1 

## MEMORANDUM

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

24, 2009 TO : Paul Shriner and Jan Matuszko, EPA FROM : Ann Roseberry

L incoln, Henry

L atimer, Blaine Snyder , and Kelly Meadows, Tetra Tech SUBJECT: Negative entrainment reductions EPA requested that Tetra Tech review two data points in the entrainment data set that appear to be counterintuitive , implying that an intake technology increased entrainment rates over a control scenario. Tetra Tech reviewed Chapter 11 of the draft Technical Development Document (TDD) from December 2008; specifically Table 11

- 8 , titled “Total Organisms: Percent Reduction of Entrainment by Slot Width and Slot Velocity

. ” There were two studies cited in the table that showed negative percent entrainment reductions

. Generally speaking, both study authors conclude that some or all of the results were not statistically significant or could not be used to draw conclusions. S pecific dis cussion for each study is presented below. Field Evaluation of Wedgewire Screens for Protecting Early Life Stage of Fish at Cooling Water Intake Structures: Chesapeake Bay Studies (EPRI 2006) This study showed a

- 7.6% reduction of total organisms for a 1 mm slot width and 0.30 m/s slot velocity. Tetra Tech believe s that Table 11

- 8 combines entrainment results from this study that were presented separately for fish larvae (which were statistically significant) with those for bay anchovy eggs (which were not statistically significant). Specifically, this study was conducted by EPRI , utilizing a test barge containing two pumps and three screens. One screen (9.5 mm square mesh) was a control screen, while the other two (0.5 mm and 1 mm) were cylindrical wedg ewire test screens. The pumps operated at two different intake velocities (0.15 m/s2

or 0.30 m/s) during testing. Water and entrained organisms drawn through the test and control screens were discharged into separate 0.75 m diameter, 335 micron mesh plank ton nets. Entrained organisms for each trial were collected, washed into a 1

- L sample jar, and preserved for analysis. The test in question was the one comprising the 1 mm screen with 0.30 m/s slot velocity (compared to results from the control screen). The authors report the results for entrained larvae (multiple species) separately from entrained bay anchovy eggs, which were the overwhelming majority of entrained eggs (148,181 fish eggs total, of which all but seven were bay anchovy eggs). They report ed a mean number of larvae entrained over 20 trials (106.3 larvae per 100 m3 entrained with the control screen, and only 49.4 larvae per 100 m3 entrained using the test screen) for a statistically significant reduction of 53.1%. However, for bay anchovy e ggs, the authors reported a mean entrainment (over 20 trials) of 271.7 eggs per 100 m3 (control) versus 356.9 eggs per 100 m3 (test) for a “reduction” of

- 31.3%. The authors specifically state that this difference is not statistically significant (p>0.05) : “The 1.0 mm screen did not provide a significant reduction in egg density compared to control samples for either slot velocity.” Brown, M.E. 1979 (as cited in Fish Protection at Cooling Water Intake Structures: A Technical Reference Manual . EPRI, Palo A lto, CA: 2007. 1014934) This study (regarding sampling conducted in the intake canal at Oyster Creek) showed a

- 24.7% reduction of total organisms for a 2 mm slot width and 0.152 m/s slot velocity. While Tetra Tech was not able to locate the original study results, a summary of the original field study was provided in EPRI (2007). This document specifically states “Organisms were not entrained in large enough numbers to draw any significant conclusions.” The citation for this study refers to an abstract published in “Passive Intake Screen Systems Workshop, Chicago, IL Dec 4

- 5, 1979 (pp. 17

- 38)” but no entrainment data are presented in that document, and the focus of the talk appeared to be on screen fouling. The observation of small, statistically insignificant differences in ichthyoplankton densities at various locations within a sampling area is common. Therefore, it is not surprising that some studies identify slightly higher, but statistically similar, densities of ichthyoplankton in water that has passed through different screens designed to limit entrainment. Such a result should not be interpreted to mean that ichthyoplankton densities are enhanced by passage through the screen. Observation uncertainty exists as an inherent characteristic of any measurement, even in the best

- designed scientific studies. This uncertainty results from a combination of causes including sampling error (the sample is an imperfect estimate of the true population because of natural variability) and measurement error (t he sample is obtained using an imperfect measuring device). Smaller sample sizes typically result in larger uncertainties. Sampling error in ichthyoplankton studies results from spatial and temporal variability in ichthyoplankton densities. Snyder (1983 ) presents uneven horizontal ichthyoplankton distribution due to either patchiness or mass movement as a problem associated with surveying fish eggs and larvae. He identifies water temperature, sample size, predation, currents, and meteorological conditio ns as factors influencing horizontal patchiness. Measurement error results from a number of sources including extrusion from plankton nets, damage to eggs and larvae from passing through pumps, and passive avoidance. Other factors may contribute to measurement error, as well, depending upon the methods employed. Conclusion Based on Tetra Tech’s assessment of the available information and our understanding of uncertainty and ichthyoplankton variability, we agree with the authors of these two studies. Sp ecifically, the reported differences were either not statistically significant (Chesapeake Bay Study) or based on too small a number of entrained organisms to be meaningful (Oyster Creek Study). As such, we do not believe that the either result should be used to quantify entrainment reductions and the results from these studies should not be included in the analysis.3

Cited Literature Brown. M.E. 1979. Preliminary Engineering and Environmental Evaluation of Fine

- Mesh Profile Wire as Powerplant Intake Screening. Pages 17

- 38. In: Passive Intake Screen Workshop, Chicago, Illinois, December 1979. Field Evaluation of Wedgewire Screens for Protecting Early Life Stage of Fish at Cooling Water Intake Structures: Chesapeake Bay Studies

. EPRI , Palo Alto, CA: 2 006 . 1012542. Fish Protection at Cooling Water Intake Structures: A Technical Reference Manual . EPRI, Palo Alto, CA: 2007. 1014934 Snyder, D.E. 1983. Fish Eggs and Larvae. Pages 165

– 197 in L.A. Nielsen and D.L. Johnson, editors. Fisheries Techniques. Ame rican Fisheries Society, Bethesda, MD.