Distribution of hydrogen and oxygen stable isotopes and pollution indicators in water during a monsoon transitional period in Min River Basin.

Based on 197 monthly river water and groundwater samples and 30 event-scale precipitation samples, our study reports the distribution of hydrogen and oxygen isotopes and pollution indicators in Min River Basin. The variation of δO and d-excess indicate that the water source in the upper main course water is more variable and that in the middle-lower part is relatively stable. Comparison between plots of δH versus δO in the river water and precipitation reflect the dominant water source is different between river water in the upper and middle-lower parts. The electrical conductivity (EC) shows a similar spatial variation trend for main course water collected in four campaigns. The pollutant concentration change at the confluences of main tributaries shows that the inflow of Heishui River and Dadu River leads to decreased NO and Cl, while that of Xi River, Pu River and Fuhe River leads to a leap in NO and Cl. A significant positive correlation is observed between EC and δO, indicating the consistent control of water sources on isotope distribution and water quality. The relationship between elevation and δO in the main course river water suggests that the factors affecting isotope distribution vary spatially. "Altitude effect" can only be observed in October and November for the upper steepest plateau zone due to the spatial variation in the precipitation stored during the wet season. The "inverse altitude effect" is observed for the upper part during the wet season and for the middle-lower part during the whole study period, which can be explained by the contribution of tributaries with different discharge regimes. Our findings show that water source with different discharge regimes can serve as the leading factor controlling the stream component in multi-tributary river basins with large spatial span and may mask the influence of spatial distribution of precipitation.