Riparian biogeochemical hot moments induced by stream fluctuations

Hyporheic exchanges in riparian zones induced by stream stage fluctuations, referred to as bank storage, can influence contaminant transport and transformation when mixing of groundwater and surface waters with distinct chemical signatures occur, which might lead to a high biochemical activity. The effect of bank storage on nutrient transport was analyzed here using a two-dimensional, variably saturated and multispecies reactive transport model, which accounted for the water flow and solute transport and reactions within riparian zones. After verification with field observations, our model demonstrated that high biogeochemical activities occurred at the near-stream riparian zone during stage fluctuation, a process referred to as bank storage hot moment (BSHM). We used Monte Carlo simulations to study the uncertainty of BSHM and related nutrient dynamics to biogeochemical and hydrological factors. The results indicated that stream fluctuations can lead to maximum bank storage volume ranging from 0 to 259 m(3) m(-1) of stream linear length (median = 9.7 m(3) and SD = 53.2 m(3)). Taking denitrification as an example, BSHM can lead to considerable NO3- removal with a median removal rate of 2.1 g d(-1) and SD of 17.2 g d(-1) per meter of stream linear length. The NO3- uptake velocity (median = 2.7 x 10(-5) and SD = 2.4 x 10(-4) m min(-1)) was comparable to that of in-stream transient storage from the literature. This result suggests that BSHM may be a significant process contributing to the nutrient budget at the ecosystem level. Finally, a theoretical framework representing the coupled hydrobiogeochemical controls on riparian hot spots was developed to help predicting when BSHM can become important in a particular stream.