WSSC Water warned residents Tuesday, Aug. 18, to avoid all water contact at Triadelphia Reservoir after detecting dangerous levels of blue-green algae, a toxin-producing bacteria that can be rapidly fatal in pets.

The health advisory will remain in effect for at least one month, meaning hikers, anglers and boaters should steer clear of the water through at least mid-September. The reservoir, created by Brighton Dam in Brookeville, straddles the Patuxent River between Montgomery and Howard counties and draws visitors from across the Potomac area for fishing, kayaking and shoreline trails.

WSSC Water stressed that tap water remains safe. Most Montgomery County customers, including those in Potomac, receive drinking water from the Potomac River, not the Patuxent reservoirs. The utility confirmed its drinking water continues to meet all Safe Drinking Water Act standards, though it is closely monitoring conditions at the Patuxent Water Filtration Plant as a precaution.

The algae, also called cyanobacteria, thrive in warm, stagnant, nutrient-rich water. WSSC Water monitors the reservoirs for blooms from July through October each year. When concentrations spike, the organisms can produce toxins including microcystin and anatoxin, which cause liver and neurological damage in humans and animals.

Dogs face especially high risk. According to Cornell University's College of Veterinary Medicine, there is no antidote for blue-green algae poisoning, and it can be rapidly fatal in dogs that swim in or drink contaminated water. Toxins can linger for weeks even after a visible bloom disappears.

This is the second Patuxent reservoir under advisory this summer. WSSC Water issued a similar warning for the T. Howard Duckett Reservoir on July 21, and that advisory also remains in effect. Together, the two reservoirs cover 1,600 acres and are surrounded by 6,000 acres of WSSC Water-owned watershed land.

Anyone who has had water contact at either reservoir should call their county health office. WSSC Water said it will lift the advisory and post an update at wsscwater.com when algae concentrations drop below threshold levels. Residents can confirm their water source at wsscwater.com/mywater.