                                       

                                  MEMORANDUM


Tetra Tech, Inc.
400 Red Brook Blvd., Suite 200
Owings Mills, MD 21117
phone	410-356-8993
fax	410-356-9005


DATE: 		November 6, 2013 (revised May 14, 2014)

TO:			Lisa Biddle and Paul Shriner
	
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 (a 281 million gallons per day (mgd) intake, 625 mega-watt (MW) coal-fired power plant on Lake Ontario). Survival of impinged fish did not include evaluation of effects of passage through the 300 meter (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 returned 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. More recently, Bigbee et al (2010) evaluated fish for post-impingement survival for 48 hours at a newly installed modified rotary disk screen (screen opening of 6.1 mm[2]) at the North Omaha Station, a coal-fired generating station on the Missouri River in Omaha, Nebraska. Also, a recent study by Shaw (2010) demonstrated that post-impingement survival of laboratory-reared catfish could be successfully monitored for seven days at the Barry Steam Generation Plant on the lower Mobile River, Alabama.

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 i.e., 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 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.

Table 1 below combines data from multiple studies and shows percent survival by life stage across several ranges of holding times. The first column of percent survival in the table shows the percentage of fish that initially survived impingement plus holding times ranging from 0 to 8 hours; the second column shows the percent of fish that initially survived impingement plus holding times between 8 and 12 hours; etc. The intervals result from natural groupings in the available study data. In general, the data show differences in survival over time for different life stages. Additionally, for some life stages and for fragile species, lower holding times appear sufficient to capture the majority of latent mortality.

Table 1. Percent survival of fish across multiple studies and holding times.

Life Stage
                                       
                            Sum of Number Collected
                           Average Percent Survival

                                       
                          Initial + 0-8 hours holding
                         Initial + 8-12 hours holding
                         Initial + 24-36 hours holding
                         Initial + 60-84 hours holding
                        Initial + >84 hours holding
Adult
                                     2,555
                                     85.11
                                     75.18
                                     60.79
                                     44.13
                                       
Juvenile
                                     1,584
                                     99.35
                                     85.73
                                     78.51
                                       
                                       
Post Yolk Sac
                                      966
                                     95.63
                                     47.01
                                     44.12
                                       
                                       
YOY[A]
                                    31,474
                                     70.16
                                     63.05
                                     53.23
                                     38.87
                                     16.85
Grand Total
                                    36,579
                                     79.42
                                     69.81
                                     59.47
                                     40.24
                                     16.85
[A] YOY  -  young of the year

It appears that a holding time between 18 and 96 hours is warranted. An 18 hour minimum would be required to evaluate latent mortality of impingement samples dominated by life stages most likely to experience latent mortality, while a maximum holding time of 96 hours would be required to ensure that the majority of initial and latent mortality associated with impingement is observed, especially for more robust life stages and species. Holding times longer than 96 hours are not recommended due to the increased mortality resulting from stresses associated with holding (and/or feeding) conditions.

As a comparison, use of a 96 hour exposure period has 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.

Conclusions

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 18 to 96 hours is necessary to adequately evaluate latent mortality associated with impingement and passage through a fish return system, with shorter holding times likely being sufficient for fragile species, and longer holding times required for more robust species. Longer holding periods are possible, but are likely associated with substantially increased effort and scientific uncertainty. Further, a 96 hour holding time is in agreement with existing EPA toxicity testing guidance regarding short-term chronic impacts to fish. 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

Bigbee, DL, RG King, KM Dixon, DA Dixon, and ES Perry. 2010. Survival of fish impinged on a rotary disk screen. North American Journal of Fisheries Management 30: 1420-1433.

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.

Shaw, WW. 2010. Effects of traveling screen operation on impingement and survivability and latent health of impinged catfish and Barry Steam Generation Plant. Graduate Thesis, Auburn University.

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.
