STORMWATER QUALITY MANAGEMENT PLANNING IN COASTAL SOUTH CAROLINA

The coastal areas of South Carolina are experiencing intense development pressures. The effect of this development on coastal water resources is the increase of nonpoint source pollutant loadings to receiving waters. The water quality condition of coastal water resources of South Carolina ranges from “outstanding resource waters,” or ORWs, to those approved for shellfishing, to South Carolina Department of Health and Environmental Control (SCDHEC) 303(d) listed impaired waters. Each water quality classification contains certain numerical standards for certain parameters as defined by SCDHEC.The regulation of stormwater quality management for new developments in South Carolina is based primarily on the traditional approach of retention/detention of a specified treatment volume within best management practices. This approach presumes that treatment of the appropriate runoff volume will result in discharges that will meet state water quality goals. Coastal counties in southern South Carolina have experienced rapid growth with a large number of planned developments either recently constructed or planned for construction in the near future. During this time, a number of shellfish areas have been closed permanently, or on a conditional basis, due to exceedences of the water quality standard for shellfish waters (14 FC/100ml for fecal coliform). These bed closings may be an indicator that the traditional treatment volume approach may not provide treatment sufficient to prevent degradation of receiving water quality. These shellfish bed closings resulted in heightened citizen awareness of water quality issues and demands for increased protection of the area's waters. In addition, those waters that have been placed on the SCDHEC 303(d) list as impaired shall not be further degraded. Proposed developments that will discharge to receiving waters on the 303(d) list are now required by SCDHEC to demonstrate that discharges from their site will not degrade receiving water quality, and in fact, must meet numerical limits for parameters that resulted in the listing of the receiving water receiving state water quality certification. Existing retention/ detention criteria and methodologies were not adequate to provided this demonstration. An SCDHEC-approved methodology for demonstrating compliance with numerical discharge limits did not exist.A methodology for computing nonpoint source pollutant loadings, required treatment efficiencies to meet numerical discharge limits, and required BMP implementation to achieve treatment efficiency goals was applied to 3 proposed multi-use developments (2 are currently under development). Two of these projects were located in the Myrtle Beach area and involved over 2500 acres of potential development. The other project was in Beaufort County and involved over 350 acres currently under development. The methodology, which was incorporated into a spreadsheet model, utilized event mean concentration data developed from literature values and NPDES MS4 sampling data to estimate pollutant loads from stormwater runoff for the proposed developments. Project BMP requirements were developed based on the required pollutant load reduction to meet the concentration limits for the receiving waters using literature values for pollutant removal efficiencies. BMP implementation plans for specific land use types and stormwater quality management guidelines for use by individual developers within the projects were developed. One of the developments evaluated using the described methodology was the first major development in South Carolina to undergo this type of demonstration of the pre-and post-development nonpoint source pollutant loadings to a SCDHEC 303(d) listed waterway. Early water quality monitoring results from one project are inconclusive that discharges from the stormwater management system are meeting numeric concentration goals.This paper and presentation will review the development of this program and how it set the standard for stormwater management planning in South Carolina.