Determining Nitrate Reduction Rates Under Simulated Turbulent Flow Conditions:a Consequence of Mississippi River Sediment Diversion.

River sediment diversions along the lower Mississippi River have been proposed to help slow the rate of wetland loss in coastal Louisiana by transferring sediment from the River to coastal marshes. Sediment diversions can deliver a sediment subsidy to eroding and subsiding Louisiana coastal wetlands. However, diversions will also deliver significant amounts of nutrients (primarily N as NO₃) to receiving basins with potential for environmental impact. The objective of this study was to determine the fate of nitrate in turbulent surface water conditions. There is a paucity of data on nitrate loss rates in the “near-field” area of diversions, where turbulent conditions impart significant shear stress on the sediment surface, suspending fine grained sediments. Low nitrate reduction in this environment could lead to algal blooms in the receiving basin. Therefore, it critical to determine the nitrate reduction in this near-field environment. Triplicate sediment cores were collected from three sites within a mudflat in an active deltaic setting in coastal Louisiana and subjected to shear stress using a flow-through erosional microcosm system for 24 hr. The erosional system can be used to mimic ranges of shear stress to the sediment surface while simultaneously flowing nitrate-enriched surface water through the system. Nitrate reduction rates were determined under “medium” and “high” shear stress conditions of 0.2 and 0.45 Pa, respectively as well as no shear-diffusion only conditions.  Nitrate reduction rates were significantly greater for the higher shear conditions (P<0.001), experiencing a reduction of 303 + 66 mg N m^-2 d^-1 compared to medium shear at 186 + 55 mg N m^-2 d^-1 compared to the significantly lower no shear, diffusion only treatment at 18.7 + 20.2 mg N m^-2 d^-1 These results indicate that turbulent flow conditions, indicative of large river diversions, can have a significant impact on increased reduction of N from the water column.  Results from this research can help inform modelers and natural resource managers in predicting potential nutrient impacts of river diversions on coastal receiving basins.