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 NO-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:NO-N levels in groundwater on NO-N reduction, and the current status of DOC limitations on groundwater NO-N reduction around 8 lakes was further evaluated. The results indicated that nearly 41 % of the groundwater NO-N concentrations exceeded the WHO threshold for drinking water (11.3 mg L) and 79 % of the groundwater DOC concentrations exceeded 5 mg L. The differences in groundwater NO-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:NO-N regulated the NO-N reduction process, and groundwater NO-N accumulation rate appeared to become limited and sharply decreased when the DOC concentration was approximately 10 mg L or when the DOC:NO-N ratio was close to 1:1, and the DOC:NO-N ratio threshold for limiting the NO-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 NO-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 NO attenuation should be combined with short-term strategies to jointly coordinate and control groundwater NO pollution.