Field and Experimental Investigations of the Effect of Hyporheic Exchange on Nitrogen Transport in Lakeshore Lacustrine Sedimentary Aquifers, SW China

In recent years, many plateau lakes have faced a high risk of eutrophication. Sewage interception engineering measures have been implemented around these lakes to reduce the discharge of nitrogen and phosphorus. However, these measures often overlook the exchange between surface water, the hyporheic zone, and groundwater, as well as the mechanisms of nutrient transport and transformation in the engineering area near the lakes. This study investigates the interaction mechanisms of surface water, hyporheic zones, and groundwater and their impact on nitrogen transport following the implementation of sewage interception engineering on the west bank of Erhai Lake in Yunnan Province, China, by combining field investigations and laboratory tracer experiments. Field monitoring results show that hyporheic flux is closely related to seasonal variations in river levels. Flow and tracer transport tests reveal significant differences between the flow patterns in the wet and dry seasons in the hyporheic zone near the lake. Processes such as cation adsorption, denitrification, nitrification, and dissimilatory reduction during the exchange of surface water, hyporheic flow, and groundwater mean that nitrification dominates nitrogen transformation in shallow soil. Denitrification and dissimilatory reduction in deep soil significantly affect nitrogen conversion. The groundwater nitrate content increases with the duration of pond or drain infiltration near the upstream of the lake. This investigation provides insights into flow and nitrogen transport within surface water-groundwater systems and offers a valuable reference for assessing the risk of lake eutrophication in similar lakeshore lacustrine sedimentary aquifers.