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 New Castle County. It is expected that approximately 3,500 outfalls will be screened as part of the permit process over the next 4 years. 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.

“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 wet weather monitoring is to collect stormwater samples for use in estimating stormwater pollutant loads in New Castle County. 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.