The complete document is available for downloading at no charge in PDF format, which requires Acrobat Reader. If you do not have Acrobat Reader installed, visit Adobe's "Free Acrobat Reader Software" to obtain it.

Click here for the 353k PDF file of NBMG Open-File Report 95-4, or here for the PDF file of the cover.

This report presents new data and the compilation of existing data on water quality at inactive and abandoned mines in Nevada. Evaluation of these data helps assess the need for additional investigations of inactive and abandoned mines. In addition, this report presents data on water quality at open pits that began filling with water within the last two decades.

This report focuses largely on water-quality issues, such as acid-mine drainage, that are the direct result of groundwater and surface water interacting with rocks exposed during mining. At the sites visited in Nevada, few localities where water flows from inactive or abandoned mines have acid-mine drainage problems, and of these, few have high flow rates.

Water quality generally is predictable from geological knowledge about ore-deposit type and mineralogy of ore and associated wall rocks. Acidic, metal-laden waters occur predictably at mineral deposits that contain significant quantities of pyrite (iron disulfide) and at which most of the acid-buffering capacity of the wall rocks had been destroyed by the natural ore-forming process. Wall rocks at many pits at which waters have been sampled contain substantial amounts of calcite, which tends to buffer the pH at slightly basic conditions.

Evidence of the occurrence of natural springs of acidic, arsenic-rich, metal-bearing waters, which flowed prior to any mining activity, is found locally in surface rocks cemented by iron oxide.

On the basis of counting adits and shafts shown on all 7.5-minute, 1:24,000-scale topographic maps covering the state, field-based estimates of the numbers of adits and shafts that do not appear on topographic maps, field measurements of rates of water flow from adits, and new chemical data in this report, we estimate that the number of inactive and abandoned mine sites that may be of concern regarding acid-mine drainage is likely to be fewer than 100, or less than 0.05% of the total number of inactive and abandoned mine sites in the state (estimated to be 225,000 to 310,000). The number is low largely because the climate in most of Nevada is arid.

The impacts on groundwater and surface water from ore processing, such as use of mercury and cyanide, and from other industrial activity at mining and milling sites, such as waste disposal, are not considered in this report but deserve further study.

Numerous sites, estimated to be approximately 50,000, present varying degrees of physical safety hazards to Nevada residents and visitors. The Division of Minerals operates programs to identify and rank those sites and secure them to avoid accidental harm to passers by.

To begin to set priorities for water-quality remediation, investigations are needed at sites not visited during this study. Follow-up measurements of flow rates and dilution downstream from sites identified in this study as having potential problems also are warranted, as are investigations of contamination from ore processing and other industrial activities at the sites. Knowledge of mineral-deposit type will be helpful in setting priorities for follow-up investigations and for possible cleanup. In particular, those deposits with high concentrations of iron disulfide and low concentrations of acid-buffering minerals are more likely to be problems than others. In Nevada, the deposits of greatest concern are quartz-alunite (high sulfidation) precious metal deposits in volcanic rocks and porphyry copper and porphyry copper-molybdenum deposits in plutonic rocks. Massive sulfide deposits, which meet these criteria as well, are uncommon in Nevada.