Abstract
In the 2011 revision to the California Department of Transportation (Caltrans) statewide stormwater permit (Tentative Order No. 2011-XX-DWQ), the California State Water Resources Control Board required Caltrans to monitor discharge flows for only three events each year and using those results, make a determination as to whether the runoff characteristics exceed water quality objectives (WQO). The process for making the determination was specified in the permit in the form of two water quality action levels: (1) three or more exceedances of a water quality objective (WQO) or (2) two or more exceedances of a WQO by more than 50 percent. The goal of this project is to assess the accuracy of these rules as a function of the number of field samples collected per year. The permit rules were applied to 100,000 randomly chosen samples of specified sizes drawn from historic Caltrans data sets for total cadmium at three sites. The California 303(d) listing process was used to determine whether the historic discharges exceeded the WQO. The results of the permit rules applied to the multiple simulated sample data sets and the 303(d) listing process (representing the “true” result) were compared. From these comparisons, the probability or frequency of incorrect conclusion arising from using the permit rules was calculated. An alternate statistical approach in which the WQO was compared to the 95 percent lower confidence limit (LCL) of the 90th percentile of the data was tested in a similar manner (i.e.100, 000 random simulations of various sample sizes).