Rainfall driven nitrate transport in runoff of hilly area by combining time-series monitoring of hydrochemistry and stable isotopes

Abstract

Runoff has declined significantly or ceased in some of the mountain sub-catchments due to climate change and human activity over the past decades. Rainfall events with high intensity generated or increased runoff again, but the effect of accumulated pollutants in land surface and soil on variation of river water quality is not well understood. In this study, time-series (daily) water quality measurements of major ions, water stable isotopes (δ²H-H₂O, δ¹⁸O-H₂O) and nitrate (δ¹⁵N-NO₃^–, δ¹⁸O-NO₃^–) were measured. Combined with the Bayesian model (Stable Isotope Analysis in R), we investigated the effect of single extreme heavy precipitation event in July 2016 and multiple continuous heavy precipitation events in July 2021 on the dynamics and transport mechanisms of nitrate in river water in a typical hilly area of North China, where intensified anthropogenic activities occurred during past several decades. The results showed that the highest concentrations of NO₃^–-N (up to 22.3 mg/L), organic nitrogen (up to 31.8 mg/L) and total nitrogen (up to 55.12 mg/L) were observed in river water in July 2016. Nitrogen concentrations decreased after the single extreme heavy precipitation and remained at low levels through the sampling period. However, the mean concentrations of NO₃^–-N (12.93 mg/L), organic nitrogen (9.01 mg/L) and total nitrogen (23.57 mg/L) during the multiple continuous heavy precipitation were higher than that in the single extreme heavy precipitation, which showed continuous discharge of groundwater to river. The results of δ²H-H₂O and δ¹⁸O-H₂O in river water showed that the runoff mainly originated from the fast flow affected by the single extreme heavy precipitation, while runoff was dominated by the continuous recharge after the multiple continuous heavy precipitation in 2021. The SIAR results revealed that manure and chemical fertilizer were the main nitrate sources after the single extreme heavy precipitation in 2016, which contributed 7.5–42.6 % and 7.9–15.2 % of nitrate in river water, respectively. However, the contribution of manure and chemical fertilizer increased and ranged from 29.6 % to 91.8 % and from 7.3 % to 28.8 %, respectively after the multiple continuous heavy precipitation in 2021. Pollution of the hilly area with intensified anthropogenic activities occurred by a flush of nitrogen input following precipitation events; precipitation intensity is therefore an important factor for the water quality management. Reducing source availability during the wet season may facilitate reduction of nitrogen loading in similar hilly areas.