Quantifying the Water Quality Benefits of a Constructed Brackish Marsh and Tidal Stream System Using Continuous Water Quality and Flow Monitoring

Based on current knowledge of marsh biogeochemical processes, marshes constructed between upstream sources of nutrients and downstream estuaries should reduce the amount of non-point source pollution reaching these sensitive waters. However, the true effectiveness of created marsh systems for improving water quality is unknown. A tidal stream and brackish marsh system was constructed between drained row crop agricultural production and an estuary in North Carolina. The system was intensively monitored to establish an accurate nutrient and suspended solids mass balance in order to quantify the water quality benefits of the created marsh. The marsh was constructed using a reference based design with the goal of reducing the load of nutrients reaching the downstream estuary while providing habitat and other ecosystem services equivalent to those in a natural marsh. The primary pollutant of concern at this site was nitrate as the adjacent estuary is nitrogen limited.;Ultraviolet-visual (UV-Vis) spectrometers were used to increase the temporal resolution of water quality measurements and decrease the error in mass balance calculations. Using recently developed instrumentation for data collection in this project resulted in multiple advancements in the use of in-situ UV-Vis spectrometers. An anti-fouling system was developed to combat optical fouling and increase confidence in the results. Partial least squares regression was used to expand the application of spectrometers to measure multiple water quality parameters including salinity, nitrate, total kjeldahl nitrogen, dissolved organic carbon, phosphate, total phosphorus, particulate organic matter, and total suspended solids in brackish marsh waters.;The mass of nutrients entering and leaving the restored system through the tidal stream was calculated using high frequency water quality measurements and continuous flow monitoring over a 20 month period. Long-term intensive monitoring allowed seasonal variations in concentrations to be observed while capturing both normal tidal fluctuations and extreme events. Results show that the marsh retained 40 kg of nitrate or 9% of the nitrate that entered the marsh from the upstream agricultural production. Despite the regular movement of water due to tides, five major events accounted for over 70% of the nitrate flux in the marsh and over 90% of the nitrate retention occurred during four of the events. These major storm events produced rainfall that mobilized nitrate in the cropland and delivered it to the marsh. This demonstrates the importance of capturing rainfall events in quantifying nutrient fluxes even in systems driven by tidal flow. The retention of nitrate was hindered by the low residence time of water in the marsh during the events. There was a net export of all the other measured nutrients from the marsh during the monitoring period. The export of total kjeldahl nitrogen (120 kg) and dissolved organic carbon (1,400 kg) were expected based on current knowledge of marsh biogeochemical processes. The export of phosphate (2 kg) was likely due to prior agricultural production on the site. The export of total suspended solids (3,000 kg), although minor, was not expected and is a concern for the marsh's ability to adjust to sea level rise. Tidal marsh construction can contribute to reducing coastal eutrophication, but other practices should also be utilized concurrently to significantly reduce non-point source pollution.