MEMORANDUM

Tetra Tech, Inc.

400 Red Brook Blvd., Suite 200

Owings Mills, MD 21117

phone	410-356-8993

fax	410-356-9005

DATE: 		August 21, 2008

TO:			Paul Shriner, EPA

	

FROM:		Henry Latimer, Blaine Snyder, and Ann Roseberry Lincoln 

SUBJECT:		Holding times to evaluate post-impingement mortality

Clean Water Act §316(b) requires an examination of impacts on the
aquatic community from impingement and entrainment related to withdrawal
of cooling water.  As part of evaluating the impact that a cooling water
intake structure (CWIS) has on the aquatic community, the number and
types of organisms (fish and shellfish) impinged and entrained by that
structure are often quantified.  Depending on the types of screens,
screen wash systems, fish return systems, and operations at the facility
in question it may also be useful to quantify impingement survival
rates.  Because impingement survival rates are influenced by the
physical design and operations of the CWIS as well as by the species and
conditions within the source water (e.g., high vs. low debris load),
impingement survival is typically facility-specific.  Therefore, the
measurement of impingement survival rates can be useful in developing a
more refined understanding of mortality associated with impingement at a
facility.  This memorandum addresses suggested holding times to use in
evaluating post-impingement latent mortality of fish and shellfish.  

McLaren and Tuttle (2000) held fish for 96 hours post-impingement to
evaluate latent mortality associated with impingement on 1 mm
Ristroph-type traveling screens at the Somerset Station (281 MGD intake,
625 MW coal-fired plant on Lake Ontario).  Survival of impinged fish did
not include evaluation of effects of passage through the 300 m fish
return pipe. Similarly, Thompson (2000) reported that fish and shellfish
were held for 96 hours post-impingement to evaluate mortality associated
with impingement and return to the source water at the Carolina Power &
Light Company’s Brunswick Steam Electric Plant in North Carolina. 
Ronafalvy et al. (2000) held fish for 48 hours post-impingement to
evaluate mortality at the Salem Generating Station in New Jersey.

A holding time of 24 hours post-impingement was used by Black (2007) to
evaluate the mortality of various fish species in a laboratory setting. 
This study was also different from some of the facility studies in that
the laboratory setting allowed for fish not subject to impingement to be
used as controls to quantify mortality associated with holding
conditions.  Laboratory studies such as this one fail to completely
replicate conditions in the field such as survival under varying
temperatures and additional physical damage related to debris loading on
the traveling screen.  Further, this study did not consider a
potentially large source of stress on impinged organisms; passage
through a fish-return system.  Although laboratory studies are necessary
and appropriate for use in evaluating the ability of various
technologies to reduce stresses to fish and shellfish related to
impingement, survival data from studies such as this should only be used
to evaluate the relative magnitude of stress related to each technology
for a given species.  Because of the site-specific nature of impingement
mortality and additional stresses to which fish and shellfish are
exposed on site (e.g., temperature, debris, etc.), laboratory-derived
survival data should not be used to estimate impingement survival at a
given facility.

A single review of post-impingement mortality studies is available in
the literature.  EPRI (2003) evaluated 65 different impingement survival
studies conducted at 29 different power generation facilities in North
America.  This review determined that these studies used
post-impingement holding times ranging from less than 24 to 108 hours
(with one study that held organisms for over 200 hours).  The plurality
of these studies (approximately 30 of 65) held fish for 96 hours to
evaluate post-impingement mortality (approximate values are used here as
data were presented graphically).  It also appears that 11 studies only
evaluated instantaneous mortality and 10 studies evaluated mortality at
48 hours post-impingement.  The remaining studies held fish for an
intermediate period between 24 and 84 hours post-impingement.  

Using data from two different studies that collected mortality data
(corrected using control organisms); EPRI (2003) also evaluated the
percentage of latent mortality observed at various time intervals.  This
evaluation indicated that most latent mortality effects can be observed
within 96 hours post-impingement.  Further, it appears that for some
species only approximately 60% of latent mortality is accounted for at
48 hours post-impingement.  For more fragile species, the majority of
mortality associated with impingement is likely to occur within the
first 24 to 48 hours.  Thus, it appears that use of a minimum holding
time of 48 hours is necessary to ensure that the majority of initial and
latent mortality associated with impingement is observed.

The primary concern related to longer holding periods is that holding
conditions may be sufficiently stressful to cause additional mortality
not related to impingement and passage through the fish-return system. 
Inappropriate crowding of organisms and/or insufficient aeration and
renewal of holding water can lead to elevated ammonia or depressed
dissolved oxygen concentrations, the stress of which increases in
relation to holding time.  Predation and aggressive interactions between
individuals can also contribute to mortality if care is not taken to
separate “incompatible” species.  Although mortality related to such
aggressive interactions would be expected regardless of the holding
period, mortality related to such interactions would likely increase
with holding time.  Therefore, regardless of the holding time selected,
it is critical that conditions in which organisms are held to evaluate
post-impingement mortality do not artificially enhance that mortality.

Use of a 96 hour exposure period has also been recommended to account
for latent mortality in acute toxicity tests with fish (e.g., Ruesink
and Smith, 1975).  This 96 hour exposure period/holding time is
consistent with the implementation of EPA’s acute whole effluent
toxicity (WET) test methods for fish (EPA 2002).  Acute toxicity testing
methodologies for fish used in NPDES permit compliance WET testing
typically require an exposure period of 96 hours, although the
methodology allows for use of exposure periods between 24 and 96 hours
and exposures of 48 hours are sometimes used.  Further, some States
(e.g., California) require the use of 96 hour WET testing in evaluation
of potentially toxic discharges (CRWQCB – San Diego, 1995).  The
evaluation of post-impingement survival is slightly different than the
evaluation of mortality related to exposure to toxic materials as the
exposure period for WET testing is integral to the amount of stress to
which the organisms are exposed (e.g., exposure to toxins continues for
the entire test period) whereas the stress of impingement and transit
through a fish return system is discrete and occurs prior to holding. 
However, the goals of WET testing and post-impingement survival studies
are similar enough that the holding times used should be similar.

Some organisms are killed outright by impingement and passage through
the fish-return system, while others are injured.  Holding organisms for
a set period following impingement and, ideally, passage through the
fish-return system allows quantification of any latent mortality
associated with these injuries.  Various holding times have been
reported in the scientific literature and in industry reports.  Based on
reviews of these studies we conclude that a minimum post-impingement
holding time of 48 hours is necessary to adequately evaluate latent
mortality associated with impingement and passage through a fish return
system.  Caution should be taken to ensure that the conditions in the
holding tank systems do not cause additional stress (e.g., predation,
aggressive interactions, low dissolved oxygen, high ammonia, etc.) which
could relate to additional mortality that could be erroneously
interpreted as the result of impingement.  If possible, control tanks
should be included with similar organisms that have not been subject to
impingement, and that control should be used to evaluate baseline
mortality in the holding tanks.  The control data should be used to
normalize the observed mortality of fish/shellfish that have been
subject to impingement and passage through the fish return.

Literature Cited

Black, J.L.  2007.  Laboratory evaluation of modified traveling screens
for protecting fish at cooling water intakes.  Graduate Thesis,
University of Massachusetts Amherst.

California Regional Water Quality Control Board (CRWQCB), San Diego
Region.  1995.  Water quality control plan for the San Diego Basin (9). 
CRWQCB, San Diego Region, San Diego, CA.

Electric Power Research Institute (EPRI).  2003.  Evaluating the effects
of power plan operations on aquatic communities: Summary of impingement
survival studies.  EPRI, Palo Alto, CA.  Report 1007821. 

McLaren, J.B. and L.R. Tuttle, Jr.  2000.  Fish survival on fine mesh
traveling screens.  Environmental Science & Policy 3 (Supplement 1):
369-376.

Ronafalvy, J.P., R.R. Cheesman, and W.M. Matejek.  2000.  Circulating
water traveling screen modifications to improve impinged fish survival
and debris handling at Salem Generating Station.  Environmental Science
& Policy 3(Supplement 1): 377-382.

Ruesink, R.G. and L.L. Smith.  1975.  The relationship of the 96-hour
LC50 to the lethal threshold concentration of hexavalent chromium,
phenol, and sodium pentachlorophenate for fathead minnows (Pimephales
promelas Rafinesque).  Transactions of the American Fisheries Society:
567-570.

Thompson, T.  2000.  Intake modifications to reduce entrainment and
impingement at Carolina Power & Light Company’s Brunswick Steam
Electric Plant, Southport, North Carolina. Environmental Science &
Policy 3(Supplement 1): 417-424.

U.S. Environmental Protection Agency.  2002.  Methods for measuring the
acute toxicity of effluents and receiving waters to freshwater and
marine organisms, fifth edition, October 2002.  EPA-821-R-02-012. 
Office of Water, Washington, DC.

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