San Diego

City Closes Black Mountain Hiking Trails Near Abandoned Arsenic Mine

Arsenic from an abandoned mine prompted the City to forbid hikers from exploring certain trails.

High arsenic levels detected near an abandoned mine in Rancho Penasquitos prompted the closure of some Black Mountain hiking trails, confirmed city officials.

Research consultants found higher than normal arsenic readings at the mine in Black Mountain Open Space Park, city officials announced on Friday. Although there is no evidence of a public safety threat or environmental hazard, nearby trails have been closed as a precaution.

The closures will allow researchers to conduct further testing, sampling and monitoring of the arsenic levels in the area, said city officials. Hikers are now prohibited from going in the mine and the surrounding trails.

Trails forbidden to hikers include a portion of the Miner's Ridge Loop Trail. According to city officials, the research and thorough testing procedures will take several months.

Due to the arsenic testing, the public was advised to carefully read signs and follow Ranger instructions when exploring the Black Mountain Ranch Open Space Park.

Hikers can still access the Black Mountain Peak along western portions of the park, with Rangers reminding them to stick to designated trails.

On the north slope of Black Mountain, the arsenic mine resides at an elevation of 1,000 feet. According to the city, it was mined for naturally-occuring white arsenic (aka arsenious oxide) in the 1920s for about eight to 10 years.

Although remnants of the mine operation remain, it has not been in use for over 80 years.

An updated map of the current trails that avoid the off-limits area are available on the City's website here.

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