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Bulkhead seals are another type of preventive or "source control" measure. The concept is that geochemical and flow equilibrium will be reached in the groundwater, whereupon anoxic conditions in the flooded workings will prevent or reduce dissolution and transport of heavy metals- Bulkhead seals are designed to prevent discharge to surface water through the adit opening by blocking the flow with an engineered hydrologic plug, flooding the mine. For most inactive mines, bulkhead seals are expensive and require considerable geologic and engineering investigation and characterization. Sites that have simple geology, sound rock, and limited subsurface workings may be amenable to this approach. Sometimes water inflow into mines can be reduced from remedial measures implemented outside of the mine workings. For example, grouting or sealing fracture azeas may be done from the surface- Amine near Eagle, Colorado, installed a well neaz a fracture zone to lower the water table. But all these hydrologic controls may not be enough or may be almost impossible to implement depending on specific chazacteristics of a site. Discharge from adits may need to be treated. There are a wide range of options, all of which have positive aspects and drawbacks. Generally, treatment involves raising pH levels if they are low and precipitating metals. Passive Treatment Techniaues Passive treatments have received a lot of attention from mine-drainage remediation specialists because of relatively low costs, low maintenance, and effectiveness. Here is a summary of passive treatment techniques from Herron et al (1999, p. 28): Anoxic limestone drains are the simplest method of introducing alkalinity into mine discharges. Anoxic limestone drains (ALD) are constructed by placing coazse limestone (3/4y - 3") inside an adit or in a fully sealed trench outside a discharging mine. In order for an ALD to function properly, the mine discharge must be devoid of oxygen. In the absence of oxygen, limestone will not become coated by iron and other metal hydroxides, which can shorten the useful life of limestone. In addition, the mine drainage should be relatively low in dissolved aluminum. Aluminum has been shown to precipitate in ALD's, causing plugging. It is theorized that very coarse limestone (4"-6") should pro~7de large enough pore spaces to minimize or prevent clogging by aluminum. The disadvantage of using larger limestone is the reduced surface area to react with the mine drainage. After the mine drainage exits the ALD, aeration causes precipitation of metals. The increase in pH due to ALD's is site specific, but generally does not exceed two standard units. Settling ponds aze often overlooked as an effective treatment method. Settling ponds aze particularly effective for treating near neutral mine drainages high in total suspended solids (TSS). Aeration of a neaz neutral pH mine drainage by means of a series of drops, followed by a settling pond can effectively remove 8