Nitrate attenuation in low-permeability sediments based on isotopic and microbial analyses.

This study investigated nitrate attenuation in low-permeability sediments (LPS) in a multi-layer aquifer by integrating hydrochemical, isotopic and microbiological molecular techniques in a field site. In the meantime, the overlying high-permeability sediment (HPS) was also examined on the nitrate attenuation for the sake of comparison. Additionally, laboratory flow-through experiments were conducted to assess the overall nitrate reduction rate in the two types of sediment. The δ15 N-NO3 - and δ34 S-SO4 2- values were more enriched by approximately 37‰ and 15‰ in the LPS than the overlying HPS associated with substantial reductions of the NO3 - and SO4 2- concentration, indicating the occurrence of strong bio-reductions in nitrate and sulfate. The microbial community diversity analyses showed a higher diversity of the denitrifiers encoding nirS- (Shannon Index SI=6.3) and nrf-type gene (SI=2.7), and the sulfate reduction bacteria (SRB) encoding the dsr gene (SI=6.4) in the LPS than in the HPS. The bacterial community structure was influenced by the groundwater hydrochemistry and the redox conditions. Due to the presence of anoxic groundwater with low levels of nutrients, the LPS featured higher abundances of nitrate reducers belonging to Alphaproteobacteria and SRB belonging to the strictly anaerobic class Clostridia relative to the HPS. Notably, chemolithotrophs were abundant in the LPS and likely coupled the reduction of nitrate with the oxidation of iron. Furthermore, the LPS was demonstrated to attenuate nitrate at a rate two times of the HPS in flow-through experiments, and denitrification accounted for approximately 93% of the nitrate reduction. The high nitrate reduction rate of the LPS was likely attributable to its high functional genes diversity. This study confirmed the occurrence of strong nitrate attenuation in the LPS. The LPS was found to play a significant role in protecting aquifers from anthropogenic contamination.