The U.S. Bureau of Mines under a cooperative agreement with Asarco Santa Cruz, Inc., has started an investigation into the feasibility of in situ leach mining of a deep copper oxide deposit near Casa Grande, Arizona. Because this investigation involves a federal agency, all requirements of the National Environmental Policy Act (NEPA) must be observed. To meet these requirements, the Bureau intends to prepare an environmental assessment (ea) for this project to evaluate the significance of anticipated environmental impacts that may result. The ea briefly provides sufficient evidence and analysis for determining whether to prepare an environmental impact statement (eis) or a finding of no significant impact (fonsi). An eis is prepared if the action is determined to have a significant effect on the human environment. A fonsi is written if it is determined that a significant effect on the human environment will not result from the proposed action. Under a fonsi, the proposed action is exempt from requirements to prepare an eis. Preliminary investigations, discussion between Bureau and industry researchers, and comments and concerns expressed at a public meeting held in Casa Grande, Arizona, in July 1990 revealed several specific issues of potential environmental concern, as follows: air quality, sulfuric acid handling and use, ground water and hydrologic modeling, surface subsidence, attenuation, and recycle of fluids.