Multi-method characterization of groundwater nitrate and sulfate contamination by karst mines in southwest China.

Groundwater contamination by nitrate and sulfate in mining areas is a significant challenge. Consequently, the inputs sources of these contaminants and their evolution have received considerable attention, with the knowledge gained critical for improved management of water quality. This study integrated data on multiple stable isotopes and water chemistry data and a Bayesian isotope mixing model to investigate the relative contributions of inputs sources of sulfate and nitrate sources to bodies of water in a karst mining area in southwest China. The outcomes indicated that hydrochemical component in the water bodies of the study area is mainly derived from the dissolution of silicate rocks, carbonate rocks and sulfate minerals as well as the oxidation of sulfides. The human and agricultural wastewater, soil nitrogen, and fertilizers were the predominant inputs sources of nitrate to the mine water environment; the predominant inputs sources of sulfide were mineral oxidation, evaporite dissolution, atmospheric deposition, and sewage. Groundwater is mainly recharged from atmospheric precipitation, and surface water is closely hydraulically connected to groundwater. Nitrogen and oxygen isotope composition and water chemistry indicative of nitrification dominate the nitrogen cycle in the study area. The oxidation of pyrite and bacterial sulfate reduction (SRB) had no significant impact on the stable isotopes of groundwater. The results of this study demonstrate the inputs of different sources to nitrate and sulfate in karst mines and associated transformation processes. The results of this study can assist in the conservation of groundwater quality in mining areas and can act as a reference for future related studies.

Spectral and molecular insights into the characteristics of dissolved organic matter in nitrate-contaminated groundwater.

The characteristics of dissolved organic matter (DOM) serve as indicators of nitrate pollution in groundwater. However, the specific DOM components associated with nitrate in groundwater systems remain unclear. In this study, dual isotopes of nitrate, three-dimensional Excitation emission matrices (EEMs) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were utilized to uncover the sources of nitrate and their associations with DOM characteristics. The predominant nitrate in the targeted aquifer was derived from soil organic nitrogen (mean 46.0%) and manure &sewage (mean 34.3%). The DOM in nitrate-contaminated groundwater (nitrate-nitrogen >20 mg/L) exhibited evident exogenous characteristics, with a bioavailable content 2.58 times greater than that of uncontaminated groundwater. Regarding the molecular characteristics, DOM molecules characterized by CHO + 3N, featuring lower molecular weights and H/C ratios, indicated potential for mineralization, while CHONS formulas indicated the exogenous features, providing the potential for accurate traceability. These findings provided insights at the molecular level into the characterization of DOM in nitrate-contaminated groundwater and offer scientific guidance for decision-making regarding the remediation of groundwater nitrate pollution.

Multi-isotope identification of key hydrogeochemical processes and pollution pathways of groundwater in abandoned mining areas in Southwest China.

Acid mine drainage (AMD) is considered one of the serious environmental issues in the mining area. Understanding the key processes and pathways of hydrogeochemical evolution is critical for the effective control of AMD pollution. Hydrogeochemical analysis along with environmental isotope tracing was utilized to provide information regarding the hydrogeochemical process of groundwater pollution by using the multi-aquifer of abandoned Dashu pyrite in Southwest China as an example. Using the deuterium excess parameter d of groundwater and the results of 2H, 18O, and T analysis, the water-rock interaction intensity was determined. The distribution characteristics of d-T revealed that the groundwater primarily originated from the Quaternary reservoir platform groundwater and that there was a close hydraulic connection among the aquifers. The results of ion analysis and sulfur isotope tracing indicated that the sulfur in groundwater was primarily derived from gypsum dissolution, whereas the sulfur in mine water was primarily derived from pyrite oxidation. The results of the hydrogeochemical inversion indicated that mining activities altered the water level and flow conditions, promoted water-rock interactions, altered the hydrogeochemical process, and caused aquifer and mine water cross-contamination. The findings provide theoretical guidance for identifying the pollution sources and critical hydrogeochemical processes that affect groundwater in depleted mining areas of multi-aquifers and also provide technical support for developing water source control and prevention techniques.