Supplemental Explanation in Support of BMPs in C&D ELG and NSPS February 15, 2012

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40 CFR 450.21(a) Erosion and Sediment Controls. Design, install and maintain effective 

erosion controls and sediment controls to minimize the discharge of pollutants. At a 

minimum, such controls must be designed, installed and maintained to: (3) Minimize the 

amount of soil exposed during construction activity. 

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(1) Minimizing soil exposed controls or minimizes the discharge of stormwater from the 

site. 

This requirement addresses two interrelated concepts – minimizing how much of the site is 

disturbed (both at one time and the total amount of the site during the entire project) and 

minimizing the duration that soils are exposed. Soil erosion rates are a function of a number of 

parameters, including soil particle size distribution, soil texture, cohesiveness, rainfall energy, 

slope, slope length, and the type and quantity of cover. Exposed soils are much more prone to 

erosion than covered or stabilized soils. Generally, projects with more land exposed and/or with 

soils exposed for longer durations would generate greater quantities of sediment, turbidity and 

other pollutants than those with lesser quantities exposed and/or exposure for shorter durations. 

Therefore, minimizing both the quantity and the duration of exposure would be expected to 

reduce the amount of pollutants contained in stormwater discharges. Even when down-slope 

sediment controls are used, such as sediment basins or silt fences, it is necessary to control upslope soil erosion. (See Supplemental Explanation in Support of 450.21(a)(1) and example 

provided there.) This is because sediment controls are not 100% effective in removing 

pollutants. Therefore, if the total quantity of sediment reaching sediment controls is higher 

because up-slope areas are exposed, then the amount of sediment discharged from the 

sediment controls will be higher. Minimizing up-slope disturbances will reduce the amount of 

sediment and other pollutants discharged from the site. The requirement to minimize the 

amount of soil exposed during construction activity can be accomplished through a number of 

practices. These include practices related to how much soil is exposed, such as phasing land 

disturbing activities, and soil stabilization practices, such as use of mulches, rolled erosion 

control products and vegetation. Permittees have flexibility to select appropriate controls to 

minimize the amount of soil exposed at their individual sites. Permittees also have flexibility to 

schedule and phase construction activities so as to limit the amount of land disturbed at one 

time and the duration of exposure.

See also the following websites for further support for this principle:

NRC Report

http://www.epa.gov/owow/NPS/MMGI/Chapter4/index.html

Runoff Characteristics for Construction Site Erosion Control Practices, J. Irrig. Drain Eng. 136, 

405 (2010); doi:10.1061/(ASCE)IR.1943-4774.0000202 (9 pages) 

http://ascelibrary.org/iro/resource/1/jidedh/v136/i6/p405_s1?isAuthorized=no (Eleven erosion 

control BMPs were evaluated at a slope of 2H:1V, three at 3H:1V, and three at 4H:1V. The 

variations in slope were used to assess the effects of slope and CECB thickness on runoff. The 

results from this study provide new insight regarding the runoff characteristics from bare soil and 

erosion control BMPs that can be used to improve construction-site storm water design. The 

results show that 2.5- and 5.0-cm-thick CECBs on top of netting or an excelsior fiber blanket 

provided a significant reduction in runoff relative to the bare soil and the other BMPs (e.g., 1.3-

cm CECBs, other blankets) due to water storage within the CECB, the mass of the CECB 
Supplemental Explanation in Support of BMPs in C&D ELG and NSPS February 15, 2012

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providing a strong bond between the soil surface and the bottom of the blanket reducing the 

potential for flowing water from coming in contact with the soil surface, and the netting/blanket 

under the CECB providing additional friction that helps keep the CECB from sliding down slope.)

In addition, it is commonly recommended as appropriate to minimize the amount of soil exposed 

during construction activity. Cities and states across the country recommend this for control of 

discharges. For example, see the following websites: 

http://www.mdt.mt.gov/pubinvolve/stormwater/construction.shtml (Montana)

http://www.deq.state.or.us/wq/stormwater/docs/indswbmps.pdf (Oregon)

http://www.belaireks.org/stormwater.cfm (Bel Aire, Kansas)

http://www.centervilleut.net/downloads/publicworks/dc_bmp_manual.pdf (Davis County, Utah)

(2) What EPA does not mean by this requirement. 

Generally, permittees should minimize the amount of land disturbed at one time, the duration of 

exposure, and should promptly implement appropriate cover as soon as practicable or 

designated by the permitting authority. Permitting authorities may develop explicit criteria all or 

some projects, such as placing limitations on the amount of land exposed at one time, the 

duration of exposure, limiting disturbance in certain areas (e.g., steep slopes, on highly erodible 

soils), limiting disturbance during certain time periods (e.g., during spring thaw or during 

seasonally wet periods), and may also require specific types and quantities of soil cover or 

stabilization measures.

(3) In its comments on the proposed C&D ELG/NSPS, NAHB expressed support for nonnumeric effluent limitations in the form of BMPs like this provision.

In its cover comment letter on the proposed C&D ELG/NSPS, NAHB stated that it “has 

consistently advocated for an ELG that allows for site-specific selection of BMPs, appropriate 

sediment basin sizing criteria and the implementation of pollution prevention practices.” NAHB 

comment cover letter at 2. NAHB stated that EPA’s proposed Option 1, a BMP-only ELG, 

“would build upon the significant environmental progress made to date by ensuring that all 

construction sites meet a minimum standard of excellence by adopting those erosion and 

sediment controls that have evolved and are now accepted across the country.” Id. NAHB 

urged EPA to broaden “best management practices and pollution prevention to ensure these 

practices can be implemented at every permitted construction site, in all circumstances across 

the country.” Id. at 3. 

In its detailed comments attached to its cover letter, NAHB continued to support a BMP-only 

ELG to control stormwater discharges from active construction sites. NAHB urged EPA “to 

remove any requirements to meet numeric effluent limits and proceed with a BMP-based ELG to 

control stormwater discharge.” NAHB detailed comments at 38. NAHB claimed that “[i]n the 

case of EPA's proposed C&D ELGs, the combination of ELG precedent and the fact that EPA is 

focusing exclusively on stormwater provides irrefutable support for a BMP-based ELG 

approach.” Id. at 42. NAHB concluded that “[t]here is substantial authority for and evidence 

regarding the success associated with permitting authorities relying on BMP-based approaches 

to achieve environmental objectives. EPA also should rely on a BMP-based approach for the 

C&D ELG rulemaking.” Id. at 46 (emphasis in original).

In addition, in its redline/strikeout “precisely crafted edits” of EPA’s proposed Option 1, the BMPonly ELG, NAHB suggested a revision to proposed 450.21(a)(3) to make that provision read as 
Supplemental Explanation in Support of BMPs in C&D ELG and NSPS February 15, 2012

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follows: “Minimize the amount of exposed soil during the construction activity.” EPA had 

proposed the following: “Minimize the amount of soil exposed for the duration of the construction 

activity as well as at any one time.” Thus, while NAHB suggested edits to this BMP, it did not 

suggest deleting the requirement, thereby acknowledging, consistent with its overall comments 

in support of a BMP-only ELG, that minimizing the amount of exposed soil during the 

construction activity is key to controlling and minimizing the discharge of stormwater from active 

construction sites. See reference to “precisely crafted edits” on page 144 of NAHB’s detailed 

comments on the proposed ELG/NSPS, as well as the attachment to EPA-HQ-OW-2008-0465-

1360. 

(4) Implementation of this provision in NPDES permits.

The construction stormwater ELG will be implemented in NPDES permits as the applicable 

technology-based effluent limitations in those NPDES permits. The ELG is not directly 

applicable to any regulated entity. If the permitting authority chooses to copy the ELG verbatim 

into its draft proposed NPDES permit as the technology-based effluent limitation, dischargers 

will have an opportunity to provide comment on the need for more specificity if they are 

concerned that the narrative nature of the limit could unfairly subject them to arbitrary 

enforcement. Furthermore, in the event that the permitting authority does not provide any 

additional specificity, it is typical in stormwater permits for active construction for the permitting 

authorities to include a requirement to prepare a storm water pollution prevention plan (SWPPP) 

in the permit. While the provisions of the SWPPP are not effluent limitations themselves, they 

indicate how the discharger intends to meet the narrative effluent limitations in the permit. As 

such, SWPPPs can be helpful to the regulated entity in demonstrating how they are meeting 

their effluent limitations. 

In the 2012 CGP, EPA has implemented this provision by requiring that permittees minimize the 

amount of soil exposed during construction activities. In the final permit fact sheet, EPA

elaborates that soil exposure can be minimized by maintaining or preserving natural vegetation 

on-site, by phasing construction activities, or by implementing soil stabilization practices on 

disturbed areas.

(5) Appropriate time for implementation of this BMP in the construction process.

This requirement applies to the entire duration of active construction. During certain time 

periods in certain climates, permitting authorities may provide some flexibility, e.g. during

seasonally arid periods where little to no precipitation (and hence little erosion risk) exists.