Diablo Canyon nuclear power plant’s impact on CA marine life

Take a dip into the Pacific Ocean along most of California’s Central Coast, and the cold water will send you running for a wetsuit.

But there’s one spot where the water isn’t so teeth-chattering and instead feels much like a lukewarm bath: Diablo Cove, the oblong 40-acre inlet that sits at the base of PG&E’s Diablo Canyon nuclear power plant near Avila Beach.

There, the ocean waters are heated by the power plant’s cooling system, which sucks in and spits out billions of gallons of seawater every day.

Water in the cove directly adjacent to the discharge structure hovers at an abnormal 70 to 80 degrees — notably warmer than the 50 to 60 degrees typical here.

That has created a unique phenomenon in Diablo Cove, significantly altering the makeup of its marine ecosystem and turning the area into a case study of how cold-water ocean species are impacted by warmer waters.

The operation of a high-energy nuclear power plant has, in effect, transformed a Central Coast ocean environment into a habitat more commonly found in Southern California.

Multiple scientists have spent lifetimes studying the cove and documenting how the native cold-water fish and algae largely died off and were replaced by more-temperate species that can tolerate the balmy ocean.

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Steve Pengilley, left, and John Steinbeck, scientists at Tenera Environmental, demonstrate how population lists and grids can be used to conduct sea life counts in Diablo Cove, on Feb. 25, 2022. They are at the Seal Haul Out tidepool, once part of the studies around Diablo Canyon nuclear power plant but no longer central to research becauase impacts were not significant there. David Middlecamp dmiddlecamp@thetribunenews.com

Such shifts in the cove’s underwater ecosystem have sparked controversy since the nuclear power plant began operating in 1985. Just last May, a $5.9 million agreement between PG&E and the regional water board settled the “differences” between the two regarding alleged violations of state water quality laws and the power plant’s permit.

After submitting requests to PG&E in the fall, The Tribune waited nearly six months to tour Diablo Cove and interview scientists about the biological changes in the cove. We analyzed hundreds of pages of reports published over a span of more than 50 years to gain a comprehensive understanding of how marine life has been impacted by the warm-water discharge.

They show that while fears of radiation leaks, nuclear meltdowns and earthquake catastrophes may capture the public’s imagination, save for the indefinite need to store radioactive spent fuel at the plant, the warm-water discharge may be the largest environmental impact of the nuclear power plant.

“It is probably the most rigorously studied area of the country,” said John Steinbeck, a principal scientist at Tenera Environmental who has been contracted by PG&E to monitor the cove since 1978.

“A lot of people thought that Diablo Cove was going to turn into a biological desert,” Steinbeck continued. “But that has never been the case.”

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A leopard shark swims alongside other fish in Diablo Cove, offshore of Diablo Canyon nuclear power plant. Courtesy of PG&E

Cooling system sends a plume of warm water out into the Pacific

To grasp the full impact of the plant’s warm-water discharge, it’s first important to understand the scope of the plume.

In a view from the ocean aboard the plant’s research boat, the cove looks like any other section of the coastline — save for the massive nuclear power plant sitting on the bluff — with waves crashing against the shore and sea lions lounging on a favorite haul-out at Lion Rock.

In the middle of the cove below the massive brown-colored turbine building, a cascade of warm water creates a frothy, highly turbulent area — like one seen at the base of a massive waterfall — that extends several hundred feet offshore as it exits the power plant.

The effect is the end result of nearly 2.5 billion gallons of water a day getting sucked through massive intake tubes in the power plant marina, located on the south end of the property. That cold ocean water is flushed through the plant and used to cool hot steam — which is generated by another system of water that has been heated by the nuclear reactors.

The steam is used to spin massive turbines that consistently produce about 18,000 gigawatt-hours of electricity, enough to provide power for more than 3 million households. Each loop of water is isolated, and the radioactive water directly heated by the nuclear reactors is in a closed system, according to PG&E.

Once the ocean water has passed through the loop and condensed the power-generating hot steam back to water, the seawater is discharged from the plant, travels over what was once a low-tide rocky beach and flows into Diablo Cove at no more than 22 degrees warmer than its original temperature, according to PG&E’s permit for the plant.

Typically, the water enters the cove at around 19 degrees warmer than the ambient ocean temperatures, said Trevor Rebel, the plant’s decommissioning environmental supervisor.

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Warm water pours out from the discharge structure of Diablo Canyon nuclear power plant as seen on Feb. 25, 2022. Each day, the plant releases 2.5 billion gallons of warm ocean water that is used to cool the plant. The water has changed the ecosystem in Diablo Cove to more resemble habitats in Southern California. David Middlecamp dmiddlecamp@thetribunenews.com

Once returned to the ocean, the plume of water dissipates into the cove and beyond the shoreline. Where it travels depends on the tides and currents, and it can range in size from about 500 to 2,000 acres on the surface, according to a 1986 analysis, the most recent available.

As the warm water dissipates, it is more buoyant than the naturally colder ocean water, so it floats on the surface.

“Sometimes when we’re diving, we can literally see the warm water sitting on top of the cold water, just an inch or so of it,” said Steve Pengilley, a scientist with Tenera Environmental who has studied the environmental impacts of the cold water at Diablo Cove since around 1996.

Pengilley said the warm water appears to shimmer on top of the colder water, similar to what one can see when heatwaves rise of hot tarmac.

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A thermal image shows how the warm-water plume disperses from Diablo Canyon nuclear power plant. The image was included in a PG&E report from a study made June 12, 1986. The warm water rises to the surface and thins out the farther it gets from the plant. PG&E

The firehose of warm-water discharge hits the bottom of the cove’s floor for about 22 to 33 acres of the cove’s 40 acres. There, the water is typically about 3.6 degrees above the ocean’s typical temperature in the area, according to a 1998 thermal effects monitoring report by Tenera scientists, one of many since 1976 that closely documents the cove’s conditions.

Intertidal zones within Diablo Cove — the shallower areas revealed at low tide — are usually exposed to water temperatures about 5 to 11 degrees warmer than normal, according to the same 1998 report.

Subtidal zones of the cove up to about 15 feet below the surface generally see water about 4 to 13 degrees warmer than the natural ocean water, the report says.

So what has cooling water discharge from the nuclear power plant done to the local ecosystem within Diablo Cove?

Nuclear power plant had immediate effects on marine life

To an extent, the marine life changes at Diablo Cove were expected to happen, and PG&E acknowledged so from the start.

A 1966 agreement between the state and power company noted that should “adverse effects accrue to aquatic life … due to plant construction or operation, Pacific (PG&E) will provide reasonable mitigation for losses incurred.”

A few years after that agreement was signed by PG&E president John Bonner and California Resources Agency administrator Hugo Fisher, scientists began their first biological assessments of Diablo Cove. These studies gathered baseline data and were vital to understanding the ecology of the cove before the millions of gallons of warm water were poured into it.

Impacts on the ecosystem near the plant began well before the nuclear reactors were turned on, and they have been dramatic, reports published in the U.S. Nuclear Regulatory Commission’s public document interface reveal.

First, during construction of the intake structure, the entire ocean floor and the organisms there were “destroyed by siltation” and likely “cannot be reestablished,” according to a 1976 report by the California Department of Fish and Wildlife (then called Fish and Game). The agency also predicted at the time that any marine community that reestablishes itself there will not “be as productive or as rich as the original rocky substrate community.”

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A 1972 photo shows construction proceeding at Diablo Canyon nuclear power plant. The discharge outlet for the once-through cooling system can be seen in the middle of the cove below the turbine building. Larry Jamison Telegram-Tribune

Some species suffered particularly fatal impacts.

In a 1988 report by Tenera Environmental, the company noted that shortly after 1974 tests of the plant’s once-through cooling system, “an estimated 1,500 red and black abalone were found dead in Diablo Cove … their death was attributed to exposures to high levels of copper ion.”

The copper was discharged from the cooling system, which first used copper-nickel alloy tubes, later replaced by less toxic titanium tubes, according to the report.

However, in the 1976 state report, PG&E reportedly attempted to dismiss the resulting abalone and sea urchin deaths and attribute them to “factors unrelated to the operation of the plant,” but Fish and Wildlife contended that “abalone and sea urchin reductions in Diablo Cove have been primarily due to factors related to the operation of the plant.”

Once the plant began operating in 1985, the most immediate effect “was the scouring of the shallow, rocky bottom (0-15 ft depth) directly in front of the discharge structure,” the 1988 Tenera report says. Most of the existing kelp, algae and invertebrates were blasted away by the sheer force of the water shooting out of the plant’s discharge structure.

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The twin reactors and the turbine building at Diablo Canyon nuclear power plant are seen in a view from the waters of Diablo Cove on Feb. 25, 2022. The plant discharges 2.5 billion gallons of cooling water into the 40-acre cove each day. The ecology of the cove has tilted toward warmer water species since it began operating in 1985 but is expected to return to previous conditions within a few years of shutdown, scheduled for 2024 and 2025. David Middlecamp dmiddlecamp@thetribunenews.com

At the time, the water in Diablo Cove skyrocketed to up to 50 degrees above normal temperatures — a veritable hot tub — or between 100 and 110 degrees, according to the Tenera report. PG&E’s permit at the time allowed the water temperature to reach that high, but only for one hour per day during “heat treatments” designed to “demussel” the intake structure.

Although PG&E’s permit still allows the discharge water to reach up to 50 degrees above the intake water for one hour per day during heat treatments, the company says it no longer uses that cleaning method.

The 1988 report also documented how Tenera scientists observed large die-offs of kelp and algal species in Diablo Cove.

“The populations of oar kelp, tree kelp and bladder chain kelp have declined significantly,” the report noted. “When the cove’s bull kelp plants grew to within 10 (feet) of the sea surface each summer and thus became exposed to the plume, the plants lost their blades.”

Deterioration of these kelp habitats in turn led to great losses of the fish that relied on them for nutrients and protection from predators.

Additionally, rock crabs seemed to avoid the cove after the plant’s startup, the report says.

The native cold-water fish — such as species in the surfperch, rockfish, sculpin, tubesnout and greenling families — abandoned the cove to avoid the warmer water, according to the report.

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Sea lions bark as they lounge on Lion Rock in Diablo Cove, off of Diablo Canyon nuclear power plant, on Feb. 25, 2022. David Middlecamp dmiddlecamp@thetribunenews.com

Later, when abalone experienced a steep decline in California in the 1980s and 1990s due to withering syndrome, a fatal disease that attacks the marine snail’s digestive organs, the impacts in Diablo Cove were even more drastic, scientists said.

“You’ve got the withering syndrome, and you’ve got the heat,” Steinbeck said. “Both of those create more stress.”

Steinbeck said scientists conducted detailed abalone surveys in Diablo Cove from 1981 through 1998.

“Surveys in 1985 and 1988, after the plant started operation but prior to the onset of withering syndrome, resulted in population estimates of about 9,000 black abalone,” he said. “The population started declining after 1988 when mortalities related to withering syndrome were first observed. The final survey in 1998 resulted in an estimate of 622 black abalone — a decline of 93% from 1988.”

The most recent surveys were conducted in 2020, when no black abalone were found, Steinbeck said, adding, “We estimate there are still some black abalone in Diablo Cove, but it is probably less than 100.”

The once-through cooling system doesn’t only affect the cove habitat at its outflow. It also kills all fish and crab larvae that are sucked through the intake, causing population declines for some nearshore species in the area, according to the monitoring reports.

The same reports documenting significant declines in kelp populations and algae and seaweed covers, along with native cold-water fish counts, show the gradual but often steady increase in warm-water-loving species that found a comfortable habitat in the cove’s turbulent waters.

“It’s not like the moon out there,” Rebel noted, dispelling early myths that the warm-water discharge would boil away marine life with heat and radiation.

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A tidepool near Diablo Canyon nuclear power plant is filled with purple sea urchins burrowing into the soft rock. Wind creates an impressionist abstract pattern on the surface of the water in a photo taken on Feb. 25, 2022. David Middlecamp dmiddlecamp@thetribunenews.com

Warm-water fish species such as “members of the wrasse, goby, blenny, pomacentrid and opaleye families” were soon found in the cove in greater numbers than before the plant began operation, according to the 1988 report.

Garibaldi fish, a marine goldfish species more commonly found in the warmer waters off Southern California and Baja California, often crowd Tenera scientists’ boats when they’re out in the cove studying.

The 1988 report also documents a great increase of certain types of red algae, “which are more tolerant of temperature and wave shear,” and “the ubiquitous black turban snail,” which can be found in tidepools up and down the Central Coast.

At the top of the food chain, young great white sharks between 9 and 14 feet long have taken to gathering in unusually high numbers in the cove.

Chris Lowe, a shark researcher and professor at Cal State Long Beach, said they’ve seen up to eight cruising in the cove at one time.

“We think it’s because the water is warm, so we call it ‘the spa,’” Lowe said. “There may be benefits of tucking into the spa: to digest your meal faster, warm your body up; it may enable you to grow faster. So for some of those smaller, younger white sharks that are still, we’ll call them on the cusp of being teenagers, that’s probably a good place to go warm up.”

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A great white shark swims in the ocean waters near Diablo Cove, off the shore of Diablo Canyon nuclear power plant. Courtesy of PG&E

Rocks stripped bare of algae, sea stars and other marine species have given way to a flourishing mussel population, Steinbeck noted.

“Even five years ago, there weren’t as many mussels in the cove as we see today,” he said.

Finally, the once-abundant bull kelp was mostly replaced by giant kelp, which can tolerate the warmer waters.

Giant kelp’s dense canopy extends in a column from the seafloor and provides habitat for fish, invertebrates and numerous other species, according to the Tenera scientists.

Because Diablo Cove exists near Point Conception — the northern- or southernmost habitat limit for many species — it has provided a unique home-away-from-home to warmer-water species that venture farther north.

“We see a lot of species in Diablo Cove that we normally see in Southern California,” Steinbeck said. “It’s not like a whole new suite of species. It’s just different amounts of those that previously existed.”

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Scientists demonstrate how they conduct tidal wildlife population studies at Diablo Canyon nuclear power plant, using grids to count sea life on Feb. 25, 2022. This area was initially a study zone but was determined not to be representative of the Diablo Cove environment. David Middlecamp dmiddlecamp@thetribunenews.com

PG&E has paid millions in settlements, mitigation measures for environmental impact to Diablo Cove

Over the years, PG&E has been subject to heavy criticism for the environmental impacts from the warm-water discharge.

From the start, early environmental impact statements by the U.S. Nuclear Regulatory Commission for the plant’s once-through cooling system were deemed inadequate by the California Department of Fish and Wildlife.

The nuclear agency often underestimated or underplayed the various detrimental environmental impacts the system would have on marine life — one such example being that the report did not properly assess how various plankton would be impacted by being sucked through the once-through cooling system, according to the 1976 report.

A separate, closed-cycle cooling system was proposed from nearly the beginning but was deemed by PG&E and the nuclear regulatory body to be too costly and infeasible compared to the ocean-water system.

In the early years of environmental monitoring and the plant’s operation, PG&E was accused of withholding certain data showing the impacts the once-through cooling system was having on marine life.

“The Regional (Water Quality Control) Board, through the California State Attorney General’s office, along with the U.S. Department of Justice on behalf of the U.S. Environmental Protection Agency (EPA), has been conducting an investigation concerning whether PG&E’s failure to include certain data in its reports on the intake structure constituted a violation of the NPDES permit,” reads one letter from PG&E to the U.S. Nuclear Regulatory Commission in 1997.

Diablo Canyon nuclear power plant is slated to close in 2025. Joe Johnston The Tribune

In that case, PG&E had allegedly failed to disclose in its 1988 environmental report on the once-through cooling system the extent of the impact of entrainment of marine organisms in the cooling system. As a result, PG&E settled with the state for $14.04 million in 1997.

PG&E’s operation of the once-through cooling system is regulated by its National Pollutant Discharge Elimination System (NPDES) permit from the Central Coast Region Water Quality Control Board, as well as the California Porter-Cologne Water Quality Control Act and the federal Clean Water Act.

Essentially, PG&E has been required to ensure that the warm-water discharge does not harm the “beneficial uses” of Diablo Cove, according to its permit.

These uses include recreation, industrial water supply, navigation, marine habitat, shellfish harvesting, preservation of rare and endangered species, wildlife habitat and ocean commercial and sport fishing, the most recent permit issued in 1990 says.

In early 2000, the California Department of Fish and Wildlife and the Central Coast water board drafted cease-and-desist orders for PG&E due to alleged violations of its permit. Had it been approved, PG&E would have been required to shut down its once-through cooling system, therefore shutting down the plant.

In October 2000, PG&E and the Central Coast water board came to a tentative agreement worth between $16 million and $26 million to “resolve receiving water impacts” from Diablo Canyon. The settlement would have set aside 2,013 acres of watersheds draining to 5.7 miles of coastline to be forever conserved, $4.05 million for projects to protect marine resources in the vicinity and $350,000 for abalone restoration, among other things.

David Oatley, nuclear plant manager at the time, said PG&E was pleased with the settlement.

“We just want to put this issue behind us and move forward,” he told The Tribune after the settlement agreement was announced.

However, in 2003, the Central Coast water board later rejected the consent judgment that would have secured the settlement agreement, and in 2004, the U.S. Environmental Protection Agency issued new regulations that required the state water board to phase out once-through cooling systems.

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Diablo Canyon nuclear power plant owned by PG&E discharges about 2.5 billion gallons of cooling water a day into the 40-acre cove along the Pacific Ocean. The ecology of the cove has shifted toward warmer-water species since it began operating in 1985 but is expected to return to previous conditions within a few years of shutdown of its two units, scheduled for 2024 and 2025. David Middlecamp dmiddlecamp@thetribunenews.com

Six years later, the state water board released a once-through cooling policy that requires PG&E and other power generation facilities to contribute funds to a program to mitigate environmental issues associated with the plant’s intake and outtake of ocean water.

PG&E’s estimated payment to the mitigation fund is about $38 million for the Diablo Canyon nuclear power plant operating years 2015 through 2025, according to the state water board.

In the meantime, the settlement discussions originally initiated in 2000 were put on hold.

PG&E and the Central Coast water board in May 2021 came to a final consent judgment agreement for the long-term alleged violations of the power plant’s permit and water quality laws — totaling $5.9 million, a far cry from the $16 million to $26 million agreed upon in 2000, which included thousands of acres of land conservation that were never realized.

However, PG&E and its affiliates have, over the years, acquired more than 12,000 acres around the power plant. These lands are currently relatively undisturbed.

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A research diver gathers information in Diablo Cove for Tenera Environmental. For several decades, PG&E has commissioned studies of the effects of warm-water discharges from the nuclear power plant. Tenera Environmental Provided

How will marine life change once Diablo Canyon shuts down? Scientists unsure

Diablo Canyon is scheduled to begin shutting down in two years as its operating licenses expire. In 2024, one nuclear reactor unit will power down, followed by the second in 2025.

That means in 2024, the once-through cooling system will begin taking in and flushing out about half the amount of water it currently does — or about 1.25 billion gallons. Such a reduction in flow has happened every time the nuclear reactors require refueling, or about once every 18 to 20 months for about 30 to 35 days at a time.

As less warm water is discharged into Diablo Cove, scientists aren’t necessarily sure what will happen.

“It’ll be interesting to see how the cove adjusts,” said Steinbeck of Tenera. “It should return to normal.”

A return to “normal” means that the warm-water species that have found a home in the cove will likely die off and be replaced by the cold-water species indigenous to the area.

“Although the natural conditions will be reestablished, this will result in the displacement and loss of the warmer-water habitat and marine life created by the power plant discharge,” wrote state water board spokesman Edward Ortiz in an emailed statement to The Tribune. “The water board will not consider the change back to the natural cold water conditions to be a violation of the permit requirements.”

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Tidepools at Diablo Cove are filled with sea life, including mussels, sea anemones, sea stars and other organisms. Courtesy of PG&E

This story was originally published March 25, 2022 5:00 AM.

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Mackenzie Shuman primarily writes about Cal Poly, SLO County education and the environment for The Tribune. She’s originally from Monument, Colorado, and graduated from Arizona State University’s Walter Cronkite School of Journalism and Mass Communication in May 2020. When not writing, Mackenzie spends time outside hiking, running and rock climbing.


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