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 Lincoln, Henry Latimer, 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.  Specific
discussion 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 believes 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 wedgewire test screens.  The pumps operated at two different
intake velocities (0.15 m/s 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 plankton 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 reported 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 eggs, 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 Alto, 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 (the 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 conditions 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.  Specifically, 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. 

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: 2006. 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. American Fisheries
Society, Bethesda, MD.

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