Figure 1-More than 3,000 mine sites were inventoried on National Forests and Bureau of Land Management Resource Areas in Montana. Of these, 336 sites had one or more adits discharging water at least part of the year. A total of 141 adit discharges were sampled.

MTDC and the Montana Bureau of Mines and Geology entered into a participating agreement to produce two reports dealing with AMD. The first report (Treating Acid Mine Drainage From Abandoned Mines in Remote Areas, 9871-2821-MTDC, 1998) summarized literature about the different types of passive treatment methods that offer the greatest chance of success for remote, inactive mines that produce flows of less than 20 gallons per minute (gal/min). This report discusses most of the Montana AMD sites with adit discharges that have potential for passive treatment. The information gathered about the mines provides a good basis to identify sites in Montana for the application of recharge control, a passive treatment method discussed in our first report. The Vindicator and Beatrice Mines in Montana are discussed in detail. These mines are considered good candidates for reducing or preventing AMD using recharge control.

The Montana Bureau of Mines and Geology (MBMG) lists 8,120 sites in its abandoned and inactive mines data base. The Bureau of Mines has inventoried and collected information for 3,678 of these sites on lands administered by the USDA Forest Service and USDI Bureau of Land Management (Figure 1). In a program developed by the Bureau of Mines and the Forest Service, the location of each site is verified and compared to a set of screening criteria that determines the action taken at each site. In general, the sites are investigated on a watershed-by-watershed basis within each National Forest or BLM Resource Area. Inventories and preliminary evaluations have been completed for the Beaverhead-Deerlodge, Helena, Kootenai, and Lewis and Clark National Forests and for BLM-administered lands statewide. Sites in the Lewis and Clark National Forest were not included in this report because all of discharging adits inventoried to date are on private land. All the information collected is stored in the Bureau of Mines Abandoned and Inactive Mines data base and Groundwater Information Center data bases. The site evaluations for each drainage are summarized in a series of reports by the Montana Bureau of Mines and Geology.

Figure 2-Of the 141 adit discharges sampled, only six were discharging more than 100 gallons per minute. Most discharges were from 5 to 10 gallons per minute.

While identifying mines and mills associated with National Forests and BLM Resource Areas, the Bureau of Mines identified 336 sites where one or more adits were discharging water at least part of the year. Of these sites, 127 were on Federal land. Fourteen of these sites had more than one discharging adit. A total of 141 adit discharges were sampled. Many other adits on private land may have an effect on National Forest lands. Those adits were not directly sampled. Their effects were measured by sampling on Federal lands upstream and downstream of the private site where appropriate. Other adits were classified as having discharges but were not sampled because they were not flowing at the time of sampling due to seasonal variations.

The majority of the adits (78 percent) discharge water at rates of less than 10 gal/min (Figure 2). Only six adits had discharges over 100 gal/min. The primary factor in controlling the rate of discharge is the relative depth and extent of the underground workings. Mines with extensive horizontal workings or with adits on the lower slopes of a watershed have larger capture zones to intercept ground water. The same is true for the horizontal extent of the workings. Sites with adit discharges greater than 50 gal/min generally have at least 1,500 feet of vertical and horizontal workings.

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