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

Rongyang Cui; Anqiang Chen; Wanli Hu; Bin Fu; Gangcai Liu; Dan Zhang

doi:10.1016/j.scitotenv.2024.170313

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

Sci Total Environ. 2024 Mar 15:916:170313. doi: 10.1016/j.scitotenv.2024.170313. Epub 2024 Jan 24.

Authors

Rongyang Cui¹, Anqiang Chen², Wanli Hu³, Bin Fu³, Gangcai Liu⁴, Dan Zhang⁵

Affiliations

¹ Agricultural Environment and Resources Institute, Yunnan Academy of Agricultural Sciences, Kunming 650201, China; Key Laboratory of Mountain Surface Processes and Ecological Regulation, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu 610299, China; University of Chinese Academy of Sciences, Beijing 100049, China.
² Agricultural Environment and Resources Institute, Yunnan Academy of Agricultural Sciences, Kunming 650201, China. Electronic address: chaq163@163.com.
³ Agricultural Environment and Resources Institute, Yunnan Academy of Agricultural Sciences, Kunming 650201, China.
⁴ Key Laboratory of Mountain Surface Processes and Ecological Regulation, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu 610299, China.
⁵ College of Resources and Environment, Yunnan Agricultural University, Kunming 650201, China. Electronic address: yidan33@163.com.

PMID: 38278230
DOI: 10.1016/j.scitotenv.2024.170313

Abstract

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.

Keywords: Dissolved organic carbon; Groundwater; Nitrate pollution; Nitrate removal.