Visual - Benchmark Stormwater Monitoring Compared To Wet Weather Inspections

Overtime, the requirements for stormwater discharge monitoring have shifted. During the 1990s, when stormwater discharge permitting requirements began to develop, “wet weather inspections” were incorporated into many State general permits. These inspections were to be conducted during a rain event and required the discharger to make certain observations about the stormwater being discharged at each of the stormwater outfalls. In most cases, wet weather inspections could be conducted without actually collecting a sample of the stormwater discharge.

Subsequently, the US EPA Multisector General Permit (MSGP) became more detailed and the Visual Monitoring requirement was developed. Overtime, State permits also incorporated a requirement to conduct visual monitoring. However, in some cases permits were not clear as to whether or not a sample of stormwater needed to be collected. Because of this ambiguity, some dischargers believed that the same protocol they were using for wet weather inspections still applied.

Recently, the EPA MSGP was revised to eliminate any uncertainty regarding the protocol to be used for Visual Monitoring:
“The visual assessment must be made:
- Of a sample in a clean, clear glass, or plastic container, and examined in a well-lit area;
- On samples collected within the first 30 minutes of an actual discharge from a storm event.”

Benchmark monitoring is also required of some industrial sectors. Benchmark monitoring becomes an extension of Visual Monitoring. Rather than documenting the visual characteristics of the sample, it is simply sent to an analytical laboratory for chemical analysis.

Therefore, in practice the same basic protocol is required to collect stormwater samples for both visual monitoring and benchmark monitoring.

[Read more about benchmark monitoring]
[Read more about selected stormwater monitoring techniques and equipment]

Caltha LLP provides a range of stormwater monitoring services, from development of a written Stormwater Monitoring Plan, to conducting stormwater monitoring training, to supplying equipment and sampling personnel to collect samples.
[Read more about Stormwater Monitoring Services]

For further information contact Caltha LLP at
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