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To better understand the decreases observed in pH levels downstream from the associated hydroelectric dam, the impounded Wujiang River in Southwest China was investigated. Study results indicate that the average pH decrease from upstream to downstream of the hydroelectric dam could be up to 0.47 units, and pH differences were particularly apparent during water thermal stratification. Notably, pH was controlled by [CO2]/[CO3 2−] ratios in this impounded river. The decrease in [CO2]/[CO3 2−] ratios and associated increases in pH at the surface of the reservoirs were mainly due to the prevalence of photosynthesis, while the reverse phenomenon was observed at the bottom due to respiration. The evidence from δ13CDIC clearly demonstrated these processes. The increase in phytoplankton biomass enhanced this pH decrease, while dissolved organic carbon had limited impacts on the pH variation. The decrease in pH downstream from the hydroelectric dam resulted from the development of pH stratification in the water profile and the dam operations that release bottom waters for hydropower generation. Consequently, the cascade in hydropower development could increase the risk of river acidification.

Inland waters, including rivers and lakes, are increasingly recognized as playing significant roles in the transport, mineralization and burial of organic carbon exported from land. However, in many areas, dissolved inorganic carbon (DIC) dominates the carbon export from catchments. Owing to different production processes and turnover times of organic versus inorganic carbon, CO2 emitted from rivers and lakes may have different impacts on global carbon cycling depending on its origin. Here, pCO2 and dissolved oxygen concentrations were determined, and the ratios of excess CO2 to O2 depletion (ΔCO2/ΔO2) were compared in spring water, river water and lake water in a carbonate catchment located in the southwestern plateau region of China. Results show that groundwater CO2 evasion, at 2.0 g C m−2year−1, is insignificant in terms of terrestrial carbon loss compared with soil CO2 emission. In the rivers, calcite precipitation due to oversaturation is an important mechanism for CO2 production in some seasons. In the lake, HCO3 − contributed approximately 75 % of the total carbon supply to organic matter production and calcite deposition during seasons favoring photosynthesis. The seasons which had high ΔCO2/ΔO2 are the main periods of CO2 emission from the lake, and the extra CO2 may be produced from HCO3 − titration by H+. Thus, lake CO2 evasion was controlled primarily by pH, not respiration. The spring, river, and lake waters mainly process DIC exported from the catchment, of which HCO3 − is primarily derived from carbonate weathering by soil CO2 that, with extraordinarily high ΔCO2/ΔO2, may originate from sources including organic matter decomposition, root respiration (autotrophic), and acid dissolution. Therefore, freshwater CO2 emission is a return pathway of catchment soil CO2 to the atmosphere more than that of net primary production and net ecosystem production.

This study analyzed the water-soluble sulfate (SSO4) and total sulfur (ST) concentrations and their isotopic signatures (δ34SSO4 and δ34ST) in needles of Pinus massoniana Lamb collected from Guiyang (seriously affected by acid rain) and Yunnan areas (acid rain did not occur), China SW. The results indicated that the SSO4 concentrations in needles of Pinus massoniana Lamb collected from several Chinese areas were found to be significantly correlated to ambient sulfur dioxide (R2 = 0.9176, p = 0.01), showing that SSO4 concentrations in needles were more reliable to indicate atmospheric sulfur. The average δ34SSO4 (−7.2 ‰) and δ34STS (−5.1 ‰) in needles in Guiyang areas were significantly lower than those in Yunnan areas (+3.9 and +5.7 ‰, respectively), which were in accordance with the lower δ34S of coals in Guiyang areas than in Yunnan areas. The δ34SSO4 and δ34STS in needles became less positive with a distance from a plant combusted 34S-enriched coals while for another plant combusted 34S-depleted coals, more positive was observed. These results indicated that δ34SSO4 in needles was also a good indicator of atmospheric sulfur sources. A very small difference between δ34 ST and δ34 SSO4 for most needle samples suggested that little isotopic fractionation accompanies sulfur assimilation processes.

The Wujiang River is an important tributary to the Changjiang River that has been intensively impounded for hydropower exploitation. To understand the potential impact of reservoir construction on the riverine inorganic carbon transport, seasonal longitudinal sampling was conducted in four reservoirs Hongjiadu (HJD), Dongfeng (DF), Suofengying (SFY) and Wujiangdu (WJD) along the Wujiang River from April 2006 to January 2007. Results indicated that damming the river induced an obvious discontinuity of water chemistry in the warmer seasons. δ 13C of dissolved inorganic carbon (DIC) ranged from −3 to −11.4 ‰, likely as the results of photosynthesis, respiration and carbonate weathering. During periods of thermal stratification, the addition of CO2 from respiration to hypolimnion and the deep water release for hydropower generation led to higher pCO2 downstream, as well as 13C depletion in DIC and undersaturated to calcite. An estimate of DIC budget indicated that only DF reservoir was the sink for DIC while reservoirs HJD, SFY and WJD were the sources for DIC. However, when the retained water was taken into account, for the reason of water storage occurring mainly in HJD and DF, all reservoirs became the sources for DIC with exporting rates of 26.68, 7.97, 6.22 and 11.80 % for HJD, DF, SFY and WJD, respectively.

We present major ion compositions for water samples from Jinshajiang, Lancangjiang, and Nujiang drainage basins of China, collected in a water-rich period. This was done to determine natural chemical weathering rates on the eastern Himalayan and Qinghai-Tibet Plateau (HQTP), where anthropogenic impacts are considered small. The major ion distribution of the mainstream samples primarily reflects the weathering of carbonates, which accounts for ~46 % of total cations in the samples of Lancangjiang and Nujiang. Evaporite dissolution prevailed in the mainstream samples of Jinshajiang, as evidenced by high total dissolved solids (TDS) (364–479 mg/L) and Cl, SO4, and Na-dominant major element composition. Silicate weathering contributed

Epikarst water, which is one of the most important water resources in karst mountain areas, is extremely sensitive to mining activities. Acid mine drainage (AMD) with high levels of heavy metals can degrade the water quality. A typical coalfield basin was chosen to research the migration process of heavy metals. It was found that the chemical compositions of the stream water in the research field were controlled by the dissolution of carbonate rocks or/and the weathering and oxidation of pyrite in the mining area. Excluding a few sites in the mining area, As(V) was dominant species of arsenic in the form of H2AsO4 − or HAsO4 2− in the research field. Based on the mass balance concept, it was found that fluxes of As, Zn, Cu and Cd in water from the mining area (site 17) affected by AMD were 18, 871, 281 and 12 kg year−1, respectively. Also, concentrations of Cd, Zn, As and Cu in the stream water decreased along the flow, because these ions deposited from the water to the stream bed as the redistribution processes in environment.

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.

There is considerable discussion and uncertainty in the literature regarding the importance of fresh litter versus older soil organic matter as sources of soil dissolved organic carbon (DOC) in forest floor. In this study, the differences of organic carbon concentration and stable isotope composition were analyzed under different background conditions to identify the origins of DOC in forest soil. The data show that there is no significant difference in SOC content between these collected soil samples (P > 0.05), but the litter-rich surface soils have relatively higher DOC concentration than the litter-lacking (P

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.