While the recent El Nino storms have dissipated toxic algae in other East Bay lakes, Lake Del Valle remains closed to swimmers and dogs.
The lake has been closed ever since toxic blue-green algae was detected in December, but experts haven't figured out exactly how usual yearly blooms in the lake -- which doubles as a regional drinking water reservoir -- turned poisonous.
"In the 80 year history of the park district, we never had any toxic blooms until two summers ago," park spokeswoman Carolyn Jones said. "At this point it's been really frustrating because we don't have enough staff to get to the bottom of this."
Recent tests confirmed the region's drinking water hasn't been affected by the toxic blooms, and chlorine is used by Zone 7 Water Agency treatment plants to kill any algae, Zone 7 general manager Jill Duerig said.
Microsystins, a group of toxins produced by cyanobacteria found in toxic algae, sicken people and kill animals by attacking the liver, according to a 2009 California Environmental Protection Agency report. It can sicken humans and kill dogs and other animals that lap up the poisonous water.
Typically, toxic algae forms after a run-off of nutrient-rich material -- such as excrement from failed septic tanks or fertilizer from agricultural farms -- but Lake Del Valle doesn't have any of those issues nearby, East Bay Regional Parks District water supervisor Hal MacLean said. While he said it's possible water that's pumped into the lake from Clifton Court Forebay in the San Joaquin River Delta could be the cause, he said he didn't know of any current toxic blooms in the source lake.
The park district's most recent tests didn't show any toxic algae, but the district is keeping the lake closed out of an abundance of caution. The only source of water going into the lake right now is runoff from the rain, California Department of Water Resources program manager Tanya Veldhuizen said.
"No water has been imported into the lake from the State Water Project aqueduct system since April 2015," she said in an email. "This indicates the bloom is a result of localized conditions. During the prolonged drought, many lakes are experiencing conditions that are ideal to support algal blooms such as warm, slow moving water."
It's rare for toxic algae to form and persist during the cooler winter months since algae typically likes dry, warm months, Jones said. This is the first time Lake Del Valle tested positive for toxic algae.
Samples of the algae were sent to Greenwater Laboratories in Palatka, Fla., which specializes in toxic algae analysis. The samples were found to contain Aphanizomenon, Dolichospermum and Microcystis genuses of algae, some of which are known to produce toxic algae at times.
The highest level of toxins found was 27 parts per billion, above the threshold of 20 parts per billion that triggers a lake shut-down.
Mark Aubel, president of Greenwater Laboratories, said awareness of toxic blooms has skyrocketed in recent years as environmental agencies grapple with how to deal with the unexpected problem.
"It's not just specific to california. It's a national issue," he said. "Now the drought out there has obviously exacerbated the situation, with water being as scarce as it is and making sure the water that is available is suitable for drinking, or in some case for recreation."
The toxic blooms have spread to the entire lake, pushed by the strong winds that accompanied El Nino storms in January, MacLean said.
"The wind can blow that stuff all the way across the entire five miles in one night if the conditions are right," he said.
Quarry Lake in Fremont, Lake Temescal in Oakland and Lake Anza in Berkeley were recently cleared of toxic algae by strong El Nino rains after the swimming spots had been closed for months.