Effluent Limitations Guidelines and Standards for the Construction and Development Point Source Category, 112-123 [2011-33661]

Agencies

• ENVIRONMENTAL PROTECTION AGENCY

[Federal Register Volume 77, Number 1 (Tuesday, January 3, 2012)]
[Notices]
[Pages 112-123]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 2011-33661]


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ENVIRONMENTAL PROTECTION AGENCY

[EPA-HQ-OW-2010-0884, FRL-9615-3]


Effluent Limitations Guidelines and Standards for the 
Construction and Development Point Source Category

AGENCY: Environmental Protection Agency (EPA).

ACTION: Notice.

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SUMMARY: The Environmental Protection Agency is issuing a notice to 
solicit data and information associated with revisions to the Effluent 
Limitations Guidelines and New Source Performance Standards for the 
Construction and Development Point Source Category issued under the 
Clean Water Act. The regulation, as originally issued on December 1, 
2009, established requirements that reduce pollutants discharged from 
construction and development sites, including requirements for a subset 
of sites to comply with a numeric effluent limitation for turbidity. On 
November 5, 2010, EPA published a direct final rule and companion 
proposal staying the numeric turbidity limitation established by the 
December 2009 rule to correct a calculation error. The Agency received 
no adverse comments regarding the stay, and therefore, effective on 
January 4, 2011, the numeric turbidity limitation was stayed. In 
today's notice, EPA is seeking data on the effectiveness of 
technologies in controlling turbidity in discharges from construction 
sites and information on other related issues. Today's notice also 
seeks comment on passive treatment data already available to the 
Agency.

DATES: Comments must be received on or before March 5, 2012, 60 days 
after publication in the Federal Register.

ADDRESSES: Submit your comments, identified by Docket ID No. EPA-HQ-OW-
2010-0884, by one of the following methods:
     https://www.regulations.gov: Follow the on-line 
instructions for submitting comments.
     Mail: Water Docket, U.S. Environmental Protection Agency, 
Mailcode: 28221T, 1200 Pennsylvania Ave. NW., Washington, DC 20460.
     Hand Delivery: Water Docket, USEPA Docket Center, Public 
Reading Room, 1301 Constitution Avenue NW., Room 3334, EPA West 
Building, Washington DC 20004. Such deliveries are only accepted during 
the Docket's normal hours of operation, and special arrangements should 
be made for deliveries of boxed information.
    Instructions: Direct your comments to Docket ID No. EPA-HQ-OW-2010-
0884. EPA's policy is that all comments received will be included in 
the public docket without change and may be made available online at 
https://www.regulations.gov, including any personal information 
provided, unless the comment includes information claimed to be 
Confidential Business Information (CBI) or other information whose 
disclosure is restricted by statute. Do not submit information that you 
consider to be CBI or otherwise protected through www.regulations.gov 
or email. The https://www.regulations.gov Web site is an ``anonymous 
access'' system, which means EPA will not know your identity or contact 
information unless you provide it in the body of your comment. If you 
send an email comment directly to EPA without going through 
www.regulations.gov your email address will be automatically captured 
and included as part of the comment that is placed in the public docket 
and made available on the Internet. If you submit an electronic 
comment, EPA recommends that you include your name and other contact 
information in the body of your comment and with any disk or CD-ROM you 
submit. If EPA cannot read your comment due to technical difficulties 
and cannot contact you for clarification, EPA may not be able to 
consider your comment. Electronic files should avoid the use of special 
characters, any form of encryption, and be free of any defects or 
viruses.
    Docket: All documents in the docket are listed in the https://www.regulations.gov index. Although listed in the index, some 
information is not publicly available, e.g., CBI or other information 
whose disclosure is restricted by statute. Certain other material, such 
as copyrighted material, will be publicly available only in hard copy. 
Publicly available docket materials are available either electronically 
in https://www.regulations.gov or in hard copy at the Water Docket, EPA/
DC, EPA West, Room 3334, 1301 Constitution Ave. NW., Washington, DC. 
The Public Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday 
through Friday, excluding legal holidays. The telephone number for the 
Public Reading Room is (202) 566-1744, and the telephone number for the 
Water Docket is (202) 566-2426.

FOR FURTHER INFORMATION CONTACT: Mr. Jesse W, Pritts, Engineering and 
Analysis Division, Office of Water (4303T), Environmental Protection 
Agency, 1200 Pennsylvania Ave. NW., Washington, DC 20460; telephone 
number: (202) 566-1038; fax number: (202) 566-1053; email address: 
pritts.jesse@epa.gov.

SUPPLEMENTARY INFORMATION:

A. Does this action apply to me?

    Entities potentially affected by this action include:

------------------------------------------------------------------------
                                                         North American
                                                            Industry
            Category             Examples of affected    Classification
                                       entities          System (NAICS)
                                                              Code
------------------------------------------------------------------------
Industry.......................    Construction activities required to
                                     obtain NPDES permit coverage and
                                   performing the following activities:
                                ----------------------------------------
                                 Construction of                     236
                                  buildings,
                                  including building,
                                  developing and
                                  general contracting.
                                 Heavy and civil                     237
                                  engineering
                                  construction,
                                  including land
                                  subdivision.
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[[Page 113]]

    EPA does not intend the preceding table to be exhaustive, but 
provides it as a guide for readers regarding entities likely to be 
affected by this action. Other types of entities not listed on the 
table could also be affected. To determine whether your may be affected 
by this action, you should carefully examine the applicability criteria 
in Section 450.10 of the December 1, 2009 final rule (74 FR 62995) and 
the definition of ``storm water discharges associated with industrial 
activity'' and ``storm water discharges associated with small 
construction activity'' in existing EPA regulations at 40 CFR 
122.26(b)(14)(x) and 122.26(B)(15), respectively. If you have questions 
regarding the applicability of this action to a particular activity, 
consult one of the persons listed in the preceding FOR FURTHER 
INFORMATION CONTACT section.

Table of Contents

I. Overview
II. Background
    A. NPDES Regulations, Construction General Permits and 
Applicability of 40 CFR Part 450 Requirements
    B. Petitions for Administrative Reconsideration and Petitions 
for Review of the Final Construction and Development Regulation in 
the U.S. Circuit Court of Appeals for the Seventh Circuit
    C. EPA's Unopposed Motion
    D. Stay of the Numeric Limitation
III. Review of Treatment Data in EPA's Current Dataset
    A. Approach to Calculating the December 2009 Turbidity 
Limitation
    B. Passive and Semi-Passive Treatment Datasets
    C. Additional Data
IV. Solicitation of Data and Comments on Numeric Effluent 
Limitations for Turbidity
    A. Control of Turbidity--Effectiveness, Cost and Feasibility of 
Different Technologies
    B. Sampling and Data Collection--Procedures and Protocols To 
Ensure Representativeness of Data; Differences in Analytical 
Equipment
    C. Effect of Storm Size, Intensity and Duration of Precipitation 
on Performance of Passive Treatment
    D. Exemptions--Design Storm Depth vs. Intensity
    E. Use of Treatment Chemicals, Disposal and Toxicity Concerns
    F. Cold Weather Considerations
    G. Small Sites That Are Part of a Larger Common Plan of 
Development or Sale
    H. Electric Utility Transmission Line Construction

I. Overview

    EPA promulgated Effluent Limitations Guidelines and Standards for 
the Construction and Development Point Source Category (hereafter 
referred to as the ``C&D rule'') on December 1, 2009 (74 FR 62995). The 
final rule established requirements based on Best Practicable Control 
Technology Currently Available, Best Available Technology Economically 
Achievable, Best Conventional Pollutant Control Technology, and New 
Source Performance Standards based on Best Available Demonstrated 
Control Technology.
    The rule included non-numeric requirements to:
     Implement erosion and sediment controls;
     Stabilize soils;
     Manage dewatering activities;
     Implement pollution prevention measures;
     Prohibit certain discharges; and
     Utilize surface outlets for discharges from basins and 
impoundments.
    The December 2009 final rule also established a numeric limitation 
on the allowable level of turbidity in discharges from certain 
construction sites. The technology basis for the final numeric 
limitation was passive treatment controls including polymer-aided 
settling to reduce the turbidity in discharges.
    Since issuing the final rule, an error in EPA's interpretation of 
the data used to establish the numeric limitation was identified in 
petitions from the U.S. Small Business Administration and the National 
Association of Home Builders (NAHB). Today's notice seeks comment in 
the form of data and information on several of the issues raised in the 
petitions, as well as other topics.

II. Background

A. NPDES Regulations, Construction General Permits and Applicability of 
40 CFR Part 450 Requirements

    EPA promulgated the Phase I National Pollutant Discharge 
Elimination System (NPDES) stormwater regulations (55 FR 47990) on 
November 16, 1990. The Phase I regulations require that dischargers 
must apply for and obtain authorization to discharge (or ``permit 
coverage''). One of the categories of dischargers that must obtain 
permits is discharges associated with construction activity, including 
clearing, grading, and excavation, if the construction activity:
     Will result in the disturbance of five acres or greater; 
or
     Will result in the disturbance of less than five acres of 
total land area that is a part of a larger common plan of development 
or sale if the larger common plan will ultimately disturb five acres or 
greater.

See 40 CFR 122.26(b)(14)(x).
    The Phase II stormwater regulations, promulgated on December 8, 
1999 (64 FR 68722) extended permit coverage to construction activity 
that:
     Will result in land disturbance of equal to or greater 
than one acre and less than five acres; or
     Will result in disturbance of less than one acre of total 
land area that is part of a larger common plan of development or sale 
if the larger common plan will ultimately disturb equal to or greater 
than one and less than five acres.

See 40 CFR 122.26(b)(15).
    Since 1992, EPA has issued a series of Construction General Permits 
(CGPs) that cover areas where EPA is the NPDES permitting authority. At 
present, EPA is the permitting authority in four states (Idaho, 
Massachusetts, New Hampshire, and New Mexico), the District of 
Columbia, Puerto Rico, all other U.S. territories with the exception of 
the Virgin Islands, Federal facilities in four states (Colorado, 
Delaware, Vermont, and Washington), most Indian lands and other 
specifically designated activities in specific states (e.g., oil and 
gas activities in Texas and Oklahoma).
    In areas where EPA is not the NPDES permitting authority, states 
issue general permits for construction activity. Many state permits 
contain requirements similar to those contained in the EPA CGP. In 
addition, a few state permits contain monitoring requirements and/or 
requirements to comply with numeric effluent limitations. For example, 
California's, Washington's, Oregon's, Georgia's and Vermont's current 
CGPs include discharge monitoring requirements. In addition, 
California's current CGP contains numeric effluent limitations for a 
subset of construction sites within the State.
    EPA issued new regulations at 40 CFR part 450 on December 1, 2009 
(the C&D Rule). The C&D Rule applies to all construction stormwater 
discharges required to obtain NPDES permit coverage. The C&D rule 
applies to the entire country, not just the areas where EPA is the 
permitting authority. Any permit issued by a state or EPA after the 
effective date of the rule (which was February 1, 2010) must include 
the requirements contained in that rule. The requirements include BMPs 
but do not include a numeric limitation which was stayed on January 4, 
2011.

B. Petitions for Administrative Reconsideration and Petitions for 
Review of the Final Construction and Development Regulation in the U.S. 
Circuit Court of Appeals for the Seventh Circuit

    Following promulgation of the December 2009 final C&D rule, the 
Wisconsin Home Builders Association

[[Page 114]]

and the National Association of Home Builders (NAHB) filed petitions 
for review in the U.S. Circuit Courts of Appeals for the Fifth, 
Seventh, and DC Circuits. The petitions were consolidated in the 
Seventh Circuit. Subsequently, the Utility Water Act Group (UWAG) also 
filed suit in the Seventh Circuit. On July 8, 2010, the petitioners 
filed their briefs.
    In April 2010, the Small Business Administration (SBA) filed with 
EPA a petition for administrative reconsideration of several technical 
aspects of the C&D Rule. SBA identified potential deficiencies with the 
dataset that EPA used to support its decision to adopt the numeric 
turbidity limitation. In June 2010, the National Association of 
Homebuilders also filed a petition for administrative reconsideration 
with EPA incorporating by reference SBA's argument regarding the 
deficiencies in the data.

C. EPA's Unopposed Motion

    On August 12, 2010, EPA filed an unopposed motion with the Court 
seeking to hold the litigation in abeyance until February 15, 2012 (see 
DCN 70084) and asking the Court to remand the record to EPA and vacate 
the numeric limitation portion of the rule. In addition, EPA agreed to 
reconsider the numeric limitation and to solicit site-specific 
information regarding the applicability of the numeric effluent 
limitation to cold weather sites and to small sites that are part of a 
larger project.
    On August 24, 2010, the Court issued its decision remanding the 
matter to the Agency but without vacating the numeric limitation. 
Subsequently on September 9, 2010, the petitioners filed an unopposed 
motion asking the Court to reinstate the litigation, hold it in 
abeyance until February 15, 2012, and vacate the numeric limitation. On 
September 20, 2010 the Court reinstated the litigation and held it in 
abeyance until February 15, 2012, but did not vacate the numeric 
limitation.

D. Stay of the Numeric Limitation

    On November 5, 2010, EPA issued a direct final regulation and a 
companion proposed regulation to stay the numeric limitation at 40 CFR 
450.22 indefinitely. The proposed rule solicited comment due no later 
than December 6, 2010. Since no adverse comments were received, the 
direct final rule took effect on January 4, 2011.
    Since the numeric portion of the rule was stayed, states are no 
longer required to incorporate the numeric turbidity limitation and 
monitoring requirements found at Sec.  450.22(a) and Sec.  450.22(b). 
However, the remainder of the regulation is still in effect and must be 
incorporated into newly issued permits. The purpose of this notice is 
to solicit new data from the public and request comment on a number of 
issues that EPA would like to consider in the context of establishing 
numeric effluent limitations for construction site stormwater 
discharges.

III. Review of Treatment Data in EPA's Current Dataset

A. Approach To Calculating the December 2009 Turbidity Limitation

    The December 2009 C&D rule established a numeric limitation for 
discharges of turbidity from construction sites. The final limitation 
was set at 280 nephelometric turbidity units (NTU) based on the 
application of polymer-aided settling, or passive treatment. The data 
used in the derivation of this limitation came from several 
construction sites that were using polymer-aided settling in 
impoundments or in channel applications. EPA's data represented 
treatment at eight separate construction sites located in Washington 
State, New York, and North Carolina.
    The data used in the calculation of the December 2009 numeric 
limitation included data from ponds that were used to pre-treat 
stormwater prior to chitosan-enhanced sand filtration (CESF) active 
treatment systems (ATS). Data representing the final effluent leaving 
CESF had been used in the calculation of the November 28, 2008 proposed 
C&D rule numeric limitation (73 FR 72562), which was based on the 
performance of full CESF.
    EPA considered effluent from the CESF pretreatment ponds as 
representing passive treatment, and used some such data in the 
calculation of the December 2009 limitation. An integral part of CESF 
and ATS is the ability to recirculate pretreated water or effluent from 
the filters back to the pretreatment ponds if turbidity levels are 
above pre-established thresholds. Although this recirculated water is 
above these thresholds, it may be lower in turbidity than the untreated 
stormwater entering the ponds, and/or water that is already in the 
ponds. The effect of recirculating water that is lower in turbidity 
than water contained in the pretreatment ponds would be to reduce the 
turbidity of the water in the pretreatment ponds. Concerns have been 
raised that such recirculation represents an additional level of 
``treatment'' that goes beyond what is otherwise understood as 
``passive'' treatment.

B. Passive and Semi-Passive Treatment Dataset

    If EPA excludes data from the ATS pretreatment ponds, the remainder 
of EPA's passive treatment dataset used in the December 2009 final rule 
consists of data from three passive treatment systems. Since 
promulgation of this rule, EPA has received additional information and 
data from several sources on the performance of passive and semi-
passive treatment approaches. As discussed below, EPA also had 
additional data in the record regarding passive treatment that was not 
used in calculating the December 2009 final rule. The following 
discussion summarizes the information and data that comprise EPA's 
currently reviewed dataset of passive and semi-passive treatment that 
is available in the docket. EPA continues to receive and review 
additional data as it becomes available. EPA may consider these data 
and any data submitted during the public comment period and collected 
by EPA in a future rulemaking to correct and remove the stay of the 
numeric turbidity limitation. Any data that EPA is considering for use 
in this rule making will be placed in the public docket once it has 
been reviewed.
    Steeltown Road and Curley Maple Road, North Carolina (DCN 70018 and 
70065). This study evaluated the performance of fiber check dams with 
polyacrylamide (PAM) on two mountain roadway projects in North 
Carolina. These data were available at the time of the December 2009 
final rule, but additional information on sample collection times and 
turbidity were submitted to EPA in 2011 (DCN 70065).
    Orange County, North Carolina Skimmer Basin (DCN 70034 and 70065). 
This paper evaluated a skimmer sediment basin with PAM at an 
institutional construction project. These data were available at the 
time of the December 2009 final rule, but additional information on 
sample collection times and turbidity were submitted to EPA in 2011 
(DCN 70065).
    Petersburg airport culvert replacement (DCN 70000). This study 
demonstrated the performance of two chitosan lactate biopolymer 
formulations in removing turbidity from pumped water at the Petersburg, 
Alaska airport. Water was semi-passively treated by pumping turbid 
water from one of five culvert locations through a cartridge applicator 
and then into sediment traps constructed of filter fabric. Additional 
treatment was accomplished by allowing the water to exit the trap and 
flow through a vegetated area (called a biofilter).

[[Page 115]]

Testing at this site occurred during March and April of 2009. Reported 
air temperatures varied between -1.0 and 10 degrees Celsius and 
reported water temperatures varied between -0.1 and 1.0 degrees Celsius 
during the study, demonstrating the effectiveness of passive treatment 
during cold-weather conditions. The study did note that chitosan 
lactate dissolution rates were slower due to the cold temperatures. The 
study noted that average daily turbidity of discharge from the sediment 
trap was 248 NTU, and discharge from the biofilter was 102 NTU. 
Influent turbidities were reported as high as approximately 5,000 NTU. 
In order to overcome the slower dissolution rate of the chitosan 
lactate due to the cold temperatures, additional cartridges were 
installed in order to deliver the appropriate dosage. In addition, the 
vendor indicated that a new formulation has been developed that 
dissolves at a higher rate specifically for use in colder climates. 
This report also provides diagrams showing various forms of passive and 
semi-passive dosing that have been developed. Additional references 
describing this project are also included in the docket (see DCNs 70001 
and 70002). EPA requests comment on whether this dataset should be 
considered representative of the BAT technology as described in the 
2009 final rule.
    Water Quality Improvements Using Modified Sediment Control Systems 
on Construction Sites (DCN 70063). This research project studied three 
types of sediment capture and treatment systems at a highway 
construction project (I-485) between 2003 and 2006 in North Carolina. 
The first type of system consisted of unlined diversion ditches with 
rock check dams leading to a standard sediment trap with a rock dam 
outlet. The second type of system added a forebay, porous baffles and 
PAM treatment in the diversion ditches and the forebay. The third type 
of system tested was the same design as the second system except the 
rock check dam was replaced with a floating outlet or skimmer. The 
author reported that the three sediment trapping systems with 
modifications including forebays, porous baffles, ditch lining, and PAM 
application had storm weighted average turbidity and peak turbidity of 
990 and 1,580 NTU, respectively.
    North Carolina State University Typar[reg] Field Test (DCN 70003). 
North Carolina State University (NCSU) conducted a field test of the 
Typar[reg] geotextile product at the university's field laboratory. The 
study evaluated the performance of the material in an in-channel 
application. The tests incorporated polyacrylamide to aid in sediment 
removal. Both total suspended solids and turbidity were evaluated. The 
study evaluated varying flow rates as well as varying sediment loading 
rates. The report contains a considerable amount of data. The report 
indicates that the system is expected to meet a 280 NTU limitation, but 
points out that field testing outside of the field laboratory setting, 
where turbidity and total suspended solids (TSS) levels may be higher, 
would provide additional insights into performance.
    Other Research at North Carolina State University (DCN 70004). 
Researchers at NCSU have conducted research on a number of passive and 
semi-passive treatment approaches. Examples include fiber check dams 
with PAM, sediment basins and traps with PAM, PAM applied to erosion 
control matting down a slope, PAM application in pipes and geotextile 
filter bags with PAM. DCN 70004 contains data from a number of 
evaluations. Additional data on one of the projects identified in DCN 
70004 is also presented in DCN 70053--70060 and 70062.
    North Carolina Department of Transportation (NCDOT) (DCN 70005, 
70006). NCDOT conducted a demonstration to evaluate the performance of 
a dual biopolymer system in removing turbidity. In this application, 
water from culvert sites and caissons at bridge construction sites that 
was impounded in a baffled skimmer basin was pumped through a manifold 
containing biopolymers. The biopolymers dissolve as water is pumped 
through the manifold, and mixing occurs in the manifold, which aids 
flocculation. The water then passes through a geotextile filter bag, 
which retains the flocculated solids. In this demonstration, turbidity 
in the water from the basin was 1,283 NTU, which was reduced to below 
100 NTU following the filter bag.
    StormKlear[reg] (DCN 70007 through 70013 and 70070 through 70080). 
StormKlear[reg]/HaloSource[reg] provided information regarding a number 
of sites using both passive and semi-passive dosing of a dual 
biopolymer system. Sites described were Annapolis, Maryland (DCN 
70007), Austin, Texas (DCN 70008), Beaverton, Oregon (DCN 70009), 
Griffin, Georgia (DCN 70010), Raleigh, North Carolina (DCN 70011), 
Memphis, Tennessee (DCN 70011), Jacksonville, North Carolina (DCN 
70011), Birmingham, Alabama (DCN 70011), Tampa, Florida (DCN 70012), 
Tennessee (DCN 70013), Huntersville, North Carolina (DCN 70070), 
Hanover, Maryland (DCN 70071), Apex, North Carolina (DCN 70072), Bonita 
Springs, Florida (DCN 70073), Staten Island, New York (DCN 70074), 
Cabarrus County, North Carolina (DCN 70075), Anne Arundel County, 
Maryland (DCN 70076), Cartersville, Georgia (DCN 70077), Central, South 
Carolina (DCN 70078), Fairview, North Carolina (DCN 70079) and Lavonia, 
Georgia (DCN 70080). The range of turbidity values reported at these 
sites is presented in Table 1.

                   Table 1--Range of Turbidity Values Reported in Dual Biopolymer Field Trials
----------------------------------------------------------------------------------------------------------------
                 Site                              Untreated NTU                         Treated NTU
----------------------------------------------------------------------------------------------------------------
Annapolis, MD.........................  300-400............................  15.
Austin, TX............................  598................................  10.5-117.
Beaverton, OR.........................  42-44..............................  14.
Griffin, GA...........................  2,189..............................  21.1-433.
Raleigh, NC...........................  2,500-3,000........................  14.
Memphis, TN...........................  1,200..............................  20.
Jacksonville, NC......................  300................................  15.
Birmingham, AL........................  1,500..............................  20.
Tampa, FL.............................  Not Reported.......................  <1.
Huntersville, NC......................  950................................  425.
Hanover, MD...........................  570................................  <50.
Apex, NC..............................  3,787..............................  297 (1.4 after basin).
Bonita Springs, FL....................  162-187............................  3.2-43.
Staten Island, NY.....................  1,057..............................  5-45.
Cabarrus County, NC...................  1,195..............................  42.

[[Page 116]]

 
Anne Arundel County, MD...............  547................................  120.
Cartersville, GA......................  >4,000.............................  51.
Central, SC...........................  687................................  32.
Fairview, NC..........................  >4,000.............................  731 (131 after basin).
Lavonia, GA...........................  >4,000.............................  32.8.
----------------------------------------------------------------------------------------------------------------

    ALPURT B2 Motorway Construction Project (DCN 70049). The Auckland, 
New Zealand Regional Council evaluated the use of polyaluminum chloride 
(PAC) to reduce sediment discharges from a motorway construction 
project. A rainfall-activated dosing system was used to deliver PAC 
prior to settling in a sediment basin. Samples were analyzed for TSS, 
particle size distribution and dissolved aluminum. This study did not 
evaluate reductions in turbidity.
    ALPURT and Greenhihte Trials (DCN 70067). The Auckland, New Zealand 
Regional Council conducted trials using alum, PAC and PAM at several 
sites. The study evaluated both rainfall-activated liquid chemical 
dosing systems as well as solid forms. This study evaluated reductions 
in TSS, but not turbidity.
    Bluffs Community Baffle Grid System (DCN 70050). This project, 
located in the metropolitan Atlanta, Georgia area, was a residential 
construction project. A passive treatment system was utilized 
consisting of a grit pit followed by a polymer mixing chamber. The 
water then flowed into another grit pit and then into a baffle grid 
system. Polymer was dosed using polymer floc logs. Polymer was also 
applied to exposed soils up-slope of the treatment system. This system 
produced an average treated turbidity of 18 NTU, according to the study 
authors. The attached data file shows a range of turbidity after the 
baffle grid ranging from 1.0 to 703 NTU.
    Cleveland Municipal Airport, Cleveland, Tennessee (DCN 70085). This 
site is a multi-year construction project that started in 2009. The 
site utilizes passive treatment including ditches lined with jute 
matting with PAM and sediment basins. Monitoring is conducted after the 
sediment basins as well as in-stream both upstream and downstream of 
the construction site. Only limited monitoring data was available for 
this site. The turbidity reported in effluent at the outfalls after 
implementation of the PAM treatment ranged from 23 to 280 NTU.

C. Additional Data

    At the time of this notice, only one state (California) has a 
numeric effluent limitation for discharges from construction activities 
that applies to a subset of construction sites statewide. Other sites 
in the state are subject to monitoring requirements and action 
levels.\1\ Between July 1, 2010 and June 20, 2011, permittees reported 
735 daily average turbidity values. The range of these daily average 
turbidity values was zero to 1,572 NTU with a median value of 42 NTU 
(see DCN 70051). EPA did not obtain information about the individual 
sites and treatment systems (such as detailed site plans, SWPPPs, 
etc.), and has not evaluated the utility of this data in the context of 
establishing effluent guidelines. EPA has not evaluated whether any of 
these facilities were subject to numeric discharge standards for 
turbidity.
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    \1\ In December 2011, the California Superior Court invalidated 
the California numeric standard of 500 NTU, which applied to a 
subset of construction projects, because the state did not evaluate 
performance data from available technologies under a variety of site 
conditions. Construction projects subject to the standard did not 
have ``reasonable assurance that the technologies are capable of 
achieving the turbidity NEL (numeric technology based effluent 
limitation).'' Decision at 16; California Building Industry 
Association v. State Water Resources Control Board, Case No. 34-
2009-800000338 (Sacramento Superior Court) December 2, 2011. See DCN 
70086.
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    As described in the December 2009 final rule preamble, Warner et 
al. evaluated several innovative erosion and sediment controls at a 
full-scale demonstration site in Georgia. In this project, polymers or 
flocculants were not utilized, but instead a comprehensive system of 
erosion and sediment controls were designed and implemented to mimic 
pre-developed peak flow and runoff volumes with respect to both 
quantity and duration. The system included perimeter controls that were 
designed to discharge through multiple outlets to a riparian buffer, 
elongated sediment controls (called seep berms) designed to contain 
runoff volume from 3- to 4-inch storms and slowly discharge to down-
gradient areas, multi-chambered sediment basins designed with a siphon 
outlet that discharged to a sand filter, and various other controls. 
Monitoring conducted at the site illustrates the effectiveness of these 
controls. For one particularly intense storm event of 1.04 inches (0.7 
inches of which occurred during one 27-minute period), the peak 
sediment concentration monitored prior to the basin was 160,000 mg/L of 
TSS while the peak concentration discharged from the passive sand 
filter \2\ after the basin was 168 mg/L. Effluent turbidity values 
ranged from approximately 30 to 80 NTU. Using computer modeling, it was 
shown that discharge from the sand filter, which flowed to a riparian 
buffer, was completely infiltrated for this event. Thus, no sediment 
was discharged to waters of the state from the sand filter for this 
event. For another storm event, a 25-hour rainfall event of 3.7 inches 
occurred over a two-day period. Effluent turbidity from one passive 
sand filter during this storm ranged from approximately 50 to 375 NTU, 
with 20 of the 24 data points below 200 NTU. For a second passive sand 
filter, effluent turbidity ranged from approximately 50 to 330 NTU, 
with nine of 11 data points below 200 NTU. In the Warner et al. study 
low levels of turbidity in discharges were achieved without relying on 
chemical flocculants or polymers or pumping of water. Although these 
data were available to EPA at the time, EPA did not use the Warner et 
al. data in calculating the limitation contained in the December 2009 
final rule because the site did not use polymers. EPA requests comment 
on whether the Warner et al. data, data from passive sand filters in 
general as described by Warner et al., and data from sites not using 
polymers or flocculants should be used in evaluating the feasibility of 
a numeric effluent limitation and whether these data should be 
considered representative of

[[Page 117]]

the BAT technology as described in the 2009 final rule.
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    \2\ The term ``passive sand filter'' in this context is used to 
describe an in-ground filter constructed by placing sand and gravel 
into an excavated area. The filter receives surface discharge from 
up-slope sediment controls which is distributed across the filter 
surface using distribution pipes. Water flows down through the 
filter bed and is collected by an underdrain system where it is 
conveyed down-slope. All flow in this application is by gravity. The 
system did not incorporate any pumps or any treatment chemicals. A 
passive sand filter differs from the sand filters which are used as 
part of CESF, which are operated by a programmable logic controller 
or onsite personnel, are pressurized and operate at much higher 
flowrates, among other differences.
---------------------------------------------------------------------------

IV. Solicitation of Data and Comments on Numeric Effluent Limitations 
for Turbidity

    The following presents the issues and areas where EPA is soliciting 
feedback, data and information.

A. Control of Turbidity--Effectiveness, Costs and Feasibility of 
Different Technologies

    On November 28, 2008 EPA issued a proposed rule that would have 
established a numeric effluent limitation for turbidity based on the 
application of what is termed active or advanced treatment, or ATS, 
specifically chitosan-enhanced sand filtration (CESF). ATS consists of 
a variety of technologies, the two most prevalent being CESF and 
electrocoagulation. The basic premise behind CESF is to collect the 
stormwater in a pond or basin, withdraw the water from the basin (using 
pumps), add a treatment chemical (in this case chitosan, although the 
technology is adaptable to other treatment chemicals), and remove the 
flocculated solids using filtration. Pretreatment with a treatment 
chemical (such as chitosan) is frequently used to reduce the turbidity 
of the stormwater withdrawn from the pond or basin to a range that will 
allow for efficient filtration. This is frequently done in dedicated 
pretreatment cells or tanks, but the configuration can depend on 
requirements specified by the regulatory agency or the operator. CESF 
typically incorporates a programmable logic controller to monitor 
turbidity and pH of the treated water continuously or during some 
specified time interval, and valves can be actuated automatically by 
the controller to recycle the treated water back to the pretreatment 
cells or storage pond if the discharge does not meet pre-established 
thresholds. Electrocoagulation does not use a polymer or treatment 
chemical, but rather uses an electrical process to destabilize the 
particles. Agglomerated particles are removed by settling and/or 
filtration. ATS, based on information available to EPA on the 
performance of CESF, appears capable of producing very low turbidity 
(generally less than 50 NTU, and in many cases less than 5 NTU) in 
treated stormwater from construction sites. Performance can be further 
enhanced by polishing the filtered water in bag or cartridge filters. 
EPA requests comment on this description of ATS.
    Costs for ATS systems include equipment rental (pumps, filters, 
generators and control equipment), fuel, chemicals, labor, management 
of residuals, piping, and miscellaneous consumables (residual polymer 
test kits, filtration media, etc.) and data management and reporting. A 
stabilized area (such as a gravel pad) may be necessary in some cases. 
In colder climates, consideration of measures to prevent freezing of 
equipment may also be necessary. The requirement to store water in 
ponds and to pretreat water can add costs. Also, managing dewatering of 
a series of large impoundments on some sites may be complicated, 
particularly during extended periods of precipitation. The costs of 
large ponds may be offset to some extent if they are converted to post-
construction stormwater water-quality or flood-control ponds. This is 
frequently accomplished by removing the accumulated sediment captured 
during the construction phase and altering the outlet structure of the 
basin to achieve the water quality and peak discharge rate control 
desired for the post-developed condition. This can result in 
considerable cost savings for the post-construction ponds, since 
significant costs are associated with excavation of the basins. 
However, recent trends toward use of decentralized stormwater 
management may be a disincentive toward utilizing large ponds (although 
the need for flood control ponds and ponds to control stream channel 
erosion may still exist). Practices such as bioretention, porous 
pavement, infiltration systems and harvest and use systems may replace, 
to some extent, centralized conveyance and stormwater detention and 
retention ponds. However, if decentralized controls are used for 
postconstruction stormwater management, then basins used during the 
construction phase may not need to be converted for post-construction 
use. In these cases, the construction phase basins may need to be 
filled in, at additional expense to the developer. In some instances, 
this may provide space where additional structures, parking or other 
amenities can be placed, which may provide a benefit to the developer.
    Passive treatment systems (PTS) in the context of construction site 
stormwater management are practices that do not rely on computerized 
systems with pumps, filters and real-time controls but do incorporate a 
treatment chemical to aid in sediment and turbidity removal. Passive 
treatment could include pumps where they are necessary to move water 
around the construction site, and pumping may be integral to properly 
dosing the water with treatment chemicals in some cases. When pumps are 
utilized to pump the water through a manifold or other apparatus to 
dose the chemical, this type of treatment has been characterized by the 
industry as semi-passive treatment. In passive treatment, polymer can 
be placed in channels that convey water on the construction site, or 
they may be used prior to basins or other practices (such as a baffle-
grid, in-ground sand filter or a geotextile filter bag) that allow for 
settling and/or filtration of the flocculated material. Treatment 
chemicals, either in solid or liquid forms, can be applied at various 
locations on the site. Common PTS include fiber check dams with PAM and 
sediment basins dosed with PAM as described by McLaughlin (see DCNs 
70018, 70034 and 70063). The Auckland, New Zealand Regional Council 
also described a PTS that utilized a rainfall-actuated system to 
deliver liquid chemical (see DCN 70049 and 70067). Minton (see DCN 
70069) described a ``pump and treat'' system whereby water was pumped 
from a basin, a treatment chemical was added, and the water was allowed 
to settle in dedicated treatment cells. Water can be re-circulated with 
the pump and additional chemical added if the settled water does not 
meet specifications. As stated above, the term semi-passive treatment 
has been used to describe practices that utilize pumped water to dose 
the chemical, or applications where the water is first held in a basin 
or other impoundment and withdrawn under more controlled conditions for 
subsequent treatment. Recent improvements to PTS incorporate the use of 
two polymers (see DCNs 70006-70013, 70070-70080), which can be placed 
in a manifold or in a channel. The use of baffles and floating outlets 
or ``skimmers'' on basins are frequently incorporated as part of PTS, 
and directing treated water to vegetated areas or ``biofilters'' can 
also provide additional sediment and turbidity removal prior to 
discharge. EPA requests comment on these descriptions of ``passive'' 
and ``semi-passive'' treatment systems and comments on what practices 
should be considered representative of the BAT technology as described 
in the 2009 final rule.
    The performance of PTS varies based on the type of system, the 
method used to dose chemicals, as well as other factors. The 
performance of simple PTS appears to be sensitive to the type and 
frequency of maintenance and system configuration, as well as the 
intensity and duration of storm events. An advantage of simple PTS, 
such as fiber check dams w/PAM, is that they are

[[Page 118]]

very inexpensive and can be easily incorporated into sites at multiple 
locations and do not require large ponds for storage prior to 
treatment. A disadvantage may be that achieving a consistent level of 
performance may be more difficult due to variations in storm flows and 
sediment loads and little control over dosage rates. The data available 
to EPA does show high levels of turbidity in discharges for some 
events, indicating that simple passive treatment systems may not 
perform well during larger and/or more intense storm events. Data 
collected at a construction site in North Carolina that used passive 
treatment measured peak turbidity in excess of 40,000 NTU during an 
intense storm event (see DCN 70064.3).
    Semi-passive approaches, which first hold the water in a basin, 
tank or impoundment and then release water either by gravity or with a 
pump to provide dosing, appear to be capable of providing lower, and 
perhaps more consistent, turbidity levels due to dampening of the storm 
flows by the basins. An advantage of semi-passive approaches is that 
since the water is withdrawn by pumping (although semi-passive dosing 
can be accomplished using gravity flow in certain cases), flowrates and 
dosing rates can be more easily controlled, allowing for more 
consistent and likely better performance. Since the water is withdrawn 
from the storage pond and dosed at a more controlled rate, the large 
variability and poorer performance that may occur under some 
precipitation conditions with simple passive treatment can potentially 
be avoided. A disadvantage may be that the stormwater must first be 
stored in ponds, tanks or other impoundments in order to provide a 
controlled release. As with ATS, these storage requirements can add 
costs and additional operational considerations to address, 
particularly during extended periods of precipitation. As described 
earlier, these costs may be offset to some extent depending on the 
nature of post-construction stormwater requirements in place.
    An integral component of ATS and PTS is the use of a treatment 
chemical to aid in removal of sediment and turbidity. However, data 
presented by Warner and Collins-Camargo (see DCN 70052) indicates that 
a comprehensive suite of erosion and sediment controls is also capable 
of producing treated stormwater with low levels of turbidity. EPA has 
little data on which to base a numeric limitation on these types of 
practices as this level of management does not appear to be typical at 
most construction sites.
    EPA is soliciting data and information on the costs, effectiveness 
and feasibility of different technologies to control TSS, settleable 
solids, suspended sediment concentration and turbidity in construction 
site stormwater discharges. EPA is also soliciting data on other water 
quality parameters, such as pH, nutrients and metals. EPA is especially 
interested in receiving data on the performance of passive and semi-
passive treatment approaches. Data collected both before the treatment 
or management practice (influent data) as well as data after the 
treatment or practice (effluent concentration) would be useful. EPA 
already has a large dataset on the performance of ATS in removing 
turbidity, but additional data on the costs of ATS would potentially be 
useful to EPA. To be most useful, EPA requests that treatment 
performance data represent multiple discharge events, that samples are 
collected over regular intervals over the course of the event (or the 
discharge), and that the data contain, if available, the following 
descriptive information:
     Site information, such as project size, project type 
(residential, commercial, road/highway, etc.), location, phase of 
construction (e.g., before, during or after grading, site 
stabilization, etc), etc.;
     Sample date(s) and time(s) of collection and date(s) and 
time(s) of analysis;
     Sample type (grab sample, flow or time-weighted composite, 
continuous turbidity measurement, etc.);
     Analytical method and/or type of field instrument used to 
measure the parameter; and
     Description of the treatment technology, including method 
of treatment chemical dosing utilized.
    Additional information that would be useful in evaluating these 
data includes:
     Estimates of the amount and intensity of precipitation for 
the time preceding and/or during sampling events;
     Drainage characteristics (predominant soil types/textures, 
drainage area, estimate of the quantity or percent of the drainage area 
that is disturbed);
     The ambient air temperature when the data is being 
collected;
     Date of last calibration if a field instrument was used; 
and
     Descriptions of any quality assurance/quality control 
procedures implemented for the data collection activity.
    In order to be most useful, data on costs should include:
     Installation costs (both material and labor);
     Operation and maintenance burden (in terms of labor hours 
and/or costs);
     Quantity, cost and frequency of treatment chemical use; 
and
     Other costs (residuals management, consumables, energy 
use, etc.).
    EPA requests comment on other factors EPA should consider other 
that those listed above in evaluating treatment performance data and 
what metadata commenters consider important to consider in the context 
of establishing effluent limitations.

B. Sampling and Data Collection--Procedures and Protocols To Ensure 
Representativeness of Data; Differences in Analytical Equipment

    EPA is aware that there are several issues associated with 
collecting turbidity data in the field at construction sites. These 
issues are associated with sampling equipment limitations, turbidimeter 
limitations, differences between turbidity measuring equipment, and 
sample handling and analysis. The following discussion presents 
information that EPA is aware of with respect to these issues and 
solicits data and comment on these issues. These issues relate both to 
collecting samples for the purposes of establishing effluent 
limitations as well as collecting samples for compliance determination.
Sampling Equipment Limitations
    Collecting samples of stormwater at construction sites can be 
accomplished using either automated equipment or by collecting grab 
samples. Automated equipment typically requires the use of a flow 
measuring device and an automated sampler. Flow measurement devices 
require that a weir, flume or other structure be installed in the 
conveyance that has a known rating curve (discharge vs. flow depth), or 
that a custom rating curve be developed for open channels based on 
surveyed channel geometry that can be used to estimate flow as a 
function of depth of water. Automated samplers can be set up to collect 
samples after a predetermined amount of flow has passed through the 
measuring device (flow-weighted) or after a predetermined amount of 
time has passed (time-weighted). In either case, the sample collection 
interval must be selected such that sufficient samples are collected 
over the course of the hydrograph to adequately characterize the 
discharge. This is frequently difficult, as it is not known in advance 
how much precipitation and flow will occur. If the sample collection 
interval is set too low, then the sampler may fill up before the end of 
the event. In this

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case, a portion of the hydrograph may not be sampled. If the interval 
is set too high, then too few samples may be collected to adequately 
characterize the event. Given the variability in stormwater flows, this 
may make the use of automated sampling challenging.
    Grab samples are easier to collect than automated samples. However, 
collecting grab samples requires that someone be physically present on 
the site. Given the variable nature of storm events and that those 
events can occur during all hours of the day, collecting grab samples 
to characterize performance can also be challenging. This is 
particularly true when the site is not located in close proximity to 
field offices of the sampling personnel.
    In the context of characterizing performance for establishing 
effluent limitations, both grab samples and automated samples are 
potentially useful. Generally, EPA believes that samples used to 
characterize performance should be collected regularly over the course 
of the event in order to capture variability in flows and associated 
pollutant parameters. This is particularly true in the case of passive 
treatment, which does not involve capture of the water in a pond or 
basin for controlled release, so that one would expect greater 
variability in sampled parameters. For treatment of water discharged in 
a controlled rate from a pond, one would expect less variability in 
flows and performance, so less frequent sample collection would likely 
be necessary in order to adequately characterize performance.
Turbidimeter Limitations
    Samples collected for turbidity can be measured in the field using 
a hand-held turbidimeter, or can be sent to a laboratory for analysis 
using a benchtop turbidimeter. Both methods are simple and inexpensive. 
However, turbidimeters only operate within specific ranges. The high-
end of the range is typically around 1,000 NTU or more. Samples with 
high amounts of turbidity may need to be diluted in order for the 
turbidity of the sample to be within the operating range of the 
instrument. This is a potential source of error, especially if done in 
the field. Another method for measuring turbidity is to use an in-situ 
meter coupled to a datalogger. In-situ meters can be programmed to 
record turbidity continuously at some specified time interval (such as 
every 15 minutes). As with other instruments, in-situ turbidimeters 
typically operate within a specific range. With these instruments, 
turbidity above the measurement range of the instrument cannot be 
determined, since a physical sample is not collected. This is a 
potential source of error, particularly during periods of peak flows 
where turbidity may be very high. This is a downside of in-situ meters 
because an average turbidity for an event cannot be determined if some 
of the data exceeds the measurement range of the instrument. In these 
cases, the use of both an in-situ meter as well as collection of a 
physical sample during peak flow periods may be necessary to accurately 
determine the average turbidity for the event. In-situ meters are also 
susceptible to failure, such as from battery failure or a piece of 
debris obscuring the detector.
    Different types of turbidimeters may provide different measurements 
of turbidity for the same sample. This is due to differences in light 
sources and differences in the orientation of the light source with 
respect to the detector. In addition, while turbidity measured in NTUs 
is the standard contained in EPA's methods, turbidity can also be 
measured in other units, such as formazin turbidity units (FTUs). While 
EPA believes that NTUs are the appropriate units in the context of 
effluent limitations for construction site stormwater, EPA solicits 
comments on the types of equipment that should be allowable and other 
considerations related to differences in measurement equipment and 
measurement units.
Sample Handling and Analysis
    EPA notes that some of the data in EPA's dataset did not follow the 
sample preservation protocols contained in EPA's approved analytical 
methods. EPA method 180.1 states that turbidity samples should be 
immediately refrigerated or iced to 4[deg]C and analyzed within 48 
hours. EPA is aware that many of the samples collected by researchers 
at North Carolina State University and described in DCNs 70004, 70018, 
70034, 70053, 70054 and 70065 were collected using automated samplers, 
and that the samples were not analyzed within 48 hours or refrigerated 
or iced. In many instances, samples were analyzed several days or weeks 
after collection. While EPA notes the deviation from approved methods, 
EPA does not believe that this deviation would produce appreciable 
changes in measured turbidity in these cases. The sample refrigeration 
and analytical timeframe guidelines are intended to minimize changes in 
turbidity that would result due to microbial decomposition of solids in 
the sample. Since EPA expects little organic material to be present in 
samples of stormwater runoff from construction sites since the solids 
are primarily composed of inert soil particles, EPA would not expect 
biological activity to appreciably change the turbidity of the samples. 
EPA does note that since these samples incorporated polyacrylamides, 
some additional flocculation could occur in the sample bottles during 
the time period between collection and analysis or during transport 
from the field to the laboratory, if residual or un-bound 
polyacrylamide was present in the sample. EPA solicits comment on the 
appropriateness of using data from samples not analyzed within 48 hours 
or otherwise not in compliance with established analytical methods in 
the context of a future regulation.
    EPA also notes that the samples collected by researchers at North 
Carolina State University were allowed to settle for approximately 30 
seconds after mixing before a subsample was collected and analyzed for 
turbidity. EPA understands that this 30-second settling period after 
mixing was to allow large flocculated particles to settle, since 
analyzing turbidity of a sample that contains large agglomerates may 
prevent the turbidity meter from producing a stable reading or may 
underestimate turbidity of the sample. The EPA approved sampling method 
does not describe an appropriate period of time between mixing of the 
sample bottle and collection of the subsample for analysis. As 
described in EPA's method 180.1 for measuring turbidity, the approved 
analytical procedure is ``Mix the sample to thoroughly disperse the 
solids. Wait until air bubbles disappear then pour the sample into the 
turbidimeter tube. Read the turbidity directly from the instrument 
scale or from the appropriate calibration curve.'' (see DCN 70083), The 
method states that ``The presence of floating debris and coarse 
sediments which settle out rapidly will give low readings. Finely 
divided air bubbles can cause high readings.'' Floating debris and 
course sediments and finely divided air bubbles are therefore 
considered sources of interference when measuring turbidity. The 
practice utilized by researchers at North Carolina State University of 
allowing mixed sample bottles to sit for 30 seconds before collecting 
the subsample for analysis, which would allow any course sediments to 
settle, may be an appropriate means of addressing possible 
interferences due to the presence of large particles. EPA also 
acknowledges that allowing the sample to settle prior to collecting the 
subsample for analysis may result in fewer particles generally being 
present in the subsample and thus an artificially low turbidity 
reading. EPA solicits

[[Page 120]]

comment on the appropriateness of using turbidity data where a sample 
was allowed to settle for 30 seconds (or some other time period) after 
mixing before collection of the subsample for analysis for purposes of 
evaluating the performance of technologies and for compliance purposes 
and the expected magnitude of the effects of varying settling time on 
observed turbidity values.
    EPA understands that the subsamples for TSS were collected by the 
researchers and analyzed immediately after mixing. As a result, there 
are certain cases where particular samples in these data had TSS 
concentrations (in mg/L) that would appear inconsistent when compared 
to the corresponding turbidity measurements (in NTU) since the large 
particles could be present in the TSS subsample. EPA notes that the 
ratios of TSS to turbidity for some samples are much higher than for 
other samples, which EPA believes can be attributed to the 30-second 
settling time prior to collection of the turbidity subsample. EPA 
welcomes comments on this topic.
    In the context of compliance demonstration, the specifics of a 
particular site (such as the location of the site, the number of 
discharge points, proximity of discharge points, accessibility of 
discharge points, etc.) are important considerations in determining the 
type of sample to be collected. Generally, both automated samples and 
grab samples are potentially useful for compliance determinations. 
However, the inherent limitations with sampling equipment and equipment 
malfunctions may be important considerations. With grab samples, 
equipment limitations and equipment malfunctions are not of concern.
    EPA solicits comment on the appropriate methods for sample 
collection in the context of both compliance sampling and analytical 
sampling for the purpose of setting limits for a turbidity effluent 
limitation for construction site stormwater discharges. EPA recognizes 
that logistics and cost are important considerations, and would like to 
better understand the potential costs and challenges of sample 
collection and analysis in these cases.

C. Effect of Storm Size, Intensity and Duration of Precipitation on 
Performance of Passive Treatment

    In establishing effluent guidelines and new source performance 
standards, proper operation of the candidate best available technology 
economically achievable (BAT) and best available demonstrated control 
technology (BADCT) should result in meeting the numeric limitation a 
very high percentage of the time. In the case of industrial wastewater, 
treatment systems typically perform well within a range of flowrates 
and influent pollutant concentrations, and systems typically operate 
within these ranges. Due to variations in manufacturing production 
cycles, the flowrates and pollutant concentrations in wastewater can 
vary over the course of a day. Industrial wastewater treatment systems 
typically incorporate equalization to dampen these diurnal variations 
in flowrates and pollutant concentrations. This dampening assures that 
high flows and/or pollutant loads do not overwhelm the treatment 
system, or that low flows and/or pollutant loads do not compromise unit 
processes.
    This same concept applies to stormwater treatment. Since 
precipitation is a stochastic process, there can be variation in 
stormwater flowrates and sediment loads during the course of a given 
precipitation event. Data available to EPA indicates that passive 
treatment with limited storage may perform well for some storm events, 
but that larger and/or more intense storm events may degrade the 
performance of these systems. The likely reasons for a decrease in 
performance include inadequate treatment chemical dosing during periods 
of higher flows, exhausting the treatment chemical during larger and/or 
longer storm events, high sediment loads during intense periods of 
precipitation that overwhelm the systems, and short-circuiting/
overtopping of controls. These occurrences are difficult to address as 
they occur on construction sites in the context of passive treatment, 
which is not based on a high level of operator involvement.
    A potential shortcoming of EPA's current dataset on passive 
treatment is that much of the data was collected during smaller storm 
events. EPA has little data available on the performance of this type 
of flow-through passive treatment during larger and/or more intense 
storm events, but the limited data available indicate that the 
performance of simple passive treatment approaches may not be as good 
for these events. The candidate BAT/BADCT should be capable of meeting 
the limitation up to whatever cutoff is established for the limitation. 
In the 2009 rule, the compliance storm event was the 2-year, 24-hour 
storm event (see Section IV.D for additional discussion of storm event 
exemptions).
    EPA does not expect this concern to arise with treatment that first 
holds the water in a pond, basin or impoundment. Impounding the water 
has two primary benefits for subsequent treatment--equalization of 
flows and reduction/dampening of sediment/turbidity levels. The amount 
of sediment and turbidity mobilized during a storm event can vary 
greatly, depending on factors such as storm intensity, storm duration, 
soil type and composition, slopes of the contributing watershed, extent 
of soils exposed, and the extent and nature of construction activities 
occurring. When water is held in a basin, a significant portion of the 
settleable materials would be expected to be removed. When water is 
withdrawn for subsequent treatment, one would expect much lower 
variability in the amount of turbidity over the course of the treatment 
period.

D. Exemptions--Design Storm Depth vs. Intensity

    The December 2009 final rule exempted discharges from compliance 
with the turbidity limitation on days where precipitation exceeded the 
local 2-year, 24-hour storm depth. The rationale for this exemption was 
that large storm events would potentially ov