Denitrification in the Stream Hyporheic Zone With In‐Stream Restoration Structure: The Effects of Stream Water Quality, Temperature, Sediment Hydraulic and Transport Properties

ABSTRACT

Changes in fluvial environments and hydrogeological conditions critically govern biogeochemical processes in the hyporheic zone, with noticeable effects on nitrogen cycling. These environmental variations further govern the migration and transformation of nutrients and contaminants within riverine systems. This study established a two‐dimensional numerical model that couples hydrodynamic flow and reactive solute transport to explore the coupled hydrological and biogeochemical behaviours of the hyporheic zone beneath in‐stream restoration structures. A series of sensitivity tests were conducted to quantify how stream water quality, ambient temperature, as well as the hydraulic and solute transport characteristics of river sediments influence the rates of nitrification and denitrification. Numerical modelling outputs reveal that shifts in river surface water quality exert regulatory controls on nitrogen translocation and biological conversion in the hyporheic zone. Specifically, dissolved organic carbon (DOC) availability dominates the source‐sink function of the hyporheic zone for nitrate, exhibiting a stronger regulatory effect than ammonium (NH ₄ ⁺ ) supply. Additionally, fluctuations in ambient temperature also serve as a key factor altering nitrogen transport and transformation processes in this critical interaction zone. As temperature increased, the penetration depth of solute fronts for four reactive species gradually decreased, nitrification rates increased by nearly two orders of magnitude, and denitrification rates first increased and then decreased. Conversely, elevated sediment hydraulic permeability accelerates hyporheic exchange flux across riverbed, which accordingly boosts the elimination efficiency of nitrate. This study deepens the current knowledge regarding the intricate coupling relationships between hydrodynamic behaviours and biogeochemical reactions in artificially modified river hyporheic zone. It provides a scientific basis for formulating adaptive river ecological restoration projects‐such as those addressing water quality regulation, temperature control andsediment property management in river systems ‐ultimately supporting fluvial system restoration and water quality improvement.