Stable isotopes reveal the water conversion and transition dynamics in a heavily-polluted plateau marginal basin: implications for aquatic ecosystem protection

Abstract

The present study investigated the hydrological connections and mechanisms of water transition based on isotopic tracing in the Baoxing River Basin, an area heavily influenced by human activity located in the plateau edge. The contrasting seasonal isotopic patterns and δD-δ18O plots between river water and groundwater and precipitation indicated that these two waters were recharged by seasonal precipitation and meltwater during the wet season, while the base flow primarily consists of stored water rather than recent input during the dry season. Both river water and groundwater exhibited a significant elevation effect in their δ18O values, however, the elevation gradient was as low as −0.05 to −0.07 ‰/100 m. The low gradient can be attributed to the combined recharge from precipitation and meltwater, which led to an increased contribution of depleted heavy isotopes to surface and groundwater flow as elevation decreases. An inverse water transition time index (ITTI) was calculated for 7 sub-basins based on isotopic damping ratios, which were identified to be positively correlated with the percentage of farmland and negatively correlated with slope. Isotope-derived hydrological information indicated that the extensive agricultural areas on gentle slopes in the Baoxing River Basin may contribute to water quality deterioration and pose increased health threat to the aquatic ecosystems, especially during the dry season with reduced discharge and high evaporation. Accordingly, various measures were proposed to strengthen the control of non-point source pollution from low-slope agricultural land.