Seasonal variations of groundwater discharge and associated nutrient fluxes in Changhu Lake

To reveal the contribution and seasonal variation of groundwater discharge to lake water and nutrients budgets, this study investigated Changhu Lake in the middle reaches of the Yangtze River. Field sampling was conducted during both wet and dry seasons using multiple tracing techniques, including electrical conductivity (EC), stable isotopes (2H and 18O), hydrochemical elements (Ca2+ and Mg2+), and 222Rn isotopes. The 222Rn mass balance model was employed to quantify LGD and associated nutrient fluxes in different seasons. Our results show that LGD rate during the wet and dry seasons was 64.52 mm/d and 14.95 mm/d respectively, with a significant difference between these seasons. Furthermore, during the wet and dry seasons, groundwater carried TN inputs of approximately 25.68×106 g/d and 5.58×106 g/d, respectively. While TP inputs were approximately 8.14×106 g/d and 0.17×106 g/d in wet and dry seasons, respectively. Differences in LGD rate between the wet and dry seasons elucidated the variation in TN and TP input flux carried by groundwater, and the input of TP during the wet season was also influenced by agricultural activities during this season. The greater precipitation and evaporation experienced during the wet season drove higher LGD intensity and increased TN and TP fluxes. Overall, our study provides important theoretical insights that can inform water resources management and aquatic ecosystem preservation efforts in the Changhu area.