SAN MATEO -- You can't swim or boat in Crystal Springs Reservoir because it's the drinking water source for more than a million people, yet illegal pot farms in the rugged land above the water might be a bigger threat than skinny dippers.

Last week authorities pulled 7,200 pounds of trash left behind by illicit marijuana growers off the steep hillsides that funnel rain and creek water into the reservoir. Among the car batteries and black plastic irrigation tubes authorities carted out by helicopter were toxic pesticides that have been banned from the United States.

Officials said the trip down the mountain dilutes the chemicals and ends with most of them being absorbed in the soil, but some of them end up the reservoir that provides water to the northern half of San Mateo County and San Francisco. So far authorities haven't turned up any evidence the toxins have harmed the safety of the water, but they are keeping watch over their system with about 100,000 tests per year.

The amount of marijuana growing varies fluctuates on the 23,000 acres around the reservoir, but in the past five years the San Mateo County Narcotics Task Force has seized 48,529 outdoor plants worth an estimated $170 million, a majority of it on the reservoir watershed. During an Aug. 27 seizure, police grabbed 1,579 pot plants said Sheriff's Lt. John Munsey, head of the task force.

Munsey said agents tearing down the gardens found pesticides Methyl Parathion and Carbofuran, which the Environmental Protection Agency has effectively banned from use in the United States. Both chemicals are considered highly toxic to people and animals. Carbofuran poisoning, in the short term, can mean headache, sweating, nausea, diarrhea, chest pains, blurred vision, anxiety and general muscular weakness, according to the EPA.

The agents also found propane tanks, fertilizer, household garbage like food waste and a handful of other chemicals, including one pesticide called Weevil-Cide.

"Weevil-Cide is a highly hazardous material and should be used only by individuals trained in its proper use," according to a warning from the product's maker.

"It was an enormous amount of garbage. We were all a little bit shocked," said Munsey, who added no one was arrested. "It just seemed like (load) after (load)."

Authorities haven't detected Methyl Parathion or Carbofuran in the Crystal Springs water, said Water Quality Engineering Manager Manoucher Boozarpour of the San Francisco Public Utilities Commission.

He said the commission tests for pesticides and has a limit of a .5 parts per billion of Methyl Parathion and one part per billion of Carbofuran. One part per billion is the equivalent of one drop of impurity, like ink, in 500 barrels of water, according to a pesticide information project that includes Cornell University and UC-Davis.

"It does end up in the water, but we haven't been able to detect it," said Boozarpour of the chemicals. "Historically we have never detected anything in the water."

The impact of outdoor marijuana gardens on the Peninsula appears to be relatively manageable when compared to the problem in the northern reaches of the state. Matt St. John, the executive director of the North Coast Regional Water Board, said they have seen areas where growers built their own roads and clear cut land. They are also noticing a bump in nutrients found in the water, which can prompt explosions of algae. The blooms are often harmless, though some types are toxic.

St. John said officials haven't drawn a direct link to the growing and water pollution. The higher levels of nutrients could be due to erosion spurred by growers who clear land to farm or the fertilizer they dump on the plants.

"We don't have any specific data on drinking water," he said. "But I think it has the potential to end up in drinking water."

Contact Joshua Melvin at 650-348-4335. Follow him at Twitter.com/melvinreport.