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This study focuses on the chemical and Sr isotopic compositions of the dissolved load of the rivers in the Liao River basin, which is one of the principal river systems in northeast China. Water samples were collected from both the tributaries and the main channel of the Liao River, Daling River and Hun-Tai River. Chemical and isotopic analyses indicated that four major reservoirs (carbonates (+gypsum), silicates, evaporites and anthropogenic inputs) contribute to the total dissolved solutes. Other than carbonate (+gypsum) weathering, anthropogenic inputs provide the majority of the solutes in the river water. The estimated chemical weathering rates (as TDS) of silicate, carbonate (+gypsum) and evaporites are 0.28, 3.12 and 0.75 t/km2/yr for the main stream of the Liao River and 7.01, 25.0 and 2.80 t/km2/yr for the Daliao River, respectively. The associated CO2 consumption rates by silicate weathering and carbonate (+gypsum) weathering are 10.1 and 9.94 × 103 mol/km2/yr in the main stream of the Liao River and 69.0 and 80.4 × 103 mol/km2/yr in the Hun-Tai River, respectively. The Daling River basin has the highest silicate weathering rate (TDSsil, 3.84 t/km2/yr), and the Hun-Tai River has the highest carbonate weathering rate (TDScarb, 25.0 t/km2/yr). The Raoyang River, with an anthropogenic cation input fraction of up to 49%, has the lowest chemical weathering rates, which indicates that human impact is not a negligible parameter when studying the chemical weathering of these rivers. Both short-term and long-term study of riverine dissolved loads are needed to a better understanding of the chemical weathering and controlling factors.

The concentration and isotopic composition of nitrate were analyzed to improve an understanding of nitrate sources and transformation processes in a typical karstic agricultural field in the Houzhai catchment, Guizhou Province, Southwest China. The results revealed that no distinct spatial pattern of content and isotopic composition of nitrate exists in this karst catchment. Nitrate in surface stream (SFS) had slightly lighter isotopic composition and lower concentration compared with nitrate in subterranean stream (STS) during the dry season. Concentrations of SFS nitrate increased to concentrations similar to those of STS during the wet season. The isotopic values indicated that nitrate were mainly impacted by manure sources during the dry season and influenced by a mix of chemical fertilizer and manure during the wet season. The denitrification rates were roughly estimated based on the isotopic compositions of nitrate after considering volatilization and ignoring assimilation. The calculated result showed that approximately one fifth of nitrate load was removed by denitrification in the catchment. Annual nitrate flux from the outlets accounted for 14.2% of applied total fertilizers used in the catchment, approximately 85% of total transported flux from the catchment in the wet season. Furthermore, chemical weathering processes were enhanced by using nitrogen fertilizer because liberated protons and enhanced HCO3− flux were produced through by nitrification. Copyright © 2014 John Wiley & Sons, Ltd.

For the first time, the river water geochemistry of the Songhuajiang Basin, the third largest river system in China, has been studied in order to understand chemical weathering and associated CO2 consumption rate in a mid- to cool temperate and monsoon-controlled climatic zone. The major ion compositions of the river waters are characterized by the dominance of Ca2+ and HCO 3 - , accounting for 46% and 74% of major cations and anions, respectively. The average total dissolved solids (TDS, 116 mg L−1) and total cation concentration (TZ+, 1388 μEq L−1) of the river waters are similar to those of global major rivers. The chemical weathering rates of carbonate, silicate and evaporites in the whole Songhuajiang Basin are estimated to be approximately 5.15, 2.23 and 0.40 t km−2 a−1, respectively. The total rock weathering rate for the whole Songhuajiang Basin is approximately 7.78 t km−2 a−1, which is at the lower end of the spectrum for global major rivers, and is comparable with that of the Amur and the Congo-Zaire River. The estimated CO2 consumption rates for the whole Songhuajiang Basin are 53.4 × 103 mol km−2 a−1 and 66.6 × 103 mol km−2 a−1 by carbonate and silicate weathering, respectively. As a sub-basin, the 2nd Songhuajiang has the highest (18.9 t km−2 a−1) while the Nenjiang River Basin has the lowest total rock weathering rate (5.03 t km−2 a−1), which indicates important controls of regional climate and lithology. Compared to the large rivers of China, total rock weathering rates increase from north to south, supporting the idea that the climate acts as major control on global chemical weathering. The CO2 consumption rate by silicate weathering within the whole Songhuajiang River Basin, though under mid- to cool temperate climate, is in the same order of magnitude as that of Huanghe (82.4 × 103 mol km−2 a−1) and is not much lower than that of Changjiang (112 × 103 mol km−2 a−1), which suggests that the role of Songhuajiang River weathering in long-term climate change cannot be neglected compared to those of the large rivers of China.

This study quantifies surface denudation of carbonate rocks by the first application of in-situ cosmogenic 36Cl in China. Concentrations of natural Cl and in-situ cosmogenic 36Cl in bare carbonates from Guizhou karst areas were measured with isotope dilution by accelerator mass spectrometer. The Cl concentration varied from 16 to 206 ppm. The 36Cl concentrations were in range of (0.8–2.4)×106 atom g−1, resulting in total denudation rates of 20–50 mm ka−1 that averaged over a 104–105 a timescale. The 36Cl-denudation rates showed roughly a negative correlation with the local mean temperature. This preliminary observation may suggest the variations of proportions of chemical weathering and physical erosion in denudation process, depending upon local climatic conditions.

Stable isotope tracing and analysis play an important role in interpretation of hydrological and ecological processes at the watershed scale and can provide information regarding the flow path, water source, nutrient loss and biogeochemical cycles of a system. In this study, environmental isotopes (δ18O-H2O, δD, δ15N-NO3−, δ18O-NO3−) and chemical compositions of surface water in Guizhou Province, China, were measured to evaluate the primary sources of nitrate and characterize the processes affecting nitrate as well as its correlation with vegetation cover in karstic areas. The δ15N and δ18O-NO3− levels ranged from +1.3 to +9.8 ‰ and +4.7 to +16.9 ‰, respectively, which indicated that nitrate in water from the investigated area primarily originated from nitrification of soil organic matter during the sampling period. There was also a wide range of isotopes in the water and high contents of nitrate in karstic areas with poor vegetation cover, indicating that water and nutrient loss were serious problems hindering plant growth in the study areas. For example, there was a positive relationship between isotopic composition and nitrate content in the natural forest and negative relationship in Libo County nearby, which suggested that the nitrate fate was affected by land use and human disturbance.

This paper presents hydrogeochemical and S isotopic characteristics of karstic surface and ground water in the karstic aquifers of Zunyi city of Guizhou Province, SW China. The average δ34S value of SO4 is +2.2‰ (n = 28) in ground waters and +7.0‰ (n = 15) in surface waters in the low flow season, while in the high flow season it is +1.0‰ in ground waters and +2.8‰ in surface waters. Natural and anthropogenic sources of the dissolved SO 4 2 - can be discriminated by a combined approach using water chemistry and stable S isotope data. A pollutant source of SO4 is estimated to have very high TDS contents and a narrow range of δ34S values (0‰ to +5.0‰). The water with a distinctively high δ34S value of +30.5‰ originates from an evaporite (gypsum and anhydrite)-bearing aquifer, and the δ34S values lower than −10‰ suggest an origin by oxidation of sulfide minerals in coal seams with intermediate TDS contents.

The sources of dissolved inorganic carbon (DIC) and rock weathering processes were studied in the Houzhai catchment, a typical karstic catchment in the Changjiang River Basin, Southwest China. The carbon isotopic compositions ( δ 13 C DIC ) of DIC in the samples collected from the catchment vary from − 13.5‰ to − 6.9‰, with a mean value of − 9.8‰. The DIC in the catchment is thereby considered to be mainly derived from soil CO 2 and weathering of carbonate rocks. The DIC concentrations and the δ 13 C DIC values show pronounced seasonal variations, with the lowest values being observed during the high flow season (from May to October). The logPCO 2 values in the waters were positively correlated with the DIC contents and negatively correlated to the saturation index of calcite (SIc) and δ 13 C DIC values. These observations indicate that CO 2 derived from organic matter oxidation plays an important role in the dissolution of carbonate for this typical karstic environment. Based on a chemical mass balance, the weathering rate of carbonate rocks in the Houzhai catchment was estimated to be approximately 133 t/km 2 /year or 584 × 10 3 mol/km 2 /year in terms of CO 2 consumption rate, which is higher than the reported values for other rivers in Southwest China.

Nitrate (NO3−) is major pollutant in groundwater worldwide. Karst aquifers are particularly vulnerable to nitrate contamination from anthropogenic sources due to the rapid movement of water in their conduit networks. In this study, the isotopic compositions (δ15N–NO3−, δ15N–NH4+) and chemical compositions(e.g., NO3−, NH4+, NO2−, K+) were measured in groundwater in the Zunyi area of Southwest China during summer and winter to identify the primary sources of contamination and characterize the processes affecting nitrate in the groundwater. It was found that nitrate was the dominant species of nitrogen in most of the water samples. In addition, the δ15N–NO3− values of water samples collected in summer were lower than those collected in winter, suggesting that the groundwater received a significant contribution of NO3− from agricultural fertilizer during the summer. Furthermore, the spatial variation in the concentration of nitrate and the δ15N–NO3− value indicated that some of the urban groundwater was contaminated with pollution from point sources. In addition, the distribution of δ15N–NO3− values and the relationship between ions in the groundwater indicated that synthetic and organic fertilizers (cattle manure) were the two primary sources of nitrate in the study area, except in a few cases where the water had been contaminated by urban anthropogenic inputs. Finally, the temporal and spatial variation of the water chemistry and isotopic data indicated that denitrification has no significant effect on the nitrogen isotopic values in Zunyi groundwater.

Natural and anthropogenic impacts on karst ground water, Zunyi, Southwest China, are discussed using the stable isotope composition of dissolved inorganic carbon and particulate organic carbon, together with carbon species contents and water chemistry. The waters can be mainly characterized as HCO3–Ca type, HCO3 · SO4–Ca type, or HCO3 · SO4–Ca · Mg type, according to mass balance considerations. It is found that the average δ13CDIC values of ground waters are higher in winter (low-flow season) than in summer (high-flow season). Lower contents of dissolved inorganic carbon (DIC) and lower values of δ13CDIC in summer than in winter, indicate that local rain events in summer and a longer residence time of water in winter play an important role in the evolution of ground water carbon in karst flow systems; therefore, soil CO2 makes a larger contribution to the DIC in summer than in winter. The range of δ13CDIC values indicate that dissolved inorganic carbon is mainly controlled by the rate of carbonate dissolution. The concentrations of dissolved organic carbon and particulate organic carbon in most ground water samples are lower than 2.0 mg C L−1 and 0.5 mg C L−1, respectively, but some waters have slightly higher contents of organic carbon. The waters with high organic carbon contents are generally located in the urban area where lower δ13CDIC values suggest that urbanization has had an effect on the ground water biogeochemistry and might threaten the water quality.

Nitrate pollution of the karstic groundwater is an increasingly serious problem with the development of Guiyang, the capital city of Guizhou Province, southwest China. The higher content of NO3- in groundwater compared to surface water during both summer and winter seasons indicates that the karstic groundwater system cannot easily recover once contaminated with nitrate. In order to assess the sources and conversion of nitrate in the groundwater of Guiyang, we analyzed the major ions, delta(15)N-NH4+, delta(15)N-NO3-, and delta(18)O-NO3- in surface and groundwater samples collected during both summer and winter seasons. The results show that nitrate is the major dominant species of nitrogen in most water samples and there is a big variation of nitrate sources in groundwater between winter and summer season, due to fast response of groundwater to rain or surface water in the karst area. Combined with information on NO3- /Cl-, the variations of the isotope values of nitrate in the groundwater show a mixing process of multiple sources of nitrate, especially in the summer season. Chemical fertilizer and nitrification of nitrogen-containing organic materials contribute nitrate to suburban groundwater, while the sewage effluents and denitrification mainly control the nitrate distribution in urban groundwater.

The chemical compositions of the surface/ground water of Guiyang, the capital city of Guizhou Province, China are dominated by Ca2+, Mg2+, HCO3-andSO42-, which have been derived largely from chemical weathering of carbonate rocks (limestone and dolomite). The production of SO42- has multiple origins, mainly from dissolution of sulfate evaporites, oxidation of sulfide minerals and organic S in the strata, and anthropogenic sources. Most ground water is exposed to soil CO2 and, therefore, the H2CO3 which attacks minerals contains much soil C. In addition, the H2SO4 produced as a result of the oxidation of sulfides in S-rich coal seams and/or organic S, is believed to be associated with the chemical weathering of rocks. The major anthropogenic components in the surface and ground water include K+, Na+, Cl−, SO42-andNO3-, with Cl− and NO3- being the main contributors to ground water pollution in Guiyang and its adjacent areas. The seasonal variations in concentrations of anthropogenic components demonstrate that the karst ground water system is liable to pollution by human activities. The higher content of NO3- in ground water compared to surface water during the summer and winter seasons, indicates that the karstic ground water system is not capable of denitrification and therefore does not easily recover once contaminated with nitrates.

Because of active exchange between surface and groundwater of a karstic hydrological system, the groundwater of Guiyang, the capital city of Guizhou Province, southwest China, has been seriously polluted by anthropogenic inputs of NO3-, SO4(2-), Cl-, and Na+. In this work, delta37Cl of chloride and delta34S variations of sulfate in the karstic surface/groundwater system were studied, with a main focus to identify contaminant sources, including their origins. The surface, ground, rain, and sewage water studied showed variable delta37Cl and delta34S values, in the range of -4.1 to +2.0 per thousand, and -20.4 to +20.9 per thousand for delta37Cl and delta34S (SO4(2-)), respectively. The rainwater samples yielded the lowest delta37Cl values among those observed to date for aerosols and rainwater. Chloride in the Guiyang area rain waters emanated from anthropogenic sources rather than being of marine origin, probably derived from HCl (g) emitted by coal combustion. By plotting 1/SO4(2-) vs delta34S and 1/Cl- vs delta37Cl, respectively, we were able to identify some clusters of data, which were assigned as atmospheric deposition (acid rain component), discharge from municipal sewage, paleo-brine components in clastic sedimentary rocks, dissolution of gypsum mainly in dolomite, oxidation of sulfide minerals in coal-containing clastic rocks, and possibly degradation of chlorine-containing organic matter. We conclude that human activities give a significant input of sulfate and chloride ions, as well as other contaminants, into the studied groundwater system through enhanced atmospheric deposition and municipal sewage, and that multiple isotopic tracers constitute a powerful tool to ascertain geochemical characteristics and origin of complex contaminants in groundwater.

A groundwater system in a karstic terrain is easily subject to pollution and its remediation is difficult once contaminated by human activities. The groundwater samples collected in both winter and summer seasons at Gulyang, SW China, show a wide range of variations in chemical composition and dominance of Ca 2., Mg 2+, SO42-, and HCO3 ions. The anthropogenic inputs include CI, NO3, SO42-, Na +, and K +, as judged from the relationships between different elemental ratios and sewage samples. C1- concentrations of all water samples in the study area vary from 0 to 3.9 mmol/L in winter and are higher than in summer. In most of the samples, SO4 zconcentrations are high, ranging from 0.1 to 11 mmol/L. The studies show that the cycling of sulfur plays an important role in controlling water chemistry and fate of contaminants in groundwater. In order to understand the source of SO42- and the geochemical cycling of some elements in the karstic groundwater environment, we have carried out a study on the variations of S and C1 isotope compositions, in addition to other isotopic tracers such as 87Sr/86Sr and ~13C studied previously. The ~37C1 values of the groundwater samples present a seasonal variation, ranging from 0.00%0 to +2.03%0 in winter and from -1.46%o to +0.29%0 in summer. The surface water samples show a similar range of variations as observed for groundwater. Two rainwater samples collected at Gulyang have a distinct isotopic signature (mean value is --3%0) compared to the ~37C1 values reported in literature. These results indicate that ground water is mainly derived from precipitation and suggest a fast recharge of the karst system by surface water. The G34S values of SO42 in the groundwater range from -20%0 to +22%0, with most of the values between -15 9/00 and +10 9/00 and no seasonality is observed for the G 34S values. Correlations between G34S and C1/HCO3, Ca/SO4, G13C and 87Sr]86Sr are in favor of mixing between at least 3 sulfur sources into the karst system: dissolution of sulfate evaporites, oxydation of sulfide minerals present in coal-containing strata and anthropogenic inputs including atmospheric inputs and domestic discharge. The covariations between G34S, G13C, and 87Sr]86Sr indicate that sulfuric acid produced via oxidation of sulfide minerals and organic sulfur probably takes a significant part in the dissolution of carbonate rocks in this karstic region.