WATER QUALITY FUNCTIONS OF NATURAL AND RESTORED FLOODPLAINS IN URBAN WATERSHEDS

The optimization of management practices to reduce pollutant loading from watersheds is limited by the knowledge of water quality processes in urban floodplain ecosystems. Therefore, we examined sediment, nitrogen (N), phosphorus (P), and carbon (C) transport, cycling, and retention in floodplains of both natural and restored urban streams of the eastern U.S. Piedmont. We measured fluxes of sediment, N, P, and C associated with bank erosion, floodplain deposition, and internal cycling. In Difficult Run, VA, natural floodplains were studied from the headwaters to the mouth of this suburban/urban watershed. Floodplain deposition increased and bank erosion decreased downstream through the watershed. Floodplain soil net ammonification and phosphate production increased and nitrification decreased in wetter soils. N and P mineralization rates were stimulated by inputs of sediment and inorganic N and P to the soil surface. Rates of N and P mineralization were similar to rates of vegetative uptake, suggesting tight recycling of inorganic N and P in the floodplain. A C budget identified that floodplain soils are losing about 2% of their C pool per year through emission of CO₂ to the atmosphere despite substantial C sedimentation inputs. The export of C is most likely due to the long-term decrease in stream baseflow due to urbanization that has dried floodplain soils and stimulated decomposition of soil organic matter. Restored urban floodplains were studied at five separate streams restored using Natural Channel Design in Charlotte, NC, with measurements at three low bench and three high bench plots in each reach. Lower elevation plots had 7x greater sediment deposition rates than higher elevation plots. Hydrologic connectivity increased inputs of NH₄⁺ and sediment to the floodplain soil surface, especially at high bench plots. Sediment deposition increased soil net N mineralization. Denitrification potential increased with NO₃^- loading to the soil surface. Restoration age was positively correlated with all nutrient cycling rates. In summary, older and frequently connected restored floodplains achieve greater water quality improvements. Designing restored floodplains that flood many times per year, rather than only during bank-full events, could increase sediment and nutrient removal from urban streams.