Later in 1978, Johnston joined the United States Geological Survey (USGS), where he monitored volcanic emission levels in the Cascades and Aleutian Arc. There he helped to strengthen the theory that eruptions can be predicted, to some degree, by changes in the makeup of volcanic gases. Fellow volcanologist Wes Hildreth said of Johnston, "I think Dave's dearest hope was that systematic monitoring of fumarolic emissions might permit detection of changes characteristically precursory to eruptions ... Dave wanted to formulate a general model for the behavior of magmatic volatiles prior to explosive outbursts and to develop a corollary rationale for the evaluation of hazards." During this time, Johnston continued to visit Mount Augustine every summer and also assessed the geothermal energy potential of the Azores and Portugal. In the last year of his life, Johnston developed an interest in the health, agricultural, and environmental effects of both volcanic and anthropogenic emissions to the atmosphere.