The stormwater program has traditionally been based on a presumptive approach— implement a set of best management practices (BMPs) as prescribed in your permit or stormwater pollution prevention plan, and you are presumed to be protecting water quality. Very little monitoring or evaluation was conducted to determine if the BMPs were actually reducing stormwater impacts.
With the prevalence of total maximum daily loads (TMDLs) and other requirements, the stormwater program is beginning to move to a demonstrative approach. In addition to selecting a set of BMPs, you will need to document that these BMPs have reduced pollutant loads to a receiving water by a certain amount.
For example, EPA requires that NPDES permits must contain requirements consistent with the assumptions of the wasteload allocation in the TMDL. NPDES permit writers are still struggling with how to address wasteload allocations in NPDES permits, but numeric limits are generally considered inevitable. As another example, a draft construction general permit in California includes site-specific numeric action levels for turbidity using the Modifed Universal Soil Loss Equation (MUSLE).
How many construction site operators have experience using MUSLE, and how many cities have developed monitoring programs to demonstrate their compliance with TMDLs? There are many good reasons to set more specific requirements and advance the stormwater program through more demonstrative approaches (and some things, like the statutory requirements for TMDLs, are out of our direct control). However, without more education, training, and knowledge about the effectiveness of BMPs and how best to affordably evaluate these BMPs, are we settling ourselves up for failure?