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Delaware Department of Transportation

Stormwater Management

Monitoring Programs


Illicit Discharge Detection and Elimination Program

A "dry weather condition" is the period following at least 72-hours after the most recent precipitation event measuring at least 0.10 inches or more. Dry weather screening involves locating, marking, and testing all stormwater outfalls in the NPDES Phase I and Phase II permitted areas of the state. Stormwater outfall coordinates are recorded with a Global Positioning System (GPS), uploaded into a Geographic Information System (GIS) and the data is used to create a map of the system.

Screening involves recording physical characteristics of the outfall such as size, shape and location, and testing for the presence of pollutants if flow is present in the outfall during a dry day. The purpose of the dry weather screening is to identify any potential illicit discharges into stormwater drainage system and to investigate and eliminate the sources of the illicit discharge.


Storm Event Monitoring

"Wet weather sampling" is the collection of stormwater samples following a dry weather condition, commencing within 20 minutes of the start of precipitation with continued precipitation for 3.0 hours. The purpose of our wet weather monitoring program is to collect stormwater samples for use in estimating stormwater pollutant loads in New Castle County. This program is conducted jointly with New Castle County government.

Five sites in New Castle County were selected for monitoring: two in residential areas, one in an industrial area, one in a commercial area, and one along a highway. Samples are collected twice a year during representative rain events. The stormwater samples are analyzed to determine the concentrations of various pollutants present in the stormwater. This concentration data is used to estimate the amount of pollutants being discharged to waterbodies in New Castle County annually via stormwater.


Best Management Practices (BMP) Performance Monitoring

Stormwater Best Management Practices or BMPs are practices that are used to reduce pollutants typically present in stormwater runoff, prior to the runoff entering streams and rivers. DelDOT has a long-term Best Management Practice (BMP) performance monitoring and research program. This includes wet weather monitoring of stormwater outfalls and BMPs, as well as chemical and biological monitoring of streams that receive stormwater discharges from DelDOT maintained BMP's. Long-term objectives of DelDOT's BMP monitoring program include the following:

  • Quantifying pollution removal abilities of BMPs
  • Identifying types and amounts of pollutants present in stormwater discharges from DelDOT maintained roads
  • Determining potential impact of stormwater discharges on water quality
  • Assuring compliance with regulatory standards
  • Provide design engineers with additional treatment options for difficult site-specific situations
  • Evaluating emerging stormwater treatment technologies
  • Determining unique problems, and solutions to problems, that occur with structural BMP retrofits
  • Integrating DelDOT monitoring with watershed monitoring already being done by the Delaware Department of Natural Resources and Environmental Control (DNREC) and other integrated biological, physical and chemical monitoring and assessment approach


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