xvEPA
           United States
           Environmental Protection
           Agency
           Office Of Water
           (EN-336)
-5- 91-100
Technical Support Document
For Water Quality-based
Toxics Control
         RESPONSIVENESS SUMMARY
                                    Printed on Recycled Paper

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                     RESPONSIVENESS SUMMARY
General
     The TSD needs to be peer reviewed.

     EPA Response:  We feel EPA has provided ample opportunity for
     review  of  the  TSD.    The  Williamsburg workgroup  met  in
     December, 1988, and was composed of a diverse group of people
     from  EPA   Regions,   States,   environmental  groups,  trade
     associations, academia, private industry, and municipalities.
     This group  was  formed to  determine what changes were needed
     in  the  document.    Based on  the workgroup's  inputs,  EPA
     produced a first draft TSD in November,  1990.  This draft was
     sent out to workgroup members,  and anyone else upon request,
     for review and comment. EPA  considered  each comment and made
     changes  to  the  document.   A  second  revised draft  TSD was
     noticed  in  the  Federal Register on May  11,  1990.  Over 2500
     copies  of the  new draft were  sent  out.   Between  the two
     drafts,  we  received  120  comments from  a  diverse group  of
     people.   EPA  considered  each  individual  comment and made
     changes to  the document where necessary.

     The TSD doesn't  apply  to CSO discharges.

     EPA Response:   We acknowledge  that the  TSD was written with
     continuous  discharges  in  mind.   However,  there is no reason
     why the  general concepts  and some of the recommendations of
     the TSD  cannot  be extended to  CSO or other rainfall related
     discharges.  The dynamic  model applications .in the TSD have
     been used over  the  last  10  years to address  water  quality
     problems related to  CSOSo  EPA believes that this model can
     also be  applied  to address toxic problems with CSOs.
Chapter 1;  Approaches to Water Quality-based Toxics Control

Overview:   Added more  information and clarification  to support
EPA's position  with regard to the major  issues  addressed in the
comments.

1.   The  cause  and  effect relationship between effluent toxicity
     and  instream impacts is not adequately documented, and other
     factors  that  may  cause  instream  impacts  have  not  been
     addressed.  EPA's use of the CETTP studies was criticized.
     EPA  Response:    The  revised draft TSD  has been  changed to
     include  more  documentation to  support  our position  on the
     cause  and  effect relationship between effluent toxicity and
     instream  impacts.    Supporting  information on a  study con-
     ducted on the Trinity River  in Texas was added as well as more

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     detailed explanation of the  CETTP studies.  EPA evaluated the
     results of the CETTP studies, the North Carolina studies, and
     the Trinity River study and  found that if toxicity is present
     after  considering dilution,  instream  impact  will also  be
     present.  EPA's finding is now clearly stated in the TSD and
     referenced.  The TSD now includes a discussion of Parkhurst's
     major criticisms of the CETTP studies and why EPA feels those
     criticisms  are  unfounded.   The  TSD also  acknowledges  that
     biological, physical, and chemical  factors of  the community
     can influence the  actual effects that  effluent toxicity may
     cause in the receiving water.

2.   Toxicity test method precision is too variable to be used in
     NPDES permits.

     EPA  Response:    The  revised  draft  includes all  available
     precision data for both acute and chronic toxicity tests; this
     includes  intralaboratory  and  all available interlaboratory
     test results.  Raw precision data is presented in table form
     in  Appendix  A  and  discussed  within  the  chapter.    EPA
     evaluations, as  well as published  literature  reviews which
     include  estimates  of  whole  effluent  precision  data  are
     presented.  EPA is comfortable with the conclusion that whole
     effluent toxicity  tests are no more variable  than chemical
     analytical   methods  and   therefore  stands   behind   the
     recommendation that  toxicity  test  methods be used in NPDES
     permits.

     The same data  contained in  the TSD has been used  by EPA in
     proposing adoption of the toxicity methods  into  EPA's  reg-
     ulations at 40 CFR 136.  The  toxicity  methods  were proposed
     for adoption on December 4,  1989.  Comments were received and
     will be answered upon notice of EPA's decision regarding these
     methods.  The decision is expected in April 1991.


3.   The biological criteria/bioassessment approach is not yet part
     of EPA's water quality regulations and should not yet be used
     in the regulatory process.  For the best assessment of sources
     and causes, EPA  should  use  a  "weight-of-evidence approach."

     EPA Response:  As  previously stated in the TSD (per section
     131.11(b)(2)  of  the Water Quality Standards  Regulation),
     biological criteria can supplement existing chemical-specific
     criteria  and provide an alternative  to  chemical-specific
     criteria  where  such criteria cannot  be  established.    To
     acknowledge the  current evolving status  of the use of  bio-
     criteria, EPA's statement in the TSD has been revised to read:
     "To  better  protect  the  biological  integrity  of  aquatic
     communities, EPA recommends that States begin to develop and
     implement biological  criteria in their water quality stand-
     ards."

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4.
EPA  does  not agree  with  use of  the  "weight-of -evidence"
approach  in this context because biosurveys are too complex
to  override the other  two methods (i.e., chemical-specific
and  whole effluent toxicity).   The TSD now explains EPA's
position  that the  concept of  "independent  application" be
applied to water quality-based situations. Since each method
has  unique  as well  as overlapping attributes, sensitivities,
and  program applications, no  single  approach for detecting
impact should be considered uniformly superior to any other
approach.   For  example,  the  inability  to detect receiving
water impacts using a biosurvey alone is insufficient evidence
to waive  or relax a permit limit established using either of
the  other methods.  The most protective  results  from each
assessment   conducted   should  be  used  in  the  effluent
characterization  process  (Chapter  3) .   The results  of one
assessment  technique  should  not be  used to  contradict or
overrule  the results of the other(s).

However,  EPA recognizes  that  there are  instances  when the
whole effluent,  chemical  specific, and biological criteria
approaches will give disparate results.  The TSD was revised
to recommend  that  permitting authorities use a more complex
way  to  assess excursions  beyond standards and establishing
permit limits to provide assurance that simplifying assump-
tions are not the cause of the apparent discrepancy.  The TSD
also  now  includes examples  of  where  the  whole  effluent
toxicity  test protocols  may conflict  with critical environ-
mental parameters  to  lead to  an apparent disparity between
the whole effluent toxicity and chemical  specific approaches.

The  problem of  false  positives  (i.e.,  instream impact pre-
dicted by toxicity  test where none exists) is not addressed.

EPA Response: The revised draft TSD now addresses the problem
of  toxicity  test  interferences  caused  by  environmental
parameters  and  explains  that there  may  be  a few  unusual
situations where the pH,  temperature, hardness, salinity, and
solids requirements of the testing procedures differ greatly
from the worst environmental conditions for these parameters.
In these  situations, the  effluent  toxicity tests  may either
over or under predict the toxicity  in the ambient receiving
water. An example of this  is where  ammonia is present and the
highest expected ambient water temperature is  20°C whereas the
chronic toxicity  test  must be conducted at 25°C.    Since  a
higher  temperature causes  more  ammonia   toxicity,   the
temperature requirements of the test may induce toxicity not
found  in the  ambient  water.    In  such  an  instance,  the
regulatory authority must  carefully look at the test protocols
and all the  data collected to determine  if the facility is
actually  contributing to  toxicity  in  the  ambient water.   A
toxicity identification evaluation (TIE) may  be necessary to

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     make this determination.  If this  analysis  shows a toxicity
     test result to be artificial due to environmental parameters,
     then that test should be overridden by  subsequent toxicity
     tests conducted.

5.   Current standard algal toxicity test methods lack the ability
     to provide useful data on the ecological impact of a discharge
     and should not be recommended as a test species.  EPA should
     recommend three species representing two different phyla be
     used  for toxicity  testing.    EPA's recommendation to  use
     surrogate (rather than resident)  species  in toxicity testing
     as being more protective seems contrary to the site specific
     nature of the permitting process.

     EPA Response:  To address  concerns  with the algal test and to
     allow more flexibility in testing,  the recommendation in the
     TSD has been changed to "fish, invertebrate, and plant."  EPA
     will not modify its  recommendation to  include  only 2 phyla.
     EPA's objective in  requiring  3 species  from 3 different phyla
     is to be predictively protective.

     The  TSD's  discussion  of  the use  of  surrogate  species  was
     expanded  to  explain that to  use a  resident organism,  a
     facility would have to  develop a protocol to  culture  the
     organism and to assess intra- and  inter-laboratory variabil-
     ity.    Such testing  is  more  costly,  more difficult,  and
     potentially subject to more variability  (disease, age, etc.)
     than standardized testing.  In any case,  organisms collected
     directly from the receiving water itself  should never be used
     because their health cannot be assured.

6.   More documentation is needed  for  the statement that the "IC25
     is approximately the analogue of an NOEC."  Before EPA makes
     such a broad ranging recommendation, there must be sufficient
     data to establish an overall relationship.

     EPA  Response:   The  language  in  the revised draft has been
     clarified to better explain what  an  IC25 is and  how it is
     calculated.   EPA believes that there  is sufficient data to
     support  the statement that  the  "IC25 is approximately the
     analogue of an NOEC."  The data  in  Appendix A is presented so
     that both hypothesis testing and IC25 calculations  of an NOEC
     can  be  compared.    A  statistical analysis  using  minimum
     significant differences is graphically  presented in Figure 1-
     1.   Figure  1-1 shows that an NOEC calculated using the IC25
     is comparable to an NOEC calculated using hypothesis testing,
     and that the relationship is statistically sound.

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 7.    The data in the TSD shows that whole effluent toxicity is  not
      additive.

      EPA Response:  The cited  article  in the TSD  and  the  TIE data
      were reviewed.  These  data  support  EPA's position that acute
      toxicity  is  additive.    However,  the  data   do  not  support
      additivity  of  chronic toxicity.    Therefore,  the  TSD  was
      changed to reflect this.

 8.    There are insufficient chemical-specific field data to support
      that exceedances of the criteria  cause instream  impacts.
      EPA Response:  The field  studies  referenced  in Chapter 1  for
      chemical specific criteria investigations were conducted over
      twenty years ago.  The  field investigators dosed a stream with
      toxicants  to measure  the response.   This  approach is  not
      possible  today because  it would violate  the  States water
      quality  standards.    Since that  time,  EPA  has  developed a
      method of using laboratory toxicity data on  specific chemicals
      to derive data.
Chapter 2: Water Quality Criteria and Standards

Overview:    Revised  introduction  to  summarize  key  regulatory
requirements;  reorganized  into clearly defined aquatic  life and
human health discussions; added more information on what should be
considered  when  allowing  mixing  zones;  added  more  in-depth
discussion of criteria for human health protection.

1.   Water quality criteria are not reliable  due  to data gaps or
     errors in derivation.  Site-specific criteria should not be
     limited to being more  stringent than the national criteria.

     EPA Response:  Changing the general procedure for deriving WQ
     criteria  is  not  within the scope  of the TSD.  In  the near
     future EPA expects  to  re-examine the general procedures for
     deriving  aquatic life  criteria,  and will  request  public
     comment thereon.  While EPA requests  public comment on all WQ
     criteria documents before publishing them in final form, EPA
     accepts comment on criteria at any time and can correct errors
     through criteria summary  documents that  it  distributes from
     time to time.  Finally, there is no Agency policy, set forth
     in the TSD  or  elsewhere,  that prevents  state-wide  or site-
     specific criteria from  being less stringent than the national
     criteria.
2.
Provide more clarity on how to prohibit lethality within the
mixing zones?

EPA Response:  The TSD has been amended to clarify the goals
of EPA's recommendations on mixing zones.  It now states that
mixing  zone  conditions  should  not be  lethal  to  organisms

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passing through it.  Survival of organisms that might wish to
reside permanently in a mixing zone  is not assured by past or
current policy.

Because the  chronic  criteria may be exceeded at the  end of
the pipe  there  is a potential  for lethality to  sensitive
organisms that  attempt to reside permanently in the  mixing
zone.  Part of the intent of the recommendations of the 1985
TSD and  1991 TSD  is to  protect  the survival of  organisms
passing through the mixing zone.   In all cases,  exposures to
concentrations  above the  CCC and  CMC  cannot be  correctly
interpreted without accounting for the duration of exposure.

The TSD is flawed because it  assumes that mixing zones exist.
The Great Lakes Water Quality Agreement precludes use of flow
augmentation as a  substitute for adequate treatment.
EPA Response:  The TSD  recommendations  in no way  authorize
mixing zones  where otherwise prohibited.  In  addition,  the
recommendations on mixing zones do not advocate management of
reservoirs for flow augmentation.

TSD  should  at a  minimum be  against mixing zones  for per-
sistent and  bioacccumulative  toxicants,  and the  burden of
justifying  mixing  zones  for non-persistent  and  non-bio-
acccumulative  toxicants should be  on  the discharger.   The
Great Lakes Water  Quality Agreement calls for zero discharge
of pollutants  and elimination of persistent and bioaccumu-
lative toxicants.  The Clean Water Act also has a goal of zero
discharge of pollutants.

EPA Response:  The TSD continues  to  note that EPA regulations
allow mixing  zones at the discretion of the  State.  The TSD
also  discusses  options  that  should  be   considered  when
determining whether to allow mixing zones for  aquatic life and
human health protection.   For protection of  aquatic life, a
mixing  zone  may  be  permitted   as  long  as  its  size  is
sufficiently  limited that it  does  not  significantly impair
the integrity  of the water body  as  a whole,  and it does not
cause lethality to organisms  passing through  the mixing zone.
For  protection  of  human health,  mixing zones   should  be
restricted such that they do  not  encroach on  areas often used
by the public for  fishing, and particularly  where stationary
species such as shellfish are harvested;  mixing zones may also
be  restricted  to  compensate  for  uncertainties  in  the
protectiveness of the water quality  criteria  or uncertainties
in the assimilative  capacity  (TMDL)  of the water body.

Bioaccumulative  pollutant problems  are not  fundamentally
caused  by mixing  zones.    Bioaccumulation  is generally a
system-wide problem that occurs when the appropriate TMDL for
a water body as a  whole is exceeded.  Consequently, EPA does
not consider mixing zone restrictions to be the best mechanism

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     for  addressing such problems.   However,  eliminating mixing
     zones can be  used as a device to further reduce the loading
     below the TMDL,  although the results are not as predictable
     as a direct reduction of the TMDL would  be.   Chapter 2 has
     been now been modified to state the factors  that should be
     considered in judging whether a mixing zone causes significant
     health or ecological risks.

     It sliould be noted that the TSD deals with WQ based effluent
     limits (i.e., those needed to protect aquatic life and human
     health), implemented primarily under the specific require-
     ments of Sections  301,  303,  and  304  of  the CWA.  Under this
     framework,  zero discharge of  pollutants  is generally required
     where the water  quality standard or  the Total Maximum Daily
     Load is set at zero.

5.   The  TSD should allow mixing  zones  for all toxicants includ-
     ing  bioaccumulative pollutants.  Furthermore, fate processes
     (such as  sedimentation or decay)  that occur  within mixing
     zones should be taken into consideration.

     EPA  Response:   The  TSD now  sets forth specific  conditions
     under which denial of mixing  zones would be appropriate.  EPA
     regulations  also  allow   (while  neither  encouraging  nor
     discouraging) States  to use  mixing  zones.   The recommenda-
     tions of the TSD implicitly discourage consideration of fate
     processes such as  sedimentation  or decay.  Mixing zones are
     an allowance  for variations  in concentration  due  to incom-
     plete mixing,  over small spatial scales,  usually  too small
     for  fate processes to significantly reduce concentrations.

6.   Limiting all dischargers to  0.3  TUa  is  akin to a  technology
     based approach  such as  a  30 mg/L BOD  or  suspended solids
     limit.   The TSD should consider the resulting instream water
     quality.

     EPA Response:  The mixing zone discussion has  been modified
     to provide different alternatives for assuring that instream
     goals and standards  are met.   Not exceeding 0.3 TUa at the
     end  of the  pipe  is  one of  the  recommended ways  to assure
     survival of  organisms passing  through  the  mixing  zone.
     Nevertheless,  the  discussion  has  been  modified  to  de-
     emphasize the  technological  requirements and  emphasize the
     attainment  of instream goals.

7.   EPA should  provided more information  on  the tests  it used to
     arrive at an LC50/LC1  ratio  of 0.3.   According to the data
     presented in the  TSD,  the 0.3 ratio is overly conservative in
     most cases.

     EPA Response:  The magnitude of  the  acute WET  criterion is
     based on data  collected from a number of  facilities  in EPA

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     Region  4.    These  data show  that  90%  of  the  facilities
     exhibited an LCI which was no  less than 0.3  times the LC50.
     This  is different  from the  0.5 factor used  to derive  a
     chemical specific acute criterion.

8.   The acute toxicity criterion of  0.3  TUa is below detection.
     How would it be implemented?

     EPA Response:  The implementation of this criterion would be
     identical to that used  for specific  chemicals.   This imple-
     mentation is expressed in Chapter 5.

9.   The 1-hour averaging period for  the  acute criteria (CMC)  is
     overly  restrictive,  does not  correspond to the  48-96 hour
     toxicity tests, and  cannot  be modeled with  existing EPA WQ
     models.   Elsewhere in  the  document, EPA  indicates  that  24
     hours  is  an  appropriate  averaging  period  for  modeling
     purposes.  Both concentration  and exposure time are import-
     ant,  since for  many toxicants the 96-hour LC50 is dramati-
     cally  higher than the  l-hour LC50.   The 1-hour averaging
     period is technically unsupported by the limited information
     presented.  EPA appears to have edited the data presented in
     Appendix D to include only those  data that  support the 1-hour
     averaging period.  EPA  should  present  all available data on
     the effect of exposure duration on toxicity.

     EPA Response: EPA agrees that both concentration and exposure
     time  are  important.   The TSD has now  been modified to note
     that the 1-hour averaging period  is based on ammonia, a fast-
     acting toxicant.  As the 1-hour averaging period was intended
     to be protective even for the fastest acting toxicants,  it may
     be overly conservative for many pollutants. Consequently, the
     TSD  recommends allowance  for  site-specific  (or chemical-
     specific) modification of the  averaging periods.  Alternative
     averaging periods  can  be  developed from  data on  the time
     course of mortality in acute toxicity tests.

     EPA expects  that  for many  pollutants,  such site-specific_or
     state-wide alternative averaging periods,  if developed using
     adequate data, may be greater  than the  period recommended for
     national  criteria.   Furthermore, EPA  recognizes  that a 24-
     hour  acute  averaging  period  may  be  appropriate  in some
     modeling contexts, where concentrations do not change rapidly
     over  short time periods.

     While Appendix  D  of   the  TSD  presents  some examples  of
     pollutants for which a  short  averaging period, on the order
     of hours, might be appropriate,  the  TSD was not intended as
     the mechanism by which  EPA would develop a rationale for the
     acute averaging period.   The averaging  periods were set forth
     in 1985 in the "Guidelines for  Deriving...National  Criteria".
     In the  near  future EPA  intends to review and perhaps modify

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     the Guidelines, and may at that time consider the feasibility
     of different acute averaging periods for different chemicals.

10.  The rationale for the 4-day averaging period is weak.  Chronic
     toxicity tests for most species require much longer than four
     days.  The field studies presented in Chapter 1 indicate that
     longer  term  excursions are  needed to  produce  ecological
     effects.  EPA inappropriately picked examples  of pollutants
     with low acute-chronic ratios to justify the 4-day averaging
     period.  A 30-day averaging period (per the freshwater ammonia
     criteria document)  should be acceptable, particularly in cases
     where concentrations do not change rapidly.

     EPA Response:  The  TSD  has  been modified  to note that EPA
     selected  the 4-day chronic averaging period  based on the
     shortest  period that  chronic  effects  may  be observed for
     certain chemicals.

     As  the  4-day  period   was  selected  for  provide  adequate
     protection in all  cases,  EPA recognizes  that longer  averag-
     ing  periods  may be appropriate  for many pollutants,  and
     recommends  using  site-specific  or  state-wide  pollutant-
     specific alternative averaging periods,  where scientifically
     supported.   EPA believes that  selection of an  appropriate
     chronic averaging period is technically difficult, with less
     applicable data  than  is  available  for  selecting  the  acute
     averaging period.  EPA  agrees that the acute-chronic ratio is
     a confounding influence in  interpreting  the duration needed
     to produce a  chronic effect endpoint.  Where an appropriate
     acute criterion is  in force,  the chronic averaging period need
     not be  shortened simply because the acute-chronic ratio is
     low.

     The TSD  does not supersede the freshwater  ammonia criteria
     document, and thus  EPA  still recommends  an averaging period
     of as  long  as 30 days for  ammonia, where  concentrations do
     not vary excessively.

11.  EPA's recommended  once in  three  year  return frequency for
     criteria  excursions is  overly  conservative.    Appendix  D
     presents  time periods  needed for  ecological recovery  from
     severe or catastrophic  stresses, not slight stresses caused
     by marginal  criteria excursions.   The frequency  of  signif-
     icant criteria  excursions,  comparable  to those  that  caused
     the measurable  ecological impacts set forth in  Chapter 1,
     would be  much less than  for  marginal criteria  excursions.
     EPA should present data  on the ecological differences  between
     sites  with   different  excursion  frequencies.    EPA  should
     develop guidance on how to establish site-specific allowable
     frequencies.

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EPA Response:  EPA has used Appendix D to set forth informa-
tion  on time  periods needed  for ecological  recovery from
severe  or  catastrophic stresses.   EPA's  recommended 3-year
return  interval  was set  forth in the  1985  "Guidelines for
Deriving  Numeric  National Criteria...",  and  a review  or
revision of this recommended  frequency was not  within the
scope of the TSD.   Nevertheless,  EPA intends to address the
excursion  frequency  during the.  upcoming  revision of  the
Guidelines.

EPA  expects that criteria  exceedances  can  cause  adverse
effects and that  the  magnitude of the effect will depend on
many  things including the magnitude and  duration of  the
exceedance.  EPA believes that all  adverse  effects are not
necessarily unacceptable,  but that pollution  should  not be
allowed to subject aquatic communities to long-term or regular
short-term  adverse effects. All dramatic adverse effects are
certainly unacceptable.

EPA believes that the 3-year return interval  can be justified
by the Appendix D data if one makes the assumption that the
type of ecological impact shown in Appendix D could be caused
by  fairly   small  criteria  excursions.   The concentrations
causing the Appendix  D impacts were in fact not known.  EPA
recognizes  that  the  chemical  and  ecological  field  data
summarized  in  Chapter 1  suggest  that successive excursions
well  above the  criteria would be  needed  to  cause  severe
impacts.  EPA  also  recognizes  that the probability of large
excursions  can be calculated  to be extremely small compared
to the probability of marginal excursions.

EPA does not have information to allow direct comparisons of
ecological quality versus criteria excursion frequency, except
possibly as could be inferred from the field  data  shown in
Chapter 1.   EPA does  not intend at this time  to set forth
guidance on developing site-specific allowable frequencies.

Nevertheless, in general, EPA recommends that ecosystems not
spend a substantial  portion of time  in a state of recovery
from  pollution stresses,  and  that  pollution  stresses  not
significantly  increase  the  total  stress  experienced  by
organisms in the  ecosystem.   If the criteria are set appro-
priately,   a marginal  excursion might  be  expected to have
little or no measurable impact, and little or no time period
needed for  recovery.  The probability of a marginal criteria
excursion nevertheless has a calculable relationship with the
probabilities of  severe  criteria  excursions.  Consequently,
a  scientifically   justified   site-specific   or  state-wide
frequency   could   be  developed   by   considering  (a)   the
probability (estimated  by  simulation  or  by  statistical
calculation) of a range of excursions of differing severity,
coupled with (b)  the estimated  ecological recovery period for

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12
13,
14,
15,
the corresponding different degrees of impact.  Based on the
total period of recovery from a full range of possible events,
compared with the sum of return intervals for such events, the
allowable frequency for the marginal criteria excursion could
be established.

The once in three year  excursion frequency does not take into
consideration the likelihood of apparent  excursions caused by
the inherent variability of the analytical tests.

EPA Response:   The allowable  frequency  for criteria excur-
sions should refer to true excursions  of  the criteria, not to
spurious excursions caused by analytical variability or error.
In evaluating data  on chemical concentrations  or  toxicity
units, it is  desirable to subtract the analytical error log
variance from the observed log variance in order to arrive at
the true log variance contributing to criteria excursions.

The IRIS (Integrated Risk Information System) data base should
be peer  reviewed before  it  is used to  update  and  generate
RACs.  The TSD  should  not imply that  a State  can update its
standards simply by inserting the latest IRIS information into
the equation used to calculate the criteria.

EPA  Response:    EPA's  IRIS  data  base  reflects the  latest
information about the Agency's health assessments for specific
chemicals.  While the material in the data base is internally
reviewed, the Agency does not plan to have it undergo external
peer review.   Use of IRIS information for  developing state
water  quality  standards  or  discharge  permits  in  no  way
relieves  the State  of applicable  requirements for  public
notice and comment.

The  fish consumption  rates  used  to  derive  residue-based
criteria and RACs  are  unrealistically  high  for  many waters.

EPA  Response:    EPA  recommends  using   site-specific  fish
consumption rates whenever such information can be obtained.
The harmonic  mean is not an  appropriate design flow.
arithmetic mean flow should be used.
The
     EPA Response:   For carcinogens it is appropriate to determine
     the long-term arithmetic mean exposure concentration.  Because
     flow is not normally distributed, using  the arithmetic mean
     flow  for  design  purposes  will  underestimate   the  mean
     concentration.

     Using the downstream harmonic mean flow will result in closely
     estimating  the  mean  concentration,  providing   that  the
     streamflow is not dominated by the effluent flow, and provided
     that the effluent input is not correlated to the streamflow.

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16.  Discussion of  sediment  criteria and biological  criteria is
     premature.   The  TSD should  not advocate  that states  and
     regions implement regulatory controls based on such criteria.

     EPA Response:  The biological criteria and sediment criteria
     sections have been modified.   EPA has undertaken development
     of biological criteria and sediment criteria with the intent
     that they would, after development,  have regulatory applica-
     tions .

     EPA does  not intend to  imply that  these approaches  can or
     should necessarily  be used at  this time to implement  con-
     trols.    Nevertheless,  EPA believes  that  the  states,  the
     regulated community, and the  interested  general public  need
     to know how  EPA  is  proceeding with  these criteria,  and  what
     the  future  regulatory  implications may be.    EPA  is  not
     advocating that sediment criteria, by themselves, be used to
     establish remediation target levels.  EPA also recognizes that
     sediment criteria cannot be used for setting discharge limits
     without first  developing  a  scientifically sound basis  for
     predicting the effect of effluent quality on sediment quality.
     EPA is not suggesting that sediment quality concerns would
     necessarily be more  limiting on dischargers than water quality
     concerns.


17.  Aquatic life protection as measured by whole effluent toxicity
     and chemical specific criteria  are  not  applicable  to waters
     without aquatic life designated uses.

     EPA Response:   The  TSD explains that numeric  water quality
     criteria are developed  by  States to protect the designated
     uses within  the  water quality standards.  However,  the  TSD
     also reiterated EPA's position,  as  expressed in the June 2,
     1989, Federal  Register  preamble on the  304(1)  promulgation
     was added, that the  narrative  criteria apply to all waters to
     prohibit acute toxicity.

18.  The  food  chain multiplier factors  in  the RAC  calculation
     should be deleted because  it  is contrary to  measured levels
     and BCF estimations.

     EPA Response:  The  differences  between  bioconcentration  and
     bioaccumulation  have  been  recognized  in  the  scientific
     literature for a number of years.  Data published by Thomann
     in Environmental Science and Technology  (June 1989)  show that
     bioaccumulation   can  be  over  100   times  higher  than
     bioconcentration.    Published  critiques  on  EPA's  dioxin
     criterion have  also expressed that bioaccumulation  is  more
     important than bioconcentration  for pollutants with log water
     octanol partition coefficients greater than 6.   EPA believes

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     that consideration of bioaccumulation through use of the food
     chain multiplier is consistent with the existing knowledge of
     bioconcentration factors.

Chapter 3; Effluent Characterization

Overview:  Simplified the chapter organization;  cited regulatory
requirements (40 CFR 122. 44 (d) ) ; revised the discussion on effluent
bioconcentration evaluation to conform with the new draft document.

1.   Determining the need for an effluent limit in the absence of
     effluent monitoring data  does not address  the required factors
     of 40 CFR § 122.44(d) (1)
     EPA Response:   EPA maintains  the position  that regulatory
     authorities may make a  finding  of reasonable potential even
     where  effluent  monitoring  data  is  not  available.    EPA
     disagrees with the commenter  on the  issue  of  whether  the
     required factors can be addressed in the absence of effluent
     monitoring data.  Box 3-1 of the TSD was revised to illustrate
     how the recommended procedure for making reasonable potential
     determinations in the absence of effluent monitoring data will
     address each of the 4 rec[uired factors.

2 .    Including bioconcentration and  bioaccumulation  recommenda-
     tions in the  TSD is  premature.   Approaches  to  this  problem
     require much more peer review,  input and development.
     EPA Response:    EPA  agrees  with this comment.   The  draft
     bioconcentration guidance  that  was referenced in  the  draft
     chapter 3 has  not  yet been released for public  comment  and
     does require  additional peer review.   The majority of  the
     chapter 3 discussion  on the specifics of  this  guidance  was
     removed; the remaining  sections  specifically state that  the
     procedures  in the  draft  guidance should  not  be  used  by
     regulatory authorities until the guidance is finalized by EPA.

3.    In determining reasonable potential,  the cumulative effluent
     discharge to a receiving water  should be  considered  instead
     of single discharges.

     EPA Response:  EPA agrees  with  this comment.  The draft of
     chapter 3 did not draw a clear enough distinction between the
     terms "cause"  and  "contributes  to"   in the context of  the
     reasonable potential determination.  Chapter 3 now states that
     where multiple discharges collectively are causing or show the
     reasonable potential to cause or contribute  to  an  excursion
     of water quality  standards, limits must be developed for each
     discharger to  protect  against  such collective  excursions.
     This is underscored by  adding the  exact  regulatory language
     of 122. 44 (d).  Finally,  the document now has  recommendations
     on  the  use  of  toxicity  testing  in  multiple  discharge
     situations.

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4.   Guidance  is needed  on  how to  demonstrate that  chemical-
     specific  limits  alone are sufficient to  achieve applicable
     water quality  standards,  thus  obviating the need  for a WET
     limit.

     EPA Response:  EPA agrees with this comment.  Regulations at
     122.44 (d) (1) (v)  provide  that WET  limits  are  not  necessary
     where the permitting authority demonstrates that  chemical-
     specific  limits  are sufficient  to protect  water  quality
     standards.  The draft of chapter 3 did not clearly recognize
     this  point and  provided no guidance  on how  to  make this
     demonstration.    Chapter  3 now  reiterates this regulatory
     provision  with a  new section entitled  "Using  a  Chemical-
     specific  Limit  to  Control  Toxicity."   This   new  section
     recommends  that the  discharger  conduct  a  TIE  to identify
     causative  toxicants.   Where  the  causative  toxicants  are
     controlled  by   chemical-specific   limits,   the  permitting
     authority may make the determination that WET limits are not
     necessary.

5.   The  multiple  conservative  assumptions   in   the   effluent
     characterization methodology (the  effluent  is  most toxic to
     the most  sensitive life  stage  at the time  of  lowest stream
     flow and peak design flow) are overkill.

     EPA Response:  EPA does not agree that multiple conservative
     assumptions amount to overkill.   This comment  implies that
     EPA recommends establishing effluent limits to protect against
     toxic impacts that are never projected to occur.  In truth,
     EPA only recommends establishing effluent limits where toxic
     impacts are  projected to occur.   Estimates of  toxic impact
     should be made assuming that the effluent is  most toxic to the
     most  sensitive  species  or lifestage at  the time  of lowest
     available dilution because these are conditions that can be
     expected  to occur.   For  the  most part  chapter  3  remains
     unchanged as a result of this comment.  However,  EPA has added
     a short discussion that suggests that the regulatory authority
     may choose to assess reasonable potential using a stochastic
     dilution model which incorporates  both ambient dilution and
     effluent variability to project toxic impact.

6.   EPA should not recommend  a 3 species minimum.  The algae test
     and the marine tests are not sufficiently studied.
     EPA Response: Chapter 3  continues  to recommend as  a minimum
     that 3  species  be tested quarterly for a minimum  of 1 year
     where toxicity tests  are  used to make decisions regarding the
     need for WET limits.  Experience  indicates  that algal tests
     can be a highly  sensitive  test species  for  some pollutants.
     Furthermore, using a plant adds another trophic level to the
     test regimen.  EPA rejects  the assertion that the algal tests
     are not sufficiently  studied.  For both freshwater and marine

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     waters, the use of  3 species is more protective than 2 species
     since a wider range of species sensitivity can be measured.

7.   EPA  should not  recommend  that  ambient  toxicity tests  be
     conducted at worst case low flow conditions.

     EPA Response:   EPA disagrees with this comment.   Chapter 3
     continues  to  recommend  that  ambient  toxicity testing  be
     conducted during appropriate low flow or worst case design
     periods.  In order for the results of ambient toxicity tests
     to  form  the  basis  for  decisions  about  whether  toxicity
     controls are needed, the test must reflect the conditions that
     such controls would be designed to protect.   If a regulatory
     authority's policy is to protect  at  the 7Q10  flow,  than the
     ambient tests must be conducted at flows  that are very near
     the 7Q10.  Otherwise, the regulatory authority will learn very
     little from the ambient test about whether toxicity limits are
     necessary for a particular discharge.

8.   Reasonable potential  determinations  should  not be based  on
     whole  effluent toxicity  data  alone.   Toxicity  data  and
     instream survey data should be used  together  in a weight  of
     evidence approach.

     EPA Response:  We disagree.   As discussed  in the response  to
     comment no. 2 in Chapter 1,  EPA considers  that water quality
     standards apply independently of each other.  Whole effluent
     toxicity measures  a different biological  endpoint  than  do
     instream survey data.

9.   One piece of  effluent data  projecting an excursion  above a
     water quality standard is insufficient to  justify setting  an
     effluent limit.

     EPA Response:  EPA disagrees.  EPA's position is  that where
     even one data point shows that an excursion  of a state water
     quality standard is projected, the Director may determine that
     permit limits  for  whole effluent  toxicity  or  for  specific
     chemicals are  necessary.    In  making such  a  determination,
     NPDES regulations requires that the Director also account for
     existing  controls  on  point  and   nonpoint   sources,  the
     variability of the pollutant parameter  in the effluent, the
     sensitivity of  the species  to toxicity testing  (for  whole
     effluent),  and where appropriate the dilution of the effluent
     in the receiving  water.   In addition,  the Director should
     consider all  other  available information  pertaining to the
     discharger to assist in making an informed judgement.
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Chapter 4;  Exposure Assessment and Wasteload Allocation

Overview:  Clarified terminology  for  mixing zone and design flow
conditions.

1.   Clarify or reevaluate Agency's position with respect to key
     mixing zone issues:  including how to establish geographical
     boundaries  of mixing  zones  and  point of application  of
     criterion for persistent bioaccumulative pollutants, and how
     to prevent lethality in the mixing zone.

     EPA Response:  The revised TSD states  as it did in previous
     drafts that the mixing zone size should be minimized and the
     dimensions should be based on the site-specific conditions.
     As previously stated, site-specific  evaluations should also
     be conducted by the permitting agency to determine whether to
     allow  a  mixing   zone   for   discharge  of  bioaccumulative
     pollutants.    The TSD   was   revised  to  expand  the  three
     approaches for preventing lethality in the mixing zone to four
     approaches.  The new approach includes submission of actual
     data to show that a drifting  organism would  not be exposed to
     1-hour average concentrations exceeding the CMC.  In addition,
     clarification was provided on the approach not requiring the
     use of a high velocity discharge  to show that the CMC is met.
     Approaches for preventing lethality in the mixing zone conform
     with the position taken in Chapter 2.


2.   Clarify  or  reevaluate  Agency's  position  on  appropriate
     critical flows for toxicants which have potential human health
     impacts and/or aquatic life impacts.

     EPA Response:  It was determined that the harmonic mean flow
     was appropriate for evaluation of  human health impacts that
     are of  concern due to  long-term exposures  (e.g.,  cancer).
     The TSD was revised to clarify the use of harmonic mean flow
     and its appropriateness for use in water quality modeling to
     evaluate human health impacts (e.g.,  a step-by-step calcula-
     tion procedure  was  added).    Additional clarification  was
     provided for the  use of hydrologically-based 7Q10  and 1Q10
     flows  for the  evaluation  of  "worst  case"  scenarios  for
     determining potential aquatic life impacts.          .

3.   EPA should present all valid modeling  approaches and deter-
     mine the  best water quality model  for each water quality
     condition.

     EPA Response:  The models  described in the text  were those
     EPA considers  to  be  comprehensive  to evaluate most  water
     quality  conditions.    Because of  all  the  potential  site-
     specific scenarios, it  is beyond  the  scope of the document to
     present all of the valid approaches that could  be  used for

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     water quality  modeling.   The permitting  authority needs to
     evaluate the data available for  the site and select the water
     quality model that is best for calculating the receiving water
     concentrations and the TMDLs.

4.   Specific caveats  should be  added  to the model descriptions.

     EPA Response:   It  is  beyond the  scope  of the  document to
     provide all of the caveats that are applicable  for each of
     the "models.    Although  the  commenters   presented  specific
     caveats for a model, they were not incorporated since it was
     beyond the scope  of revising the  document to  determine all
     the cases that the caveat could apply.

5.   Commenters want additional  explanation  or justification for
     the assumptions and applications for the equations presented
     in the TSD.

     EPA Response: The TSD already contained a sufficient explan-
     ation  or  referenced  the  documents  which  contained  the
     rationale.

6.   The water quality model "DYNTOX" and the software program HHD
     FLOW are not available.

     EPA Response:  DYNTOX  is not currently  available but should
     be accessible by early 1991.  HHD FLOW is not available, but
     DFLOW is available, and the  TSD  has been changed to reference
     this software package.

7.   Guidance on regulating  nonpoint  sources should be included in
     the TSD.

     EPA Response:   It  is  beyond the  scope  of the  document to
     include information on regulating non-point sources.

8.   Guidance should be given in  the  discussion on design flow for
     persistent pollutants.

     EPA Response:   Persistent  pollutants should be  assessed in
     the same way as bioaccumulative  pollutants.

9.   The human health  section does not  provide direction regard-
     ing the percentage of fish that  are taken from a  given area.

     EPA Response:   The percentage of fish should  be determined
     based on site-conditions.   The WLA criteria are conservative
     estimates based on heavy consumption of fish or a  potentially
     large contaminated area.
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10.  The TSDs  discussion on mixing zones'  contribution  to human
     health is overemphasized.

     EPA Response:  The  TSDs  discussion  on mixing zones  has been
     revised and does not overemphasize the contributions to human
     health.

11.  EPA should revise its  discussion of modeling WLAs for human
     health toxicants because  it  is misleading since it  suggests
     that  the  method  should be used  whether  the  applicable
     criterion is  a drinking water standard  intended  to prevent
     acute effects  on humans  or  criterion established  for Deriv-
     ing Numerical National Water Quality Criteria.

     EPA Response:   The  TSD does not discuss  acute human health
     effects; therefore, it was determined that the discussion is
     not misleading.


Chapter 5; Permit Requirements

Overview:   Added   clarifications  and  examples  to support  the
existing text; gave equal weight  to developing limits  based on a
dynamic and steady  state wasteload  allocations; added  discussion
on metals,  average  and  maximum  permit  limits,  single  dilution
tests, variability, and mass-based limits.

1.   The two value, steady-state model approach  to permit limit
     derivation  is overly  conservative with  too  many  built-in
     safety factors.   As a  result  the  limits derived   are  too
     stringent.

     EPA  Response:    EPA has  revised  Chapter  5 of the  TSD  to
     emphasize the development of permit limits that are  as exact
     as possible to attain and maitain  water  quality  standards.
     Chapter 5  now recommends the  use  of  the  statistical limit
     derivation procedure  which  provides  for  two  options.   The
     first option uses a steady state model approach for  develop-
     ment of the wasteload allocation (WLA) and long term average
     (LTA).   Since  this approach relies  on  critical condition
     assumptions regarding effluent characteristics and receiving
     water characteristics, this  approach  may  derive limits that
     are more  restrictive than the second  option.   The  effluent
     characteristics of  importance  are  pollutant concentrations,
     pollutant concentration  variation,  and effluent  flow.   The
     receiving water characteristics of  importance are pollutant
     concentrations and receiving water flow.

     Where a discharger or permitting authority believes  that the
     steady state  model approach results  in  overly restrictive
     permit  limits, under  the  revised  TSD  the  discharger  or
     permitting authority has  the alternative  of using a dynamic

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     model approach.  Under the dynamic model approach worst case
     assumptions are minimized  and more accurate receiving water
     concentrations of a pollutant can be  calculated.  In general,
     dynamic  models  account  for  the  daily  variations  of  and
     relationships  between   flow,   effluent  and  environmental
     conditions and therefore directly determine the probability
     that a water quality standards exceedance will occur.  Because
     of  this,  dynamic  models can be  used to  develop  wasteload
     allocations which  more  exactly  maintain the  water  quality
     standards  at  the  return  frequency  requirements  of  the
     standards.  The WLA is first developed by iteratively running
     the dynamic model with successively lower LTAs until the model
     shows compliance with the water quality  standards.  With this
     approach now a recommended option in the TSD,  this major issue
     is resolved.  A disadvantage  of  using dynamic model outputs
     to develop permit  limits is the lack of  necessary  data for
     effluent variability and receiving water flows.

2.   The discussion of  below  detection  levels  is confusing.   EPA
     should  not be  setting  water  quality-based  limits  below
     detection  levels,   especially  since  variability  of a  test
     method is greater the closer the results are to the detection
     level.

     EPA Responses   The  discussion  of below detection level limits
     in the draft  TSD was unclear.  Since the time of  the draft
     TSD, EPA issued its guidance  on  setting permit limits below
     the detection  level for dioxin.  This guidance uses a minimum
     level (ML)  to ascertain compliance with  limits set  below
     detection levels.   The TSD was revised  to follow the dioxin
     guidance  (May 21,  1990 Memorandum  from LaJuana  Wilcher,
     "Strategy for  the Regulation of Discharges of PHDDs and PHDFs
     from Pulp and Paper Mills to  Waters  of  the United States").
     Specific  values  for  the minimum level  are  found  in  the
     description of methods  1624 and  1625  for  some  organic
     compounds.  (See  the appendix to 40 CFR 136.)

3.   EPA's criteria recommend that limits be derived for toxicant
     in the soluble form or biologically available form,  yet many
     permit limits are  being  written and compliance being  based
     upon the total recoverable  form  of metals.   The TSD should
     address  this issue and give guidance on developing limits only
     for bioavailable  forms of pollutants.

     EPA Response:   The  TSD has been revised to  cite the  regula-
     tory requirements  at  40  CFR 122.45 (c)  and to  provide  three
     options  for use where a  state has not developed a method for
     determining total   recoverable  permit  limits  based  on  a
     dissolved or acid soluble water quality criterion.  The three
     options  are 1)  assume  complete  availability  of the  total
     recoverable metal,   2) use  the method  in the  EPA  wasteload
     allocation guidance manual for toxics in rivers to relate the

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     two, and 3)  use site  specific data to develop a relationship.

4.   EPA's regulations do  not allow permit limits based on one day
     maximum concentrations for POTWs.

     EPA  Response:    The  NPDES  regulation  at  40 CFR  122.45(d)
     require the use  of a 7-day  average  unless  impractical.  The
     discussion on the expression of permit limits now states that
     EPA  considers  the   7-day  average  limit  for  POTWs  to  be
     impractical for the purposes of controlling the discharge of
     toxics.  The reason for this statement is that control of the
     7-day average  in lieu of control of the 1-day  maximum will
     allow for unmeasured short-term excursions  of an acute water
     quality standard.

5.   TREs should not be required as a permit condition to respond
     to a violation of a whole effluent toxicity  limit.  The proper
     response  by  the permitting  authority  is  through  use  of
     enforcement mechanisms.

     EPA  Response:    The  discussion  on  TREs  was changed  to not
     recommend that the TRE or accelerated monitoring be included
     in the permit to respond to permit limit violations but rather
     be part of the enforcement  response.  This was  done to help
     distinguish between monitoring only provisions and enforcement
     of  permit  limits.    This  should reduce confusion  about the
     difference between enforcement on a  single event violation of
     a limit and the need for multiple violations before a TRE is
     warranted.

6.   Permit  limits  should  be increased to consider  analytical
     variability.    EPA  should  consider using  the  approach  it
     proposed in the Amelia River study.

     EPA Response:  EPA disagrees with the proposal.  A discussion
     was added to explain  how EPA considers analytical variability
     in developing permit  limits.  The discussion states that since
     this variability is an intrinsic part of all data collection
     (effluent monitoring,  wasteload  allocation development, and
     water quality standards development)  , and that the variability
     can go both ways (higher or lower) , that EPA does not separate
     it out from all other variability factors.   EPA's Amelia River
     study is not final; the approach cited may not be included in
     the final report.

     Although  difficult,  it  may be  possible  to determine what
     proportion of  the observed  variability  can be attributed to
     sampling error,  and what proportion can be attributed to the
     method of measurement.   Regardless, the TSD makes  use of a
     coefficient  of  variation  that  includes   both  sources  of
     variability.   This  is  not  unreasonable since  sampling for
     monitoring purposes  also results in the inclusion of these

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     two sources  of  variability.   Any  consideration of upstream
     concentrations  in  the  WLA  will  also  include  analytical
     variability.  There  is  rio  "true"  calculation in the process
     of  developing  permit   limits  because there is  analytical
     uncertainty throughout the process.  Instead, LTAs should be
     calculated from the WLAs using the same CV that is also used
     to calculate permit limits  from the most limiting LTA.  It is
     unimportant  exactly  what  CV  is  used  because  the  most
     restrictive  LTA itself is used  only  for  determining  the
     desired treatment performance level.  However, this procedure
     assures (99 or 95% confident)  that the permit limits will be
     less than or equal to the more limiting LTA.

7.   Since  permit limits were  derived based  on 95th and  99th
     percentile  probabilities,  that occasional  exceedances  of
     permit limits should be allowed on the same basis.
     EPA Response:  In statistics,  the selection of an acceptable
     probability  level  reflects the level of confidence  that is
     desired of the results.   As such, an acceptable level must be
     defined prior to  performing  any statistical procedure.   As
     stated in the TSD, the probability basis of  0.99 for the daily
     maximum limit,  and 0.95 for  the average monthly  limit have
     been used historically in connection with development of the
     effluent guideline  limitations  and have been well accepted
     upheld in legal challenges to the  guidelines.   These values
     are tied to monitoring  frequencies that are required for each
     limit.  There is no mixing of two probability  bases since they
     are distinct and separate limits.  The goal in  establishing
     these levels is  to  allow the regulatory agency to distinguish
     between adequately operated wastewater treatment plants with
     normal variability from poorly operated treatment plants.


8.   Permit  limits  should   vary  with  flow  conditions  in  the
     receiving waters  since exposure is  based  on dilution  with
     receiving water  flow.

     EPA Response:   EPA partially  agrees  but only to  the extent
     that the limits  are seasonally based.   The  seasonal approach
     has been used by  permitting  authorities for setting permit
     limits to protect against excursions  of dissolved oxygen and
     ammonia standards.   However,  seasonal limits are  different
     than limits which  vary  dciily based on river flow.  EPA is not
     convinced that a daily variable approach would be universally
     practical given wastewater treatment response and performance;
     for this  reason EPA has not  included  procedures for  this
     approach.   In addition, the discharger has the option of using
     dynamic modeling  to develop  permit  limits.   Since  dynamic
     modeling considers  all receiving  water  flows,  this  option
     would provide the discharger  a  less restrictive  permit limit
     than would be obtained by using steady state modeling.


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9.   If limits were derived that were overly stringent due to the
     worst case assumption  of the steady state model,  and if in
     the future  the permittee  conducted dynamic  modeling which
     resulted in less stringent limits, permittees would be bound
     to the previous limits due to the anti-backsliding require-
     ments .

     EPA Response:   There   is  no absolute  prohibition.   EPA's
     September 1989 guidance document on antibacksliding contains
     EPA's. interpretation  of the  Clean  Water Act  provisions in
     §402(o)  and §303(d)(4).  This guidance also does not contain
     an absolute prohibition.  It is EPA's position that the CWA's
     anti-backsliding provision and EPA's existing regulations do
     not uniformly prohibit  the  incorporation into a permit  of less
     stringent  limits,  standards,  or  conditions.   In  certain
     situations (i.e.,  under Sections 402(o)  and 303(d)(4)  of the
     CWA) ,  less stringent  limits or conditions may be permissible.

     Section  402(o)(l)   provides  that  backsliding  from  water
     quality-based limits is prohibited except in compliance with
     Section 303(d)(4).  Section 303(d)(4)(A)  only allows estab-
     lishment of less stringent limits in  a  permit for discharge
     into a non-attainment  water only  if two conditions are met:
     1) the existing permit limit must have  been based  on a TMDL
     or other WLA established under Section 303, and 2) attainment
     of  water  quality  standards  must  be  assured.    Section
     303(d)(4)(B)  allows establishment  of less stringent limits in
     a permit  for  discharge into an  attained water only where
     relaxation  is  consistent  with  a  State's  antidegradation
     policy.

     Section  402(o)(2)  also  outlines  exceptions to  the general
     prohibition  against  backsliding  from  water  quality-based
     permit limitations. Under Section 402(o)(3), backsliding may
     be allowed:   1) where there have been material and substantial
     alterations or additions to the facility; 2) where good cause
     exists due to  events beyond the permittee's control and for
     which there is no  reasonably available  remedy;  3)  where the
     permittee has installed and properly operated and maintained
     required treatment facilities; and  4) where new information
     justifies  backsliding  from  water  quality-based  permit
     limitations and other Section 301(b)(1)(C) limitations.

10.  EPA does not  have  guidance on how  to conduct  a  chronic TRE
     and therefore limits  should not be derived based upon chronic
     endpoints.

     EPA Response:   EPA  is  aware  of  the need for  guidance on
     conducting TREs for chronic toxicity.   EPA's Duluth labora-
     tory is near  completion of a draft guidance  document.   The
     guidance document  will  be widely available  when  finished.
     Regardless,  the lack of a  finished  guidance document is not

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     a  valid reason  to allow  for  excursions above  a narrative
     standard  as measured by  chronic whole  effluent toxicity.
     Some  dischargers  have been able  to  comply with  chronic
     toxicity  limits  and identify and control sources of chronic
     toxicity.   EPA  experience has  shown that portions  of the
     published  TRE procedures  for  solving incidences  of  acute
     toxicity   (EPA/600/2-88/070,   EPA/600/2-88/062,   EPA/600/3-
     88/034)  can  be  used  for resolving  incidences  of  chronic
     toxicity.

11.  EPA should  allow adequate time  for facilities  to come into
     compliance with water quality-based permit limits.
     EPA Response:  EPA  is aware that facilities may  need time to
     comply with newly established effluent limits.  This has been
     accomplished  in  the  past  in   NPDES permits   by  allowing
     compliance schedules within the permit.  Regulatory agencies
     use of compliance schedules for water quality-based effluent
     limits are governed by  recent decisions regarding the Star-
     Kist Caribe ruling by EPA's Chief Judicial Officer (CJO).  On
     March 8,  1989, in  review  of the evidentiary hearing request
     by Star-Kist Caribe, the CJO ruled that compliance schedules
     for water quality-based effluent limits  may not be included
     in NPDES permits  unless explicitly authorized by  the State in
     its water  quality standards or  implementation  regulations.
     The  ruling  was  based  on  an  interpretation  of  section
     301(b)(1)(C) of the Clean  Water  Act.   Later, on Septermber 4,
     1990,  the CJO granted a stay of the ruling  to allow EPA and
     States to  use compliance   schedules for  water quality-based
     limits where such schedules are  consistent with State policy.
     In any case, the allowance for compliance schedules is a State
     decision which may  ultimately need to be expressed in water
     quality standards.

12.  Effluent limits should be  set within the ability of treatment
     technology.

     EPA Response:  EPA is aware that there may be  a number  of
     water quality-based  permit limits for  toxics which may  be
     presently  unachievable  with  existing wastewater  treatment
     technology.    However,  the  NPDES  regulations  at  40  CFR
     122.44(d) require  that  effluent limits more  stringent  than
     those established based on a treatment technology basis must
     be set to achieve water quality  standards.

13.  The limit derivation procedures  for human health should use
     the same statistical procedures  as used  for  deriving limits
     for aquatic life.  Permit  limits should  be derived from the
     harmonic mean effluent concentration.

     EPA Response:   Since compliance with permit limitations is by
     regulation determined on  a daily and monthly basis, it  is
     necessary  to   set   permit  limitations  expressed  in  these

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     contexts that meet a given WLA every month.   The statistical
     procedures for permit limit derivation in the TSD are designed
     to accomplish this for aquatic life protection where the use
     of shorter  term  averaging  periods  is  consistent with  two
     number aquatic life criteria.

     However, if the  TSD procedures were directly used for setting
     permit limits on  bioconcentratable  pollutants,  both maximum
     daily and  average  monthly  permit  limits  could exceed  the
     wasteload allocation necessary to meet the criterion.  These
     two permit  limits would assure  that the long  term average
     effluent discharge would comply with the human health derived
     WLA only if  the assessment of the  effluent  variability was
     precise.  With  bioconcentratable pollutants  where  exposure
     duration ranges up to  70 years,  EPA  believes that  effluent
     variability cannot  be reliably estimated  from existing data
     for exposure periods a year.  If the effluent variability was
     over-estimated when establishing the permit  limits,  then a
     facility could be  discharging in compliance  with the permit
     limits but would  be exceeding the  wasteload  allocation for
     human  health  protection.     This  approach   is   clearly
     unacceptable.

     This problem  does  not  arise when using the  TSD statistical
     procedure for setting  permit  limits  for  protecting against
     aquatic toxicity.   In this case, the monthly average and daily
     maximum permit limits are more closely related to the four day
     average and one  hour maximum used as exposure periods for the
     criteria.  Any imprecision in assessing effluent variability
     would therefore not have as great  an effect on  the permit
     limits.

14.  Effluents may not  always demonstrate a  log-normal distribu-
     tion.   The TSD  should present  procedures  for using other
     distributions.

     EPA Response:  EPA believes, after reviewing the database used
     to  establish  effluent  guidelines,  that   the  log-normal
     distribution best characterizes effluents.  EPA's analysis of
     these  data  are  provided  in Appendix  E.     The  general
     characteristics of the lognormal  distribution (it  is  only
     positive and  is skewed towards extreme  high  values)  make it
     an  appropriate  distribution  for   dealing   with  effluent
     concentrations.   According  to Gilbert (1987), the lognormal
     distribution is the only available 2-parametric distribution
     that can routinely  be applied to environmental data.   Since
     the  Agency  is  not  providing  derivation  procedures  for
     alternate data distributions,  it  is  not necessary to test for
     lognormality  of effluent data.   Permitting  authorities can
     develop  their   own  methods  using   other   probabilistic
     distributions.
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Chapter 6; Enforcement

Overview:    No  major changes  made  in  recommendations;  added
clarifications and explanations.

1.   One  effluent test  failure  should  not  equal a  violation;
     provisions should be made to delay punitive enforcement action
     where legitimate efforts are being made; permittees should not
     be held in violation of their toxicity limit while conducting
     a TRE.

     EPA  Response:    The  current  draft  still  emphasizes  the
     principle that  any single exceedance  is a violation  and is
     subject to a full range of enforcement responses.   However,
     the draft has been caveated with discussion on EPA's guidance
     which  outlines  a systematic  review  of all  violations  to
     determine  the   appropriate  level  of  enforcement.    EPA's
     enforcement guidance  is  included  in  an appendix.
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