MPCA review of submittals using the MPCA Simple Estimator
If a permittee is claiming to meet one or more wasteload allocations (WLA), they must submit documentation to the Minnesota Pollution Control Agency, using an approved method, that they are meeting the WLA. If a permittee uses the MPCA Simple Estimator, it is recommended they submit either of the following:
- the entire Excel spreadsheet, or
- the worksheet called Summary sheet
- a printout or legible image of the worksheet called Summary sheet.
In reviewing the information in the Estimator, MPCA staff will focus on the following.
- Comparing the reduction (lbs or %) or final loading rate (lb/ac/yr) with the WLA, depending on how the WLA is expressed in the TMDL
- Changes in default values for event mean concentration (emc), runoff coefficients, bmp removal efficiencies, or the fraction of runoff that is treated and infiltrated by bmps
- Changes in adjusted loads compared to unadjusted total loads
- Land use distributions
- BMPs implemenated
These are discussed briefly below.
Comparing the reduction (lbs or %) or final loading rate (lb/ac/yr) with the WLA, depending on how the WLA is expressed in the TMDL
Wasteload allocations are typically expressed as a total load, a mass load reduction (e.g. pounds), a percent mass reduction, or a loading rate (lb/ac/yr). The value reported in the Estimator will be compared to the WLA as reported in the TMDL or as interpreted by the MPCA.
Changes in default values for event mean concentration, runoff coefficients, bmp removal efficiencies, or the fraction of runoff that is treated and infiltrated by bmps
Default values for these inputs are based on information from the literature and are therefore considered defensible. However, local information may exist to support changing these values. This includes water quality monitoring data, site-specific land use information, bmp performance monitoring, or some similar information. The Estimator spreadsheet contains cells for describing the rationale for changes to these inputs. The Stormwater Manual provides guidance for adjusting emcs and runoff coefficients. The adjacent image provides an example of some changes that MPCA staff would examine in determining compliance with a wla. Examples of changes that may merit further investigation by MPCA staff include the following.
- Improved bmp removal efficiency
- increased fraction of water infiltrating for a specific bmp
- changes in emc or runoff coefficient greater than 10 percent compared to the default value