Stormwater pollutant concentrations can be estimated from local or regional data, or from national data sources. Table 4 identifies pollutant concentration values for Phosphorus, Nitrogen, COD, BOD, and some metals for more generalized land use categories. In general, the selected data sources are nationwide in scope, or are summaries of several regional studies. In these instances, the data are reported as the same concentration for each land use in Tables 1 through 3. Fecal coliform is more difficult to characterize than other pollutants.
Data are extremely variable, even during repeated sampling at a single location.
Because of this variability, it is difficult to establish different concentrations for each land use. In the Simple Method, the runoff coefficient is calculated based on impervious cover in the subwatershed. This relationship is shown in Figure 1. Although there is some scatter in the data, watershed imperviousness does appear to be a reasonable predictor of Rv. The model uses different impervious cover values for separate land uses within a subwatershed.