall amount in the months with more rainfall. The water temperature, electrical conductivity, sodium, magnesium, calcium, bicarbonate, nitrate and chloride concentrations of S2 were higher than that of D1, while pH value, potassium and sulfate concentrations of S2 were lower than that of D1. The pH value, electrical conductivity, sodium, calcium, magnesium and bicarbonate concentrations were lower in May and June, and potassium concentration was higher in September and October, and nitrate and phosphate concentrations showed rising-falling variation obviously in May and June. Hydrochemical variation of the subterranean river was influenced by geological background and karstification intensity, flushing eluviation and dilution effects of rainfall, land use and seasonal farmland fertilization and other anthropogenic inputs. The karst groundwater dynamics are highly sensitive to the changes in the external environment, and the water of subterranean river is bad for drinking directly during the rising-dropping process of water level.

P CO 2 variations of cave air and cave water in a subtropical cave, SW China

Cave CO2 is an important part of the carbon cycle in a karst system. From 2008 to 2009 the partial pressure of CO2 (PCO2) of the cave air and cave water (cave stream and dripwater) in Xueyu Cave was studied. The PCO2 for cave air and cave water over two years showed that CO2 concentrations vary seasonally in all field sites, with higher CO2 concentrations in the wet season and lower concentrations in the dry season. These seasonal CO2 variations of cave air and cave water are attributed to seasonally variable soil CO2 production that is controlled by outside temperature fluctuations. Cave ventilation controls synchronous sharp variations of PCO2 from wet season to dry season or dry season to wet season in cave water and cave air. In the wet season, there were no obvious spatial differences of mean PCO2 in cave stream and cave air from upstream to downstream cave. Conversely, there were significant spatial differences of mean PCO2 in cave stream and cave air from upstream to downstream cave in the dry season because of cave ventilation. Mean dripwater PCO2 in D1 site where near cave entrance is much higher than that of the D2 and D3 sites where located in inner cave because D1 has a shorter flowpath from cave ceiling to soda-straw tip and lesser CO2 degassing along the flowpath. Further work in Xueyu Cave needs to clarify the quantitative relationship of CO2 cycle between cave air CO2 and cave water and focus on the influence of CO2 cycle on speleothem form.

Hydrogeochemical characteristics in karst subterranean streams: a case history from Chongqing, China

Karst subterranean streams are important water resources and life-support systems in karst areas and hydrogeochemistry research is an indispensable tool for karst aquifer protection. There are approximately 380 known subterranean streams in the Chongqing area, which are a fundamental groundwater resource. Hydrogeochemical analysis of 84 karst subterranean streams in Chongqing indicated that the solutes mainly came from the dissolution of carbonate rock (using the Gibbs diagram) and most subterranean streams are influenced by the anthropogenic processes. Also, the hydrochemical characteristics showed clear regional effects in natural background chemistry and human impacts. The calcite content decreases and the dolomite content increase from Triassic to Cambrian carbonate rock (except in the Permian carbonate rock), so that subterranean streams that developed in the Cambrian carbonate rock have the highest Mg2+ concentration. Karst subterranean streams that developed in the Permian strata displayed the lowest Mg2+ concentration due to the lowest dolomite concentration in Permian strata. Under the same geological settings, major hydrochemical parameters of karst subterranean streams showed significant diversity across regions as a result of the varying intensity of human activities. Utilizing the Gibbs diagram, it is concluded the karst subterranean streams are deteriorating under the impact of industry activities, agriculture activities and city sewage.