Since the Gold Rush of 1849, tens of thousands of mines have been dug in California. Many of these mines were immediately abandoned when insufficient minerals were found, others were abandoned later when poor economics of the commodity made mining unprofitable, while still others were abandoned in 1942 after the issuance of War Production Board Order L-208. The result is that California’s landscape contains tens of thousands of abandoned mine sites, many of which pose health, safety, or environmental hazards. Every year people fall victim to the hazards of abandoned mines. Many sites possess serious physical safety hazards, such as open shafts or adits (mine tunnel). Thousands of sites have the potential to contaminate surface water, groundwater, or air quality. Some are such massive problems as to earn a spot on the Federal Superfund list.

In the interest of environmental and public health and safety, the Department of Conservation (DOC) undertook a three-year effort to determine “the magnitude and scope of the abandoned mine problem in California.” An inventory of abandoned mines was accomplished, culminating in this report to the Governor and Legislature. Prior to this effort, the number of abandoned mines reported was based solely on legacy databases and ranged from a low of 7,000 to a high of 20,000 abandoned mines. To get a more accurate picture of the nature and extent of this problem, existing literature and data were collected, input, and spatially analyzed through the implementation of a Geographic Information System (GIS). Data gaps were identified, and a field program was implemented to acquire site specific information. Data were collected at selected abandoned mine sites, by watershed, in various bioregions throughout the state. Significant mine features were photographed and precisely located by differentially corrected Global Positioning System (GPS). A standardized assessment and ranking protocol were applied to potential physical and chemical hazards observed. Field data, in addition to information collected from existing sources, were entered into a relational database and spatially and statistically analyzed for this report.​