MPCA review of submittals using the MPCA Simple Estimator

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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

Changes in adjusted loads compared to unadjusted total loads

The bmps included in the Simple Estimator have approved pollutant removal efficiencies. There are other actions that may reduce pollutant loading, or in some cases increase loading. Examples include changes in land use and street sweeping. Because there are not documented reductions for these activities, permittees can adjust emcs and/or runoff coefficients based on local data or information. MPCA encourages these adjustments when they are well supported and provide more realistic pollutant loading estimates. These activities should be documented in the spreadsheet. The spreadsheet is designed so that these adjustments will be noted and MPCA staff may request additional information about the basis for these adjustments.

Land use distributions

MPCA staff will not conduct a comprehensive analysis of land use detailed in the Estimator spreadsheet. However, staff may request additional information under certain circumstances such as the entire area being in commercial land use.