Riverine sulfate sources and behaviors in arid environment, Northwest China: Constraints from sulfur and oxygen isotopes

Using sulfur and oxygen isotope values to partition riverine sulfate sources and illustrate their responses to hydrological processes

Orogenesis contributes to the elevation of highly eroded rock strata, whose interactions with the atmosphere, water, and biota result in the release of dissolved substances. The fluvial transport of dissolved sulfate (SO42-) from mountains to oceans is a critical component of the global sulfur cycle. However, anthropogenic activities have significantly increased the concentrations of sulfate and altered isotope compositions. It is imperative to delineate the impacts of anthropogenic disturbances and clear their transport mechanisms. To address this issue, spatial and temporal water samples were collected from the Qin River Basin (QRB) between 2012 and 2015. Sulfate sulfur and oxygen isotope values (δ34S-SO42- and δ18O-SO42-), alongside Bayesian isotope mixing models (BIMMs) were employed to identify pathways of anthropogenic inputs and quantify their contributions. The average mainstream SO42- concentration, δ34S-SO42- and δ18O-SO42- values in the upper reaches (n = 18), middle (n = 9), and lower reaches (n = 44) were 1.09 mmol/L, 1.5 ‰ and 5.6 ‰; 1.34 mmol/L, 6.8 ‰ and 6.9 ‰; 2.31 mmol/L, 8.5 ‰ and 7.7 ‰, respectively. BIMMs results from spatial water samples indicated an increasing trend in contributions from gypsum, loess, sewage and chemical fertilizer but a decreasing trend from coal mine drainage (CMD) and pedogenic sulfate sources to riverine sulfate along the river. Results from temporal water samples at the outlet indicated that pedogenic sulfate, CMD, and loess sulfate were transport-limited, conversely, gypsum and chemical fertilizer were source-limited, and sewage has chemostatic behaviors. Despite a significant reduction in annual water discharge since 1956-2000, the average annual sulfate flux from 2013 to 2015 exceeded historical values, with approximately 45 % of riverine sulfate derived from anthropogenic input, and the flow-weighted average δ34S-SO42- and δ18O‒SO42- values changed to 7.9 ± 1.2 ‰ and 6.4 ± 0.2 ‰. These findings illuminated the profound impacts of anthropogenic inputs on riverine sulfate flux in Qin River and offer a robust methodology for partitioning aqueous pollution sources and delineating their transport mechanisms in the complex environmental settings.

Sulphur and oxygen stable isotopes in acid mine drainage impacted rivers of Meghalaya (India): deciphering the sulphide oxidation pathways

Sulphur and oxygen stable isotopes of sulphate have been used to trace the sources of sulphur into aquatic systems. These isotopes have also been used to understand the transformation and fate of sulphur in the water bodies contaminated by AMD discharge from active and/or abandoned mines. Stable isotopes of oxygen in dissolved sulphate (δ18OSO4) and water (δ18OH2O) have helped to decipher the sulphide oxidation pathways and estimate their contributions. The present study is focused on analysing the composition of sulphur and oxygen stable isotopes of sulphate and oxygen stable isotope in AMD and Lunar-Lukha River water flowing through the coal mining area of the East Jaintia Hills District, Meghalaya, in order to decipher the sulphide oxidation pathways. The results showed that the sulphur stable isotope of sulphate (δ34SSO4) ranged between -12.5 and -8.0 ‰ (VCDT). The oxygen isotope of sulphate (δ18OSO4) ranged between 1.4 and 2.0 ‰ (VSMOW). The oxygen isotope of water (δ18OH2O) was distributed between -6.2 and -4.2 ‰ (VSMOW). Pyrite oxidation was found to be the dominant source of sulphate in the Lunar-Lukha Rivers. The results of the stoichiometric isotope balance model showed that 68-83 % of sulphate derived Fe3+ oxidation pathway, with a high portion of sulphate oxygen derived from water. The sulphite-water oxygen exchange model revealed the release of intermediate sulphoxyanions, suggesting the presence of an oxidation pathway of sulphide minerals to sulphate via sulphoxyanions. The results from this study will be helpful in defining effective remediation strategies to mitigate AMD impacts.

Deciphering dissolved organic matter characteristics and its fate in a glacier-fed desert river-the Tarim river, China

The bioavailable diverse dissolved organic matter (DOM) present in glacial meltwater significantly contributes to downstream carbon cycling in mountainous regions. However, the comprehension of molecular-level characteristics of riverine DOM, from tributary to downstream and their fate in glacier-fed desert rivers remains limited. Herein, we employed spectroscopic and high-resolution mass spectrometry techniques to study both optical and molecular-level characteristics of DOM in the Tarim River catchment, northwest China. The results revealed that the DOC values in the downstream were higher than those in the tributaries, yet they remained comparable to those found in other glacier-fed streams worldwide. Five distinct components were identified using EEM-PARAFAC analysis in both tributary and downstream samples. The dominance of three protein-like components in tributary samples, contrasting with a higher presence of humic-like components in downstream samples, which implied that the dilution and alterations of the glacier DOM signature and overprinting with terrestrial-derived DOM. Molecular composition revealed that thousands of compounds with higher molecular weight and increased aromaticity were transformed, generated and introduced from terrestrial inputs during downstream transportation. The twofold rise in polycyclic aromatic and polyphenolic compounds observed downstream compared to tributaries indicated a greater influx of terrestrial organic matter introduced into the downstream during water transportation. The study suggests that the glacier-sourced DOM experienced minimal photodegradations, with limited influence from human activities, while also being shaped by terrestrial inputs during its transit in the alpine-arid region. This unique scenario offers valuable insights into comprehending the fate of DOM originating from glacial meltwater in arid mountainous regions.

Transformation of sulfur in the sediment-water system of the sewage pipeline under different hydraulic retention time

Transformation of sulfur in sewage pipeline was affected by water flow, and the transformation laws at different locations in the sediment-water system were different. This work studied the changes of sulfur in sediments, sewage, and upper space of the sewage pipeline, analyzed the differences in microbial community under different hydraulic retention time (HRT) and depth, and focused on the transformation law of sulfur. Results showed that sulfate and sulfide concentrations in sewage were higher than those in sediments under anaerobic conditions. Moreover, sulfate and sulfide concentrations in sediments decreased with depth. When HRT decreased from 3 h to 1 h, H2S concentration increased evidently, whereas sulfate concentration decreased in the sewage and sediment, and sulfide concentration increased in sewage and surface sediment. Those differences were related to the relative abundances of the two microbial communities. The relative abundances of sulfate-reducing bacteria (SRB), such as Desulfobacter, Desulfovibrio, and Desulfomicrobium, were higher in surface sediment. Correspondingly, those of Thiobacillus, Bacillus, and other sulfur-oxidizing bacteria (SOB) and Smithella were higher in deep sediment. The decrease of HRT might worsen the mass transfer effect of dissolved oxygen, thereby increasing the production rate of sulfur and causing H2S to easily escape from sewage.