Feeling Reasonably Assured? Potentially Spending of Billions Based on Models
Since the term “Reasonable Assurance Analysis” (RAA) first appeared in the 2012/2013 Municipal Separate Storm Sewer System (MS4) Permit, approximately forty watershed groups have performed and submitted RAAs in the Los Angeles and San Diego regions. These modeling analyses form the basis for watershed-based BMP implementation plans that potentially represent billions of dollars in proposed capital improvement projects, many of which lack identified funding sources. The models have proven to be useful planning tools upon which major investment decisions are being contemplated. The models involve calibration for hydrology and water quality, and are then used to demonstrate future compliance with TMDL Water Quality Based Effluent Limits (WQBELs) and other Receiving Water Limits (RWLs) resulting from various virtual BMP implementation scenarios. The models are driven by GIS-based and water quality monitoring datasets which vary in degree of robustness for each watershed. Based on a review of a subset of RAA applications, a high-level perspective of the results, implications, and lessons learned will be provided.
Specific issues that must be addressed include the extent to which natural variability and uncertainty are inherent in the water quality and cost forecasts that are being predicted, and how an optimism index can be used to quantify uncertainties and manage risk. These uncertainties extend to seasonal conditions, and the differences in approaches for dry weather and wet weather.
Of significant interest are the predicted costs associated with implementing BMPs that, based on the RAA analysis, are expected to result in regulatory compliance as it relates to the MS4 system. Typical ranges of unit costs (implementation cost per area, by BMP type and scale) can be used to support programmatic estimates. These cost ranges also can vary based on beneficial uses, and by extension, pollutants of concern (e.g., zinc vs bacteria). As such, the cost implications when considering site specific objectives (e.g., copper in the Los Angeles River) and other beneficial use modifications (e.g., high flow suspension for bacteria) are significant.
Finally, though coordinated integrated monitoring efforts, new data will be collected. It is important to address how future data (e.g., collected through coordinated monitoring programs) can affect RAA accuracy and can support adaptive management for continuous improvement.