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Stormwater Runoff in South Carolina: Best Management Practices

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Stormwater Runoff in South Carolina
Performance of Four Best Management Practics for Highway Runoff

Picture of the inside of a BMP.

Interior of a BMP

As part of National Pollutant Discharge Elimination System (NPDES) stormwater program mandated in the Clean-Water Act, the South Carolina Department of Transportation (SCDOT) is required to address the quality of stormwater runoff from state-maintained roadways. Stormwater discharges from state roadways are considered a large municipal separate storm-sewer system (MS4) by the NPDES program and require development of a proposed storm-water management program that would meet the standard of "reducing pollutants to the Maximum Extent Practicable (MEP)" (South Carolina Department of Health and Environmental Control, 2001). To mitigate the effects of runoff from state roadways to area water bodies, the SCDOT has installed structural Best Management Practices (BMPs) throughout South Carolina. These BMPs include grassed waterways, detention ponds, and vendor supplied systems. Many of these systems incorporate some combination of filtration media, hydrodynamic sediment removal, oil and grease removal, or screening to remove pollutants from stormwater.

Previous research has assessed the ability of stormwater treatment BMPs to reduce pollutant concentrations and loadings in stormwater. However, the ability to relate findings from past research to SCDOT sites is limited because of noted inconsistencies in performance studies (Strecker and others, 2000). At this time, the SCDOT has no quantitative data to evaluate the effectiveness or performance of these BMPs at enhancing stormwater runoff quality. The U.S. Geological Survey (USGS), in cooperation with the SCDOT, conducted an investigation to evaluate the performance of 3 representative BMPs in the Beaufort, S.C. area and one BMP at the I-95 rest area near Walterboro, Colleton County, S.C. A subset of data collected in this investigation also may be used by the SCDOT for the NPDES permit requirements to characterize stormwater quality at these sites. In addition, the results of this investigation should be applicable to sites throughout the southeast where similar conditions exist.


Picture of the interior of a stream gage house .

Interior of a gage house.

The purpose of this investigation is to determine the effectiveness of the BMP devices in reducing suspended sediment, selected metals, nutrients, and organics concentrations by comparing estimated removal efficiencies. The results provide the South Carolina Department of Transportation (SCDOT) with quantitative data to evaluate the effectiveness of these BMPs at enhancing storm-water runoff quality. This information can be used by the SCDOT and other State, Local, and Federal agencies in the selection of appropriate BMPs for future installation.

Objectives of this investigation are to:

  1. determine event-mean concentrations entering and leaving the BMPs,
  2. compute event-mean loads entering and leaving the BMPs,
  3. estimate the removal efficiency of the BMPs for selected parameters,
  4. compare removal efficiencies among BMPs,
  5. evaluate the relation of water-quality constituents to average daily traffic (ADT) data by correlation analysis.


Picture of a BMP Monitoring Station.

A BMP Monitoring Station

The approach of this investigation included an initial data-collection period followed by the final data compilation and analysis. From March 2005 through November 2006, the USGS operated data-collection stations at the four sites. At each of the four sites, flow and water level (stage) were monitored continuously. Flow was measured by an electromagnetic flowmeter installed in the inlet pipe upstream from the device. Flow velocity and pipe dimensions were used to calculate flow. Water levels in the treatment chamber were measured by a pressure transducer. Rainfall data were collected at one site near Beaufort, S.C. and at the Colleton County, S.C. site.

Refrigerated automatic samplers were used to collect fixed-time interval water samples of the inlet waters to and outlet waters from the BMPs during selected events. All samples were collected from discharge resulting from rainfall exceeding 0.05 inch and at least 24 hours from the previously measurable (greater than 0.05 inch rainfall) storm event. Flow-weighted volumes of each fixed-time-interval sample were computed and placed in a compositing device to produce a flow-weighted composite sample. Flow-weighted composite sampling means that a volume of subsample is collected in proportion to the volume of water in the inlet and outlet. The composite sample thus represents the average constituent concentration during a runoff event (Waschbusch, 1999).

Over the 20-month data collection period, a total of 49 sample sets (a sample set includes an inlet and outlet water sample) were collected among the four BMPs. Water samples were collected over a range of seasons. Inlet and outlet samples were collected and analyzed for pH, conductance, oil and grease, turbidity, total suspended solids, chemical oxygen demand, selected total and dissolved nutrients, selected total and dissolved metals, major ions, five-day biochemical oxygen demand, suspended sediment, grain-size distribution, and bacteria. Eight samples were analyzed for base/neutral extractable organic compounds (polyaromatic hydrocarbons) and compared to the oil and grease samples. In addition, prior to the start of data collection, the sediment previously retained by the devices was removed. At 6-month intervals, the bed sediment retained by each structural device was quantified and core samples were collected and analyzed for grain-size distribution and inorganic constituents.

The SCDOT provided ADT counts for several time periods during this investigation as ancillary data that were used in the data analysis. A database that includes USGS water-quality, flow, and precipitation data, SCDOT traffic data, and other ancillary data was compiled and reviewed for accuracy.

A tiered statistical approach will be used in the data analysis. Performances of the 4 structural BMPs will be assessed individually based on how well the BMPs were able to reduce selected constituents listed above. Descriptive statistics and nonparametric Wilcoxon signed-rank tests will be applied to event-mean concentrations and loads in the water entering the inlet and the water leaving the outlet of each BMP for each constituent to identify if significant reductions occurred. If significant reductions exist, the BMP is considered efficient at reducing that constituent. A simplistic approach will be applied that computes mean and geometric mean efficiency ratios for the significantly reduced constituents in each BMP and each BMP performance will be ranked based on the computed efficiency ratios. Further, a more complex approach will use statistical comparison tests to percent reductions computed for individual storms (a modified removal efficiency of individual storm load approach) to determine if differences in event-mean concentrations, loads, and percent reductions in significantly reduced constituents existed among the 4 structural BMPs.


The results of this investigation will provide the SCDOT with information from a comprehensive dataset that evaluates the effectiveness of these BMPs at enhancing stormwater runoff quality. This information can be used by the SCDOT in determining the appropriate BMP for future installation. The results also should be applicable for similar conditions throughout South Carolina and in other states.


A draft USGS Scientific Investigations Report has been written and is currently in review.


South Carolina Department of Health and Environmental Control, 2001, Water Pollution Control Permits: 2000 Code of Regulations, State Register, vol. 25, issue 1, R61-9

Strecker, E.W.; Quigley, M.M.; and Urbonas, B.R., 2000, Determining Urban Stormwater BMP Effectiveness: Proceedings from the National Conference on Tools for Urban Water Resources Management and Protection, on February 7-10, 2000 in Chicago, Illinois, U.S. Environmental Protection Agency, EPA/625/R-00/001, p. 175 - 185.

Waschbusch, R.J., 1999, Evaluation of the effectiveness of an urban stormwater treatment unit in Madison, Wisconsin, 1996-97: U.S. Geological Survey Water-Resources Investigations Report 99-4195, 49 p.

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