Assessing occurrences, transformations, and source apportionment of nitrate in a riverine area (Southwestern China) using multi-parameter approaches
Academic Article in Scopus
Study region: Luolouhe (LLH) catchment in the western Yungui Plateau of southwestern China Study focus: Investigating the transformation pathways and source apportionment of nitrate in regional riverine systems remains challenging due to spatial variations in topography, hydrogeology, and land use patterns. Thus, stable isotopes, the Bayesian stable isotope mixing model (MixSIAR), positive matrix factorization (PMF), and local indicators of spatial association (LIAS) were applied to elucidate the sources and contributions of nitrate (NO3¿). New hydrological insights for the region: High NO3¿ concentrations in surface water and groundwater were observed in the alluvial terrace in the LLH catchment. Nitrification was identified as the predominant biochemical process in both water bodies. Two water bodies in our study presented different nitrate sources. Nitrate source contributions ranked as domestic sewage and manure (DSM) (54.00% by SIAR and 62.15% by PMF)> chemical fertilizer (CF) (27.00% by SIAR and 25.51% by PMF)> soil nitrogen (SN) (17.70% by SIAR and 12.30% by PMF) in surface water. For groundwater, nitrate sources ranked as SN (52.40%)> CF (27.30%) > DSM (19.20%) by SIAR, while ranked as SN (58.74%)> DSM (21.43%)> CF (19.65%) by PMF. The non-perfect sewage discharge facilities and the surface runoff contributed to the nitrate sources in surface water. SN dominated the nitrate source due to the high background levels of nitrate in the soil profile. A proportion of nitrate fertilizer was transported into rivers while the rest percolated through the unsaturated zone and accumulated in the groundwater. The soil nitrate input hotspots identified by LISA further illustrated the spatial heterogeneity of the nitrification process. The hotspots scattered in the lower reach indicated an intense nitrification process in this region, facilitated by favorable oxidation conditions. Overall, this study enhances nitrate source apportionment methodologies and provides novel insights into the spatial variability of biochemical processes influencing nitrate concentrations at a regional scale.
