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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 
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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 
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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.
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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 postimpingement 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 
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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.