Spatiotemporal Variations and Driving Factors of Inorganic Nitrogen in Groundwater in Tidal Riparian Hyporheic Zones

Abstract:

The hyporheic zone is a place where river water and groundwater frequently interact， whose internal nitrogen cycle is of great significance for maintaining the ecological health of rivers. Taking tidal rivers in the Shanghai as the research object， the seasonal variations in inorganic nitrogen concentrations in groundwater in the riparian hyporheic zone and their key driving factors were investigated using on-site monitoring， sampling tests and statistical analysis. The results show that ammonia nitrogen （NH??—N） and nitrate nitrogen （NO??—N） concentrations were significantly higher during the hyporheic flow （HF） net inflow period than those during the HF net outflow period， while nitrite nitrogen （NO??—N） concentration was extremely low. With increasing distance from the river， the dominant form of inorganic nitrogen shifted from NO??—N to NH??—N， and NH??—N concentration initially increased and then decreased while NO??—N concentration gradually decreased. The key factors influencing the changes in inorganic nitrogen concentrations in groundwater in the hyporheic zone were hyporheic exchange flux， Eh， and pH during the HF net inflow period ， and Eh and hyporheic exchange flux during the HF net outflow period. The function of hyporheic zone in tidal riverbanks tends to be a nitrogen sink during the HF net inflow period and a nitrogen source during the HF net outflow period.