November 20, 2019
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California Water Agency Concerned as Neighboring Geothermal Plant Expands

Volcanically active Mammoth Mountain in California has been home to a productive geothermal energy operation for decades. Now, expansion plans have the local water agency concerned about the future of its groundwater.

by Matt Weiser

THE TOWN OF Mammoth Lakes, in California’s eastern Sierra Nevada, is generally known for two things: epic skiing in winter, thanks to the very high elevation of its ski mountain; and volcanic activity, because the mountain is a simmering volcano. It’s normal to hike or ski around Mammoth and smell the sulfurous gases venting from gurgling magma deep under the mountain.

That magma is also a rich source of geothermal power. Ormat Technologies, based in Reno, Nevada, has been tapping that power source for three decades. Now it wants to double its energy output to 60 megawatts by drilling more geothermal wells.

Not so fast, says the Mammoth Community Water District. The agency is concerned that Ormat’s proposal to drill as many as 16 new geothermal wells could cause boiling hot geothermal fluids to mix with the town’s groundwater wells.

The water district’s general manager, Pat Hayes, says that raises a risk that the town’s domestic water supply could be contaminated with poisonous arsenic, which occurs naturally in the geothermal fluid.

“Because of the arsenic content, it’s a very big worry,” said Hayes.

Ormat won approval to expand its Mammoth geothermal operation in 2014 from the United States Bureau of Land Management, from which it leases land, and the Great Basin Air Pollution Control District. The latter was designated as the “lead agency” under the California Environmental Quality Act, even though the project has no significant air emissions.

The company taps geothermal fluid deep underground – groundwater, essentially, that has been superheated by volcanic activity – and routes it to the surface. It’s a closed-loop system in which the geothermal fluid heats a secondary fluid, which is then used to create steam and drive turbines to make electricity. The geothermal fluid is then routed back underground to be heated again by the idling volcano.

When Ormat’s expansion was approved, the water district wanted the company to be required to drill multiple monitoring wells as an early-warning mechanism. Pressure or volume changes in the monitoring wells could indicate that geothermal fluid might be leaking into drinking water aquifers.

The request was denied, largely based on Ormat’s claim that its geothermal fluid and Mammoth’s groundwater were separated by an impermeable barrier underground and could not possibly mix.

The water district, however, recently commissioned a study based on groundwater data collected by the U.S. Geological Survey. The consultant’s study found higher levels of chemicals linked to geothermal activity in two of the water district’s wells, including arsenic, boron and chloride. The water in these two wells was also warmer than the district’s seven other wells, suggesting a geothermal connection.

As a result, the water district has asked the air pollution control district to amend the environmental impact report approved in 2014. The water district wants safeguards added requiring Ormat to drill more monitoring wells, and to require the project to be halted if any threat to groundwater emerges.

Hayes said he is particularly concerned about the water district’s well No. 17, which provided as much as 20 percent of Mammoth’s total water supply during California’s recent long drought. That well already has elevated arsenic levels that are treated by diluting it with water from other wells, so it cannot tolerate additional arsenic contamination that might leak into the aquifer from geothermal activity.

The district provides drinking water to about 8,000 full-time residents in Mammoth Lakes, and as many as 35,000 people on busy tourist weekends.

“These new data tell us that an impermeable barrier does not exist,” Hayes said. “If that well gets knocked off line in a drought period, we’d be hard pressed to find a replacement.”

Ormat looks at the situation differently. Company spokesman Paul Thomsen noted the city’s water wells are 2 miles away and about 1,000ft higher in elevation, so it’s unlikely they are linked in any way to the geothermal fluid Ormat uses to make electricity.

He also said Ormat tested its well field by adding a chemical tracer to the geothermal fluid. If there was any connection to the city’s groundwater aquifer, that tracer would have shown up in drinking water wells. The well water was tested, he said, and the tracer never appeared.

As for the arsenic in the water district’s well, Thomsen said that’s no surprise.

“It kinda makes sense, because Mammoth Mountain has active geothermal fissures all over it,” he said. “There are naturally occurring geothermal constituents that have been present in Mammoth Community Water District’s wells as far as records go back. What they can’t say is that those constituents come from the activity at Ormat’s geothermal powerplant.”

Thomsen added that any “leakage” from the geothermal field into surrounding groundwater would represent an inefficiency that could compromise power generation. If that was happening, the company wouldn’t proceed with the project.

“We’re looking to invest close to another $150 million in the Mammoth area to bring online another 30 megawatt project,” he said. “They really seem to be pointing a finger at Ormat. We’re trying to figure out why.”

Thomsen said Ormat objects to reopening the environmental impact report. The company hopes to fire up its expanded geothermal energy project by 2021.

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