About one-third of US States include chemical monitoring in their general stormwater discharges permits. Over time, this percentage has been increasing as States revise their permits as they expire.
For States that require chemical analysis of stormwater samples, “benchmark” concentrations or values are often included. Benchmark concentrations can be specified for a number of different chemical parameters, but the more common chemicals which are assigned benchmarks include total suspended solids (TSS), biochemical oxygen demand (BOD), chemical oxygen demand (COD), nutrients and heavy metals.
Benchmark values differ from permit limits. In a typical wastewater NPDES permit, limits may be specified for chemical parameters; if any of the limits are exceeded, it becomes a violation of the permit and may be subject to enforcement action.
Benchmark values are intended to provide a measurement of the effectiveness of the stormwater pollution prevention plan (SWPPP). Exceeding a benchmark does not directly result in a permit violation. However, permits typically require facilities to reevaluate their SWPPP and to take prompt corrective action after a benchmark value is exceeded. Failure to take prompt corrective action if a benchmark value is exceeded can be a permit violation and subject to enforcement action.
Benchmark values tend to be fairly stringent. For example, a common benchmark concentration for total suspended solids (TSS) is 100 mg/L. To put this concentration into perspective, it is roughly equivalent to 1 teaspoon of sediment added to 14.5 gallons of water.
As a sector example, US EPA tested the stormwater discharge at 185 transportation and warehousing facilities. The average TSS was reported as 466 mg/L and half the facilities sampled exceeded 159 mg/L (Federal Register Vol. 60, No. 189). Based on these data, over one-half of transportation and warehouse sites would not meet the 100 mg/L benchmark.
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