MICRO-WETLAND IN URBAN TOPOGRAPHY TO MITIGATE SALT CONTAMINATION

Boone Creek is a gaining stream draining a 5.2 km² watershed in Boone, North Carolina. Salt applied to roads throughout cold periods directly enters the stream via surface-water runoff during melt or precipitation events. Stage increases during these runoff events reverses aquifer gradients near the stream, causing salt transport into the aquifer. This salt is released back to the stream during baseflow conditions, yielding chronically-elevated stream salinity values.

Over the past four years, a wetland measuring 21 m by 5 m in area and 0.30 m in thickness has formed within a concrete culvert draining to Boone Creek. This wetland has become large enough to function as a small-scale riparian aquifer that reduces peak runoff salinities by storing salt during these times and releasing it throughout the year as seepage. Electrical conductivity, an analog for salinity, was measured in the wetland using hand-held probes, continuous datalogging, and water samples analyzed with ion chromatography. Our data suggest that the mini-wetland acts as a temporary buffer, decreasing peak salinity levels in water discharging from the wetland to Boone Creek.

We modeled groundwater flow and solute transport using the finite-element model FEFLOW. The 2D model domain was parallel to the culvert and utilized field-measured fluctuating water level and salinity boundary conditions at the upstream end of the aquifer, seepage at the downstream end, and a salt-free initial condition. We ran the model for 3.5 years, roughly corresponding with the lifespan of the wetland. Simulation results demonstrate that peak salinities are reduced at the seepage outlet by approximately 90% from the inputs, and these peaks lag source peaks by over 100 days. The results also show that over the course of the simulation, salinity values in the wetland have risen to approximately 0.5 kg/m³, demonstrating that the wetland stores the salt.

The wetland in our study formed organically, but our data demonstrate that the creation of mini-wetlands along storm sewer systems has strong potential to reduce acute chloride contamination (one-hour average Cl- values of 860 mg/L) and may also help to attenuate chronic chloride contamination (four-day average Cl- values of 230 mg/L) to receiving streams.