IDENTIFYING POTENTIAL TIME LAG RESPONSES THROUGH LONG-TERM WATER QUALITY TRENDS IN A RESTORED RIPARIAN WETLAND STREAM COMPLEX IN THE PIEDMONT OF NORTH CAROLINA

Restoration of impaired ecosystems often experience hydrological and biogeochemical time lags between the restoration implementation and ecosystem recovery and water quality improvements, but there is a lack of long-term studies that have adequately evaluated completed projects to identify these time lags. Water quality is strongly tied to biogeochemical and hydrological functions, so one the best ways to see delayed responses of streams or rivers to nutrient and sediment changes is to assess data pre- and post- restoration.

Duke’s SWAMP project has been monitoring pre- and post- construction responses, but water quality has not been fully analyzed since SWAMP’s initial five-year assessment in 2011. Since 2011, Duke campus has expanded and SWAMP underwent two additional phases. Water collection and monitoring has been ongoing since 2000, providing enough time and data to identify lag times or water quality trends. This report analyzes SWAMP from 2008 to 2020 to determine water quality, ecosystem efficiency in retaining nutrients or sediments, and seasonal trends.

The first section of the report is a site analysis of water quality variables at the SWAMP site that compares Pre-2008 and Post-2008 water quality based on the Richardson (2011) study. Results showed Pre-2008 and Post-2008 water quality was not statistically different, which may be evidence of a lag time in ecosystem response to nitrogen and phosphorus or a change in input concentrations. Comparing individual site mean differences indicate SWAMP may be receiving worse water quality inputs despite treating nutrients.

The second section of the report is a water quality assessment for SWAMP. Fecal coliform and total suspended solids are greatly reduced in SWAMP, primarily due to the completion of Phase 5 built in 2014. Nitrogen and phosphorus have large loading rates which is negatively impacting the removal rate of nutrients. Nutrient sources for SWAMP are from inlet water from Duke campus, primarily athletic fields carrying stormwater and fertilizer.

The third section of the report analyzes seasonal trends for water quality variables at the inflow and outflow of SWAMP. Total nitrogen had an increasing monotonic trend that was not significantly influenced by seasonality and Phase 3 was the main source of nitrogen. Phase 3 caries Duke Campus water, so it is likely runoff transported by rain events is the cause of nitrogen’s upward trend. Total phosphorus has a downward monotonic trend with seasonal influences likely due to rain events. Not all phosphorus trends were significant, because phosphorus concentrations have changed depending on stream flow and rain events.

Ultimately, SWAMP water quality has improved. There has been significant storage of nitrogen and phosphorus in the restored wetlands and holding pond, but high loading rates prevent increased nutrient removal efficiency values. However, total suspended solids are being stored or deposited along SWAMP and SWAMP is effectively treating fecal coliform. After the construction of Phase 5 in 2014, fecal coliform and total suspended solids dramatically decreased, indicating no lag period. Phosphorus and nitrogen have complex cycles, so their response is unclear without flow data and mass nutrient budgets.