Enhancing nitrate attenuation in groundwater via selectively applying surface agricultural practices: A novel and sustainable strategy for non-point source pollution mitigation

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.

Microbial enhanced manganese-autotrophic denitrification in reactor: performance, microbial diversity, potential functions

Autotrophic denitrification technology has gained increasing attention in recent years owing to its effectiveness, economical, and environmentally friendly nature. However, the sluggish reaction rate has emerged as the primary impediment to its widespread application. Herein, a bio-enhanced autotrophic denitrification reactor with modified loofah sponge (LS) immobilized microorganisms was established to achieve efficient denitrification. Under autotrophic conditions, a nitrate removal efficiency of 59.55 % (0.642 mg/L/h) and a manganese removal efficiency of 86.48 % were achieved after bio-enhance, which increased by 20.92 % and 36.34 %. The bioreactor achieved optimal performance with denitrification and manganese removal efficiencies of 99.84 % (1.09 mg/L/h) and 91.88 %. ETSA and 3D-EEM analysis reveled manganese promoting electron transfer and metabolic activity of microorganisms. High-throughput sequencing results revealed as the increase of Mn(II) concentration, Cupriavidus became one of the dominant strains in the reactor. Prediction of metabolic functions results proved the great potential for Mn(II)-autotrophic denitrification of LS bioreactor.

Appropriate stoichiometric ratios of dissolved organic carbon and nitrate can trigger a transition in nitrate removal in groundwater around plateau lakes, Southwest China

Increasing dissolved organic carbon (DOC) in groundwater as a carbon source for microorganisms that stimulate nitrate attenuation is considered a sustainable strategy to mitigate nitrate pollution in groundwater. However, little is known on the stoichiometric ratio of DOC and nitrate required in groundwater nitrate reduction processes, which has become an obstacle for evaluating the current status of DOC limitations on nitrate reduction. Here, the NO3--N and DOC concentrations in groundwater around 8 plateau lakes were investigated, and a microcosm experiment was performed to elucidate the effects of different DOC:NO3--N levels in groundwater on NO3--N reduction, and the current status of DOC limitations on groundwater NO3--N reduction around 8 lakes was further evaluated. The results indicated that nearly 41 % of the groundwater NO3--N concentrations exceeded the WHO threshold for drinking water (11.3 mg L-1) and 79 % of the groundwater DOC concentrations exceeded 5 mg L-1. The differences in groundwater NO3--N and DOC concentrations among the 8 lakes were controlled by the intensity of agricultural and human activities and hydrogeological background. The stoichiometric ratio of DOC:NO3--N regulated the NO3--N reduction process, and groundwater NO3--N accumulation rate appeared to become limited and sharply decreased when the DOC concentration was approximately 10 mg L-1 or when the DOC:NO3--N ratio was close to 1:1, and the DOC:NO3--N ratio threshold for limiting the NO3--N reduction process was approximately 2.25. Based on this threshold, >33 %-86 % of the groundwater samples around the 8 plateau lakes were strongly limited in the reduction of groundwater NO3--N due to a lack of sufficient DOC provides energy for heterotrophic microorganisms. Additionally, we highlight that the sustainable strategy of increasing DOC to stimulate groundwater NO3- attenuation should be combined with short-term strategies to jointly coordinate and control groundwater NO3- pollution.

Field evaluation of carbon injection method for in-situ biological denitrification in groundwater using geochemical and metataxonomic analyses

This study focuses on the bioremediation of nitrate-contaminated groundwater, which has become a significant environmental problem due to the increasing usage of fertilizers and sewage disposal. The nitrate reduction efficiencies of biological denitrification by injection of carbon source in a pilot-scale treatment system setup were investigated at a groundwater contamination site. The field test was conducted using acetate as a carbon source for 22 days to assess the nitrate reduction efficiencies of in-situ treatment. Geochemical parameters and microbial community analysis using next-generation sequencing were performed before and after carbon source injection. After 12 h of reaction time, nitrate concentration decreased from 31.6 to 4.2 mg-N/L at PC-2, and then remained stable at 3.9 mg-N/L. The nitrate reduction rate when acetate was injected was 29.0 mg-N/L/day. Aquabacterium commune, pseudomonas brassicacearum, dechloromonas denitrificans, and Massilia FAOS were dominant species after acetate injection. Predictive metabolic pathway analysis indicated that nitrate reduction metabolisms during injection of acetate were denitrification and assimilatory nitrate reduction to ammonium. The evaluated hazard quotient of nitrate-contaminated groundwater significantly decreased after acetate injection (non-carcinogenic risk decreased from 1.176 to 0.134 for children). This research could provide fundamental information for decision-makers in nitrate-contaminated groundwater quality protection and management.