flotation. It is interesting to contrast this number with the only other published estimate we could find, for 1985, of 1,400 MMT (Science Applications International Corporation, 1985). The SAIC figure is far too low because it supposedly included the coal mining sector, which we discuss below.
Coal mining is the largest single source of waste materials. For every short ton of coal moved, 6.5 tons of mostly overburden wastes are produced (Anonymous, U.S. Department of Energy, personal communication, 1994). Because national soft coal production in 1988 was 862.1 MMT (weight at the mine), total materials handled in coal mining, exclusive of the coal itself, were on the order of 5,600 MMT. This is more than three times the amount of topsoil lost by erosion. Coal mining is also a source of methane; methane is trapped in the coal seams and released when the coal is pulverized. Later, we consider methane production from all fossil-fuel-related activities.
Some utility coal is washed to remove pyrites and ash, resulting in a significant further production of waste. In 1975, about 41 percent of soft coal was cleaned, resulting in 16 tons of coal refuse for every 100 tons of coal washed, for an 84 percent yield.22 By 1988, more low-sulfur western coal was mined and only about 30 percent of midwestern coal was cleaned. In 1988, therefore, we assume that 4.8 tons (0.3 x 16) of sulfurous refuse were produced at the mine per 100 tons of coal mined. Given that 862 MMT were shipped, approximately 900 MMT must have been mined, generating beneficiation wastes of about 47 MMT, give or take 10 MMT. Sulfurous refuse is a significant cause of acid mine drainage. The 1983 Census of Manufactures reported that coal mines discharged 470 MMT of water, including washing water (Bureau of the Census, 1983). Combustion emissions are discussed below.
In 1988, 86.2 MMT of U.S. coal were exported and 1.94 MMT were imported. Roughly 42 MMT of coal went to coke ovens, producing about 30 MMT of finished coke, 7.6 MMT of coke oven gas, 3.9 MMT of tar and breeze, and small quantities of other by-products, including ammonium sulfate. There were also minor fugitive emissions, mainly from the coke quenching process. The mass flows and wastes in the coal system are shown in Figure 4.
Petroleum and natural gas production involve relatively little waste, except water. During 1988, the U.S. oil and gas industry drilled 25,000 wells encompassing 124 million feet of holes. Assuming 6-inch pipe for the holes, and 5.5 liters of material removed per linear foot, this drilling would have generated 682 million liters total. A liter of water weighs 1 kg, and we assume drilling wastes displaced by pipe have an average specific gravity of 3, so the weight of displaced materials was about 2 MMT. Another 4-5 MMT of material was removed and displaced by the drilling mud (allowing for water content). We therefore estimate a total of 6-7 MMT of drilling wastes.