Researchers at the United States Geological Survey and the Arkansas government announced on Monday that they had found a trove of lithium, a critical raw material for electric vehicle batteries, in an underground brine reservoir in Arkansas. With the help of water testing and machine learning, the researchers determined that there might be five million to 19 million tons of lithium — more than enough to meet all of the world’s demand for the metal — in a geological area known as the Smackover Formation. … Federal researchers also have identified other potential resources that could produce large quantities of lithium, including the Salton Sea in Southern California, where Berkshire Hathaway Energy and other companies are working to extract lithium from hot liquid pumped up from an aquifer more than 4,000 feet below the ground by geothermal power plants.