Tracing the Impacts of Ecological Water Replenishment on the Sources and Transformation of Groundwater Nitrate Through Isotope and Microbial Analysis

Abstract

Ecological water replenishment (EWR) changes the recharge conditions, flow fields, and physicochemical properties of regional groundwater. However, the resulting impacts on mechanisms regulating the sources and transformation of groundwater nitrate remain unclear. This study investigated how EWR influences the sources and transformation processes of groundwater nitrate using an integrated approach of Water chemistry analysis and stable isotopes (δ15N-NO3− and δ18O-NO3−) along with microbial techniques. The results showed that EWR resulted in a decrease in groundwater NO3-N from 12.98 ± 7.39 mg/L to 7.04 ± 8.52 mg/L. Water chemistry and isotopic characterization suggested that groundwater nitrate mainly originated from sewage and manure. The Bayesian isotope mixing model (MixSIAR) indicated that EWR increased the average contribution of sewage and manure sources to groundwater nitrate from 46% to 61%, whereas that of sources of chemical fertilizer decreased from 43% to 21%. EWR exacerbated the impact of sewage and manure sources on groundwater nitrate. Microbial community analysis revealed that EWR resulted in a substantial decrease in the relative abundance of Pseudomonas spp. from 13.7% to 0.6%. Both water chemistry and microbial analysis indicated that EWR weakened denitrification and enhanced nitrification in groundwater. This study showed the effectiveness of integrated isotope and microbial techniques for delineating the sources and transformations of groundwater nitrate influenced by EWR.

Keywords: isotope, Microbial, Ecological Water Replenishment, Nitrate, Groundwater