Evaluation of Structural Best Management Practices for Highway Runoff in Beaufort and Colleton Counties, South Carolina,

Study Description

As part of the National Pollutant Discharge Elimination System (NPDES) permit 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. From 2005 to 2006, the SCDOT and the U.S. Geological Survey (USGS) worked cooperatively in Beaufort and Colleton Counties, South Carolina (SC), to evaluate the performance of four different structural devices that served as best management practices (BMPs). These structural devices were installed to lessen the potential effects of stormwater runoff on water quality in waterways near state roads.

The purpose of this Fact Sheet is to summarize results published in the USGS Scientific Investigations Report 2008-5150. The report documents the ability of these four BMP devices to remove suspended sediment, metals, nutrients, and organics compounds in stormwater runoff. The quantity of rainfall and stormflow and quality of stormwater entering and leaving the BMPs were monitored during 12-13 storms over a 21-month period. The results provide the SCDOT with quantitative information to evaluate whether or not the BMPs effectively enhanced stormwater 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.

Three of the four BMP sites were located on Ladys Island, which is south of Beaufort, SC (see location map). Storm runoff in this part of the study area flows to the Beaufort River, which is an estuarine system. One of the four BMP sites was located at the I-95 South rest area, south of Walterboro, SC (not shown on location map).

Findings from the Study

The difference between the inlet and outlet constituent concentrations during each storm event was used to quantify the amount of constituent removed by a BMP. Overall, the four BMPs were capable of signficantly reducing suspended sediment in the stormwater entering the inlets of the BMPs before discharging the stormwater at the outlets (see chart). The cumulative load of suspended sediment in stormwater entering the BMPs from all the storms sampled during the data-collection period was 1,026 kilograms (1.13 tons). The BMPs removed over half that amount as the cumulative suspended-sediment load of 558 kilograms (0.62 ton). However, the BMPs tended to trap sand-size sediment, thereby releasing a greater proportion of silt- and clay-sized sediment in the water discharging from the outlet.

Three of the four BMPs released significantly lower oil and grease concentrations and all four BMPs released significantly lower total trace-metal concentrations in the stormwater discharged from the outlet. In general, the BMPs were not capable of significantly reducing fecal bacteria, nutrient, and total organic carbon concentrations.

The four BMPs also were compared and were determined not to be significantly different in their capacity to remove suspended sediment, total suspended solids, or oil and grease in stormwater runoff. However, BMP4 removed significantly more total cadmium, lead, copper, and zinc than the other BMPs during this study.

In general, multiple interacting environmental factors

Authors

appeared to influence the amount of reduction of constituent concentrations by the BMPs during individual storms (see error bars on chart). These environmental factors included the rainfall intensity of the storm, the amount and frequency of rainfall between sampling events, seasonal traffic-density patterns, and the period of time since the last maintenance (clean out) of the BMP.

Where this page came from

This page was imported from U.S. Geological Survey. Published by the U.S. Geological Survey and, as a work of the United States government, in the public domain.

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