Drilling muds constitute a much larger source of waste. Drilling muds are, on average, 86 percent water, much of which is taken from the wells themselves, 3 percent oil, 2 percent polymers, and 9 percent other materials (United States Congress, Office of Technology Assessment, 1992). The latter include clay, barite, and chrome lignin sulfonates. It should be noted that petroleum drilling accounts for virtually the entire national consumption of the mineral barite, or 1.4 MT. Consumption of clays, notably bentonite and fullers' earth, accounted for 1 MMT. On this basis, drilling muds must have weighed at least 25 MMT. EPA has estimated that drilling fluids used in 1985 weighed 57 MMT (United States Congress, Office of Technology Assessment, 1992). Additional ''associated wastes" amounted to 2 MMT, most of which are stored in ponds, where the water gradually evaporates.

EPA estimated that 3.7 billion tons of "produced waters" were generated by drilling activities in 1985 (United States Congress, Office of Technology Assessment, 1992). About 62 percent of this water was reinjected in oil and gas recovery operations, leaving 1.4 billion tons, or 1,270 MMT. Produced waters are usually saline and therefore constitute a disposal problem of some magnitude. (Most of the water is injected into wells.) The excess of produced waters would help to reconcile the water data for the mining sector as a whole. The 1983 Census of Manufactures reported that about 700 MMT of water were used for cooling in the gas-fractionation process (Bureau of the Census, 1983).

Natural-gas distribution by pipeline is a source of methane leakage to the atmosphere. In the United States in 1988, it is estimated that 17.15 MMT, equal to 3.5 percent of total production and 4.6 percent of the total quantity transported, disappeared from the system (International Energy Agency, 1991). We estimate that half of this was used to drive compressors whereas the other half, approximately 8 MMT, leaked into the atmosphere. Further losses occur in local distribution.

Petroleum Refining23

Total output of crude oil in the United States in 1988 was 402.6 MMT. Exports were 0.6 MMT, and imports were 269.05 MMT, for a total domestic crude oil supply of 671.0 MMT. Reported inputs of crude oil to domestic oil refineries were 680.687 MMT, leaving a discrepancy of 9.1 MMT. In addition, refineries consumed 17.15 MMT of natural gas as fuel, 16.230 MMT of natural gas liquids, and 22.585 MMT of intermediate feedstocks. Of the latter, 16.166 MMT were imported and 6.319 MMT were internal transfers of "gasoil" to be upgraded. Subtracting the gasoil amount from both inputs and outputs leaves a total of 730.23 MMT of net inputs (Table 5).

Crude oil is desalted before refining. Water pollution from this process contains emulsified oil as well as salts, ammonia, sulfides, and phenols. Also, this process involves considerable water use. Refinery products include noncon-

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