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BACKGROUND Eco-functional areas play an important role in soil and water conservation and biodiversity conservation. The Wushui River is a watershed located in the Nanling ecological function area. However, study on the content and potential ecological risk of heavy metals in the soil of the upstream area of the watershed is scarce. OBJECTIVES To investigate the degree of heavy metals pollution and the potential ecological risk of soils in the upper reaches of the Wushui River. METHODS The soil samples of five land utilization types in the upper reaches of the Wushui River Basin, including mining, industrial, and cultivated land and forest, were collected to analyze the contents of seven heavy metals Cd, As, Cu, Hg, Ni, Pb and Zn in the soil. The Nemerow comprehensive pollution index was used to evaluate the degree of heavy metal pollution, and the Hakanson potential ecological risk index was used to evaluate the potential ecological risk of heavy metals in soil. Principal component analysis was used to investigate the source of heavy metal pollution. RESULTS The results show that the average concentrations of Cd, As, Cu, Hg, Ni, Pb and Zn in the upper reaches of the Wushui River were 1.28, 72.44, 54.62, 0.27, 68.32, 72.29 and 158.42mg/kg, respectively, which higher than the soil background values. The contents of the heavy metals, except mercury in mining land, were higher than those in other types of soils. The single factor pollution index of heavy metals in the mean state show that Cd and As were heavy pollution elements with factors of 5.07 and 3.25, respectively. In mining land, the Cd single factor pollution index reached 13.59. Heavy metal pollution in mining land was the most serious, followed by industrial land. Forest was in a safe state. Potential ecological hazard index evaluation results showed that Cd reached a very strong ecological hazard and As was a strong ecological hazard in mining land. Mining land and industrial land reached a strong ecological hazard, industrial land reached a moderate ecological hazard, and other types of soils were slightly ecologically harmful. The results showed that As, Cd, Cu and Zn came from industrial and mining activities, Ni and Hg mainly came from parent materials, and Pb came from transportation. CONCLUSIONS Heavy metal pollution of soil is relatively serious in the upstream area of the Wushui River Basin. Cd and As are the main risk factors in the area, mainly deriving from mining and industrial activities.

Carbonate weathering and the CO 2 consumption in karstic area are extensive affected by anthropogenic activities, especially sulfuric and nitric acids usage in the upper-middle reaches of Wujiang River, China. The carbonic acid would be substituted by protons from sulfuric and nitric acids which can be reduce CO 2 absorption. Therefore, The goal of this study was to highlight the impacts of sulfuric and nitric acids on carbonate dissolution and the associated deficit of CO 2 uptaking during carbonate weathering. The hydrochemistries and carbon isotopic signatures of dissolved inorganic carbon from groundwater were measured during the rainy season (July; 41 samples) and post-rainy season (October; 26 samples). Our results show that Ca 2 + and Mg 2 + were the dominant cations (55.87–98.52%), and HCO 3 – was the dominant anion (63.63–92.87%). The combined concentrations of Ca 2 + and Mg 2 + commonly exceeded the equivalent concentration of HCO 3 − , with calculated [Ca 2 + + Mg 2 + ]/[HCO 3 − ] equivalent ratios of 1.09–2.12. The mean measured groundwater δ 13 C DIC value (− 11.38‰) was higher than that expected for carbonate dissolution mediated solely by carbonic acid (− 11.5‰), and the strong positive correlation of these values with [SO 4 2 − + NO 3 − ]/HCO 3 – showed that additional SO 4 2 − and NO 3 − were required to compensate for this cation excess. Nitric and sulfuric acids are, therefore, suggested to have acted as the additional proton-promoted weathering agents of carbonate in the region, alongside carbonic acid. The mean contribution of atmospheric/pedospheric CO 2 to the total aquatic HCO 3 – decreased by 15.67% (rainy season) and 14.17% (post-rainy season) due to the contributions made by these acids. The annual mean deficit of soil CO 2 uptake by carbonate weathering across the study area was 14.92%, which suggests that previous workers may have overestimated the absorption of CO 2 by carbonate weathering in other karstic areas worldwide. [ABSTRACT FROM AUTHOR]

The Wujing River, the largest river in Guizhou Province, is one of the most important water resources for social and economical development. Recently, with the fast population proliferation and rapid economic growth, the drainage basin is intensively interfered by anthropogenic activities. The hydrochemistry of surface water was analyzed from the upper-middle reaches of Wujiang River for investigating the hydrochemical characteristics and their main influencing factors. The results showed that the major cations of the four rivers were Ca²⁺ and Mg²⁺, accounting for more than 70%, and the main anions were HCO₃⁻ and SO₄²⁻, occupying more than 85%. The hydrochemical characteristics in the four rivers were found to be of HCO₃-Ca type, and mainly determined by the carbonate rock dissolution, while only a small proportion of them were of HCO₃ · SO₄-Ca type, reflecting the influence of SO₄²⁻ from anthropogenic activities. Compared to hydrochemical data in 1999, there was an obvious increase in cations and anions concentrations, majorly in NO₃⁻, SO2- ion concentrations, which were significantly affected by human activities. The Na⁺, K⁺ , Cl⁻ in the river mainly came from atmospheric precipitation, and Ca²⁺, HCO₃⁻, Mg²⁺, mainly came from carbonate rocks dissolution, while NO₃⁻ and SO₄²⁻ mainly came from human activities. According to principal component analysis and correlation analysis, hydrochemical composition of Liuchong River was affected by human activity, and that in the upstream of Sancha River was controlled by atmospheric precipitation and the dissolution of carbonate rocks, that to the downstream was enhanced by human activities. The main ion of Maotiao River was controlled by atmospheric precipitation and carbonate rocks dissolution, and also affected by human activity. The Nanming River, the Qingshui river's tributary, was mainly affected by human activity, while the middle and lower reaches of Qingshui River were affected by both the atmospheric precipitation and human activity.

Groundwater and surface water from the Upper and Middle Reaches of the Wujiang River were sampled and analyzed for the hydrochemistry and Carbon isotope in DIC. Then hydrochemical characteristics and the main influencing factors were investigated, and the contributions of carbonate dissolution by sulphuric acid to total(Ca2+ + Mg2+) and HCO3 were calculated using the stoichiometry method. The results showed that the advantage cations of groundwater and surface water is Ca2+, which accounted for more than 50% and the advantage anions is HCO3- and SO(4)2-, which accounted for more than 85%. The hydrochemical characteristics of most samples were of Ca-HCO3 type, and a small part of HCO3.SO4-Ca, reflecting a few samples affected by SO(4)2- from human source. The δ13CDIC of groundwater and surface water, varying from -12. 98%o to -6. 36%o with a changeable molar ratio between (Ca2+ + Mg2+) and HCO3- of 1. 11 to 1. 90, indicated that sulfuric acid has an important influence on hydrochemistry and δ3CDIC. The contributions of carbonate dissolution by sulphuric acid to total (Ca2+ + Mg2+) and HCO3 in groundwater ranged from 20. 59% to 92. 87% (average 51. 50%), and from 11. 47% to 86. 69% (average 36. 90%). While the contributions of carbonate dissolution by sulphuric acid to total (Ca2+ + Mg2+) and HCO3- in surface water ranged from 56. 14% to 94. 55% (with an average of 76. 89%), and 39. 02% to 89. 66% (with an average of 64.24%), respectively, demonstrated that sulphuric acid is an important agent of carbon rock weathering. The results of this study have a great significance for the protection and development and utilization of water resources in the Wujiang River basin and for karst carbon cycle research.