Potential impacts of coal mining activities on nitrate sources and transport in a karst river basin in southwest China.

Typical sources of nitrate pollution in the fragile ecological environment of karst areas, such as agricultural production activities and domestic sewage, have long attracted serious concern. However, coal development can play an equally significant role in releasing the nitrogen fixed in coal into surface watersheds in the form of nitrate, nitrite, or ammonia, consequently threatening the water quality of surface water systems in mining areas. In this study, a typical karst surface watershed system affected by coal mining activities was selected for an in-depth investigation with the aim of realistically assessing the potential contribution of coal mining to nitrogen pollution. The results reveal increasingly concerning nitrate pollution from August 2020 to November 2021 in the Huatan River watershed under the influence of anthropogenic activities, especially mining development and agricultural production. Given that the nitrogen and oxygen isotope compositions of nitrate do not support the presence of denitrification, the variation in the NO3-/Cl- ratio and the relatively stable Cl- concentration may be a reflection of nitrification. Although the leaching of atmospheric precipitation on the strata in the basin promoted the release of nitrogen associated with coal mining, the higher rate of nitrogen cycling in the oligotrophic mine water environment limited the contribution of coal mining to nitrogen pollution in the surface watershed. Specifically, the contribution of coal mining activities to nitrogen pollution in surface karst river is mainly NH4+-N, which contributes 10% or less to the nitrate input to the waters of the Huatan River. The findings thus highlight the necessity of further uncovering the geochemical cycling process of nitrogen during the transport of mine water in the coal mining environment.

Effect of iron cyclic transformation on the natural purification of antimony in contaminated reservoirs of mines.

To further understand the purification mechanism of antimony (Sb) in reservoirs, samples of stratified water and bottom interface sediment were collected in this study. The cross-flow ultrafiltration technique was used to separate the truly dissolved (<1 kDa) and colloidal (1 kDa-0.45 µm) phases of water, and two modified sequential extraction techniques were used to determine the Sb and Fe mineral forms in sediment, respectively. The results showed that the total Sb concentration could decrease from 142.2 µg/L in surface water to 98.6 µg/L at 16 m; this was contributed to by the removal of truly dissolved Sb. In comparison to particulate Sb (>0.45 µm), the formation of colloidal Sb played a greater role in the purification process. There was a positive correlation between Sb and Fe in the colloidal phase (r = 0.45, P < 0.05). The generation of colloidal Fe could be promoted by higher temperatures, pH values, DO, and DOC in the upper layer (0-5 m). However, the complexation of DOC with colloidal Fe inhibited the adsorption of truly dissolved Sb. After entering the sediment, the secondary release of Sb could not increase the Sb concentration in the lower layer obviously, while the supplementation of Fe(III) could further enhance Sb natural purification.

Biogeochemical transformation of sulfur and its effects on arsenic mobility in paddy fields polluted by acid mine drainage.

This study aimed to study the biogeochemical behaviour of sulfur and its potential impact on arsenic in farmland polluted by acid mine drainage (AMD). Soil samples were collected from paddy fields that were near arsenic-rich coal mines. Different geochemical analysis technologies, combined with the study of microbial populations, were used to analyse the enrichment and transformation characteristics of exogenous sulfur in the soil profile as well as their coupling effect with arsenic transformation. The results showed that sulfur, iron, and arsenic were obviously enriched in the soil, and sulfur easily migrated to deeper layers. The content of arsenic in contaminated soil was about 1.30-3.14 times higher compared to standard Chinese soil quality. Polluted soil had a higher content of bioavailable sulfur and adsorbed sulfur accounted for about 29% of the total sulfur. This promoted an increased microbial population abundance of Desulfosporosinus. The δ34S values of the polluted soil were lower than the background, ranging from -0.69‰ to 11.44‰, but were higher than the theoretical values. There was evidence of dissimilatory sulfur reduction and enrichment in 34S. A significant positive correlation was observed between the contents of adsorbed sulfur and total arsenic. The biogeochemical transformation of sulfur was conducive to the transformation of iron oxide-bound arsenic into sulfide minerals, which enhanced its stability. These results provide theoretical support for understanding the cycling transformation and environmental impact of sulfur in paddy fields polluted by AMD.

Using stable isotopes and major ions to identify hydrological processes and geochemical characteristics in a typical karstic basin, Guizhou, Southwest China.

The investigation of hydrological processes is very important for water resource development in karst basins. In order to understand these processes associated with complex hydrogeochemical evolution, a typical basin was chosen in Houzai, southwest China. The basin was hydrogeologically classified into three zones based on hydrogen and oxygen isotopes as well as the field surveys. Isotopic values were found to be enriched in zone 2 where paddy fields were prevailing with well-developed underground flow systems, and heavier than those in zone 1. Zone 3 was considered as the mixture of zones 1 and 2 with isotopic values falling in the range between the two zones. A conceptual hydrological model was thus proposed to reveal the probable hydrological cycle in the basin. In addition, major processes of long-term chemical weathering in the karstic basin were discussed, and reactions between water and carbonate rocks proved to be the main geochemical processes in karst aquifers.

Occurrence and behaviour of nonylphenol and octylphenol in Nanming River, Guiyang City, China.

Occurrence, variation and behaviour of nonylphenol (NP) and octylphenol (OP) were studied in surface water and groundwater in Guiyang, Guizhou Province, southwestern China. Discharge of wastewater from Guiyang City was the main source of alkylphenols (APs) entering the aquatic environment. The concentrations of NP and OP in river water ranged from 40 to 1582 ng L(-1) and from below the lowest limit of detection (LOD) to 67 ng L(-1), respectively. NP and OP were also detected in groundwater. Both NP and OP exhibited spatial and temporal variations in river water and groundwater. It was found that concentrations of NP and OP in river water was low upstream and dramatically increased downstream, and higher concentration of NP was found in winter compared to that in summer. Proportions of NP and OP were trapped by suspended particulate matter (SPM), which accounted for 7.6-50.0% and 3.4-25.6% of their total concentration in the river water system, respectively. Seasonal changes in water flow were responsible for the temporal variations of APs. To determine the behaviour of APs along the river, a mass balance equation based on chloride was used. The results showed that a mixing process was the predominant factor to determine upstream APs concentrations; while the discharge of wastewater controlled the concentrations of APs downstream. Considering the adverse effect of APs on organisms, combined effect modeling was used to assess the toxicity to fish. It was found that the predicted mixture effect for APs in river water on fish vitellogenin induction was low upstream and medium downstream, respectively.