Assessing sources of nutrients in small watersheds with different land-use patterns using TN, TP, and NO3--N

The Seohwa watershed, South Korea. Agricultural practices and urbanization in watersheds increase nutrient loads (total nitrogen (TN), total phosphorus (TP), and nitrate-nitrogen (NO 3 - -N)) in streams and deteriorate water quality. Between October 2017 and October 2018, water samples were collected from a small stream (MS), main tributary (TS), and sub-tributaries (ST) characterized by different land-use patterns. The objective of this study was to identify the sources that determine water quality in the respective streams and to estimate whether water quality was dependent on land-use patterns. TN in MS progressively decreased downstream, whereas such spatial variation was not observed in TS and ST. For TP, TS and ST samples showed elevated concentrations during storm events, but not in MS. These observations resulted from TN and TP derived from natural and anthropogenic sources associated with land-use patterns. Interestingly, TN/TP ratios were strongly negatively correlated with the proportion of residential area (R 2 = 0.92) rather than other land-use patterns such as agricultural area and forest. In addition, statistical analysis and dual isotopes of nitrate (NO 3 - ) showed that the water quality in MS and the one in TS and ST were predominantly affected by natural sources and anthropogenic sources, respectively. Considering the effects of land-use patterns and rainfall on water quality, anthropogenic sources should be precisely identified to manage stream water in small watersheds more effectively. • TN was peaked during winter and declined in summer across main streams. • Agricultural (A) and residential (R) areas were normalized by forest (F). • TN and TP were correlated with A/F and AR/F ratios, respectively. • TN/TP ratios were influenced by rainfall intensity and the proportion of R. • Statistical analysis and isotope data identify main sources for the nutrients.