Parking Deck's First Flush

The goals of storm-water management have shifted from a flood control to a holistic and sustainable strategy, emphasizing the relationship between rainfall event size and pollutant loadings. The first flush concept is the first part of a rainfall event that contains the largest pollutant loading. Questions have been raised on the concept's validity for storms across different land uses and pollutant types. The existence and magnitude of the first flush impacts sizing of best management practices used to meet pollutant reduction goals, assessment sampling methodologies, and state storm-water management strategies. Current concepts support the use of distributed control measures focused on smaller storms off impervious surfaces. Runoff from a small impervious parking area was sampled incrementally during multiple storm events to measure pollutant concentration with respect to storm depth. This sampling routine established the existence of a first flush for a single use paved parking area. Total suspended solids, nitrate, chloride, dissolved copper, and dissolved cadmium exhibited a first flush up to a rainfall depth of 25.4 mm; total dissolved solids, total nitrogen, total phosphorus, nitrite, phosphate, and dissolved chromium did not exhibit a first flush.