Water Quality and Hydrologic Mitigative Potential of an Undersized Subsurface-flow Gravel Wetland Treating Industrial Runoff

Natural (undisturbed) landscapes yield sustainable hydrology with distinct subsurface pathways, predominately shallow interflow and groundwater surge. A subsurface flow gravel wetland (SSGW) promotes the removal mechanisms of media filtration and sedimentation while storing pollutants in the submerged gravel surface inter-event. The subsurface water held interstitially in the gravel is analogous to shallow interflow and is similarly discharged into the stream during subsequent storm events when it is replaced by polluted event water. During shallow interflow, pollutants are subjected to biogeochemical transformations, uptake, immobilization, and other treatment processes.
In Greensboro, North Carolina, water quality and hydrologic monitoring of an undersized SSGW (able to detain and fully treat up to the 13-mm storm) began in September 2019. The undersized SSGW treated industrial stormwater runoff and vehicular wash-off. Formation of a schmutzdecke clogged the surface with up to 85% reduction in surface infiltration rate within the first four months of operation. The surface was raked twice during the first year after construction to maintain the intended hydrology/ infiltration rate. Raking increased the surface saturated hydraulic conductivity by up to 214%. Thereafter, macrophyte establishment maintained intended subsurface wetland hydrology. After 2.3 years of operation, the wetland surface saturated hydraulic conductivity reduced to 40 mm/hr, within the current design guidance for filtration-based practices in North Carolina 25 to 50 mm/hr.
The median concentration percent removals (n = 28 storms) were 92% for TSS, 43% for TN, 47% for organic-N, 43% for TKN, 54% for NOx, 40% for NH3, 67% for TP, 53% for Ortho-P, and 75% for Particle-P. The median percent reductions were 77, 23, and 50% for TSS, TN, and TP loads, respectively. The areal annual loading rates were 388, 7.58, and 1.92 kg/ha/yr for TSS, TN, and TP inflow, respectively.