SONGS coastal view

Project Background

  • Overview
  • SONGS Operations
  • Coastal Impacts
  • Mitigation Requirements
  • Project Evaluation
San Onofre Nuclear Generating Station

San Onofre Nuclear Generating Station.


In 1973, the California Coastal Zone Conservation Commission (CCZCC, now the California Coastal Commission or CCC) denied a permit for the construction of the San Onofre Nuclear Generating Station (SONGS) Units 2 and 3. In 1974, the CCC issued a coastal development permit to Southern California Edison company (SCE) for operation of SONGS Units 2 and 3.  A condition of the Permit required the establishment of a three-member independent Marine Review Committee (MRC) to direct comprehensive monitoring to assess potential impacts of the operation of Units 2 and 3 on the marine environment offshore from San Onofre, and mitigation of any adverse impacts.  As a result of the MRC monitoring studies the CCC added new conditions in 1991 and 1997 for mitigation of impacts identified in these monitoring studies.  These conditions require: (1) restoration of a southern California tidal wetland to mitigate for in-plant losses to fish, (2) construction of an artificial reef to replace kelp bed resources lost as a result of SONGS’ cooling water discharge, (3), provision of funds for a marine fish hatchery, and (4) installation of fish barrier devices to reduce fish impingement at the power plant.  The new conditions also require SCE to fund monitoring of the mitigation effort by scientists working under the direction of the Executive Director of the CCC.  The monitoring program for SONGS mitigation is being administered by scientists at the Marine Science Institute, University of California, Santa Barbara.  The three major areas of SONGS mitigation monitoring involve biological and physical monitoring of wetlands and kelp forests, and assessments of fish impingement at SONGS units 2 and 3.


Songs Operations

The San Onofre Nuclear Generating Station (SONGS) is located on the coast in north San Diego County. Construction of SONGS Units 2 and 3 was completed in 1981. Operation of Units 2 and 3 began in 1983 and 1984, respectively and each unit generated up to 1,100 MW of electric power. Both reactors were shut down in January 2012 due to excessive wear in the cooling tubes of the steam generators, and in June 2013 both units were permanently retired. Full retirement of the units prior to decommissioning is expected to take several years in accordance with customary practices; actual decommissioning will take many years until completion.

Schematic of SONGS operations
Schematic of SONGS single pass cooling water intake and discharge pipes.

The SONGS Unit 2 and 3 reactors are cooled by a single pass seawater system and have separate intake lines, each 18 feet in diameter, that are located in about 30 feet of water offshore of the power plant. The volume of water taken in each day by these two intake lines when Units 2 and 3 were fully operational was about 2.4 billion gallons, equivalent to a square mile 12 feet deep. The intake volume has been reduced by about 50% since the reactors were shut down in 2012.

The discharge pipe for Unit 2 terminates 8,500 feet offshore, while the discharge pipe for Unit 3 terminates 6,150 feet offshore. The last 2,500 feet of the discharge pipes for Units 2 and 3 consist of a multi-port diffuser that rapidly mixes the cooling water with the surrounding water. The diffusers for each unit contain 63 discharge ports angled offshore that increase the velocity of the discharge. Under normal operations the discharge water is approximately 19°F warmer than the intake water temperature. To cool the discharge water, the diffusers draw in ambient seawater at a rate about ten times the discharge flow and mix it with the discharge water. The surrounding water is swept up along with sediments and organisms and transported offshore at various distances, depending on the prevailing currents. All the impacts to the marine environment caused by the operation of SONGS Units 2 and 3 result from the seawater cooling system’s intakes and discharge.

Aerial view of kelp forest to diffuser
Aerial view showing the proximity of the San Onofre kelp forest to the diffuser
lines of SONGS Unit 2 and 3. 

Coastal Impacts

The Marine Review Committee (MRC) submitted its Final Report to the California Coastal Commission in August 1989. The report concluded that the operation of SONGS Units 2 and 3 was causing substantial adverse effects to the organisms in the San Onofre kelp bed, the fish stocks in the Southern California Bight, and to local midwater fish populations. These effects are summarized below.

San Onofre Kelp Bed:

  • The discharge plume from SONGS Units 2 and 3 results in a substantial reduction in the abundance and density of kelp plants.
  • The discharge plume results in a substantial reduction in the abundance and biomass of most of the kelp bed fish species that the MRC studied.
  • The discharge plume results in a substantial reduction in the abundance of large invertebrates inhabiting the kelp reef.

Fish stocks in the Southern California Bight:

  • Intake loss of immature fish is projected to cause substantial reductions in Bight–wide adult fish populations.

Local midwater fish populations:

  • Substantial reductions in local abundance of midwater fish populations were measured out to a distance of 3 km from SONGS.


Fish swimming in kelp forest

Mitigation Requirements

The California Coastal Commission (CCC) concluded that a compensatory mitigation program was the most cost-effective means of addressing the adverse impacts caused by the operation of SONGS Units 2 and 3. The CCC further conditioned SCE’s coastal development permit for SONGS to require the following mitigation elements:

  • Create or substantially restore at least 150 acres of Southern California wetlands, as compensatory mitigation for Bight-wide fish losses;
  • Construct an artificial reef that is large enough to sustain at least 150 acres of giant kelp and associated biota along with payment of $3.6 million to Ocean Resources Enhancement Hatchery Program (OREHP) to fund a fish hatchery program as compensatory mitigation for adverse impacts to the San Onofre Kelp community;
  • Installation of fish behavioral barrier devices in the sea water intake systems at SONGS as avoidance mitigation for losses of local midwater fish; and
  • Provide the funds necessary for technical oversight, monitoring, and performance assessment of the mitigation projects, to be done by independent contract scientists working under the direction of the Executive Director the CCC.

Evaluation of Mitigation Projects

Physical and biological standards are identified in the SONGS coastal development permit that specify how the wetland and reef mitigation projects should perform to be considered successful, and the timing and level of monitoring that is needed to evaluate their performance. The specific requirements for attaining compliance with these standards and the schedules for the different levels of monitoring that are required are discussed in various sections throughout the SONGS permit and have been summarized in a working compliance document.

Two different types of physical and biological performance standards are being used to judge the success of the wetland and reef mitigation projects: (1) fixed standards that are measured against criteria set in the SONGS Permit, and (2) relative standards that require certain features of the mitigation projects to be similar to natural reference sites that are removed from the adverse impacts of SONGS. Evaluating compliance with the relative performance standards requires the use of an objective approach to determine whether the values of the performance variables at the mitigation sites are similar to those at the reference sites. The elements considered in developing the method for assessing similarity between the mitigation and reference sites include: (1) Effect size or the amount of difference between the mitigation site and the reference sites that we wish to be able to detect, (2) Type I error which involves concluding that the mitigation site and the reference sites are different when in fact they are similar, (3) Type II error which amounts to concluding that the mitigation site and the reference sites are similar when in fact they are different, and (4) Sample size, which not only affects the cost of monitoring, but also the level of certainty in the estimated performance values for the mitigation sites and reference sites (and hence the effect size, and Type I and Type II errors). The methods for determining similarity are detailed in the SONGS Compliance document.