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• Another indicator of the potential for producing acid is the paste pH.. This is measured by forming a slurry of the solids in distilled water. While this test is not quantitative in terms of amount of acid that potentially could be produced, it reflects the presence or absence of reactive pyrite. An important factor in the production of acid by oxidation of pyrite is the rate that ferrous iron is oxidized to ferric iron. At neutral pH's and higher, the reaction occurs quickly. At lower pH's, bacterial catalysis is required for the reaction to proceed rapidly. The bacteria become active at pH's below approximately 4.5 to 5. Within the approximate pH range of 5 to 7 there is a "kinetic valley" where the reaction proceeds slowly. The neutralization capacity of the rock, primarily provided by calcite, keeps the pH above the range where bacterial catalysis is effective. The calcite serves primarily to prevent the acid from being produced, and secondarily to neutralize the acid that is produced. Nucla East Core Data. Paste pH's and ABP's were measured on samples of overburden and interburden from the Nucla East area. Figure 17-14 is a plot of ABP versus paste pH • for data from core holes in the Nucla East area. Note that not all samples with ABP's less than zero produce paste pH's less than 7.0, rather many with negative ABP's have pH's greater than 7.0. The presence of pyrite, or a negative ABP, is not itself an indicator that acid problems are likely. There appear to be two separate groups or populations in the plot. Group A samples form a horizontal trend line where the pH does not seem to be a function of ABP. The pH in this group is generally above 6.5 or 7.0. The second group (B) consists of samples where the pH does appear to be a function of ABP. The overburden, upper interburden, and lower interburden contain samples in Croup B, though not exclusively. For example, the overburden samples fall into both groups. Based on Caruccio's work, the distinction betneen the two groups may be the presence or absence of framboidal pyrite. The figure further illustrates that only a few of the samples had low pH's. As previously discussed, acidic zones (pH 4.5) were found in the upper and lower interburdens (Units 2 and 4) in Holes #872E, #886E, and #871E. No acidic zones were found in the underbu rden. A thin acidic horizon (0.7 ft) occurred in the overburden in Hole . #882E. Of the 448 ft of core from rocks overlying the lower Dakota coal in the Nucla East area, 14 ft (3.1 percent) exhibit paste pH's lower than 4.5. 17-40 Revised 04/11/88