Water district gears up for another battle over Mammoth Lake’s drinking water

Fresh from one battle over drinking water, Mammoth’s water district is now plunging into another—this time over a proposed new geothermal plant that would be located to the northeast of the existing geothermal plant on Casa Diablo Road.

The project, called Casa Diablo IV, would double the current geothermal generating capacity of the site, which also includes an older plant still in operation.

A few weeks ago (see MT Aug. 29, p. 15, “Mammoth’s water district fears expanded geothermal plant will hurt town’s water supply”), the district appealed the plan on the basis that the two local federal agencies that approved the project plan had done so without assuring Mammoth’s water supply would not be impacted negatively, and/or setting up a mitigation and monitoring plan to ensure it was not.

Since then, and following a public information meeting last week at the district office, the district has been working with Ormat Technologies, the project’s proponent, toward such a plan, even as the legal process continues, according to Pat Hayes, the district’s general manager.

“I am hopeful we can put together a monitoring and mitigation plan,” he said.

Another meeting before the board of the Great Basin Unified Air Pollution Control District, which must approve the air quality plan for the project before it can more forward, will be held this coming Monday, Sept. 16, in Mammoth (see box).

In the meantime, there is still a large gap between what the district wants to see studied and what was studied in the project’s environmental review documents (Final Joint Environmental Impact Report/Environmental Impact Statement for the Casa Diablo IV Geothermal Development Project) for the project that was approved by the Inyo National Forest and the Bishop office of the Bureau of Land Management earlier this year, he said.

“At the heart of this is their premise that the coldwater and geothermal water systems are not connected,” Hayes said. “Based on that premise, they made a finding that our drinking water wells would not be impacted.”

The problem is, he said, the water district is not convinced the wells are connected-—or that they are not.

“Whatever scientific evidence that would have supported their claim was held as proprietary,” he said. 

The district is thus asking Ormat to conduct a series of “stress tests” to determine if the new geothermal energy plant would impact the district’s drinking water wells, he said.

The coldwater aquifer that the water district taps into for its wells can be reached with a well that is about 400 to 500 feet deep.

The proposed geothermal wells, according to Hayes, are much deeper, where they can tap into geothermal, heated aquifers.

What Hayes wants to know is if the two can “talk to each other,” or in other words, is there is any connection between the two?


What the water district claims: According to the district: “the mining of heat in the hot water below the coldwater aquifer could reduce pressures and increase the likelihood of coldwater migrating downward through fractured rock into the geothermal zone. This could be catastrophic for Mammoth’s groundwater supplies.”

What the water district wants: “a comprehensive, robust monitoring and mitigation plan with ‘trigger points.’ The plan would protect the community’s water resources through the drilling of monitoring wells, aquifer stress tests ahead of project construction, inclusion of ‘trigger points’ to reduce or cease operations if the project impacts the district’s groundwater and surface water supplies, collection of additional water chemistry data, and the implementation of a long-term monitoring program prior to construction and operation of the proposed project.”


  • The proposed plant, called the Casa Diablo IV plant, would be located northeast from the current geothermal plant facilities.
  • The project would also include as many as 16 new wells, and the pipeline to bring the geothermal brine from those wells to the plant.
  • An electric transmission line to connect the power generated from this plant would also be constructed, according to the environmental review documents for the project.
  • The U.S. Forest Service and the Bureau of Land Management are cooperating lead agencies for the project, as the project is on forest service land as well as BLM land.
  • According to MCWD, water use at the Casa Diablo IV plant would increase from 19,000 acre-feet to about 29,000 acre-feet per year.  MCWD currently extracts about 2,000 acre-feet of groundwater per year, or about 70 percent of the daily water used in Mammoth.



Great Basin Unified Air Pollution Control District must also approve the plan. That has not occurred yet and a district board meeting regarding this issue, as well as other items, will be held in Mammoth on Sept. 16 at the Town of Mammoth offices above Vons at 10:30 a.m.

For more information, call Ted Schade at 760-872-8211 or visit www.gbuapcd.org.