Pollution and variation of stream nitrate in a protected high-mountain watershed of Central Taiwan: evidence from nitrate concentration and nitrogen and oxygen isotope compositions

Abstract

This study analyzed the concentration and stable nitrogen (δ15N) and oxygen (δ18O) isotopic compositions of water NO3−, as well as NO3−concentration and δ15N values of soils and manure-sourced fertilizers to assess pollution and variation in stream nitrate at the watershed of the Chi-Chia-Wan Stream (CCWS), a protected high-mountain stream in Central Taiwan. Results indicate a gully (G1) that contributes significantly high NO3− concentration water (up to 122 mg/L) to trunk water as the major pollution source of CCWS. The high NO3− concentration gully water has a close relationship with manure-sourced fertilizer with both having compatible enriched δ15N values. Results also indicate that water mixing over isotopic fractionation processes such as denitrification or assimilation is the major process accounting for variations in concentrations and isotopic values for stream NO3−. Incorporation of gully/tributary water of high NO3− concentration increases both the concentration and isotopic values of trunk water and vice versa for the incorporation of low NO3− concentration tributary water. Despite G1 contributing high NO3− concentration water to the trunk water of CCWS, the concentration of the trunk water is only slightly elevated and is still lower than the required water quality standard due to much lower drainage of the gully water compared to trunk water’s runoff. In addition to gully or tributary water and rainwater, NO3− derived from soil is another important contributor to trunk water. The NO3− contribution of soil to trunk water is greater in summer than in winter. Additionally, NO3− concentrations in soil from ex-cultivated land are significantly lower than that in cultivated land. This means that NO3− contribution from ex-cultivated land soil to trunk water is small and demonstrates that the land-recovery plan that has been underway in the studied watershed for sometime is effective.