THE RELATIONSHIP BETWEEN DELTA FORM AND NITRATE REMOVAL FUNCTION REVEALED BY NUMERICAL MODELING EXPERIMENTS

River deltas display a wide range of morphologic patterns that influence how nutrients flow through channels and wetlands on their way to the coast. In order to quantify the role of delta morphology on nitrate fate, we simulated steady state (no tides) reactive nitrate transport for six morphologically different synthetic deltas created using various incoming grainsize distributions in Delft3D simulations. We parameterized our models based on observed relationships between first-order removal kinetics and elevation on Wax Lake Delta (Louisiana, USA), and we validated our approach by simulating nitrate transport in the delta using nitrate observations over a week. Total nitrate retention across the six simulated deltas and Wax Lake Delta, ranged from <1-6%, suggesting a limited capacity of similar river-dominated deltas to remove nitrate from incoming river water due to the inefficiency of nitrate exchange with highly reactive wetlands at greater elevation. Synthetic delta simulations show that nitrate retention increases as channel network complexity and delta topset slope increases. Thus, deltas with complex channel configurations and large proportions of wetlands at higher elevation are likely most efficient at removing nitrate from incoming rivers. Because these factors tend to be associated with coarse-grained deltas, an important implication for sediment diversion projects that aim to reclaim coastal land is that the nutrient removal capacity can be increased by diverting coarser sediment loads.