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1.3 PREPLANNING TOTAL MINING OPERATION 1.3.1 Acid-Base Account In the humid areas of the United States, the toxicity associated with acid results largely from the oxidation of iron disulfides. This process takes Place when earth disturbance activities such as mining (Temple and Koehler, 1954; Hill, 1970) and highway construction (Miller et al. , 1976) expose iron disulfides to the atmosphere. Since the public in the United States has supported legislation that acid-toxic or potentially toxic materials (a source of pollution) will not be left exposed, the need for a basis to evaluate overburden materials arose. Acid-base accounting is a dependable criterion by which overburden materials can be evaluated. An acid-base account consists of two measurements: (1) total or pyritic sulfur and (2) neutralization potential. The accounting balances maximum potential acidity (from immediately titratable sources plus sulfuric acid equivalent calculated from total sulfur) against total neutralizers (from alkaline carbonates, exchangeable bases, weather- able silicates or other rock sources capable of neutralizing strong acids as measured by the neutralization potentials). The total or pyritic sulfur content (see 3.2.4) accurately quantifies �. potential acidity of materials when all sulfur is present as a pyritic mineral. When gypsum is found in an overburden sample or the materials are weathered, sulfur occurs in the form of sulfates. Samples high in . organic carbon usually contain organic sulfur. When part of the sulfur . occurs in nonacid-producing forms, the maximum potential acidity as calcu- lated will be too high. It is for this reason that such calculations are referred to as maximums and that in doubtful cases appropriate acid and water leachings should be made to rule out those forms of sulfur which do not produce .acid (see 3.2.6). Then from the stoichiometric equation of pyrite oxidation, the maximum potential acidity can be calculated in terms of calcium carbonate equivalent. Overburden material containing O.1% sulfur (all as pyrite) yields an amount of sulfuric acid that requires 3.325 tons of calcium carbonate to neutralize one thousand tons of the material. The neutralization potential (see 3.2. 3) of overburden materials, the second component of a net acid-base account, measures the amount of neutralizers present in the overburden materials. This measurement is found by treating a sample with a known amount of standardized hydrochloric acid, heating to assure complete reaction, and titrating with a standardized base. The result is then expressed in calcium carbonate equivalents.. When balanced against acidity from the total sulfur measurement, a net acid-base account can be made. From the acid-base account, potentially toxic material is defined as any rock or earth material having a net potential deficiency of 5.0 tons of _+ calcium carbonate equivalent or more per 1000 tons of material. The 1000 tons is based on the assumption that an acre plow-layer contains 2 million pounds of soil. Regardless of the acid-base account, materials which have a pH of less than 4.0 in a pulverized rock slurry in distilled water are i defined as being acid-toxic. 3 ti }