Urbanization and agriculture intensification jointly enlarge the spatial inequality of river water quality.

Rivers are severely polluted by multiple anthropogenic stressors. An unevenly distributed landscape pattern can aggravate the deterioration of water quality in rivers. Identifying the impacts of landscape patterns on the spatial characteristics of water quality is helpful for river management and water sustainability. Herein we quantified the nationwide water quality degradation in China's rivers and analyzed its responses to spatial patterns of anthropogenic landscapes. The results showed that the spatial patterns of river water quality degradation had a strong spatial inequality and worsened severely in eastern and northern China. The spatial aggregation of agricultural/urban landscape and the water quality degradation exhibits high consistency. Our findings suggested that river water quality would further deteriorate from high spatial aggregation of cities and agricultures, which reminded us that the dispersion of anthropogenic landscape patterns might effectively alleviate water quality pressures.

[Sources and Biogeochemical Processes of Nitrate in the Laolongdong Karst Underground River Basin, Chongqing].

Samples of sewage, well water, and underground river water of the urbanized Laolongdong karst underground river basin in Chongqing, China were collected during July 2019 and October 2020 and measured to determine the nitrate origin and biogeochemical processes based on geochemistry and dual nitrate isotope (δ15N-NO3- and δ18O-NO3-) data. The results showed that:① the isotopic nitrate compositions of sewage ranged from -3.3‰ to 14.6‰ for δ15N-NO3- and from -5.2‰ to 20.6‰ for δ18O-NO3-, which indicated that nitrate originated from manure and sewage, fertilizer, and soil organic nitrogen. The δ15N-NO3- and δ18O-NO3- of well water varied from 3.1‰ to 12.6‰ and 2.9‰ to 8.9‰, respectively, suggesting nitrate was mainly from soil organic nitrogen and manure and sewage. For the underground river water, the δ15N-NO3- and δ18O-NO3- ranged from 5.6‰ to 28.6‰ and from -2.0‰ to 15.7‰, respectively, suggesting that municipal sewage and manure were the dominate nitrate sources. ② Based on the MixSIAR model, manure and sewage were the primary nitrate source of the underground river water, accounting for 89.1% of the total contribution, whereas the contributions of soil organic nitrogen, fertilizer, and atmospheric precipitation were 4.4%, 3.4%, and 3.1%, respectively. ③ In the basin, the concentration ratios of COD:ρ(NO3-) from low to high were as follows:well water (0.14-5.15), underground river water (0.50-9.36), and sewage (4.08-89.50). Only 50% of well water samples with COD:ρ(NO3-) were slightly higher than 0.65, which is the minimum stoichiometric ratio for denitrification occurrence. This indicated that there were insufficient COD concentrations to support that denitrification occurred in the well water. This was further verified by no significant enrichment of nitrogen and oxygen isotopes. As much as 90% of underground river water samples had a COD:ρ(NO3-) higher than 0.65, and the dual nitrate isotopes were simultaneously enriched with a δ15N:δ18O of 1.8, which is within the ratios ranging from 1.3 to 2.1, indicating that denitrification occurred. The COD:ρ(NO3-) for all wastewater samples was much higher than 0.65, of which 25% were higher than the stoichiometric ratio (29.34) for the occurrence of dissimilation reduction nitrate to ammonium (DNRA). The δ15N-NO3- and ρ(NH4+):ρ(NO3-) of sewage increased simultaneously, indicating that DNRA may have occurred in the sewage.

Nitrate fate and decadal shift impacted by land use change in a rural karst basin as revealed by dual nitrate isotopes.

Nitrate pollution in oxygenated karst aquifers is common due to nitrification and anthropogenic inputs. However, the shift of nitrogen sources influenced by enhanced rural tourism activities and land use changes are not well understood. In this study, hydrochemistry and dual nitrate isotopes of water samples from a rural karst basin in Chongqing, southwestern China were employed to investigate the nitrate fate and its decadal change during the periods from 2007-2008 and 2017-2019. The results showed that δ15N-NO3 and δ18O-NO3 values at the groundwater basin resurgence averaged 9 ± 3.4‰ and 2.5 ± 3.4‰, respectively, with a mean NO3- concentration of 19.7 ± 5.4 mg/L in 2017-2019, clearly exceeding natural background levels. The dual isotope results suggested that nitrification occurred at the sampled sites. From 2007-2008 to 2017-2019, the mean δ15N-NO3 values from the primary sink point and the resurgence of the underground river water samples increased from -0.2 ± 2.1 to 11.2 ± 4.8‰, 4.2 ± 0.9 to 9.0 ± 3.4‰, respectively. A Bayesian mixing model in R (MixSIAR) based on the isotopes revealed that soil organic nitrogen, and manure and sewage proportions for the groundwater increased by 34% and 23%, respectively, while chemical fertilizer and atmospheric precipitation proportions decreased by 32% and 25%, respectively. These decadal changes resulted from reforestation practices and enhanced rural tourism activities in the basin, which were evidenced by the change of land use patterns. The elevated nitrogen load from the rapid development of rural tourism is likely to increase this contamination in the near future if the infrastructure cannot meet the demands. The results from this study could contribute to minimizing environmental health risks in drinking water when rural tourism activities are increasing.

Integrated understanding of the Critical Zone processes in a subtropical karst watershed (Qingmuguan, Southwestern China): Hydrochemical and isotopic constraints.

Improving the management and protection of karst groundwater resources and addressing karst-related environmental and ecological problems still face challenges raised from the limited knowledge on the entire karstic Critical Zone (K-CZ), including soil, epikarst, the vadose and saturated zones. Particularly, there is still a lack of integrated understanding of K-CZ properties and major CZ processes across space and time. In this study, we measured and analyzed the hydrochemical and multiple stable isotopic compositions of soil water, surface- and groundwaters from various compartments of the K-CZ in a typical subtropical karst watershed - Qingmuguan (QKW), Southwestern China, in order to explore the source and spatiotemporal variations of water and solutes (C, N, S) within the K-CZ; thereby elucidating the hydrological and biogeochemical processes and their affecting factors. The results show that (i) the K-CZ of QKW is characterized by high heterogeneity and permeability, with fast and strong hydrologic variations in response to rainfall variability; (ii) water-CO2­carbonate interactions (i.e. carbonate weathering) are remarkably active in different zones and are significantly modulated by hydrologic dynamics and seasonal change in biological activities; (iii) efficient migration of nitrate and sulfate occurs across the surface toward the saturated aquifer zone, which is affected by the source availability, elemental transformation and flow transport processes; (iv) human activities have clearly influenced groundwater quality and the natural K-CZ processes, for example, exogenic acids of anthropogenic origin (e.g. acid precipitation and nitrogenous fertilizers from crop lands) have been proven to be involved in the carbonate weathering, with a contribution of ~20%-30%. Our study highlights the strong coupling of hydrological and various biogeochemical processes and the interactive connection among various layers of K-CZ. Thus, systematical monitoring along the CZ profile and a process-based dynamic approach to elucidating climatic and anthropogenic forcing are necessary to better understand the K-CZ properties and functions.

Nitrate sources and biogeochemical processes in karst underground rivers impacted by different anthropogenic input characteristics.

Nitrate is one of the most common pollution sources in groundwater, particularly in highly vulnerable karst aquifers. The potential for nitrification and denitrification within karst aquifers varies in different settings depending on the extent of anthropogenic inputs, so that accurate identification of nitrate sources can be difficult. Geochemical data and dual nitrate isotopes were measured in this study, incorporating a Bayesian isotopic mixing model, and used to identify nitrate sources, nitrification and denitrification, and quantitatively determine nitrate sources under different extents of anthropogenic inputs in three karst catchments within Chongqing Municipality, SW China: Laolongdong (an urbanized area), Qingmuguan (a suburban village), and Shuifang Spring (a protected natural area). At the Laolongdong catchment, the groundwater was in a reducing condition and enriched in δ15NNO3 (averaging 18.9 ± 6.9‰) and δ18ONO3 (averaging 8.5 ± 4.6‰). Manure and sewage waste were the main contributing nitrate sources. A slope of 1.8: 1 of the dual isotopes suggested a denitrification process occurring in anaerobic conduit flow. Within the Qingmuguan catchment, groundwater had average δ15NNO3 and δ18ONO3 values of 9.7 ± 3.5‰, and 1.9 ± 3.4‰, respectively. The data showed evidence for nitrification, and the contribution of soil organic nitrogen was 52.1%, followed by a contribution of 44.8% from manure and wastewater. At the Shuifang Spring catchment, the mean δ15NNO3 and δ18ONO3 values in groundwater were 8.8 ± 2.9‰, 2.3 ± 4.6‰, respectively. Nitrification was the dominant process and most of the nitrate was derived from soil organic nitrogen. This study suggests that karst underground rivers overlain by urban land use undergo denitrification, while the suburban and relatively pristine karst aquifers are dominated by nitrification, allowing development of a conceptual model for nitrate sources and transformations in karst aquifers from the categories of land use (i.e., urban, suburban, and pristine areas). MAIN FINDING: Anthropogenic activities can change biogeochemical nitrogen dynamics of vulnerable karst aquifers, such that the groundwater overlain by an urban settlement has undergone denitrification, while suburban and pristine areas have been dominated by nitrification.

Nitrate migration and transformations in groundwater quantified by dual nitrate isotopes and hydrochemistry in a karst World Heritage site.

Karst aquifers are extremely vulnerable to pollution, including from nitrate. This research advances a systematic evaluation of water quality dynamics and processes in a relatively pristine karst flow system impacted by seasonal tourism, in particular migration and transformation of nitrate. Water samples from the Shuifang Spring basin (Jinfoshan Karst World Heritage Site, Chongqing, China) were collected for analysis of ion concentrations and dual nitrate isotopes. The principal sampling sites included untreated tap water from the Jinfoshan Holiday Hotel (JHH), JHH septic system effluent, discharged effluent at Sinkhole #1, and groundwater at Shuifang Spring (SFS, the resurgence of the basin). Sampling from April 2017 through April 2018 showed that higher nitrate concentrations were observed at SFS during busy tourist seasons that occurred in the summer drought and winter dry seasons. Between the septic system and SFS, nitrate was diluted by rainwater and infiltration from the matrix regions of the karst aquifer in the wet season. The MixSIAR model quantitatively revealed that nitrate from manure and sewage (M&S), soil organic nitrogen (SON) and atmospheric precipitation (AP) contributed roughly 55%, 41% and 4% on average, respectively. The mixture of effluent and oxygen-rich tap water from the JHH underwent nitrification at the septic system and Sinkhole #1. After the nitrification and decomposition of organic matter with oxygen consumption in the effluent, denitrification was the dominant response to nitrate attenuation in the effluent of Sinkhole #1 during peak tourism periods. The septic effluent sinking into the aquifer deteriorated the groundwater quality, which was controlled by nitrification in the summer drought and winter dry seasons. In contrast, microorganisms in groundwater assimilated nitrate from effluent in the wet season. Because of the relatively pristine ambient background conditions and seasonal tourism, these results can inform interpretation of background and impacted nitrogen processes in a range of karst settings.

[Comparison of the Geochemical Characteristics of Karst Springs of a Vertically Zoned Climate Region under Human Activity: A Case of Shuifang Spring and Bitan Spring in the Jinfo Mountain Area, Chongqing].

To investigate the hydrochemical variation of karstic groundwaters in a vertically zoned climate region affected by human activity, Shuifang Spring and Bitan Spring in the Jinfo Mountain area of Chongqing were selected as a study site. Based on the differences between the natural state and intensity of human activity of these two springs, their hydrogeochemical characteristics and the controlling factors on karstic groundwaters were analyzed by means of independent sample t tests, the Gibbs graphic method, principle component analysis (PCA), and geochemical susceptivity analysis. The results show that differences in karst development in the vertical climatic zone leads to higher total ion concentrations in Bitan Spring than in Shuifang Spring. The hydrochemical types of Shuifang Spring and Bitan Spring are HCO3-Ca and HCO3-Ca·Mg, respectively, which reflect the lithology of their different elevations. Carbonate rock dissolution is the main source of Ca2+, Mg2+, and HCO3- in karstic groundwaters. Hotel sewage discharge supplies SO42-, NO3-, PO43-, K+, and Na+ in Shuifang Spring, which peaked in winter and summer, while hydrochemical parameters of Bitan Spring changed smoothly throughout the year. The water quality of Bitan Spring is better than Shuifang Spring (Shuifang Spring water is classified as Class Ⅳ). PCA shows that the water-rock interaction was the first controlling factor. Hotel sewage discharge and ions from precipitation had important effects on Shuifang Spring and Bitan Spring, respectively. In addition, the effects of soil erosion and leaching caused by precipitation also impact on the water quality of two springs to some extent. The geochemical susceptibility of Shuifang Spring was greater than that of Bitan Spring; therefore, corresponding measures should be formulated according to the characteristics of these differently elevated karst systems when exploiting groundwater resources. This is especially the case for the treatment of hotel sewage.

Coupled hydrogeochemical evaluation of a vulnerable karst aquifer impacted by septic effluent in a protected natural area.

Karst aquifers are highly vulnerable to pollution from human activities. Among sources of these contaminants, septic tank effluent can easily pollute karst aquifers, especially concentrated inputs such as those, for example, from tourist hotels. However, the impacts of septic effluent from relatively large, concentrated inputs on karst aquifers have seldom been assessed previously and therefore provide the focus of this study. Artificial tracer tests, geochemical analysis, and dual nitrate stable isotopes were employed to evaluate the impacts of a concentrated input of septic effluent from the Jinfoshan Holiday Hotel (JHH) on the vulnerable Shuifang Spring (SFS) karst aquifer in a remote mountainous area, the Jinfoshan Karst World Heritage Site within Chongqing Municipality of southwest China. The results of artificial tracer tests showed that the underground flow mainly occurred in a primary conduit with a pooled or bifurcated flow path that connects a sinkhole input to SFS. The high tracer recovery rates suggest that the karst aquifer was characterized by high intrinsic vulnerability to contamination. Chemographs at SFS responded rapidly to the episodic release of effluent from JHH. Decreased pH and dissolved oxygen and elevated turbidity, specific conductance and NH4+ concentrations of SFS resulted from the episodic release of septic tank effluent from the JHH during high-use periods. Although the nitrate concentrations were far below the guideline value of the Standard for Groundwater Quality of China, the isotopes of δ15NNO3 and δ18ONO3 suggest that nitrate flowing from SFS was primarily derived from manure and sewage, in addition to soil organic N. Thus, episodic release of septic effluent provides a challenge to the sustainability of karst groundwater management. The results of this study may be relevant to other remote and mountainous karst environments where tourism provide otherwise scarce economic resources and particularly to protected sites throughout the world.

[Nitrate-Nitrogen Pollution Sources of an Underground River in Karst Agricultural Area Using 15N and 18O Isotope Technique].

The objectives of this study were to reveal the sources of nitrate and the ratio of karst in an agricultural basin based on a 15N and 18O isotope technique and quantitative calculation of the IsoSource model. From May to October 2017, six sampling points in the Qingmuguan river basin, Chongqing, were monitored every 24 d. Results showed that there was a great risk of nitrate pollution in the underground river system, because most NO3--N concentrations of the sampling points exceeded the threshold. Spatially, NO3--N concentrations in the underground river increased from upstream to downstream. Temporally, NO3--N concentrations of Fishpond and Yankou Ponor upstream and Jiangjia Spring downstream were impacted by agricultural fertilizer from May to June and fluctuated from June to September due to precipitation. With decreased agricultural activities, NO3--N concentrations gradually decreased after September. NO3--N concentrations were high in midstream soil water. Daluchi, in the middle and lower reaches, maintained relatively low NO3--N concentrations with stable fluctuations. Dual 15N and 18O isotopic compositions suggested that the upstream nitrates were derived from soil organic nitrogen and a mixture of manure and sewage. The midstream nitrates originated from soil organic nitrogen and NH4+ from fertilizer and rain. Nitrates in the middle and lower reaches were derived from the mixing of manure and sewage, soil organic nitrogen, and NH4+ from fertilizer and rain. Jiangjia Spring, the outlet of the underground river, was seriously polluted by nitrates. It is believed that soil organic nitrogen, NH4+ in fertilizer and rain, the mixing of manure and sewage, and NO3- in precipitation were the main nitrate sources in the outlet. Nitrate source contribution of the outlet was calculated with the IsoSource model. The calculation results showed that manure and sewage, soil organic nitrogen, NH4+ in fertilizer and rain, and NO3- in precipitation contributed 46.4%, 32.6%, 18.6%, and 2.4%, respectively.

[Geochemical Characteristics of Lateral Hyporheic Zone Between the River Water and Groundwater, a Case Study of Maanxi in Chongqing].

The hyporheic zone is a place where river water and groundwater mutually exchange and mix. It plays an important role in protecting the ecology and water quality of river water and groundwater. In order to study the geochemical characteristics of lateral hyporheic zone in river and ground water, water temperature, dissolved oxygen, pH, electrical conductivity were measured automatically at the hyporheic zone of Maanxi in Chongqing. The concentrations of ions in water and elements in sediment within the hyporheic zone were also analyzed. The results showed that the hydrochemical species of lateral hyporheic zone in Maanxi was HCO3-Ca·Mg. Affected by the infiltration of river water, the coefficient variations of water temperature, dissolved oxygen, pH and electrical conductivity in the hyporheic zone were lower than those observed in the river under the buffer action. Along with the farther distance from the riverbank, an anoxia redox environment was formed in the hyporheic zone due to a physical, chemical and biological interactions. An acid and alkali environment was also formed with a decreasing pH trend near the riverbank and hyporheic zone. Under its influence, concentrations of K+, NH4+-N, NO3- and SO42- decreased. Mn, electrical conductivity, and the concentrations of Ca2+, Mg2+, Ba2+ and Sr2+ firstly increased and then decreased, while the concentrations of Fe, Al3+ were elevated. Affected by the long-time interaction of river water and groundwater, the elementary concentrations in the sediment were relatively high at the place of about 30 cm away from the riverbank. This consequently formed a hydrogeochemical gradient in the hyporheic zone. The boundary of the hyporheic zone was inferred at 30 to 50 cm away from the riverbank, whereas the boundary of shallow hyporheic zone was located at 10 cm away from the riverbank. In the process of river water recharging groudnwater, hyporheic zone of river and groundwater played an important role in the purification of water quality.

[Comparison on the Hydrogeochemical Characteristics of Typical Karst Groundwater System in Southwest China, a Case of Qingmuguan and Laolongdong in Chongqing].

To study the hydrogeochemical characteristics of southwestern typical karst underground river influenced by different land-use types and human activities, underground rivers of Qingmuguan and Laolongdong, which have similar geological background in Chongqing, were contrastively analyzed from the scale of monthly and rainfall event variation. By the means of independent t-test and principal component analysis(PCA), the results showed that ions of Ca2+, HCO3-, Mg2+, K+, NO3-, Na+, SO42-, Cl- and conductivity were distinctly different at the outlet of the two observed underground rivers, Jiangjia spring and Laolongdong. Compared with Laolongdong, Jiangjia spring showed larger monthly variation range and mean concentrations of K+, NO3-, but ions of Na+, SO42-, Cl- showed an opposite trend. Water-rock interaction played an important role in the hydrogeochemical characteristics and variations of two observed karst underground rivers. Qingmuguan underground river was mainly affected by agricultural activities, while Laolongdong underground river was dominantly influenced by urban and industrial activities. Owing to the difference in manner and intensity of human activities, the two observed karst underground river systems responded differently during a single rainfall event. The variation of water chemical indicators that highly corresponded to discharge at Jiangjia spring were relatively disordered at Laolongdong. It was shown that the hydrogeochemical characteristic of Qingmuguan was primarily influenced by soil erosion and agricultural activities, followed by water-rock interaction, while the hydrochemical characteristic of Laolongdong was dominantly influenced by water-rock interaction, followed by urban activities, industrial activities and soil erosion.

[Influences of Biological Processes on Geochemical Characteristics: An Example of a Mountain Karst Pool in Spring Season].

Karst hydrologic system is quite sensitive to the surrounding environment, which leads to changes in the quality of karst water within diel, hours even minutes. Many surface water undergoes changes of pH value, dissolved gas, trace elements, nutrition and other hydrochemical parameters in a daily timescale. The Shuifang spring and its draining pool are located in Jinfo Mountain in Chongqing, the middle karst mountain belonging to temperate climate with an elevation of about 2050 m a. s. l. Diel cycles of geochemistry were measured for three days and nights to investigate the influence of biological processes on the geochemistry of the karst pool. Results showed that the geochemistry of Shuifang spring didn't exhibit diel variations, while the pool water appeared diurnal change, even if the variation amplitudes of water temperature, pH value, dissolved oxygen and specific conductance were slight. Under different weather conditions, variation amplitude of the geochemical parameters in the pool appeared discriminatory. pCO2 and concentrations of Ca2+, dissolved inorganic carbon (DIC) decreased during the day time and increased in night, while calcite saturation index (SIc) showed an inverse trend. Those phenomena might be attributed to water temperature change, calcite precipitation or dissolution and the process of metabolism by the aquatic plants in the pool. It was found that the influence of water temperature change on pCO2 accounted for only 0.79% to 10.01% by means of calculation of Henry constants. DIC loss contributed by physical factors, such as temperature and calcite precipitation, accounted for 39%, whereas the metabolism of aquatic plants accounted for 61%.

[Variation Characteristics and Sources of Heavy Metals in an Urban Karst Groundwater System during Rainfall Event].

The groundwater discharge and heavy metal concentrations (Mn, Pb, Cu and As) at the outlet of Nanshan Laolongdong karst subterranean river, located at the urban region in Chongqing, were observed during the rainfall events. Analysis of flow and concentrations curves was employed to study their responses to the rainfall events and explore the internal structure of karst hydrological system. Principal component analysis (PCA) and measurements were used to identify the sources of heavy metals during rainfall. The result showed that the discharge and concentrations of the heavy metals responded promptly to the rainfall event. The variation characteristics of flow indicated that Laolongdong subterranean river system belonged to a karst hydrological system including fractures together with conduits. Urban surface runoff containing large amounts of Mn, Pb and Cu went directly to subterranean river via sinkholes, shafts and karst windows. As a result, the peak concentrations of contaminants (Mn, Pb and Cu) flowed faster than those of discharge. The major sources of water pollution were derived from urban surface runoff, soil and water loss. Cave dripwater and rainwater could also bring a certain amount of Mn, Pb and As into the subterranean river. Urban construction in karst areas needs scientific and rational design, perfect facilities and well-educated population to prevent groundwater pollution from the source.

[Characteristics and Resources of Fly Ash Particles in the Snowpack of Jinfo Mountain, Chongqing].

Snow can preserve the atmospheric information, which makes it become a good media in studying regional environment. Jinfo Mountain with an elevation of 2251.1 m, located at the transition zone between Sichuan basin and Yunnan-Guizhou Plateau, is deeply affected by human activities, and snowfall is the main form of precipitation during the winter. While the literature focus on single spherical particles in this area is uncommon. Five snow samples were collected, and determined morphology and chemical composition of 132 single spherical particles by the scanning electron microscope couples with energy dispersive X-ray spectrometer (SEM-EDS). Results show that snowfall in Jinfo Mountain includes the massive fly ash particles with 1.64 µm in average diameter and 1.09 in average roundness which contains smooth particles, rough particles and soot particles, accounting for 80. 31% , 14. 39% and 5.30% of statistical particles respectively. Furthermore, on the basis of chemical information obtained from EDS, the fly ash particles counted in this research can be classified into 5 types, namely, Si-dominant particles, C-dominant particles, Fe-dominant particles, Al-dominant particles and Ti-dominant particles, which make up 34.09%, 49.24%, 12.88%, 2.27% and 1.52% respectively. In conclusion, it can be inferred, based on the analysis of meteorological information, the properties of fly ash particles, and backward air mass trajectory and dispersion analysis, that C-dominant fly ash mainly comes from daily life and industry activities, Si-dominant fly ash particles may originate from the plant industry located in west Chingqing, north of Guizhou province, central of Hunan province, Zhejiang province, Jiangxi province and the west of Guangdong province, while the activities of foundry and iron or steel plants in the west of Chongqing, the north of Guizhou province and the central of Hunan province may be the main sources of Fe-dominant fly ash particles in our samples.

[Hydrogeochemical characteristics of a typical karst groundwater system in Chongqing].

The two-year hydrologic process, hydrochemistry, and a portion of deltaD, delta18O of both the surface water at the inlet and the groundwater at the outlet, were investigated to identify the spatial and temporal variations of hydrogeochemistry in the Qingmuguan karst groundwater system. Research results show that there are wet and dry periods in the groundwater system owing to the striking influence of seasonal rainfall. The evolution of the chemical compositions in the groundwater is significantly influenced by the water and rock interaction, anthropogenic activities and rainwater dilution. The variations of the chemical compositions in the groundwater exhibit obvious spatiality and temporality. The deltaD and delta18O of the surface water beneath the local Meteoric Water Line of Chonqing indicate that the surface water is strongly evaporated. Furthermore, the deltaD and delta18O of the surface water are more positive in the dry period than in the wet period, showing a distinct seasonal effect. The deltaD and delta18O of the groundwater are quite stable and much negative compared with those of the surface water, which suggests that the rainwater recharge the groundwater via two pathways, one directly through sinkholes and the other via the vadose zone.

[Investigation of nitrogen, phosphorus and microbial contamination in Laolongdong underground river system of Chongqing].

With urbanization, groundwater in China has been widely polluted. Karst groundwater is important in southwest China, and would be difficult to recover once contaminated. NO3(-), PO4(3), NH4(+), total coliform, total E. coli and fecal coliform were chosen as indexes in the study of groundwater of Laolongdong Underground River System in Nanshan Mountain, Chongqing. After a few years of survey, the results showed that NO3(-), NH4(+) and PO4(3-) concentrations in the water were all above the nature value, especially NH4(+) and PO4(3-). The NO3(-) concentration of Guihuawan spring ranged from 19.78-68.55 mg x L(-1), in some months, above the recommended water quality guideline (50 mg x L(-1)) according to Standards for Drinking Water Quality set by World Health Organization. NH4(+) and PO4(3-) concentrations in Laolongdong underground river varied from 2.71-12.92 mg x L(-1) and 0.16-11.22 mg x L(-1). The NO3(-) concentration in Laolongdong underground river was lower than in karst spring; however, the concentrations of NH4(+) and PO4(3-) were higher than in the spring. It seemed that the NO3(-) concentration tended to decrease from 2008 to 2013 in the underground river caused by urbanization, reduction of farmland and reducing environment. However, waste water with a high PO4(3-) concentration led to an increasing trend in the PO4(3-) concentration in underground river. Microbial contamination was extremely serious, and even far exceeded class V of water quality standards of China. For example, the concentration of fecal coliform in the groundwater ranged from 3.4 x 10(4)-3.68 x 10(4) CFU x mL(-1). Because of the special hydrogeological structure, karst depressions, skylights and sinkholes can lead pollutants easily to the underground water. Agriculture activity, sewage from towns, enterprises and residential areas were the major sources of nitrogen, phosphorus and microbial contamination.

[Variation characteristics and environmental significant of trace elements under rainfall condition in karst groundwater].

Chemical dynamics of Qingmuguan karst groundwater system were continuously monitored during the rainfall events. A series of high-resolution concentrations data on trace elements, such as barium, strontium, iron, manganese, aluminum, and other major elements were acquired. Correlation analysis and analysis of concentration curve were employed to identify the sources and migration path of the trace elements. And the formation process of trace elements in groundwater was discussed with the geological background of underground river basin. Research shows that barium and strontium derived from carbonate dissolution appeared to be stored in features such as fissures and pores. These two ions were recharged into the underground river by diffusion during precipitation, which resulted in small changes in the their concentration. However total iron, total manganese and aluminum derived from soil erosion varied relatively widely with strong response to rainfall, attributing to the migration of total iron and aluminum with overland flow to recharge the subterranean river directly via sinkholes while total manganese via soil-rock porous media. The results showed that concentrations of all the five trace elements were below 1 mg x L(-1), and the highest concentrations of total iron, total manganese and aluminum exceeded the limit of drinking water. To some extent, the concentrations of total iron and aluminum may be an indicator for soil erosion and water quality.

[Contamination and ecological risk assessment of polycyclic aromatic hydrocarbons in water and in Karst underground river catchment].

Water samples in Laolongdong underground river catchment were collected to determine the concentration, compositional profiles, and evaluate ecological risk of 16 priority polycyclic aromatic hydrocarbons (PAHs). PAHs were measured by GC/MS. The total concentrations of 16 PAH ranged from 81.5-8019 ng · L(-1) in underground river, 288.7-15,200 ng · L(-1) in karst springs, and 128.4-2,442 ng · L(-1) in surface water. Affected by waste water from Huangjueya town, concentrations of PAHs in underground river were higher than those in surface water and waste water from sinkhole. The PAHs profiles were dominated by 3 ring PAHs. There were differences of monthly variations of PAHs contents in the water, due to waste water, season and different characteristics of PAH. Surface water and waste water from sinkhole played an important role on contamination in the river. The levels of ecological risk were generally moderately polluted and heavily polluted according to all detected PAH compounds in the water.

[Research on the sensitivity of geochemical of underground river in Chongqing Xueyu Cave].

Quoted geochemical susceptivity index and isosensitive line on geochemical susceptivity, analyzed the data of underground rivers of Xueyu Cave in Chongqing from September 2010 to August 2011, we found that the chemical composition of the underground river was controlled by the bedrock, due to the composition of high concentration of Ca2+ and low concentration of Mg2+. Owing to the effects of the monsoon, water chemistry was different between drought season and rainy season: the value of [Mg2+]/[Ca2+] was 0.018-0.051 in the rainy season, but in dry season the value was 0.038-0.064. The value of [HCO3(-)]/[SO4(2-)] was 4.86-36.62 in the rainy season, and 6.23-46.67 in the dry season. The seasonal change of Karstification made Ca2+ and HCO3(-) become the most sensitive ion. As a result of the special hydrogeological structure in Karat area, rain, surface water and groundwater transformed rapidly, which caused the underground river was sensitive to agricultural activities, especially for Cl- and NO3(-), and their sensitive indices were 0.286 and 0.022 respectively. The influence of tourism activities on the underground river was less than the management. The management work of ecological system should be strengthen in the recharge area, thus the largest economic and environmental benefits in the Karst area could be achieved.

[Nitrate storage and transport within a typical karst aquifer system in the paralleled ridge-valley of east Sichuan].

In order to investigate the nitrate storage and transport in the karst aquifer system, the hydrochemical dynamics of Qingmuguan underground river system was monitored online by achieving high-resolution data during storm events and monthly data in normal weather. The principal component analysis was employed to analyze the karst water geochemistry. Results showed that nitrate in Jiangjia spring did not share the same source with soluble iron, manganese and aluminum, and exhibited different geochemical behaviors. Nitrate was derived from land surface and infiltrated together with soil water, which was mainly stored in fissure, pore and solution crack of karst unsaturated zone, whereas soluble iron, manganese and aluminum were derived from soil erosion and directly recharged the underground river through sinkholes and shafts. Nitrate transport in the karst aquifer system could be ideally divided into three phases, including input storage, fast output and re-inputting storage. Under similar external conditions, the karstification intensity of vadose zone was the key factor to determine the dynamics of nitrate concentrations in the groundwater during storm events. Nitrate stored in the karst vadose zone was easily released, which would impair the aquatic ecosystem and pose seriously threats to the local health. Thus, to strengthen the management of ecological system, changing the land-use patterns and scientifically applying fertilizer could effectively make a contribution to controlling mass nutrient input from the surface.