Characterizing the Geothermal Lithium Resource at the Salton Sea
The energy transition towards a more sustainable and renewable future is a pivotal global endeavor. Central to this shift for the United States is the critical role of domestically sourced lithium, a key mineral in the production of high-performance batteries essential for electric vehicles and renewable energy storage systems. This has driven the United States to invest heavily in a domestic supply chain for battery-grade lithium to enhance energy security, reduce supply chain vulnerabilities, and foster economic growth by tapping into local resources. A notable example is the Biden Administration’s “American Battery Materials Initiative,” which was included in the $2.8-billion Bipartisan Infrastructure Law (The White House, 2022). The “Salton Sea Known Geothermal Resource Area” in Imperial County, California has been identified as a potential domestic U.S. resource of lithium due to the brine-hosted lithium in the deep subsurface geothermal reservoir. An analysis funded by the U.S. Department of Energy provides an overview of opportunities and challenges associated with developing the lithium resource in the Salton Sea geothermal reservoir, as well as potential environmental and societal impacts to the county and surrounding region. The geologic history of the region suggests that lithium in the subsurface brines could have come from multiple sources, including water and sediments from the Colorado River, which have been periodically deposited over the past several million years; rocks from the mountain ranges surrounding the Imperial Valley; and lithium-bearing volcanic rocks and igneous intrusions from past geologic events. Further, several processes may have concentrated lithium in the brine over time, including evaporative concentration of lithium-bearing water that flowed into the basin and leaching of lithium from the sediments and rocks by the circulating geothermal brines. Geothermal brine production at the Salton Sea Geothermal Field, the area with existing geothermal power plants, has averaged just over 120 million metric tons per year since 2004. Using an approximate lithium brine concentration of 198 parts per million (ppm), the amount of dissolved lithium contained in these produced brines is estimated to be 127,000 metric tons of lithium carbonate equivalent (LCE) per year. The total dissolved lithium content in the well-characterized portion of the Salton Sea Geothermal Reservoir is estimated at 4.1 million metric tons of LCE, and the estimated total resource increases to 18 million metric tons of LCE if assumptions for porosity and total reservoir size are increased to reflect the probable resource extent. Analysts measured lithium concentrations in the reservoir rocks, which were shown to vary with depth and mineralogy. These data were used to help refine conceptual and computer models of the reservoir; specifically, two complementary computer models of the reservoir were developed. Analysts used the first model to simulate the approximate 30-year history of geothermal power production in the area using historical production and reinjection data, then used that model to simulate a 30-year forecasting period. This forecast assumed continued production and reinjection rates at current levels but removes 95% of the lithium from the produced geothermal brine starting January 1, 2024. The model found that lithium recovery declines by more than half, from 0.8 to 0.3 kilograms per second (kg/s). Forecast scenarios that are optimized to both recover lithium and harness geothermal energy are expected to sustain lithium production rates much more effectively. The second model included more detailed simulations of the movement of brine and chemical reactivity of lithium within the reservoir. It showed that the reactions of relatively stable lithium-bearing minerals are slow, and that the primary replenishment mechanism for lithium in the brines is the upward flux of convecting lithium-rich brine from below the producing reservoir. However, these replenishment rates are not fast enough to produce significant increases in lithium, which could limit the long-term sustainability of the lithium resource. It is important to note that these models are preliminary and are based on current understanding of fluid replenishment rates, the minerals present in the geothermal system, and their chemical properties and reactivity. Further work should be undertaken to improve them and the associated predictions. The report also considered potential impacts on regional water resources, air quality, chemical use, and solid waste disposal needs, as well as the seismic risk associated with geothermal power production and lithium extraction activity. These investigations highlighted the need to proceed with good monitoring and verification systems and with appropriate mitigation technologies. However, the analysis illustrates that if these things are done properly, lithium development is not likely to create significant negative environmental impacts. Specifically, expanding geothermal energy production and lithium extraction will have a modest impact on water availability in the region. Initial estimates suggested that ~3% of historically available water supply for the region would be needed for currently proposed geothermal energy and lithium recovery operations; the majority of current water usage is for agriculture. It is not anticipated that expanding geothermal capacity or lithium production would impact the availability or quality of water used for human consumption and will not directly affect the water quality of the Salton Sea. However, the long-term drought conditions in the western United States may restrict future availability of water to the region, which is sourced from the Colorado River. In terms of regional air emissions of all pollutants identified in the analysis (particulate matter, hydrogen sulfide, ammonia, and benzene, expanding geothermal energy and adding lithium extraction overall have a small impact. Chemical use involved in geothermal power production and lithium extraction is consistent with chemical use in industrial settings, and the analysis did not identify any persistent organic pollutants or acutely toxic chemicals among those currently being used. Moving fluids within the subsurface can impact subsurface pressures and stresses, potentially triggering seismic activity. Early in geothermal energy production, increasing seismicity rates in the Salton Sea Geothermal Field correlated strongly with energy production activity; however, that correlation weakened after 1996. Even following the onset of geothermal energy production, seismic hazard in the Salton Sea Geothermal Field has not increased beyond that of the surrounding region. In addition to technical outcomes from the analysis, the report describes an initial effort to incorporate community engagement into lithium research by understanding the local context and priorities and identifying how to effectively communicate to share information and gather feedback. The report includes information about the social and historical context of the region to enable a more holistic understanding of the resource and its potential impact, and identifies key community questions by observing public meetings, visiting the region, and consulting with local organizations. The report provides recommendations about how future research efforts can address community concerns and implement more community-engaged practices. These include developing formal partnerships with local organizations and establishing a community advisory board to facilitate ongoing dialogue and opportunities for feedback. The future work will build on and further refine the models and scenarios presented in the report and strive to deepen engagement with local communities.