In California, it is closing time on the 2015-2016 stormwater year. This means that some facilities are moving from “baseline” status to Exceedance Response Action (ERA) Level 1 due to exceedances of Numeric Action Levels (NALs). We have talked before about what it means, but not the math behind stormwater exceedances. After multiple Qualified Industrial Stormwater Practitioner (QISP) workshops and emails from confused facilities (plus, ahem, some really confused consultants), I realized we should break it down for California facilities. The way it works is at the end of the year (aka right now), you take all your sample results for a particular industrial stormwater parameter and average them together (except for pH). For example, if Total Suspended Solids (TSS) results were 100, 200, 300, 450, and 500 mg/L for the year, your average TSS concentration is 310 mg/L. You then compare each parameter’s average concentration against that parameters’ NAL from Table 2 in the California stormwater Industrial General Permit (IGP). The NAL for TSS is 100 mg/L. If your average is greater than the annual NAL then you have “exceeded” and are now moving to Exceedance Response Action (ERA) Level 1 this upcoming year and need a QISP. Instantaneous NALs (e.g. 400 mg/L for TSS) are compared against each individual sample result. So for our example above, two of the results (450 and 500 mg/L) were over the instantaneous NAL of 400 mg/L. If any two samples exceed the instantaneous NAL then that also puts a facility into ERA Level 1 for the IGP. If only one stormwater sample result was over the instantaneous NAL (say you had a pH of 9.5 and the NAL for pH is 9), then you stay in baseline status as long as all other stormwater samples were below pH 9 and above pH 6.