Table 1-Partial list of Environmental Protection Agency water-quality standards. The acronym MCL stands for maximum contaminant level.

	Primary MCL(1) (µg/L)	Secondary MCL(2) (µg/L)	Aquatic Life Acute(3,4) (µg/L)	Aquatic Life Chronic(3,5) (µg/L)
Arsenic	50	-	360	190
Cadmium	5	-	3.9/08.6(6)	1.1/2(6)
Copper	-	1000	18/34(6)	12/21(6)
Lead	50	-	82/200(6)	3.2/7.7(6)
Zinc	-	5000	120/210(6)	110/190(6)
pH (Standard Units) 6.5-8.5 (1) 40 CFR 141; revised through 8/3/93. (2) 40 CFR 143; revised through 7/1/91. (3) Priority pollutants, EPA Region VIII, August 1990. (4) Maximum concentration not to be exceeded more than once every 3 years. (5) 4-day average not to be exceeded more than once every 3 years. (6) Hardness dependent. The first value is calculated at a hardness of 100 mg/L; the second is calculated at 200 mg/L.

Figure 3-Discharges from 78 adits at inactive or abandoned mines in Montana were within Environmental Protection Agency water quality limits for the six standards studied.

Sixty-three of the 141 adit discharges exceeded one or more of the six water-quality criteria (Figure 3). The adits with the higher discharge rates generally had better water quality, but overall, there was no apparent correlation between water quality and discharge rate. As would be expected, there is a strong correlation between water quality and the the ore deposit and the mineralogy of the country rock. For the sites studied, there did not appear to be a relationship between water quality and the condition of the adit (open or closed).

The impacts of abandoned and inactive mines (Figure 4) can be classified using several factors, such as individual metals being contributed by the discharge, the acidity of the water, and the total flow. The rock type with which the deposit is associated may be used to predict water chemistry to a limited extent. The cumulative effect of several mines to the waters of an individual drainage should also be examined. The screening and sampling system used by the Bureau of Mines Abandoned and Inactive Mines Program is based on individual factors that may be used in any combination to rank mine sites according to the needs of the investigator. The total effect or total daily load of dissolved contaminants should be compared using several factors. In addition to adit discharges and flooded shafts, other factors include tailings or waste in the flood plain, seeps at the base of waste dumps, and springs associated with the site. These factors are not discussed in this report but should be considered in a site's overall assessment.

Figure 4-Although vegetation in the riparian area is healthy downstream from the Vindicator Mine in southwestern Montana, pore spaces in the stream substrate are filled with precipitation from acid mine drainage.

Direct treatment of adit discharges poses many limitations. Most sites are remote and without power. Year-round access may be limited. Any treatment design must be passive. Similarly, most adits have only a small area, often formed by waste rock, to serve as a staging area when setting up a treatment facility. Reclamation by removing or regrading waste rock (Figure 5) near the adit further limits the area available for passive, "end-of-pipe" treatment. The wetlands approach to treatment generally requires about 60 square feet of wetland for each gallon per minute of treated water, depending on the concentration of dissolved metals. An adit discharging 10 gal/min, would need 600 square feet of wetlands for treatment, more area than is available for treating most adit discharges. Direct control of adit discharge by plugging, grouting, or recharge control may require temporarily disturbing the portal area, but probably offers the best long-term solution to most adit discharge problems.

Figure 5-Waste rock was dumped in the stream channel at the Beatrice Mine site near Helena, MT. The stream channel now contains large amounts of iron hydroxide, called Yellow Boy by the miners, that is precipitated by acid mine drainage.

Controls on the rate and quality of the adit discharge are a function of the mineralogy, geology, and hydrogeology of the rock affected by the mine. Each of these three factors controls the quality, direction, and quantity of ground water flowing into and around the workings. The most common approach to eliminating discharge from an adit is to construct an adit plug. This option has proven to be expensive and, in several cases, has resulted in catastrophic failure. The adit discharge can be reduced or eliminated in many cases by reducing groundwater flow into the workings. Backfilling and recontouring mine-related excavation, mechanically reducing soil permeability, and diverting storm water and snowmelt can significantly reduce the infiltration of water into the workings. Detailed information of the extent of workings, the mineralogy, and production history of a mine is needed, but is often not available. Field reconnaissance and literature pertaining to the general area can be used to identify sites that may be good candidates for reducing or eliminating adit discharge by reducing groundwater flow into the workings.

Of the 63 adits discharging water that exceeds one or more water-quality criteria, about 47 would be good candidates for recharge control. Small open pits and cat-cuts uphill and above the workings act as catchment basins for snowmelt and storm-water runoff. Adits that are good candidates for recharge control are generally near the drainage divide. The effective recharge area for groundwater entering the mine is probably limited to a few thousand square feet. The remaining sites would probably require a combination of recharge control, grouting, and adit plugging. In some cases, only a reduction in flow could be expected, but that reduction may allow end-of-pipe treatment methods to be used.

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