| 其他摘要 | Wujiang River catchment is the typical karst landform, also is one of the most seriously acid rain areas in our country. Elevated atmospheric input of SOx into catchment is detrimental, because it accelerates the depletion of nutrient basic cations from soils, and increases the concentrations of acidic cations (H+, Al3+) in soil porewater and surface water, resulting in the destruction of vegetation and ecology. Even after the acid deposition stop, the soil degeneration can also continue, and the pH may decrease continually. Therefore, it is extremely necessary to strengthen the research on the soil and ecosystem destruction caused by acid deposition in Wujiang River catchement. In the middle and upper reaches of Wujiang River catchment, sulfur-enriching (mainly pyrite) coal and deposit sulfide are widespread. The sulfide may be oxidized to produce massive free metal cations and sulfuric acid, which pollute environment and accelerate the carbonate weathering.
This research study has been carried out under the financial support of Chinese Academy of Sciences, through the research grants KZCX2-105 (Hydro-geochemical cycling of substances in karstic terrain (Wujiang River catchments and its environmental effect) and KZCX3-SW-140 (A geochemical study on nutrient cycling in karstic soil-plant ecological system of the Wujiang River catchment). Some typical rivers in the karst regions of Guizhou Province have been selected as the objects of study. By making use of chemical mass equilibriums and isotope geochemical approaches, the influence of carbonate weathering by sulfuric acid on chemical composition of river water, sources of dissolved sulfate and its temporal and spatial variations of sulfur isotopic composition are discussed / investigated. The weathering of carbonate by sulfuric acid and it’s influence on the release of CO2 to Wujiang River catchment during high-flow period have been calculated on quantitative basis. Meanwhile, the sulfur isotopic compositions of total sulfur (S) and inorganic sulfate in solid phases are used as a tag on the pathways for sulfur biogeochemistry transformations in soil. The results obtained in this work are of significances for better understanding of the factors controlling the sulfur biogeochemistry cycle and their cycling role of sulfur in chemical weathering rate of carbonate, which can be used as an important scientific theoretical base for understanding sulfur evolutions and its environment impact in karstic terrain. The main conclusions have been summarized as follows:
The influence of carbonate weathering by sulfuric acid on chemical composition of river water
The sulfate (SO42−) accounts for more than 25% of the total anions in Wujiang River water; the average SO42− concentration is 0.65mmol/L during low-flow period, 0.17mmol/L higher than that during high-flow period. The dissolved sulfate concentration of mainstream decreases gradually from upstream to downstream. Spatial variations in SO42− concentration of tributaries over the catchment area are obvious: 0.80mmol/L and 0.26mmol/L in upstream and downstream tributary water during low-flow period, respectively. River water samples collected during high-flow period also have similar characteristics. The good positive correlation between SO42−/Na+ and NO3−/Na+, and also SO42−/Na+ and Cl−/Na+ during high-flow period, suggest that the areas may be partly affected by human activities. The SO42− concentrations of Yuanjiang River water are 0.22mmol/L and 0.14mmol/L during low-flow and high-flow period, respectively, far lower than that of Wujiang main stream river water.
Carbonate weathering by both sulfuric acid and carbonic acid are the most important factor that affects the Wujiang River water chemistry. The solutes in Wuyang River come mostly from weathering of dolomite; there is no indication of carbonate weathering by sulfuric acid. In Qingshui river carbonate weathering by sulfuric acid, by carbonic acid and silicate weathering by carbonic acid contribute the water chemistry. Thus it can be seen that the influences of carbonate weathering by sulfuric acid on the chemical composition of river water are different from rivers in karstic region.
The CSI (Calcite Saturation Index), which is computed by water-rock interaction simulation, ranges between −0.2 and 1 in Wujiang River water sampled during low-flow period, great majority of the samples are calcite supersaturated. The average CSI of high-flow period samples are 0.3 higher than that of low-flow period samples, and all the samples are calcite supersaturated. Generally the water samples are supersaturated with PCO2 in river relative to the atmosphere. The results indicate that both calcite depositing and CO2 outgassing from river water are possibly. The calcite and dolomite in all the samples from Wuyang River are supersaturated and average DSI (Dolomite Saturation Index) is far higher than average CSI, whereas the CSI and DSI of most of the samples from the Qingshui River lower than 1, namely, unsaturated. The CSI and DSI differences of both rivers are mainly related to different lithology of drainage catchments, dolomitic limestone in Wuyang River catchment and detrital sedimentary rocks in Qingshui River catchment. In general, the CSI and DSI interrelated with lithology of drainage catchments firstly, and the PCO2, river secondly.
After deducting the solute contributed by carbonate weathering by sulfuric acid from chemical composition of Wujiang River water sampled during high-flow period, the deposition-dissolution characters of calcite and dolomite changed significantly. In partial samples CSI and DSI become < 1 from being > 1 before. Thus it can be concluded that the process of carbonate weathering by sulfuric acid, via changing the chemical composition, has significant influence on the chemical stability of river water.
Sulfur isotope geochemistry of river water and carbonate weathering
The δ34S values of Wujiang River range from −15.7‰ to 18.9‰ during low-flow period, the wide spread in δ34S values suggests that the sulfur load in different segments of the river is controlled by primarily inputs from small streams draining geological terrains with isotopically distinct signatures. The δ34S values vary in a lesser extent during high-flow period than during low-flow period, from −11.5‰ to 8.3‰. In both seasons the δ34S value decreases with the increasing sulfate concentration. A pronounced seasonal variation in isotopic composition of sulfate characterizes the Wujiang River. The δ34S values of the mainstream range from −6.7‰ to −3.9‰ in summer, which is on average 3‰ lower than winter. Spatial variation in sulfur isotopic composition of tributaries over the catchment area is obvious. Wuyang River is in rich of 34S, whereas Qingshui River is characteristic by enriching 32S. The temporal and spatial variations of δ34S values of both rivers are far less significant than Wujiang River.
According to the variations in sulfate concentration and isotopic composition, it can inferred that the sulfate ions in the upper-reach of Wujiang River waters may have three distinct sources, rain water, sulfate produced by the oxidation of pyrite in coal, and sulfate from sulfides deposits. In the lower reaches, the S isotopic composition of the samples lie mainly on a mixing trend between evaporite sulfate and rainwater sulfate, the contribution of sulfate from oxidation of pyrite being lesser. The sulfur isotopic composition of different end members suggests that when the discharge increases during high-flow period, the contribution of sulfate produced by the oxidation of pyrite in coal increases, which results in the decrease of δ34S values of river water. The δ34S values seasonal variation of rain is the minor reason for the Wujiang River.
The SO42− export flux of Wujiang River is calculated approximately 172×1010 g/a. The upper-reach export flux accounts for 80% of the total. The high-flow period export flux accounts for 72% of the total, with contributions from pyrite oxidation in coal, rain, sulfides deposits and evaporite of 45%, 27%, 24% and 4%, respectively. Sulfide oxidation (sulfuric acid production) and subsequent weathering of carbonate are important processes in the study area. During high-flow period, the weathering rate of carbonate by sulfuric acid is on the order of 35.1t/(km2•a) (17.5mm/ka) and the total CO2 releasing flux is approximately 8.1 t/ (km2•a). By deducing the equation of carbonate dissolution, we speculate that 52% of the weathered carbonate comes by in contact with sulfuric acid during high-flow period.
Soil sulfur biogeochemistry in karstic catchment
Total S concentrations in yellow soil are usually lower than 1% throughout the entire profile, whereas in limestone soil the values are always higher than 1%. In the same soil depth of a profile, total S concentration in yellow soil is higher in summer (the growing season) than in winter (the dormant season), but the situation of limestone soil is quite contrary. In general, total S concentrations in soil have significant relation with soil type, and then, the aboveground vegetation type. The variations of total S concentration with depth are not uniform even in different profiles of same soil type.
Inorganic soil sulfate (free plus adsorbed SO42−) contents are markedly relevant to soil types, distinctively higher in yellow soil than in dolomite soil. In topsoil of yellow soil profiles, inorganic sulfate form 2.4% to 6.4% of total soil S. In yellow soil profiles, the proportion of inorganic sulfate increases with increasing soil depth and attains maximum, more than 20% of total S, in the depth interval between 0 ~ 45cm, most probably arising from the adsorption by free iron and aluminum oxides or hydroxides, and then decreases. Sulfate contents differ with different aboveground vegetation type in yellow soil profiles, the seasonal variations of it may be caused by the same reason. Sulfate-S in dolomite soil accounts for no more than 3% of total S, even if the aboveground vegetation is in good condition, it can be washed away easily from soil because of eluviation.
Total S of all the samples are enriched with 34S and the δ34S values are always higher than that of inorganic sulfate. With increasing depth in yellow soil profiles, the δ34S values of total S increase, this may be the result of organic S cycle in which organic S become more and more enriched in 34S.
The sulfur isotopic composition suggests that the inorganic sulfate in topsoil of yellow soil profiles is mainly from atmospheric deposition. The δ34S values of inorganic sulfate in topsoil are higher in summer than in winter and distinctively higher than the δ34S values of rainfall sulfate in summer, suggesting that the inorganic sulfate came from total deposition, not only wet but also dry depositions. It is also possible that the organic S mineralization increases in summer, which results in the δ34S values differences of inorganic sulfate mentioned above. The δ34S values of inorganic sulfate increase with yellow soil depth both in summer and winter. There is a remarkable positive correlation between the δ34S values and contents of inorganic sulfate in the upper horizon soil, whereas in the lower horizon soil, the δ34S values and contents of inorganic sulfate become significantly negative correlate. The former may be caused by the addition of sulfate from organic S, and the latter most probably is the result of dissimilatory reduction of sulfate.
The inorganic sulfate in topsoil of dolomite soil profiles become more and more enriched in 32S from upper part to lower part of Wujiang River catchment. The sulfur isotopic composition suggests that the inorganic sulfate is mainly from rainfall, except the upper part samples in which the sulfur from mine may be another source.
The contribution of soil sulfur to Wujiang River water is still not unclear but it can be concluded that soil sulfur is not an important sulfate sources to Wujiang River water. |
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