EFFECTS OF STORMWATER MANAGEMENT PRACTICES ON NONPOINT SOURCE BOD LOADING TO COASTAL WATERS IN SOUTH CAROLINA

The occurrence of low dissolved oxygen (DO) is the primary water quality impairment with respect to supporting aquatic life in South Carolina’s coastal zone. In order to maintain sufficient DO in the state’s coastal waters, effective management of both point and nonpoint source discharges of oxygen demanding substances is critical. Oxygen demanding substances are operationally measured as a five-day Biochemical Oxygen Demand (BOD5) defined as the concentration of DO consumed by microbial decomposition of organic matter and the conversion of nitrogen compounds to the stable forms of nitrite and nitrate over a five day incubation at 20°C. Much is known about the BOD loads of point sources, in particular wastewater, since BOD5 has been routinely measured for decades. Non-point source BOD loading, however is ambiguous as it has been combined into the natural BOD loads of receiving waters. To determine controls on the magnitude and variability in BOD5 in stormwater in South Carolina’s coastal zone, samples were collected during the summer of 2018 from seventeen different stormwater catchments in Georgetown and Horry counties across six different rain events of varying magnitude. Catchments were selected to represent a range of stormwater management conveyances and best management practices (BMPs) as well as undeveloped forested and wetland drainages. To elucidate the mechanisms driving individual site BOD loading, the particulate and dissolved fractions of organic matter (organic carbon, nitrogen, and phosphorus) were quantified in addition to the dissolved inorganic nutrients. Further, the dissolved organic carbon was optically characterized using a variety of absorbance and fluorescence measures as proxies for organic matter source and compositional state. Preliminary results show BOD5 values varied greatly among sites with varying drainage basin land cover and land use. Large within site variability was also observed between individual rain events, suggesting the importance of both event size and antecedent conditions in influencing the magnitude and lability of organic matter export. These results suggest understanding stormwater derived non-point sources of BOD is vital for effective management of DO in South Carolina’s coastal waters.