Nonparametric Statistical Tests Comparing First Flush and Composite Samples from the National Stormwater Quality Database
The University of Alabama and the Center of Watershed Protection, as part of an EPA 104(b)3 project, has collected and reviewed phase I NPDES (National Pollutant Discharge Elimination System) MS4 (Municipal Separate Storm Sewer System) stormwater data. The database contains more than 3700 event data sets from 66 municipalities in 17 States. In 1990, communities larger than 100,000 were required to monitor and control the pollutants that reach surface waters from stormwater runoff. Some communities collected grab samples during the first 30 minutes of the event to evaluate the “first flush” effect in contrast to the flow weighted composite samples.
There were 417 paired samples representing both first flush and composite samples from eight communities, mostly located in the southeast USA. Box and probability plots were prepared for 22 constituents including TSS, TDS, BOD5, COD among others. Nonparametric statistical analyses were used to measure differences between sample sets. This chapter shows the results of this preliminary analysis, including the effects of storm size and changes in land use. First flush effect was not present in all the land uses, and certainty not for all constituents. More detailed analyses will be performed as additional data are received.
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