In the rapidly urbanizing South Carolina coastal zone, intensive landscape maintenance and turf management are significant sources of nonpoint source pollutant loadings.  The stormwater best management practice of choice in this region is wet detention ponds, the majority of which are brackish lagoons.  Typically, stormwater is piped directly into the ponds, and their capacity for processing pollutants is limited.  These highly eutrophic brackish ponds are “hot spots” for harmful algal blooms – over 200 blooms from 23 different species were documented over the last four years, many associated with measured toxins, fish kills or shellfish health effects. Pond nutrient accumulations may also impact estuarine eutrophication through surface or groundwater transport.   SCHABP personnel, through a SC Sea Grant-funded project, will test the use of constructed wetlands as a supplementary BMP to process stormwater and groundwater nutrients prior to entering detention ponds. The constructed wetland is designed to enhance denitrification, reducing nitrogen input to the pond, improving pond water quality, and decreasing stormwater and groundwater nutrient transport to downstream estuaries.  Wetland construction is targeted for December 2006.  Findings thus far from the baseline study include:

 

a)      characterization of a 5-month long Microcystis bloom (see previous section);

b)      high denitrification potential in watershed soil and pond sediments;

c)      high nutrient fluxes into and out of the pond by stormwater and groundwater. 

 

Because these ponds exchange with tidal creeks, they are sources for harmful algal bloom dispersion into adjacent estuaries.  Furthermore, flux measurements indicate that groundwater acts as both a source of nutrients to the ponds and a mechanism for transporting nutrients from the ponds.  These findings imply that manmade ponds as presently designed along the South Carolina coast may contribute to estuarine eutrophication and harmful algal bloom prevalence.