1

MEMORANDUM

Tetra Tech, Inc.

400 Red Brook Blvd., Suite 200

Owings Mills, MD 21117

phone 410-356-8993

fax 410-356-9005

DATE: August 13, 2008

TO: Paul Shriner, EPA

FROM: Ann Roseberry Lincoln, Henry Latimer and Blaine Snyder

SUBJECT: Impingement and Entrainment Sampling Methods

We’ve prepared this memorandum in response to your request for information on 

“typical” impingement and entrainment sampling methods. There are no published 

“standard methods” for impingement and entrainment sampling; however, there are some 

fundamental study elements and parameters that are universally essential for the 

completion of impingement and entrainment surveys. The following method descriptions 

may serve as general guidance and assume that the reader has a general understanding of 

impingement/entrainment definitions and impacts.

IMPINGEMENT ABUNDANCE STUDIES

Study Design

Impingement studies, whether for assessment of abundance or survival, should be 

designed to evaluate annual, seasonal, and diel variability exhibited by potentially

impacted organisms, and hydrographic variability that may influence the numbers or 

organisms present near the intake. Young and Dey (2003) point out that “Life histories of 

many of the affected species are complex, involving only temporary occurrence near the 

power plants and/or long annual migrations, making them extremely difficult to sample 

for some parts of the life cycle. Invariably, all fish in a cohort do not follow the same life 

history pattern.” Unless historical data indicate otherwise, this typically requires a yearround weekly or bi-weekly sampling program. Historical data may show that fish and 
2

shellfish activity diminishes during part of the year and sampling effort may be less 

intense, or (for example) may show that a particular species of concern is present during a 

particular time and sampling effort should increase.

During sampling events, data collection should cover the entire daily and (where 

appropriate) tidal cycles. Typically, impingement samples are collected continuously for 

6 or 8 hours, and repeated to cover an entire 24-hour period. Stratifying collection in this 

manner allows an analysis of the diel variation exhibited by many aquatic organisms. The 

specific stratification scheme should be selected to fit the specific objectives of the study

and patterns exhibited by all of the species of concern. For example, Spicer et al. (2000) 

stratified sampling by “day” (8 AM until 8 PM) and “night” (8 PM until 8 AM) in studies 

conducted at the Comanche Peak Steam Electric Station in Texas, a facility located on a 

man-made reservoir with little impingement due to poor habitat quality near the intake. In 

contrast, Heimbuch et al. (2007) collected samples for four 6-hour collection periods for 

the Charles Poletti Power Project to investigate potential impacts to the New York-New 

Jersey estuary and Long Island Sound.

The study design must include an integrated quality assurance/quality control (QA/QC) 

management plan that presents sampling design and method specifics, and addresses data 

quality objectives, measurement performance criteria, analytical methods, and field and 

laboratory QC requirements.

Sampling Location

Samples should be collected from the screen wash discharge trough or fish return system. 

If fish return survivability studies also will be conducted, the sampling location should be 

as close to where fish enter the receiving water body as feasible, to ensure that the 

survivability accounts for all impacts that fish would incur if not collected for study. If 

survivability studies will not be conducted or fish are typically (by design) not returned to 

the water body, collection can occur at any accessible point in the screen wash system.

Samples should not be collected from the water body area ahead of the intake screens

(unless the intent is to obtain data describing the numbers of dead or moribund fish and 

shellfish that may subsequently be impinged).

Sampling Apparatus and Collection

Samples should be collected in nets or debris baskets fitted with suitably small mesh size 

(at a minimum equal to, and preferably slightly smaller than the intake screen mesh size). 

The collection device should be of sufficient size to filter the entire wash stream and to 

contain the volume of debris, detritus, and impinged organisms likely over the total 

collection duration. If the collection device cannot contain the entire 

debris/detritus/organism volume, the collection device must be checked and emptied at 

sufficient intervals to eliminate the potential for overflow or loss of impinged organisms 

from the sample. Moreover, if survival studies will be conducted, the impinged 

organisms should be transferred to a holding area regularly to avoid additional damage by 

compaction within the collection device. Live fish should be handled as little as possible 
3

to minimize stress, and should be held in conditions close to, if not identical to, their 

source waters. Water should be kept oxygenated and holding tanks should be large 

enough to hold the fish with minimal stress (Kelsch and Shields, 1996).

Prior to sampling, the intake screens and screen wash discharge trough or fish return 

sluiceway should be carefully washed to remove any previously impinged and trapped 

organisms. The collection device should be inspected for holes or other damage that 

would allow impinged organisms to escape, and then secured in place in the discharge 

stream. The time that sampling begins should be noted, along with initial conditions. If 

the volume of impinged organisms (and/or detritus) is very large, it may be necessary to 

empty the collection device during the sampling event to prevent overtopping of the 

device and loss of organisms. At the conclusion of the sampling event, the intake screens 

should again be carefully washed to ensure that all organisms impinged during the 

sampling time frame are collected. The end time and conditions for the sampling event 

should be recorded. 

The sample should be carefully transferred to an appropriate holding area for immediate 

taxonomic identification. Impinged organisms can then be identified (species-level 

identification is preferred; or the lowest feasible taxon for damaged organisms), 

measured, weighed, and their condition (live, dead, injured, etc.) recorded before being

returned to the receiving water body. In rare cases, sample sizes may be too large to 

measure and weigh each individual. Only in those cases, it may be acceptable to 

subsample; subsampling should be avoided under most circumstances. If subsampling 

may be required, a statistician should be consulted to ensure that the subsampling design

provides statistically useful results.

Data collected may vary depending upon the specific facility and study requirements, but 

should include some description of environmental conditions, hydrologic conditions,

intake system operational conditions, and organism-specific information. Example 

primary data elements or parameters include the following:

 Environmental data: water temperature, dissolved oxygen, pH, conductivity or

salinity (if applicable), and turbidity

 Hydrologic conditions: river flow or tide stage

 Intake system operational conditions: intake pump flow rate, screen spray wash

pressure and schedule, traveling screen change rate

 Organism data: species, lifestage, length, weight, and condition (live or dead)

Additional data might include: air temperature, cloud cover, wind speed, wind direction, 

current direction, precipitation levels or recent precipitation events, degree of icing, and 

general observations such as debris loading, debris type, and whether or not confounding 

conditions occur (such as unrelated local fish kills observed in the vicinity).

Representatives of each species and lifestage should be preserved (or photographed) as 

vouchers for QA evaluation purposes.
4

Measuring Collection Efficiency and Re-impingement Rates

Measurements of impingement abundance may be underestimated if the study’s 

collection efficiency is less than 100 percent, or overestimated if re-impingement occurs. 

Fortunately, both can be measured relatively easily with some forethought.

If collection efficiency is to be measured, it is important to obtain a suitable sample prior 

to the start of the study. The collection efficiency sample should represent the variety of 

species and organism lifestages anticipated in the impingement study. Ideally this control 

sample will be collected about the same time as the impingement study, in proximity to

the intake structure. The sample should be collected using collection gear types suitable 

for the capture of species likely to be impinged (e.g., trawl, seine, or electrofishing unit). 

Organisms should be identified, measured, counted, and marked with fin clips, dye, or 

another method that allows for rapidly identifying the organism as a collection efficiency 

organism. The marked individuals are then released immediately ahead of the intake 

screens under operating conditions identical to those anticipated during the impingement 

study. The sample is collected using the same apparatus and methods as for the 

impingement study, and the marked individuals identified to species, counted, and 

measured. The collection or recovery results are compared to the released records to 

determine the collection efficiency.

Reimpingement may occur if tidal or other currents transport fish from the discharge or 

fish return system back to the intake. Reimpingement rates can be determined by marking 

impinged individuals, releasing them into the discharge or fish return system, and 

determining the number that are collected a second time. 

ENTRAINMENT ABUNDANCE STUDIES

Study Design

Like impingement studies, entrainment studies for abundance or survival should be 

designed to evaluate annual, seasonal, and diel variability exhibited by potentially

entrained organisms, and hydrographic variability that may influence the numbers or 

organisms present near the intake. According to Snyder (1983), “Most larval fish 

investigations require sampling from twice a week to twice a month depending on 

objectives. Except for exploratory purposes, monthly or less-frequent sampling is of very

limited value—entire species or cohorts may be missed, even when sampling is 

somewhat more frequent.” On the other hand, regional fish spawning information 

suggests that entrainment may be very low during cold months in northern and inland 

areas in the United States (see attached figures). Therefore, unless historical data indicate 

otherwise, a well-designed entrainment abundance study requires a year-round weekly or 

bi-weekly sampling program, with increased sampling frequency when egg and larval 

entrainment increases.

During sampling events, data collection should cover the entire daily and (where 

appropriate) tidal cycles. Entrainment samples are collected over an entire 24-hour 

period, although collection of each individual sample may last as little as 5 to 10 minutes 
5

in the case of towed plankton nets or plankton trawls, or from 1 to 3 hours for pumped 

samples. For example, Heimbuch et al. (2007) collected at least 100 m3of water using a 

gasoline-powered trash pump during 1 hour of pumping, and repeated that effort for a 

total of four samples over 24 hours at the Charles Poletti Power Project in New York. On 

the other hand, Melton and Serviss (2000) conducted paired-net plankton tows lasting 30 

minutes each in the discharge canal of the Florida Anclote Power Plant, which exhibits 

low flow conditions and required lengthy tows to ensure adequate sample collection.

The study design must include an integrated quality assurance/quality control (QA/QC) 

management plan that presents sampling design and method specifics, and addresses data 

quality objectives, measurement performance criteria, analytical methods, and field and 

laboratory QC requirements.

Sampling Location

Samples should be collected from the cooling water intake system, downstream of the 

intake screens, if possible. Sampling directly after the intake screens ensures that the 

sample contains organisms actually entrained and that the organisms have suffered the 

least possible physical damage, thereby facilitating proper identification. However, if 

entrainment survivability studies also will be conducted, the sampling location should be 

as close to where the cooling water discharges as is feasible, to ensure that the 

survivability accounts for all impacts that entrained organisms would incur if not 

collected for study. Samples should not be collected from the water body area ahead of 

the intake screens (unless the intent is to obtain data describing the numbers of dead 

fish/shellfish eggs and larvae that may subsequently be entrained). For all entrainment 

sampling it is important to collect samples from all depths (e.g., by oblique net tows or 

pumping from multiple depth intervals), as ichthyoplankton frequently exhibit patchy

distribution vertically (as well as horizontally and temporally).

Sampling Apparatus and Collection

Samples may be collected directly from the intake screen discharge stream, or water 

(taken downstream from the intake screens) may be diverted and pumped to the sampling 

location. In either case, samples should be collected in plankton nets with suitably small 

mesh size and sufficient length to adequately filter the water and to collect the entire 

sample. Care should be taken to minimize damage to the sample due to extrusion or 

overloading, resulting in compaction of the collected organisms. Kelso and Rutherford 

(1996) recommend a minimum net area to mouth area ratio of at least 3:1 and preferably 

5:1 to minimize net clogging and extrusion. Typical mesh sizes range from 333 to 505 

μm. If the sample is collected directly from the discharge stream, the net should be fitted 

with a flow meter in its mouth so that the volume of water filtered can be calculated. The 

net may be towed within the stream or it may be fixed to a suitable physical structure, if 

available. If a pump is used instead, the piping leading from the pump to the net should 

be fitted with an in-line flow meter, and the pump should be selected to minimize damage 

to entrained organisms (Kelso and Rutherford 1996). Bowles and Merriner (1978) present 

a thorough review of entrainment sampling gear.
6

Prior to sampling, the collection nets should be carefully washed to remove any attached 

organisms, inspected for holes or other damage, and a removable collection cup should 

be attached to the terminal end of the net. Flow meters should to be set to zero or their 

initial readings recorded. If a pump is used, the pump should be flushed thoroughly with 

clean water prior to sampling, to ensure that no ichthyoplankton remain within the 

chamber. The pump is then fitted into place with the inlet withdrawing from the intake 

screen discharge stream. If the pump cannot be flushed with clean water, then it should 

be run for several minutes prior to starting collection to ensure that the sample collected 

represents the current conditions. The net is then positioned to collect the pumped water 

sample. To minimize any additional damage to the entrained organisms, the net is placed 

vertically into a container of water with an overflow outlet. The sample flow is directed 

into the center of the net. During the sampling period, the pump inlet should be 

raised/lowered periodically to obtain a depth-integrated sample. The time and operating 

conditions are recorded when sample collection begins and ends. 

If a towed plankton net is used, the clean net (with flow meter attached) is deployed in 

the intake screen discharge stream and towed against the current for a known time (e.g., 

from 5 to 10 minutes at 1 to 2 meters per second). Towing speed should be fast enough to 

minimize larval avoidance, but slow enough to ensure adequate filtering. Multiple depths 

should be sampled, either by deploying multiple nets at specified depths or by employing 

stepped oblique tows.

At the completion of the pump or net tow sampling event, the time and flow meter 

readings are recorded (to allow calculation of sampling duration and sample volume) and 

the collection net washed so that the entire sample flows into the removable collection 

cup at the end of the net. The collection cup is removed and all contents are placed into a 

sample jar (or the sample transferred). Samples are fixed or preserved by adding 5-10% 

formalin.

Preserved entrainment samples are sent for laboratory identification by an 

ichthyoplankton taxonomy specialist. Entrained organisms are picked from detritus in the 

laboratory using side lighting and a high-contrast background for better visibility, and are 

stored in 5% buffered formalin. Ichthyoplankton are identified with the aid of a dissecting 

or low-power microscope. All fish and shellfish eggs and larvae are identified to the 

lowest feasible taxon (species preferred).

Data collected may vary depending upon the specific facility and study requirements, but 

should include some description of environmental conditions, hydrologic conditions, 

intake system operational conditions, and organism-specific information. Example 

primary data elements or parameters include the following:

 Environmental data: water temperature, dissolved oxygen, pH, conductivity, and 

turbidity

 Hydrologic conditions: river flow or tide stage

 Intake system operational conditions: intake pump flow rate
7

 Organism data: species, life stage, length

Additional data might include: air temperature, cloud cover, wind speed, wind direction, 

current direction, precipitation levels or recent precipitation events, degree of icing, and 

general observations including any confounding conditions. 

Measuring Collection Efficiency and Re-entrainment

Entrainment rates can be underestimated if collection efficiency is less than 100%, and 

overestimated if re-entrainment occurs. It is possible to measure both collection 

efficiency and re-entrainment if suitable fish eggs and larvae are available for the study. 

The ichthyoplankton sample used to measure these potential sources of error can be 

collected from source waters and should be stained with a suitable biological stain (e.g., 

rose Bengal) prior to the study.

If collection efficiency is to be measured, a stained and counted ichthyoplankton sample 

is released immediately in advance of the intake screens, such that 100% entrainment will 

occur. Entrainment sample collection proceeds as described above and the number of 

collected organisms compared to the number released to determine collection efficiency.

To measure re-entrainment, stained and counted ichthyoplankton are released into the 

discharge stream near its terminus so that the sample discharges to the receiving 

waterbody. Entrainment sample collection proceeds as described above and the number 

collected subsequently is the number re-entrained.

REFERENCES

Bowles, R.R. and J.V. Merriner. 1978. Evaluation of Ichthyoplankton Sampling Gear 

Used in Power Plant Entrainment Studies. Pages 33–43 in L.D. Jensen, editor. Fourth 

National Workshop on Entrainment and Impingement, 5 December 1977, Chicago, 

Illinois. Sponsored by Ecological Analysts, Inc., Melville, NY. Published by EA 

Communications, Melville, NY.

Heimbuch, D.G., D.J. Dunning, Q.E. Ross, and A.F. Blumberg. 2007. Assessing 

Potential Effects of the New York–New Jersey Harbor Estuary and Long Island Sound.

Transactions of the American Fisheries Society 136: 492–508.

Kelsch, S.W. and B. Shields. 1996. Care and Handling of Sampled Organisms. Pages 

121–144 in B.R. Murphy and D.W. Willis, editors. Fisheries Techniques, 2nd Edition. 

American Fisheries Society, Bethesda, MD.

Kelso, W.E. and D.A. Rutherford. 1996. Collection, Preservation, and Identification of 

Fish Eggs and Larvae. Pages 255–302 in B.R. Murphy and D.W. Willis, editors. Fisheries 

Techniques, 2nd Edition. American Fisheries Society, Bethesda, MD.

Melton, B.R. and G.M. Serviss. 2000. Florida Power Corporation-Anclote Power Plant 

Entrainment Survival of Zooplankton. Environmental Science and Policy 3: S233–S248.
8

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.

Spicer G., T. O’Shea, and G. Piehler. 2000. Entrainment, impingement and BTA 

evaluation for an intake located on a cooling water reservoir in the southwest.

Environmental Science and Policy 3: S323–S331.

Young, J.R. and W.P. Dey. 2003. Uncertainty and Conservatism in Assessing 

Environmental Impact under §316(b): Lessons from the Hudson River Case. Pages 30–39 

in D.A. Dixon, J.A. Veil, and J. Wisniewski, editors. Defining and Assessing Adverse 

Environmental Impact from Power Plant Impingement and Entrainment of Aquatic 

Organisms. Swets & Zeitlinger B.V., Lisse, The Netherlands.