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

1 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 s controls must be designed, installed and maintained to: (1) Control stormwater volume and velocity within the site to minimize soil erosion.

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(1) C ontrolling stormwater volu me and velocity within the site to minimize soil erosion

relate s to controlling or minimizing the discharge stormwater from the site

.

Control of volume and velocity of stormwater in conveyances where concentrated flow occurs as well as control of volume and velocity of sheet flow on site are necessary to reduce mobilization, transport and discharge of sediment and other pollutants. EPA n otes that this requirement reflects common practice for water handling on construction sites. The need for effective erosion control practices is an important component of stormwater management on construction sites and is well

- known and described in avail able references. For example, t he Virginia Erosion and Sediment Control Handbook (Third Edition, available at http://www.dcr.virginia.gov/stormwater_management/e_and_s

- ftp.sh tml ) describes the need for appropriate water handling practices. At page II

- 14, the Handbook states :

“The removal of existing vegetative cover and the resulting increase in impermeable surface area during development will increase both the volume and vel ocity of runoff. These increases must be taken into account when providing for erosion control. ”

The handbook describes the practices that are appropriate for managing the volume and velocity of stormwater.

“Keeping slope lengths short and gradients low and preserving natural vegetative cover can keep stormwater velocities low and limit erosion hazards. Runoff from the development should be safely conveyed to a stable outlet using storm drains , diversions, stable waterways, riprapped channels or similar measures.”

“Conveyance systems should be designed to withstand the velocities of projected peak discharges. These practices should be operational as soon as possible after the start of

construction.”

Examples of appropriate controls to address this provi sion include management of concentrated flows through the use of channel liners or other stabilization measures to minimize erosion caused by flowing water in channels, use of pipe slope drains to move water down slopes to minimize erosion, use of check da ms in channels to reduce flow velocities and minimize erosion, and use of sediment basins and traps to provide detention and reduction in peak flowrates which minimizes downslope erosion. Examples of practices to reduce volume and velocity of stormwater wi th respect to sheet flow or other non

- concentrated flow on site include the use of slope breaks such as berms to slow water as it flows down slopes and the use of cover materials such as mulches and vegetative stabilization on slopes to reduce the velocity of stormwater flowing down the slopes. During construction, the volume and rate of runoff increases, which increases the capacity of the stormwater to transport pollutants to receiving waters (see Controlling Runoff and Erosion at Urban Construction Sites

, University of Missouri

Columbia Extension, http://extension.missouri.edu/explorepdf/agguides/agengin/g01509.pdf ). Erosion of soil particles is caused by both rainfall impa ct energy as well as the energy of flowingSupplemental Explanation in Support of BMPs in C&D ELG and NSPS February 15, 2012

2 water. Water flowing over soil as sheet flow as well as concentrated flow overland and in conveyances, such as channels, causes detachment of soil particles and transport of these particles downslope. These partic les can be discharged from the construction site along with the stormwater. While removal of some particles in downslope sediment controls (e.g., sediment basins) can be accomplished, these sediment controls are not 100% effective in removing

entrained soi l particles. Therefore, some portion of soil that is mobilized (and the pollutants associated with those soil particles) can be discharged from the construction site. Controlling stormwater volume and velocity within a construction site reduces the amount of erosion caused by flowing water, and therefore can reduce the amount of sediment, turbidity and other pollutants discharged from the site. For example, a particular sediment basin may be capable or removing all particles above 40 microns in diameter thr ough settling. If the stormwater flowing to the sediment basin during a particular storm event contains 1,000 pounds of soil, 80% of which is above 40 microns, then the basin would remove 80% (or 800 pounds) of the sediment while 20% (or 200 pounds) would not be removed and would be discharged. However, if during this same storm event upslope controls to reduce erosion due to the volume and velocity of stormwater were not implemented, then one would expect a larger quantity of sediment to be

eroded and tran sported to the sediment basin. For this example, say the total quantity of sediment transported to the basin for this event is twice as much, or 2,000 pounds , because upslope volume and velocity controls were not implemented . Again, the distribution of soi l particle sizes is such that 80% of the particles are above 40 microns while 20% is below 40

microns in diameter . In this case, the amount of sediment not removed by the basin is 20% of 2,000 pounds, or 400 pounds. This is twice as much as discharged from the example where upslope controls to reduce erosion were implemented. Therefore, reducing the volume and velocity of stormwater, which reduces the amount of erosion, can directly reduce the quantity of sediment and associated pollutants that are discharg ed.

As EPA has noted in the preamble (see pg. 63011), prevention of erosion is an important pollution prevention consideration at construction sites. This is especially important on sites that include fine silts and clays since these particles, once erode d and mobilized by stormwater, are difficult if not impossible to remove in conventional sediment controls utilized on construction sites. The specification of minimum erosion control and water handling practices is appropriate in the effluent guideline. I f these erosion control techniques are not utilized, one would expect higher amounts of pollutants to be discharged from sites since down

- slope sediment controls have variable performance and may not remove these smaller particle size fractions (see the example provided above). The performance of sediment controls can be improved through the

use of practices such as applying chem ical flocculants or other enhanced sediment removal techniques specifically designed to remove all particulate size fractions that are present

. Some sites may implement chemical treatment or other enhancements to improve sediment control performance. Reaso ns for implementing chemical treatment or other enhanced sediment controls include receiving water quality standards, numeric discharge standards, and/or benchmarks/action levels specified by the permitting authority. However, even if chemical

flocculants or other enhanced sediment removal techniques are implemented at a site, this does not negate the need for implementing up

- slope erosion prevention techniques. This is because there may be periods of time (for example, during larger storm events and/or dur ing periods of construction where the amount of disturbed acreage is below the established thresholds for applicability of a numeric standard or benchmark/action level ) when the standard or limitation would not apply. In these cases, the BMPs would serve a s the only requirements controlling the discharge of pollutants. In addition, enhanced sediment controls typically involve first holding the stormwater in a pond or basin before treatment. During larger storm events or extended periods of precipitation, th e storage capacity of these basins may be exceeded. If up

- slope erosion controls are not implemented, then the stormwater flowing into the basins would have a higherSupplemental Explanation in Support of BMPs in C&D ELG and NSPS February 15, 2012

3 concentration of pollutants. Therefore, the quantity of pollutants in these basin overflow s would be much larger than would have occurred had the up

- slope erosion controls been implemented. Due to these (and other) reasons, the requirement to control stormwater volume and velocity within the site in order to reduce erosion and transport of poll utants is necessary on all sites, even those subject to numeric discharge standards or benchmarks/action levels.

(2) W hat EPA do es not mean by this requirement

.

EPA does not intend for this requirement to apply once construction has ceased and sites have been stabilized. This requirement only applies during the construction phase, and does not apply to post

- construction conditions.

( 3 ) In its comm ent

s on the proposed C&D ELG /NSPS, NAHB expressed support for non

numeric effluent limitati ons in the for m of BMPs like this BMP

.

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 implementat ion of pollution prevention practices.” NAHB

comment c over 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 th at 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 li mits and proceed with a BMP

- based ELG to control stormwater discharge.” NAHB d etailed 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 stormwate r 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 enviro nmental 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 BMP

only ELG, NAHB did not su ggest any change whatsoever to proposed 450.21(a)(2), which is

identical to final 450.21(a)(1). Thus, NAHB acknowledg ed , consistent with its overall comments in support of a BMP

- only ELG, that controlling stormwater volume and velocity within the site to

minimize soil erosion 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 t o 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 limitation s in those NPDES permits. The ELG is not directly applicable to any regulated entity . If the permittin g authority chooses to copy the ELG verbatimSupplemental Explanation in Support of BMPs in C&D ELG and NSPS February 15, 2012

4 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 e vent tha t the permitting authority does not provide any additional specificity, it is t ypical 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 pe rmit. 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 ho w they are meeting

their effluent limitations.

In the 2012 CGP , EPA has implemented this provision by requiring that construction stormwater conveyance channels be designed to avoid unstabilized areas on the site and to reduce erosion, unless infeasible . The provision requires that erosion of channels and their embankments, outlets, adjacent streambanks, slopes, and downstream waters to be minimized through the use of erosion controls and velocity dissipation devices along the length of any

constructed stormwater conveyance channel, and at any outlet to provide a non

- erosive flow velocity.

(5 ) A ppropriate time for implementation of this BMP in the construction process

.

The proper time for implementation of controls to control both the total volume and velocity of stormwater within the site depends on the nature of the control. Some practices (such as sediment basins) should be installed very early in the construction process so that they are functioning and able to accept runoff from up

- slope distur bed areas. Other practices may be installed later in the construction process as they are needed. For example, a sediment basin may be designed to accept water from several catchments in a project, all of which may not be

disturbed at the same time. Prior to disturbance of an area, it may be appropriate to install a channel to divert runoff from the disturbed area to the basin. When this channel is installed, the need for velocity control measures such as a channel lining or check dams would dictate that

th ey be installed when the channel is constructed. The need for specific controls is site

- specific, and will vary based on the nature of the construction activity.