Oil and grease were nearly always undetectable in water samples. Nutrient and metal concentrations were generally low enough to be considered at background levels, in inlets and outlets. Nitrate/nitrate was the only parameter to show statistically significantly lower concentrations at outlets compared to inlets. Copper, lead, and zinc concentrations were higher in wetland centers than at inlets, suggesting sequestering of metals by the large amounts of algae biomass commonly present in the wetland centers. This difference disappeared during stormflow, probably because of dilution. During baseflow conditions, the urban study wetlands output copper and lead into Walnut Creek, possibly because of beaver dam interference with the free-flowing status of the outlet tributaries. However, the concentrations being output still amounted to extremely small portions of the total loadings in the Creek itself. Lead levels in the wetland soils were high in some wetlands, indicating that legacy lead from leaded fuel had been sequestered in wetland sediments, but lead was not proportionately being suspended into the water. Total suspended solids were significantly higher in the water at outlets than inlets at baseflow times, but a detailed sediment movement assessment at one study wetland suggested that overall sediment coming into the wetland during storms was a much greater volume than that exiting the outlet through bank erosion.

Concentrations of measured contaminants were quite low in general, making uplift between inlet and outlet difficult to detect. However, these low concentrations underscore the function and importance of intact vegetated buffers on streams and waterbodies in urban areas. Additionally, we recorded significant accumulation of sediment (about 80 dump truck loads over 24 months), only 7% of which was eroding at the outlet to the mainstem Walnut Creek. The urban wetlands in this study were functioning to capture sediment which otherwise would have stayed in or entered Walnut Creek and the Neuse River. Large quantities of discarded trash, mostly plastic, have also been trapped by the urban study wetlands, providing a capture mechanism that would be absent without them.

A literature review of performance of constructed stormwater wetlands is included in the final report. Constructed stormwater wetlands are generally sized and designed for the specific inputs they receive, which makes their contaminant removal effectiveness high in the first years after construction. Secondly, because constructed stormwater wetlands tend to fill up with sediments over time and they are seldom maintained in the long term, their effectiveness often dramatically decreases over time. Thirdly, any unexpected contaminant inputs may not be able to be absorbed, since the original design tends to be limited to specific contaminants. Natural urban wetlands tend to be larger in size and have the vegetation and structural capacity to capture a range of contaminants, especially sediment and plastic. That said, functioning constructed wetlands can deal with much higher contaminant loads than what we encountered coming into our natural study wetlands.

The results from this data collection cannot indicate at what high levels of contamination (amounts typically not found in relatively unurbanized or otherwise undisturbed areas but also not found in this study area), introduced into the natural wetlands of Raleigh through surface water, would overwhelm these wetlands and make them unable to keep these contaminants from entering Walnut Creek. Therefore, it is not possible to directly compare stormwater uplift capability of these wetlands with operating constructed stormwater BMPs for a similar suite of contaminants.

However, it is clear that Raleigh City wetlands are performing invaluable ecologically and water quality-enhancing services to the City and without any direct or indirect cost. Tons of nuisance and even potentially dangerous public waste materials, hundreds of dump truck loads worth of cubic meters of eroded sediment, and high amounts of heavy metals in the soils are being kept from the lower Walnut Creek watershed and by extension the Neuse River basin by the self-maintaining, trapping action of these forested marshes and ponds. In addition, millions of gallons of flood waters are being slowed, if not stored long-term, from entry into the downstream watershed by these sites every year, through both growing and non-growing seasons. These wetlands and the local catchment buffers associated with them, should not be infringed upon or impaired to protect these cost-free, self-sustaining services for the City, Wake County, and the other downstream municipalities connected to the Neuse River.