Contrasting nutrients input along with groundwater discharge to east Dongting Lake, central China: A geological perspective

Abstract

The spatial patterns of lacustrine groundwater discharge (LGD) and associated nutrient inputs are crucial for effective management and protection of lakes. Multiple factors have been found to influence the spatial differences in LGD rates and associated nutrients loads, but the influence of geological conditions on the differences have not been well understood. In this study, we quantified LGD rates and associated nutrients loads on the west and east of the lake with contrasting geological conditions of East Dongting Lake (EDL) within central Yangtze catchment and discuss the influence of geology on the spatial differences, through 222Rn mass-balance model, water chemistry coupled with existing geological data. The results showed that LGD rates were 34.76 ± 23.36 mm/d in the east EDL which is characterized by hilly geomorphy, deep/fast/narrow flowing, coarse-grained lakebed and large hydraulic gradients (0.004–0.006). Surprisingly, LGD rates were higher (71.47 ± 52.16 mm/d) in the west EDL which is characterized by alluvial-lacustrine plain geomorphology, shallow/sluggish flowing, clayey or silty lakebed and low hydraulic gradients (0.0002–0.0015). The remaining factor determining the higher LGD rates in the west EDL is the permeability of the porous aquifer connected with the lake. The groundwater around the east EDL existed in a less confined environment, and frequent flushing led to low concentrations of nutrients. By contrast, rapid burial of sediments and deposition of paleo-lake sediments since Last Deglaciation formed an organic-rich and reducing environment, which facilitated the enrichment of geogenic nutrients. As a result, the loads of LGD-derived nutrients in the west generally exceeded that in the east by one order of magnitude. The study provides new understanding of the spatial differences in LGD and associated nutrients input to the large lakes resulting from variations in geological conditions, thus serving as a reference for ecological protection of lakes.