Hydrochemistry and source apportionment of boron, sulfate, and nitrate in the Fen River, a typical loess covered area in the eastern Chinese Loess Plateau

Fen River Basin (FRB) is water-deficient and strongly influenced by human activities in the eastern Chinese Loess Plateau. The spatio-temporal variation and controlling factors of hyrochemistry and quality, sources of high boron, sulfate, and nitrate of surface waters in FRB were unclear. Major ions, delta 11B, delta 15N, and delta 18O in surface waters in dry season and wet season of FRB were analyzed and correlation analysis (CA), principal component analysis (PCA), self-organizing map (SOM), forward model, and Bayesian isotope mixing model (MixSIAR) were used to solve above problems. Results showed that average riverine delta 11B, delta 15N, and delta 18O of FRB was 7.8%o, 11.2%o, and 1.3%o (1SD), respectively. Dissolved solutes ranked midstream > downstream > upstream with water type of Na +-Cl-, Ca2+-Mg2+-Cl-, and Ca2+-HCO3-, respectively. Low dissolved solutes were in forest areas while high values were in cropland and city areas. SOM analysis indicated that hydrochemistry was both influenced by natural (upstream) and pollutional input (midstream and downstream) and variation between dry season and wet season was minor. The abnormally high boron concentrations were mainly from silicate weathering (43%) and evaporites dissolution of loess (32%), urban and industrial input contributed 15% of riverine boron. High SO42- (207 +/- 267 mg/L, 1SD) was mainly from sulfates. delta 15N and delta 18O analysis indicated that nitrification was the primary N cycling process. Further, MixSIAR showed that NO3- was mainly from municipal sewage (-67%) and the total contribution of chemical fertilizer and soil nitrogen was -30% with slightly higher values in upstream and wet season. Influenced by land-use types, evaporite dissolution, and anthropogenic input, water quality below midstream was worse and strict sewage reduction policies must be developed. This study highlights the significant influence of evaporite dissolution of loess and anthropogenic input (urban and industrial input for B and sewage for NO3- ) on hydrochemistry and water quality.

Automated summary

The Fen River Basin in the eastern Chinese Loess Plateau faces water deficiency and significant human impacts on hydrochemistry and water quality. Analysis revealed that water quality was influenced by both natural and pollution inputs, with lower dissolved solutes in forest areas and higher values in cropland and city areas. High boron levels were mainly attributed to silicate weathering and evaporite dissolution, while SO42- concentrations were mainly from sulfates. Additionally, urban and industrial inputs played a role in riverine boron levels. Nitrification was identified as the primary nitrogen cycling process. Municipal sewage was found to be the main source of NO3-, highlighting the importance of sewage reduction policies for improving water quality.