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

Area of Science:

• Environmental Science
• Hydrogeology
• Microbiology

Background:

• Nitrate contamination poses a significant threat to groundwater quality and ecosystem health.
• Aquifer sediments play a crucial role in natural attenuation processes, including nitrate reduction.
• Understanding the differences in nitrate attenuation between sediment types is vital for effective groundwater management.

Purpose of the Study:

• To investigate and compare nitrate attenuation in low-permeability sediments (LPS) versus high-permeability sediments (HPS) within a multi-layer aquifer.
• To elucidate the hydrochemical, isotopic, and microbiological factors controlling nitrate reduction in different sediment types.
• To assess the potential of LPS in protecting aquifers from anthropogenic nitrate contamination.

Main Methods:

• Field investigations integrating hydrochemical, isotopic (δ¹⁵N-NO₃^-, δ³⁴S-SO₄^2-), and microbiological molecular techniques.
• Laboratory flow-through experiments to quantify nitrate reduction rates in LPS and HPS.
• Analysis of microbial community diversity and structure, focusing on genes involved in denitrification and sulfate reduction.

Main Results:

• LPS exhibited significantly higher enrichment in δ¹⁵N-NO₃^- and δ³⁴S-SO₄^2-, indicating strong bio-reduction of nitrate and sulfate compared to HPS.
• Microbial analysis revealed higher diversity of denitrifying and sulfate-reducing bacteria in LPS, with Alphaproteobacteria and Clostridia being more abundant.
• LPS demonstrated a two-fold higher nitrate attenuation rate than HPS in flow-through experiments, with denitrification accounting for 93% of the reduction.

Conclusions:

• Low-permeability sediments possess a high capacity for natural nitrate attenuation, primarily through microbial denitrification.
• The enhanced microbial community and favorable redox conditions in LPS contribute to their superior nitrate removal efficiency.
• LPS play a critical role in safeguarding aquifers from anthropogenic nitrate pollution, highlighting their importance in groundwater protection strategies.