Although Ventura County's largest lakes are in good shape, a new study shows that two smaller lakes near Thousand Oaks have collected unhealthful levels of pollutants over the years from surrounding lawns, houses and streets.
Largemouth bass caught in Lake Sherwood for the study contained more mercury than the Food and Drug Administration allows in fish sold at market.
Water samples taken from Lake Sherwood and the nearby Westlake Lake had concentrations of lead and copper that exceeded state standards for inland waters. And bass pulled from Westlake Lake had elevated levels of selenium, which is toxic to wildlife and humans in anything more than minute doses.
The urban lakes study deems the health of Lake Sherwood and Westlake Lake to be at least "moderately impaired" because of fertilizers, chemicals and heavy metals that have drained into them.
In sharp contrast, Lake Casitas near Ojai and Lake Piru were among the healthiest of the 23 lakes studied by UC Riverside researchers for the California Regional Water Quality Control Board, Los Angeles Region.
Both of these lakes in western Ventura County receive their water from national forest land largely removed from human-created pollution.
"It is a lot easier to manage a large lake in a pristine environment compared to a small lake that is surrounded by man's activities," said Lanny J. Lund, a UC Riverside professor who wrote the two-volume report.
Pesticides and fertilizers are regularly washed from golf courses and yards into urban lakes, he said. And everything from oil drippings, dog feces and bits of tires can get flushed from city streets.
"If you have a drainage into the lake," Lund said, "that's where all this is going to end up."
Lake Sherwood covers 168 acres at the heart of a gated community of about 150 houses just south of Thousand Oaks. The study found that the lake had the highest concentration of ammonium of any of the 23 lakes studied in Ventura or Los Angeles counties.
Although researchers visited the lake a dozen times to take water samples, they did not attempt to pin down the sources of ammonium, which can be toxic to aquatic animals.
Rotting vegetation, fertilizers or leaking septic tanks could be potential sources, as could the many horses corralled in the area, researchers said.
"Any time you have urine and manure around, you can have ammonium," Lund said.
The study also found elevated levels of the toxic metals lead and copper in water samples taken from Lake Sherwood.
Lead often comes from the residues of lead-based paints, lead solder or leaded gasoline that linger in soils or city streets. The copper is attributed to copper sulfate sprayed on the lake to control algae blooms.
As part of the study, state officials caught six largemouth bass in the lake in 1991 and again in 1992 and analyzed the fish tissue for contaminants.
In 1991, a state laboratory found elevated levels of mercury. The levels more than doubled to 1.6 parts per million in 1992--exceeding FDA limits of 1 part per million.
"It is not possible to say if this is a trend based only on these two samples," the report stated.
Toxicologists recommend that people, especially pregnant women, avoid eating fish tainted with mercury. A reproductive toxin, mercury can impair the mental development of unborn children and it attacks the kidneys and nervous system of adults.
"We are concerned about this, but not to the extent that we should be issuing a health warning," said Manjunath Venkanarayana, an environmental specialist with the water quality control board.
Instead, he said, the state agency will send the study to lake managers so they are aware of the problem, do follow-up studies or take steps to correct them.
Carol Witchell, an on-site administrator for billionaire David Murdock who owns Lake Sherwood, referred all inquiries regarding the lake to a consultant. The consultant could not be reached for comment.
Jim McCleod, a Lake Sherwood resident and chairman of a lake management committee, said he was surprised at the elevated levels of mercury.
McCleod said he and other residents of the older Lake Sherwood community are waiting for Murdock to carry out a long-term lake management plan as was promised when Murdock won the right to build 450 more houses near the lake.
Currently, there are no restrictions on Lake Sherwood residents, who can swim, boat or fish on the lake and eat what they catch from the bass stocked by Murdock.
"Given all of the fertilizers and all that drains into the lake, I would be very afraid to eat them," said Jack Adams, one of the lake's most avid fishermen. "But I think there are people who do."
Westlake Lake, a 156-acre body of water that straddles the Ventura-Los Angeles County line, also has contamination problems, according to the study.
But local homeowners' associations tightly control recreation on the lake and thus minimize any potential human health impact.