LID RETROFITTING OF STORMWATER SYSTEMS TO MITIGATE SALT CONTAMINATION

Boone Creek is a gaining stream draining a 5.2 km² watershed in Boone, NC. Road salt is a chronic and acute issue for streams like Boone Creek in many cold weather regions, such as the Southern Appalachians near Boone. Catchments with high runoff ratios that produce flashy conditions, such as headwater and urban streams, can drive the road salt into the riparian aquifer during meltwater events. Residence times are further prolonged with large reverse gradients that occur during flashy summer convective storm events.

In previous studies, salt transport was measured through a naturally-formed wetland in a stormwater culvert draining to Boone Creek. Measurements indicated that natural retardation processes in the wetland delayed the arrival of the salt plume entering Boone Creek while also reducing the peak salinity levels. This natural wetland suggested the idea of modeling the influence of hypothetical LID wetlands, which could be constructed at the outlets of storm drains, on the stream salinities following meltwater events.

We modeled solute transport through an LID wetland using the Ogata-Banks solute transport solution to generate breakthrough curves for typical meltwater conditions. Next, using the Ogata-Banks output, we used a simple mixing model to generate hypothetical stream salinities under different implementation strategies that assumed baseflow and meltwater discharge rates as well as a set percentage of LID wetlands in the catchment. The mixing model ignores groundwater storage and assumes that all water added to the system is runoff.

Model output was compared to input salinities at values ranging from no enhanced wetlands to 100 percent enhanced wetlands. The addition of the augmented culverts decreased peak salinities from meltwater events by up to 20% of the original peak values and delayed the arrival of the road salt to the stream by at least 20 days. We think that LID wetland construction, if implemented properly, will help urban stream water quality by reducing the potential for acute and chronic chloride contamination. Although the LID wetlands will not remove salt from the system, they are effective at limiting peak concentrations, which will improve the water quality in the stream.