1. Introduction and Environmental Significance The May River is an essential ecological and recreational resource in South Carolina, designated by the state as an Outstanding Resource Water. It is particularly valued for its oyster production, aesthetic beauty, and recreational opportunities. The entire May River watershed spans 13,477 acres, with the Headwaters—comprising the Duck Pond, Palmetto Bluff, Rose Dhu Creek, and Stoney Creek subwatersheds—accounting for the majority of the area. However, this vital ecosystem is impaired due to rising fecal coliform (FC) bacteria levels in the Headwaters, which forced the closure of shellfish harvesting beds in this portion of the river in 2009.
2. The Impact of Rapid Development and Impervious Cover The Town of Bluffton has experienced rapid population growth, escalating from 794 residents in 1990 to an estimated 25,557 people in 2019. This explosive development has led to significant land-use changes and a sharp increase in impervious surfaces in the Headwaters, rising from 5.78% in 2002 to 15.31% in 2018. The Rose Dhu Creek and Stoney Creek basins are the most urbanized, with impervious areas reaching 19.74% and 15.49% respectively. The report identifies increased stormwater runoff volume associated with this development as a key contributor to both the volume of freshwater discharge and the pollutant (FC) loading into the May River.
3. The Water Quality Modeling Framework To understand and address the impairment, a comprehensive Water Quality Model (WQ Model) was developed using the XPSWMM software to analyze baseline (2002) and current (2018) conditions for the Headwaters. The purpose of this model was to identify project locations and types for the May River Watershed Action Plan Update (Action Plan Update). The model integrates various factors, including land use, impervious cover, soil infiltration, groundwater flow, and meteorological data, to simulate stormwater runoff and FC concentrations.
4. Baseline and Current Land Use Conditions The analysis revealed a stark contrast between the two modeled periods. In the 2002 baseline condition, the Headwaters subwatersheds were predominantly natural, with Evergreen Forest (35.55%) and Woody Wetlands (33.35%) as the main land covers, and a total of 10.67% developed land. In this baseline, 97 out of 123 subcatchments were less than 10% impervious.
5. Dramatic Shift in 2018 Development By the 2018 current condition, the total amount of developed lands in the Headwaters subwatersheds had increased dramatically to 30.72%. This growth pushed the majority of subcatchments into higher imperviousness categories: 26 subcatchments were found to be between 20-30% impervious, and seven were over 30% impervious. The Rose Dhu Creek subwatershed experienced the most significant transformation, with its developed land use soaring to 47.52%.
6. Sources of Fecal Coliform Pollution The sources of fecal coliform are complex and include both human and animal contributors. The Town’s Microbial Source Tracking (MST) program has detected human, deer, and dog markers within the river. In response, five failing septic systems in the Headwaters have been eliminated, and plans are underway for further septic-to-sewer conversions in partnership with local water authorities. Additionally, studies confirm that increased freshwater input from developed and deforested lands reduces salinity, which in turn elevates FC levels in the estuary.
7. Model Results and Hotspot Identification The calibrated XPSWMM model was used to calculate the annual total and normalized FC loading for each subcatchment. The model results showed the average daily maximum FC concentrations in the outfalls of the subwatersheds. By analyzing the normalized FC load (load per acre), the report was able to pinpoint specific “bacterial hotspots” where pollution contribution per unit area is highest, thereby guiding the prioritization of future mitigation efforts. 8. Evaluating Past Best Management Practices (BMPs) The Town had previously implemented the 2011 May River Watershed Action Plan, including a volume-based stormwater ordinance, and secured grants for several projects. However, evaluation of completed projects, such as the New Riverside Stormwater Pond, yielded mixed results. While the pond successfully reduced FC concentrations by over 95% at the point of treatment, this improvement did not result in a statistically significant decrease in FC concentrations 1,700 linear feet downstream in the receiving waterway. This finding underscored the necessity for continuous evaluation and refinement of BMP technologies to ensure actual downstream water quality improvement.
9. Proposed Projects: Septic-to-Sewer and Stormwater Retrofits The 2020 Action Plan Update proposes a new list of projects focusing on two main strategies: Septic to Sewer Conversion and Stormwater BMP Retrofits. The report provides a detailed analysis, including estimated bacteria load reduction for specific septic-to-sewer conversions in areas like Cahill and Pritchardville. Furthermore, it proposes numerous stormwater retrofit projects at large impervious areas like schools (e.g., Bluffton High School, McCracken Middle School) and community centers, modeling their potential to manage stormwater retention volume and reduce runoff.
10. Future Strategies: The Four Ps and Policy Updates The report’s final recommendations are structured around the “Four Ps”: Partnerships, Policies, Programs, and Projects. The overarching goal is to utilize Better Site Design principles to conserve natural areas, reduce impervious cover, and manage stormwater through infiltration and filtration techniques. Key policy and program recommendations include adopting a new regional Stormwater Ordinance and Design Manual , establishing enhanced long-term monitoring locations for bacteria and flow , and continuously updating impervious surface data. This comprehensive strategy is intended to be a “living document,” ensuring the ongoing protection of the May River and the restoration of shellfish harvesting.