Stormwater Runoff in South Carolina

Performance of Four Best Management Practices for Highway Runoff in South Carolina

Introduction

image of storm drainage station

Storm Drainage Station

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, is currently conducting 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.

Objectives

image of storm drainage

BMP Interior

Objectives of this investigation are to:

  1. collect storm-event samples over a range of storm intensities and seasons to identify the variability in storm-water quality,
  2. statistically analyze event-mean concentrations entering and leaving the BMPs for all storm events,
  3. statistically analyze computed constituent loads entering and leaving the BMPs for all storm events,
  4. estimate the removal efficiency of the commercially available BMPs for selected constituents such as suspended sediment, metals, oil and grease, and fecal indicator bacteria, in roadway runoff based on statistical analysis, and
  5. evaluate the relation between water-quality constituents to average daily traffic Average Daily traffic (ADT) data by correlation analysis.

Project Description

image of the cooler inside the storm drainage station

Gage House Interior

At each of the four gaging sites, flow and waterlevel are monitored continuously. Flow is measured by an electromagnetic flowmeter installed in the inflow pipe upstream from the device. Flow velocity and pipe dimensions are used to calculate flow. Water levels in the treament chamber are measured by a pressure transducer. Rainfall data are collected at one site near Beaufort, S.C. and at the Colleton County, S.C. site.

Refrigerated automatic samplers are being used to collect flow-weighted composite water samples of the inflow and outflow from the BMPs during selected events. All samples are 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 composite sampling means that a volume of subsample is collected in proportion to the volume of water in the inflow and outflow. The composite sample thus represents the average constituent concentration during a runoff event (Waschbusch, 1999).

Data collection began March 1, 2005, and will continue through September 2006. Inflow and outflow samples will be collected for 12-15 events distributed by season—spring, summer, fall, and winter, assuming adequate hydrologic conditions, 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 will be analyzed for base/neutral extractable organic compounds (polyaromatic hydrocarbons) and compared to the oil and grease samples. According to Environmental Technology Verification Program protocols (U.S. Environmental Protection Agency, 2002), the outflow sample should be taken after the by-pass water is combined with the effluent from the BMP. 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 will be quantified and core samples will be collected and analyzed for for grain-size distribution and inorganic constituents.

The SCDOT has provided ADT counts for several time periods during this investigation as ancillary data to be used in the data analysis. A database that includes USGS water-quality, flow, and precipitation data, SCDOT traffic data, and other ancillary data will be compiled and reviewed for accuracy. The USGS will use these data and similar data from other investigations and attempt to correlate types of constituents measured with the ADT for each site.

Initial data analysis will be done concurrently during the data collection phase. Final analysis will commence once data collection is completed. Using measured flow and event-mean concentrations for each storm event, loads will be calculated for the inflow and outflow for each structure. Removal efficiences attained for each storm event will be computed as percent differences between event-mean concentrations and loads in the inflow and outflow of each BMP. Removal efficiences will be used to determine the effectiveness of the devices in reducing selected constituent concentrations, such as suspended sediment and trace metals. Descriptive statistics (for example, mean, median, standard deviation) of the precipitation, flow, event-mean constituent concentrations and loads, and removal efficiences will be computed. Analysis of variance (ANOVA) statistical technqiues will be used to determine if observed differences in removal efficiences, event-mean concentrations and event loads among BMPs are statistically significant. Similar ANOVA techniques will be applied to event-mean concentrations and event loads of inflow and outflow samples of each BMP to determine if significant differences exist. The USGS also will use the compiled dataset to evaluate the potential relation of constituent concentrations and loads to the changes in traffic patterns for each site using statistical correlation techniques (for example, Spearman rho or Kendall tau correlation tests) with the ADT data.

Water-Quality Characteristics

Figure 1: Event loads for total and dissolved zinc and copper

Figure 1: Event loads for total and
dissolved zinc and copper.

The present goal of this investigation is to collect sufficient data that represent as much of the variability in storm water quality as possible to allow a more accurate evaluation of the effectiveness of the 4 BMPs. Variability in storm water quality is influenced by many factors, including seasons, traffic density, antecedent conditions (number of days since the last rainfall), and rainfall intensity. As previously stated, since March 1, 2005, the USGS has collected data from selected storm events. However, the present data are not complete enough to make an initial evaluation of the performance of the BMPs. Therefore, only an example of data analysis is provided, including a BMP's responses to one rain event by computing the event loads of selected constituents in the stormwater runoff entering and leaving the selected BMP.

On April 7, 2005, an approximately 100-minute event produced 0.13 inch of precipitation. This precipitation resulted in a peak discharge of 1.2 cubic feet per second (ft3/s ) and a mean discharge rate of 0.73 ft3/s for the inflow and 0.77 ft3/s for the outflow sample. No recorded rainfall occurred within 72 hours prior to this sampling event. Ten samples where collected from the inflow and outflow runoff of the BMP throughout the range of the hydrograph (figure 1).

The event loads for oil and grease showed a 27-percent reduction from the inflow to the outflow of the BMP for this storm (figure 2). The computed oil and grease loads were 11 kilograms per day (kg/d) for the inflow sample and 8 kg/d for the outflow sample. The suspended-sediment load for the inflow sample was 88 kg/d compared to the outflow sample of 77 kg/d. The event loads for suspended sediment showed a 12-percent reduction from the inflow to the outflow. This reduction in suspended sediment corresponded well with turbidity levels which exhibited a 15-percent reduction from the inflow to the outflow sample of the BMP.

The event loads for total zinc, which includes the particulate and dissolved forms of zinc, showed a 23-percent reduction from the inflow to the outflow of the BMP (figure 3). The total zinc load for the inflow sample was 176 grams per day (g/d) compared to the outflow sample of 135 g/d. However, dissolved zinc exhibited a different pattern. Dissolved zinc showed a 21-percent increase from the inflow to the outflow. Total copper event loads showed a 21-percent reduction from the inflow to the outflow of the BMP (figure 3). The load for the inflow sample was 33 g/d compared to the outflow sample of 26 g/d. Dissolved copper showed no reduction from the inflow to the outflow. Total cadmium showed a 3-percent increase from the inflow sample (0.33 g/d) to the outflow sample (0.34 g/d). Dissolved cadmium showed no change between inflow and outflow samples of the BMP. The loads for the total forms of zinc and copper had slightly higher reductions from the inflow to outflow than turbidity and suspended sediment loads.

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.

BMP Site Location Maps

Contacts

  • Project Contacts
  • South Carolina Center Contacts
  • USGS - South Carolina Water Science Center
    720 Gracern Road Suite 129
    Columbia, SC 29210
    (803) 750-6100