PB84-220326
Use of Probabilistic Information in the
Water Quality Based Approach
(U.S.) Environmental Research Lab.-Duluth, HN
1984
J
of Commerce
Information Service
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PB84-220326
EPA-600/D-84-192
1984
USE OF PROBABILISTIC INFORMATION IN THE WATER QUALITY BASED APPROACH
Norbert A. Jaworskl and Donald I. Mount
Environmental Research Laboratory-Duluth
U.S. Environmental Protection Agency
6201 Congdon Boulevard
Duluth, Minnesota 55804
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TECHNICAL REPORT DATA
(Pliae naa Jrumtcnoni on the rtvtnt btfon committing)
1. REPORT NO.
EPA=£00/D-8d-192
3. RECIPIENT'S ACCESSION NO.
Ppg L 220326
K TITLE ANDSUBTITLE
USE OF PROBABILISTIC INFORMATION IN THE WATER
QUALITY BASED APPROACH
5. REPORT DATE
1984
6. PERFORMING ORGANIZATION CODE
7. AUTHORIS)
N. A. Jaworski, and D. I. Mount
. P&RFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
U.S. Environmental Protection Agency
Environmental Research Laboratory-Duluth
6201 Congdon Boulevard
Duluth, MN 55804
10. PROGRAM ELEMENT NO.
II. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Duluth, MN 55804
13. TYPE OF REPORT AND PERIOC COVERED
14. SPONSORING A5ENCY CODE
EPA-600/03
15. SUPPLEMENTARY NOTES
Aquatic Toxicology: Seventh Symposium, ASTM, 1984
^During the past two decades, implementation procedures in vastevater manage-
ment often resulted in a large margin of safety being incorporated into the use of
water quality criteria for the protection of aquatic life and its uses. Wasteload
allocation design conditions, such as the use of seven-day, ten-year low flow, gave
assurances of instrearn concentrations well below the water quality criteria for a
large percentage of time. Present-day economic conditions and the Increasing cost of
advanced wastewater treatment are necessitating a re-examination of how water quality
criteria are being used in the water quality based approach for establishing effluent
limitations.
The relationships between water quality criteria and other components of
the water quality based approach are identified. The need for a better defined and
more consistent use of statistical information is suggested not only in the develop-
ment of water quality criteria but also for the entire water quality based approach.
Intensity, duration, and frequency of occurrence (return period) appear to be three
common statistical parameters of the six-step water quality based approach.' Research
is identified, which if successful, would allow wat=r quality managers better insight
in determining pollutant exposures that more adequately simulate receiving water ""
conditions resulting from variable stream flows, wastewater discharge rates, and
pollutant concentrations.
17.
KEY WOROb AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
COSATI Field/Croup
IB. DISTRIBUTION STATEMENT
Release to public
IB. SECURITY CLASS ITnuHtporli
Unclassified
21. NO. Of PAGES
20
20. SECURITY CLASS IThu pojtj
Unclassified
22. PRICE
EPA FMIW 2220-1 (R«». 4-771 »M(viOU» KOITION it OMOLCTC
1
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Or
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: ABSTRACT: During the past two decades, Implementation procedures in
wastewater management often resulted in a large margin of safety being
incorporated in the use of water quality criteria for the protection of
*"' aquatic life and its uses. Wasteload allocation design conditions,
such as the use of seven-day ten-year low flows, gave assurances of
instream concentrations well below the water quality criteria for a
. large percentage of the time. Present-day economic conditions and
increasing cost of advanced wastewater treatment are necessitating a
i
i re-examination of how water quality criteria are being used In the
i
! water quality based approach in wastewater management implementation
j procedures.
; The interrelationships of water quality criteria and other steps of
I the water quality based approach are identified. The need for a better
i
' defined and more consistent use of probabilistic Information 1s suggested
: not only In the development of water quality criteria but for the
entire water quality based approach. Intensity, duration, and frequency
of occurrence (return period) appear to be three common statistical
parameters of the six steps of the water quality based approach.
Research is Identified which, if successful, would allow water quality
managers more insight Into the better selection of pollutant exposure
histories that more adequately simulate receiving water conditions.
resulting from.variable stream flows and wastewater discharge rates. It
Is also suggested that more aquatic toxiclty research resources be focused
on those areas where greatest variability and lack of knowledge currently
I
exist, such as in species selection, incorporating fluctuating exposures, i
• - considering multi-pollutant effects, field applicability, etc.
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Introduction
During the 1950's and 1960's, water quality criteria for the
• protection of aquatic life prescribed a maximum pollutant concentration
for the receiving water. The criterion was usually presented as a "no-
I effect level" which would protect aquatic life and its uses under most
ambient conditions. This was readily interpreted as applying to all
stream flow and wastewater discharge conditions.
This "no-effect level" ambient concentration concept 1s analogous
to a speed limit for our highways. That 1s, a posted highway speed
limit is in essence a maximum "safe" speed for traffic under optimum
i
| road conditions. For both the water quality criteria and the speed
limit applications, the "safe" limits have no time period restrictions.
That 1s, the no-effect concentration can be maintained 1n the receiving
water "forever"; likewise, one can drive forever at the maximum speed
! limit. The only safety factors Incorporated are those asserted 1n
j developing the no-effect concentration or the maximum speed limit.
The early intended use of water quality criteria was, in general, to
i
| allow the determination of whether the concentration of a pollutant 1n the
! receiving water was above, and/or below a level deleterious to aquatic life;
simply stated, "safe" or "unsafe".
The purpose of this paper is to demonstrate how the use of water
j quality criteria/standards has gone beyond the "speed limit" stage and
j Us use expanded in the water quality based approach. The Interrelationships
j of water quality criteria and other steps of the water quality based
; approach are identified. The need for a better defined and more consistent or7or.-.u
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use of probabilistic Information 1s suggested for the entire water quality
based approach. Research areas that require further study are Identified.
i ' i I
•Expanded Use of Water Quality Criteria In the Water Quality Based Approach
i A major departure from the early Intent (safe or unsafe) occurred when
i
.•water quality standards were used to establish wastewater treatment requirements
as part of the water quality based approach. The water quality based
i
: approach, as envisioned today, encompasses many technical, socioecor.omic,
land judgmental Issues which previously were not rigorously addressed. It
consists of the following components:
1. Establishment of water quality standards, Including:
a. Use attainability analysis;
b. Development of water quality criteria, Including site-
specific modification of the national criteria;
c. Impact analysis.
2. Wasteload allocation process.
3. Development of wastewater discharge permit limits.
4. Design of wastewater treatment facilities.
5. Operation and maintenance of wastewater treatment facilities.
6. Compliance monitoring of both effluent and ambient receiving waters.
Water quality standards today take on more meaning, and therefore th« use
i
i of standards needs to be better defined and be more consistent with the water
i
! quality based approach. In the establishment of water quality standards.
i
j both water quality criteria and use attainability analysis are Integral
aspects. In this Interpretation, criteria and use attainability are
linked through exposure and water-body use attainability analysis. Water
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quality standards, once established, are Incorporated In the wasteload
allocation process.
To illustrate this point, let us examine how receiving stream flow
» characteristics are utilized along with water quality criteria 1n the
wasteload allocation process. One part of the wasteload allocation process
Includes a statistical analysis of hydrologic stream-flow discharge rates.
This analysis was usually accomplished Independently of the water quality
criteria setting process. For example, for a wastewater discharge
containing a toxic pollutant, the analysis can result in design conditions
In which the "criterion" concentration 1s applied to a relatively severe
hydrologic condition which may occur less than five per cent of the time. At
all other times, the concentration 1n the receiving water will be much less
than the prescribed "criterion" level. This use of water quality criteria
in conjunction with the hydrologic analysis has led to the establishment
of wastewater permits and the design and operation of waste treatment
plants that are based on the "worst case" analysis.
The degree of rigor involved in hydrologic analysis in the late 1950's
has greatly expanded through the use of operations research techniques.
The use of advanced statistical techniques Including synthetic hydrology
has given the water quality planner greater insight into the sensitivity
of stream-flow conditions and wastewater treatment requirements. Nevertheless,
the two processes of development of water quality standards for the protection
of aquatic life and analysis of hydrologic factors were done independently,
and this has lead to the Incorporation of unnecessarily large safety factors
under some stream-flow conditions.
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1 Stream Flow and Exposure Considerations
Let us further examine the selection of stream-flow design conditions.
Historically, the water quality based approach has been based on the
selection of stream-flow design conditions such as 7Q10, which 1s the
lowest seven-day stream flow that occurs, on the average, once every
ten years. The specification of a stream flow (Intensity) for a seven-
day period (duration) that occurs, on the average, once 1n ten years
(frequency of occurrence or return period) was the basis for calculating
the permit limitations. These three statistical parameters: Intensity,
duration, and return period, are major input variables in the six steps
of the water quality based approach. However, many times the three
parameters are either not well defined or not used in a consistent
manner.
In many wasteload allocation processes [1] especially for BOO-DO
analysis, there often Is a coupling using relationships between stream
flow characteristics and DO targets. Nevertheless, the basic design
parameters are usually developed mainly from hydrologlc considerations,
not from toxlcologlcal, proposed use, or water quality criteria consider-
ations.
Statistical analysis of the exposure of aquatic systems to toxic
pollutants can bridge the gap between the development of standards and their
use In wasteload allocation. Pollutant exposure analysis has three basic
properties which are very analogous to precipitation analysis [2]. In
precipitation analyses, one requires probabilistic Information concerning
;the intensity of rain storms of various frequencies and for specific
!
durations. From these analyses one can determine the average Intensity «.•».< or
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"'of a given frequency of rainfall for any desired duration. The same
probabilistic techniques can be used to develop the three pollutant exposure
statistical parameters for a stream reach or lake segment. The three
^parameters are: Intensity, as measured by the concentration of a pollutant;
duration, as measured by the exposure period; and return period, as measured
by frequency of occurrence. These parameters can facilitate the selection
of design conditions for an ambient receiving stream's "pollutant exposure"
which can be expressed. ly to the stream flow design condition,
as "7C10" where 7 1s jurdi'on of exposure 1n days, C is the water quality
[criterion, and lp retir iriod in years. This simple analysis provides
ja mechanism fr ae probability of events or exposure periods
i
j during which t. .19 water concentration may be above or below the
.no-effect level. This ability to analyze these exposure periods or events
.during which the toxicit.y of a pollutant may be deleterious — depending, of
i
course, on the Intensity, duration and frequency or return periods of
•the events — adds greatly to our understanding of how toxidty can occur
and thereby helps to ensure appropriate protection.
Water Quality Criteria Interrelationships
, «_«^H^HH_^^_^_^.^».^.^H^_^^_H_^.^M^^_^HBi^.^^^HH__^_aiM_B_MM^«^
j As our understanding of aquatic toxicological science became more
sophisticated in the 1970's 1n the development of water quality criteria
for the protection of aquatic life, there evolved a two number criterion.
i
i The criterion Includes a maximum concentration, usually for protection against
I
n.ost acute toxic effects, and an average concentration for protection against
ir.ost chronic toxic effects. The National Guidelines [3] for deriving water
quality criteria, which were developed in 1978 and updated In 1930, reflected
[ this refinement In establishing national water quality criteria.
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To Investigate tne Interrelationships of water quality criteria,
wasteload allocation limits, return period, stream flow, statistics,
time averaging periods, and pollutant toxidty Information (acute/chronic
ratio), a Joint study was undertaken between the Environmental Research
Laboratory-Oulutn and Manhattan College [4]. Preliminary study results
suggest the Following:
1. For toxic chemicals with an acute/chronic ratio less than 100
and being discharged Into a stream with high flow variability, the
wasteload allocation limits are mainly controlled by the maximum
water quality criterion with return periods having a major
Impact. For chemicals with an acute/chronic ratio above 100,
the wasteload allocation is controlled by the average water
quality criterion.
: ?.. For toxic chemicals with an acute/chronic ratio less than 15
'. and being discharged into a stream with low flow variability,
: the wasteload allocation limits are mainly controlled by the
maximum water quality criterion, with return and ti-ne averaging
period being of lesser importance. For chemicals wiuh an
i
| acute/chronic ratio greater than 15, the average water quality
! criterion is restrictive.
I
i 3. When the third condition 1s added to prevent significant
• excursions between the maximum and average concentrations, it
i
: becomes the most restrictive in the wasteload allocation
i
I process for toxic chemicals which have an acute/chronic ratio
! of about 8 or more, and stream flow variability and return
j
! period considerations have small Impact on restricting
V •
~ wasteload allocations.
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PAGE X* '.'.. -;
The preliminary analysis clearly demonstrates the Interrelationships
and the need to integrate fully the various processes of the water quality
bi.sed approach.
Use Attainability Considerations
It is important to review how the water quality criteria are established.
The basic building blocks for the criteria are laboratory tests—acute,
chronic, and bioconcentration--at steady-state exposures, and generally in
clean test waters. The algorithm that has been developed for the National
Guidelines brings together the information required to derive criteria,
Including a maximum concentration and an average concentration, that must
be maintained to protect aquatic life and Its uses. Th<* intent of the
two-number criteria was to give the planner more information and degrees of
freedom in the water quality based approach. A major question to be asked
1s: are use attainability considerations and analyses congruent with the
entire water quality based approach?
I If the analogy of the speed limit for traffic control 1s further
.examined, additional considerations arise. If the maximum concentration
1s analogous to the maximum speed limit that cannot be exceeded, the
i
{average concentration has no direct analogy 1n traffic control, unless
iwe begin to distinguish between "maximum" or "safe" speeds. This dis-
tinction adds significantly to the process and Involves other technical,
jsodoeconomic, and judgmental Issues. For example, how many accidents
jwithln a given time period and of what type are we willing to accept?
jLikewise, 1n the protection of aquatic life, how many exposure events
iand for what duration within a given time period and at what Intensity .
j ''CM O"
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The current water quality based approach suggests that the amount of
wastewater treatment provided should consider not only the establishment of a
numerical water quality criterion for a given pollutant for the protection of
aquatic life, but also "use attainability" considerations. Use attainability.
Including level of protection for a given site, can have a significant Impact
on the other component steps of the water quality based approach. For example,
the selection of wasteload design conditions for duration and return period
should be based more on the use attainability analysis than on hydro!ogle and
exposure considerations alone. The selection of design conditions for
return period, above a given design exposure concentration (Intensity) and
duration, should consider the assemblage of the aquatic organisms to be
protected; that 1s, their biological value to the aquatic comnnnlty and
their ability to repooulate. One further point Is worth mentioning here.
If two deleterious events occur back-to-back, t!;e Impact of the No events
may be additive. Therefore, we may need also to Include an additional
parameter for this effect, especially when small periods of return are
considered.
i I
i I
ToxicoToqlcal Considerations
In reviewing the National Water Quality Criteria Documents, one can
readily see that for certain pollutants the slope of the ranked acute
toxlcity plot for the species tested 1s very steep, suggesting that all
of the species tested with that chemical have similar toxidty responses.
For other pollutants the species rank curve Is relatively flat, suggesting
that some species are greatly more sensitive than others. A key question
here 1s how can we best take advantage of the information already known
iabout the differences 1n species sensitivity to given pollutants? Can we
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•take advantage of this Information by treatment of these relationships 1n
a more probabilistic nature?
Receiving stream data clearly demonstrate that the ambient concentration
of pollutants can vary significantly. Influenced nalnly by effluent and
stream flow variability. The preponderance of available toxlclty data has
been developed using clean water and constant toxicant concentrations. We
are just beginning to study fluctuating exposures In the laboratory.
•Models [51 have been suggested for certain pollutants which can Incorporate
the tox1f1cat1on/detox1f1cat1on process, mainly on an acute basis. Laborator>
data for certain compounds, such as ammonia [6], suggest that even short
exposure periods (hours) can have severe biological Impact. With the high
cost of data generation using existing laboratory test methods, the need
exists to make better use of existing data bases by extrapolating from
steady-state to fluctuating exposures. It 1s paramount 1n this extrapolation
process to keep 1n perspective the receiving water pollutant exposure
statistical parareters—Intensity, duration, and return period.
All of the above Items suggest more extensive and expensive testing
and analysis requirements 1n the water quality based approach. Is this
really true? Some preliminary studies [7] suggest that ecosystems may be
more similar than dissimilar with respect to toxlcologlcal endpolnts.
LAST LIlNE JSf-
- TLXT
when comparative toxlclty analyses are made that couple the derivation of
site-specific water quality criteria and analysis of use attainability
requ1rc«nents. If co, the process should collapse, not expand, into
multl-toxlcologlcal endpolnts. More extensive comparative toxlcological
analysis Including probabilistic considerations in use attainability analysis
should aid In answering this fundamental question.
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* Studies [8] have Indicated that most wastewater discharges contain
multiple pollutants near or above the concentrations required to protect
against chronic effects of aquatic life. Many of the pollutants are not
3readily removed by wastewater treatment, and are persistent in surface
receiving waters. The ability to Incorporate multiple pollutant effects
in wasteload allocation, even at steady-state conditions, is not well
developed at the present time. The inclusion of multiple pollutants under
fluctuating conditions is even more poorly understood. Although the use of
laboratory toxicity tests on complex effluents 1s promising [9l, much more
research is needed in use of the complex effluent toxicity testing approach
as an alternative to single pollutant testing 1n the water quality based
i
iapproach, Including Incorporating appropriate Intensity, duration, and
i
I return period considerations.
The selection of wasteload design conditions for intensity and duration
of pollutant exposure should be based more on toxicological considerations
(I.e., water quality criteria/standards) than on hydrologic conditions
alone. The pollutant exposure design conditions should reflect both the
characteristics of community assemblage to be protected and of the toxicological
characteristics of the toxicant.
Need For An Integrated Approach
Inherent in all six steps of the water quality based approach are design
conditions which contain time variables like intensity, duration of exposures,
recruitment time, return period, permit limits, wastewater treatment design
I i i
conditions and monitoring frequency. Although different terminoloyy Is
often used in each of the steps, one can grossly aggregate time considerations
Into three parameters: intensity, duration, and return period.
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Studies [10] have shown that the concentrations of pollutants 1n effluents
and from non-point sources vary considerably but can be represented by a
probability function. Likewise, studies [11] have shown the ambient stream
concentrations of most pollutants vary considerably but a",so can be
expressed by a probability function. Although the robustness of the functions
1s debatable, probability functions, for the most part, can be developed.
Such analyses suggest that most pollutants are "event" oriented
with a determinable Intensity, duration, and return period. In the development
of this argument further, the need for aggregating common time units becomes
jmore obvious as Indicated below:
1. Water Quality Standards*
. What properties of the aquatic systems are to be protected
or not protected, and to what extent?
. What frequency and duration of Impairment should be used for
use attainability analysis?
2. Waste!oad Allocation
. What are the major factors that Influence exposure and Impact In
receiving water: wastewater effluent variability, stream flow
variability, or physical, chemical, and biological transport
processes?
3. Wastewater Discharge Permit Limits
. What are options for expression of loading, 1n terms of Intensity
(concentration), duration, and return period time units?
4. Wastewater Treatment Design
. Are the design parameters such as hydrologlc loadings and
treatment removal effectively based on time parameters similar to ur..l0F
C AHEA
those used 1n setting water uses? l^utl
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5.
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Wastewater Treatment Operation and Maintenance
ME:
. Are the operational procedures resulting 1n effluents with
the prescribed pollutant concentration expressed in Intensity,
duration, and return periods?
6. Monitoring
. Is the monitoring program of both effluent and ambient receiving
water properly addressing time-event conditions?
What does this mean In terms of the state of the establishment of water
quality criteria 1n the water quality based approach for tha protection of
aquatic life? The message 1s very clear. No longer can we consider water
quality criteria for the protection of aquatic resources as a simple speed
limit. We must better understand how It Is used 1n the entire process of the
water quality based approach. We have taken a major first step, In the authors'
opinion, by developing a two number criteria; that 1s, maximum and average
concentration numbers. However, these terms need to be better defined 1n
relation to the entire water quality based approach. For example, what 1s
maximum? What 1s average? What return periods do these two number criteria
suggest 1n wasteload allocation, 1f any? Should the duration and return
periods be the same or different for the maximum and the average concentration?
Better defined and more consistent use of the probabilistic parameters are
needed for all steps of the water quality based approach.
Safety Factor Considerations
The need for safety factors 1n the water quality based approach can be
debated, but it 1s of paramount Importance to determine when safety factors
are being employed, either Intentionally or unintentionally. If a better
.defined and more consistent use of probabilistic Information were Incorporated TQM OP
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*1nto the water quality based approach. It would be much easier to determine
what safety factors are being employed and when the factors are or are not
additive.
, In the development of water quality criteria and standards, the
i
speed limit concept does not Incorporate any safety factors. The
Incorporation of safety factors for protection of aquatic life results
1n the selection of design conditions such as in wasteload allocation
for calculating permit limitations from water quality standards.
In the authors' opinion, much of the margin of safety provided by
these Intentional or unintentional safety factors has provided a "cushion"
for scientific uncertainty 1n developing water quality criteria for the
protection of aquatic life. We should be well aware of the consequences
to the environment when we begin to reduce these margins of safety In the
water quality based approach.
Conclusions
In summary, intensity, duration, and return period of a pollutant
.exposure event appear to be the cannon statistical parameters also used
in the other component steps of the water quality based approach. We
need to understand better how these common statistical parameters can be
utilized 1n the entire water quality based approach, so that consistency
can be obtained and maintained; that Is, consistency in criteria development,
use attainabllty determination, wasteload allocation, Impact analysis,
wastewater treatment permitting, treatment design and operation, and
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Specifically:
1. An approach for statistical defining receiving water pollutant
exposure 1s suggested consisting of the parameters of Intensity,
duration, and return period.
2. More emphasis should be placed on the consistent .~e of intensity,
duration, and return period, not only in defining acute and
chronic toxldty and bioaccumulation required In developing
the watv. • quality standards, but 1n the entire water quality
based approach.
3. It is well established that ambient concentrations fluctuate
due to many reasons, as indicated earlier. We must be able to
incorporate into our current aquatic toxicology research, either
directly or indirectly, the ability to handle fluctuating
exposures.
4. Most systems have more than one pollutant. We must begin to
give more attention to chemical Interactions of pollutants
and to multl-pollutant toxic effects. Currently we have a
very limited capability for handling multl-pollutant effects under
both laboratory and ambient conditions. This could be Improved
in a number of ways: use of simple models, multl-pollutant
models, or toxldty tests on complex effluents or instream toxldty
determinations.
5. The species sensitivity comparative analysis must be coupled to
use attainability—that 1s, what use are we trying to protect-
not only the specific communities, but for what Intensity, duration,
and return period.
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6. More rigorous comparative toxlcologlcal studies to determine
species sensitivity differences for a range of organisms and
compounds. These relationships should be examined probabilistically
and 1f possible be Incorporated Into the water quality standards
process.
The above Indicates that although we have come a long way In aquatic
toxicology research 1n support of the water quality based approach.
there are many areas in which we can become more effective, efficient, and
consistent 1n developing a scientific basis for the water quality based
approach. We need to focus our research efforts on these areas. We need to
determine what are the key parameters that have the greatest Impact on the
entire process and focus our research effort on answering the key unknowns
for these parameters, so that the effectiveness of the water quality based
approach can be maintained.
References
[1] Krenkel. P.A.. Journal of the Water Pollution Control Federation,
Vol. 51, No. 8. Aug. 1979, pp 2168-2183.
[2] Wisler, C.O. and Brater, E.F., Hydrology, 2nd ed.,
Wiley. New York. 1963. Chapter 4. pp. 57-101.
[3] Environmental Protection Agency, "Guidelines for Deriving
Numerical National Water Quality Criteria for the Protection
of Aquatic Life and Its Uses," Washington, D.C., Sept. 1982.
[4] DIToro, D.M., "Preliminary Probabilistic Interpretation of
Proposed Two Number Criteria," Progress Report Manhattan College,
Bronx, New York, April 1982.
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EPA Form 23'XM (4-80)
|HMI.VIUUS_V CIN CPA FOI4M 287)
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[51 Manclnl, J.L., "A Method for Calculating Effects, on Aquatic
Organisms, of Varying Concentrations," (Journal Artele,
In press, 1983).
[61 Environmental Protection Agency, "Ammonia Water Quality Criteria
for the Protection of Aquatic Life and Its Uses," Washington, D.C.,
Final Draft, Jari. 1983.
[7] Slouff, W., "Biological Effects of Chemical Pollutants In the
Aquatic Environment and Their Indication Value," Utrecht!ar.nt
Vol. 43, Ck Lage Zucluwe, Nether!and, Apr. 1983, p. 4926
[8] Environmental Protection Agency. "International Memorandum on
Predicted Performance of Less-Than-Secondary Treatment Processes."
Washington, D.C., July 1582.
[9] DIToro, D.M., "Exposure Assessment for Complex Effluents -
Principles and Possibilities," presented at the Hazard Assessment
for Complex Effluent Workshop, Cody, Wyoming, Aug. 1982
[10 1 Yake, W.E. and James, R.K., Journal of the Water Pollution
Control Federation. Vol. 55, No. 3, Mar. 1983, pp. 303-309.
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DIToro, D.M., "Probability Model of Stream Response to Runoff,"
presented at the American Geophysical University Spring Meeting
Symposium on Impacts of Urban Runoff on Receiving Waters,
Philadelphia, Pennsylvania, June 1982.
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(PREVIOUSLY CIN. CPA FORM 287)
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