KELSEYVILLE, Calif. - As many as a dozen people luxuriating in the ''spiritual, healing'' effervescence of Northern California's Soda Springs may have been asphyxiated over the years by carbon dioxide that bubbled out of the hot spring's water.
The pool is encircled by a natural rock border, but a few years ago someone built up the wall with bricks and concrete, making it about 2 feet higher. The higher wall made the basin cozier, but it also apparently trapped the gas close to the surface of the water.
The most recent victim was 31-year-old Stephen Kastner of Napa, who died alone in the pool July 28, said Phil Damon, an assistant field manager with the federal Bureau of Land Management, which owns the land.
''From our documentation, and it's minimal, we know for sure there have been three deaths and probably there have been four times that many,'' Damon said last week. ''I don't have much doubt about that.''
The earliest documented death by asphyxiation at the springs was John ''Pop'' O'Shea, a former Lake County coroner who died in 1878, Damon said. Another man, who was not identified, died in 1981.
Soda Springs sits at the edge of a small island about 200 feet off the shore of Clear Lake, about 120 miles north of San Francisco. The water in the rectangular 6-by-8-foot pool smells vaguely of rotten eggs and the basin's natural back walls are encrusted with a multicolored patina of chemicals.
Carbon dioxide comes up through inactive volcanic vents, making the water bubble like a hot tub, or a bottle of soda.
Damon said locals erected the wall to cloister the springs from the lake's waters, which tended to lap in and cool the 90-degree pool.
The BLM tore down the wall last week, and the springs are now off limits. Warning signs sit in the effervescent waters, strung together by a chain.
The plan now is to reopen the springs when the BLM determines carbon dioxide levels are safe, then install a wall that can be adjusted to the changing lake levels to allow for ample circulation, Damon said.
Without proper air flow, dangerous levels of carbon dioxide hover over the surface, said Cathy Janik, a U.S. Geological Survey geochemist. ''I'll tell you, I wouldn't want to sit in the spring, just because there's so much gas,'' she said.
Stephen Kastner's father, Bill Kastner, said all mysterious deaths at Soda Springs should have been taken seriously.
''We've been going there with the boys since they were 8 or 10,'' said Kastner, who has a summer house nearby. ''Back then there was no wall. In the past few years, someone built the wall, then the BLM knocked it down and then someone built it back up again and BLM knocked it down again.''
Officially, the cause of Kastner's death has not been determined and lab results are expected to take several weeks, according to Russell Perdock, Lake County's coroner. Preliminary autopsy results showed that Kastner drowned, he said.
But Damon said he believes the young man was asphyxiated, and Kastner's father said drowning as the cause of death is ''basically impossible as far as I'm concerned, unless someone was holding his head down.'' He said Stephen was an accomplished water skier, swimmer and diver.
After his son's death, Kastner hired a chemist who said the carbon dioxide level was so high, his meter could not register it.
At first, the BLM was ready to fill the basin with boulders to make it unusable, but two dozen locals appealed for a compromise at a public meeting earlier this month.
The waters of Soda Springs have long been prized for their supposed medicinal qualities. Locals have bathed in the waters for generations. Pomo Indians gathered at the spot, teen-agers met there for romance, and families soaked together in the steaming mineral water.
''As a kid, this was adventure, this was discovery,'' said Rolf Kriken, 55, who spent youthful summers at Clear Lake and moved back nine years ago to raise his two sons. ''It's a very spiritual, healing place.''
The waters attract legions of visitors, especially during the summer. Residents talk of boats idling 30 at a time, their passengers waiting for a chance to take a dip.
''The bubbles are great,'' said Thelma Dangel, 75, who has lived on the shore nearby for 27 years. The fact that her husband once revived a man he found choking in the basin never deterred Dangel.
''I took my mother out there when she was in her 80s,'' she recalled. ''You just sit there and have a lot of good chats and fun.''