The Bureau of Mines, under a cooperative agreement with Santa Cruz joint venture, owned by Asarco and Freeport Copper Company, is investigating the feasibility of in situ leach mining a deep copper oxide deposit near Casa Grande, Arizona. In keeping with the National Environmental Policy Act, the Bureau intends to prepare an environmental assessment for this project, which will provide in brief sufficient evidence and analysis to determine whether to prepare an environmental impact statement (eis) or a finding of no significant impact (fonsi). An eis is required if the action is determined to have a significant effect on the human environment, whereas a fonsi is written if there is no such effect. Preliminary investigations, discussions with industry researchers, and comments and concerns expressed at a public meeting held in Casa Grande in July 1990 revealed several specific issues of potential environmental concern: air quality, sulfuric acid handling and use, groundwater and hydrologic modeling, surface subsidence, attenuation, and recycle of fluids. Each of these will be the subject of future Open File Reports, whereas the present report summarizes the results of the investigation into the natural attenuation capacity of rock from the Santa Cruz site. Overall process description, geology and hydrology, mineralogy and petrology, rock quality and structure, and geophysics will be the subject of other background documents.