The discovery of 22,740 marijuana plants growing in and around Point Reyes National Seashore last week wasn't only the biggest pot seizure ever made in Marin County. It was an environmental mess that will take several months and tens of thousands of dollars to clean up.

The crops seized on the steep hillsides overlooking Highway 1 were planted by sophisticated growers who cleared vegetation, terraced land, drew water from streams through miles of irrigation hoses and doused acres of land with hundreds of pounds of fertilizer and pesticides.

Such operations are turning up in greater numbers within state and national parks throughout California. Federal officials estimate the state produces half of all the marijuana seized on public lands nationwide.

Officials at Point Reyes National Seashore have only begun to assess the resulting damage to an area that is habitat for the spotted owl, steelhead trout and coho salmon, and they said it could be months before they know the long-term implications for the ecosystem.

"We've seen some really nasty damage," National Park Service spokesman John Dell'Osso said Tuesday. "And there's a very good possibility there are sites we haven't even found yet."

Cultivating marijuana on land managed by the Park Service, the National Forest Service and other agencies is a multibillion-dollar industry. So far this year, authorities have found more than 940,000 marijuana plants growing on state and federal land in the Golden State. With the harvest season beginning, officials expect to find more pot farms and surpass last year's haul of 1.1 million plants.

Federal officials believe as much as 80 percent of the marijuana on public land is grown by Mexican drug cartels that have turned to places like Point Reyes National Seashore, Sequoia-Kings Canyon National Park and Whiskeytown National Recreation Area in this era of tightened border security; growing the drug here is far easier than smuggling it in. The plants found in Point Reyes last week were valued at around $50 million, Dell'Osso said.

The federal Office of National Drug Control policy estimates that growing 1 acre of marijuana damages 10 acres of land. Repairing that land is a costly, time-consuming process, and because the National Park Service does not allocate money specifically for the task, the funds come from each park's operating budget -- leaving less money for things like park programs and improvements.

"We have no budget for this," Dell'Osso said, noting that it is a problem "the powers-that-be need to start discussing."

The pot seized last week was growing on six sites scattered along Bolinas Ridge between Stinson Beach and the Randall Trail just south of Olema. About half of it was on land managed by the Marin Municipal Water District. Marin County sheriff's deputies discovered it during a routine aerial search. Investigators have not made any arrests.

Lt. Scott Anderson of the Marin County Sheriff's Department said the pot farm's similarities to those found in other national parks suggests it was the work of a Mexican cartel that probably employed undocumented immigrants.

The sites in Marin County are tucked away in remote canyons, sheltered beneath madrone and oak trees and surrounded by thick brush hacked away haphazardly. Trees have been stripped of their limbs to make room for the plants, leaving only a canopy of branches to hide the illicit crop.

Irrigation hoses as long as a mile each drew water from pools dug into the ground and fed by the springs and streams that course through the Tomales Bay watershed. The steep hillsides have been terraced, much like a vineyard, and are dotted with hundreds of deep holes that held as many as four marijuana plants apiece. The land is littered with empty 50-pound bags of fertilizer and gallon jugs of pesticide.

Investigators believe as many as three people tended each plot, and the amount of trash -- empty soda and beer cans, food wrappers, propane canisters and clothing -- suggests they'd been living there for at least several weeks but fled before officials reached the site. Authorities found animal traps, pellet guns and a rabbit hutch, leading them to believe the growers hunted for food.

With the last of the crops cleared away, park officials have begun assessing the damage. Once the trash is removed, the biggest priority will be protecting the land with straw and new ground cover to prevent the winter rains from washing it away. Beyond that, though, it's not yet known exactly what must be done to restore the land and what it will cost.

Sequoia-King's Canyon National Park has spent more than $72,000 during the past two years to clean up 81 cultivation sites that covered 10 acres, said Athena Demetry, a restoration ecologist at the park. Authorities have seized more than 100,000 marijuana plants within Sequoia-King's Canyon since 2004. The latest seizure came Aug. 9, when authorities found 2,152 marijuana plants growing within view of Moro Rock, a popular park destination.

Over the course of six weeks during the winter of 2005 and 2006, park rangers hauled almost 5 tons of trash and debris out of the park, removed 13 miles of irrigation hose, and repaired deep cuts and terraces made to 35 hillsides, Demetry said. Empty bags and bottles revealed the growers used at least 8,031 pounds of fertilizer, 15 pounds of rodenticide and 7.6 gallons of pesticide. An additional 80 grow sites still must be repaired.

"We chose the sites that are easiest to reach and did those first," she said. "The ones that are left to do are more remote, and on steeper terrain."

Demetry said the land will recover quickly as new vegetation grows but the effect on wildlife will be harder to measure. No one knows how much fertilizer and pesticide is polluting the land, or how much of it made its way into the streams that feed the East Fork of Kaweah River.

"It's got a huge impact on the stream ecology," she said. "It has the potential to ripple through the food chain."

Park rangers have for years stumbled upon small stands of marijuana, but the problem has exploded within the past five years and reached a point where they're having difficulty keeping up, Demetry said. Although individual cultivation sites rarely cover more than an acre, the growers have taken to scattering them over hundreds of acres to evade detection. That, she said, spreads the destruction over a far broader area, with far graver results.

"When we first saw them, we thought they were pretty small," she said. "But then we realized how many there were, and it became staggering. And there's a lot more out there."