Chapter 10: Mining, Sand and other Backfill Wells.
Mine backfill wells are used in many mining regions throughout the country to inject a mixture of water and sand, mill tailings, or other materials (e.g., coal combustion ash, coal cleaning wastes, acid mine drainage (AMD) treatment sludge, flue gas desulfurization sludge) into mined out portions of underground mines. On occasion, injection (in low porosity grout form) also occurs into the rubble disposal areas at surface mining sites. Mine shafts and pipelines in an underground mine, as well as more “conventional” drilled wells, used to place slurries and solids in underground mines are considered mine backfill. Such wells may be used to provide subsidence control (the most common purpose), enhanced ventilation control, fire control, reduced surface disposal of mine waste, enhanced recovery of minerals, mitigation of AMD, and improved safety.
The physical characteristics and chemical composition of the materials that are injected into backfill wells vary widely depending on the source of the backfill material, the method of injection, and any additives (e.g., cement) that may be included. Data from leaching tests (e.g., USEPA Method 1311 Toxicity Characteristic Leaching Procedure (TCLP)) of backfill materials indicate that concentrations of antimony, arsenic, barium, beryllium, boron, cadmium, chromium, lead, mercury, molybdenum, nickel, selenium, thallium, sulfate, and zinc frequently exceed primary maximum contaminant levels (MCLs) or health advisory levels (HALs). Concentrations of aluminum, copper, iron, manganese, total dissolved solids (TDS), and sulfate, as well as the pH, frequently exceed secondary MCLs.
At sites where water is present in the injection zone (the previously mined ore body), the mine water may already exceed MCLs or HALs prior to injection either as a result of mining activity or natural conditions. At such sites, one objective of injection often is to improve the already poor quality of the mine water by reducing the availability of oxygen in the mine workings and/or neutralizing AMD.
In other areas, water from coal beds may be used to supply domestic wells. No incidents of contamination of a USDW have been identified that are directly attributable to
injection into mine backfill wells. Although ground water contamination is not uncommon at mining sites, it is generally difficult to identify the specific causes. The chance that backfill injection will contribute to ground water contamination is highly dependent onsite conditions, including mine mineralogy, site hydrogeology, backfill characteristics, and injection practices. Some studies of the effects of backfill injection on mine water quality show that concentrations of some cations and anions can increase in mine water following injection, whereas concentrations of trace metals generally are relatively unaffected or decline over time. Other studies (at other sites) show an increase in selected