Lateral hyporheic exchange throughout the Mississippi River network

River water circulates vertically through the river bed and laterally through bank sediments(1-4). This hyporheic exchange brings river water into contact with microbes in the adjacent sediments, otherwise known as the hyporheic exchange zone. As such, hyporheic zones act as hotspots for the biogeochemical cycling of carbon, metals and nutrients in rivers(1,2,5-8), and can, for example, influence the export of nitrogen to downstream ecosystems(5,9,10). Here, we calculate the extent and duration of lateral hyporheic exchange throughout the Mississippi River network, using a physics-based numerical model that takes into account the distribution of groundwater baseflow, river discharge, alluvium permeability and river morphology(3,4). We find that in our simulations, lateral exchange occurs throughout the network: practically all of the river water that reaches the mouth of the Mississippi River network has circulated through the lateral hyporheic zone. River water residence time in the hyporheic zone ranges from less than an hour in headwaters to over a month in larger channels. Comparison of the residence times that we derive to a previously reported residence time threshold for denitrification suggests that around one quarter of the lateral hyporheic zones in the Mississippi River network favour denitrification, and thus nitrogen loss. Given that river water can circulate many times through the river bank before reaching the river mouth, and can undergo vertical exchange with the river bed, we suggest that our estimates serve as a lower limit for potential nitrogen loss.