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EPA’s Proposed Rule for Cooling Water Intake Structures at Existing and Phase I 

Facilities; 316(b) Impingement Requirements

RE: The need to set a de minimis standard

Summary: 

Pacific Gas & Electric Company (PG&E) urges EPA to consider including a de minimis

standard for impingement, so that facilities that substantially minimize impingement with 

existing technologies are provided an exemption. Large affected stakeholder 

organizations, including the Edison Electric Institute and the Utility Water Act Group, 

have expressed support for the concept of a de minimis exemption as well.

PG&E believes such a determination should be made based on site-specific factors and 

based on criteria that are consistent and verifiable. The different water bodies, species of 

concern, and seasonal dynamics affect impingement such that an appropriate de minimis 

exemption for one facility may be too restrictive at another and too large at another 

facility. Nevertheless, we would urge EPA to provide guidance to permitting authorities 

to ensure appropriate de minimis standards are developed. The Diablo Canyon Power 

Plant provides a sound example of a facility that can be instructive to the design of a 

metric to apply on a site-specific basis, or if EPA determines appropriate, could also 

apply as the basis for a national standard. 

Background: 

The Diablo Canyon Power Plant (DCPP) is located on the Pacific coast in San Luis 

Obispo County, California. The plant has a very high capacity factor and an intake 

design volume of 2,500 million gallons per day (mgd). The plant also demonstrates the 

lowest impingement biomass per million gallons circulated of all of the coastal plants in 

California that use once-through cooling. 

There are 14 coastal power plants in California with 17 separate structures that have some 

level of fish impingement as a result of intake operations. The overall data from 

California demonstrates the wide variation in levels of impingement at facilities - from 

over 32,000 pounds per year to as little as 77 pounds per year, as displayed in the table 

below (See Attachment 1, Table E1-1 in August 18, 2011 PG&E comments to EPA):
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This variation in data from a single state clearly demonstrates the importance of sitespecific impingement assessments. 

A facility such as DCPP, with average daily impingement of a dozen predominately

juvenile fish, makes a strong case for a de minimis exemption. The intake at Diablo 

Canyon was designed to minimize impingement and the impingement numbers 

demonstrate its success – the plant’s annual impingement losses are approximately 710 

pounds per year (2 pounds or 12 fish per day), while circulating approximately 2.5 billion 

gallons of water per day. The open ocean environment means that most species are 

adapted to large changes in tides, as well as significant swells. Additionally, the plant’s 

low through-screen velocity (average centerline between 0.78 and 1.02 fps) allows most 

fish to swim in and out of the bar racks. 

Establishing the de miminis standard for annual losses should be based on estimates of 

impingement from power plants within a region withdrawing water from similar 

environments. Such a baseline can also help address any concerns for sensitive fish 

species. Further, developing additional criteria that account for the large differences in 

water intake is also appropriate. If EPA determined it appropriate to finalize a national 

de minimis standard, PG&E supports the Clean Energy Group’s 316(b) Initiative 
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comments filed related to the proposed rule and suggested an annual impingement level 

of 2,000 pounds. Using such a number is reasonable when one considers the wide range 

of impingement levels across the country. 

Conclusion:

The low impingement losses measured at DCPP and the characteristics of fishes collected 

during associated studies demonstrate that the facility’s current design achieves a level of 

impingement that should qualify for a de minimis exemption. EPA should consider a

facility-specific de minimis exemption for the impingement mortality standard that is

based on a benchmark average impingement rate (biomass per volume of water 

circulated) and/or a general benchmark for annual total impingement losses of 2,000 

pounds. We would be happy to discuss the design of such a benchmark further with EPA 

staff.