1988-04-11_PERMIT FILE - C1981008A ‎(15)‎

• Permit No: C1981008A
• IBM Index Class Name: PERMIT FILE
• Doc Date: 4/11/1988
• Doc Name: Probable Hydrologic Consequences
• Section_Exhibit Name: Tab 17
• Media Type: D
• Archive: Yes

percent GW-N20 water (pH 4.68) and 20 percent GW-N21 (pH 7.17) water would have a pH of
5.85; a minor amount of dilution increases the pH by more than one unit. When the low-pH
water is allowed to equilibrate with calcite (which is very common in the overburden), the
pH can exceed 7.0.
Summary. The available data indicate that a small proportion of the overburden may
produce acid through the oxidation of pyrite. Based on laboratory tests on overburden
cores, calcite is almost ubiquitous. Calcite serves two functions. First, it buffers the
pH of the water, which overall tends to slow the oxidation of pyrite, slowing the
production of acid. Second, it will neutralize the acid that is produced. The core
samples that exhibited low paste pH's (using distilled water) are surrounded by non-acid
producing, calcite-bearing rocks. The water that contacts the low-paste pH materials will
have first reacted with calcite, and therefore developed a pH-buffer capacity of its own;
the paste-pH test may overestimate the potential for acid production because of the use of
distilled water in the test.
The ground water monitoring data indicate that mixed overburden and interburden waters
have near-neutral pH's. Sample pH's less than 6 are associated only with the lower Dakota
coal . Where the coal 's permeability is high enough to produce about 5 gpm during
sampling, the acid-producing reactions do hot appear to be fast enought to maintain the pH
of the water less than 5.
Oxidation rates may increase because of the mining process. However, the supply of
oxidation is only one of the constraints on the production of acid. Other constraints are
imposed by the quantity of calcite present, and the reactivity of the pyrite. The
paste-pH test, conducted under oxidizing conditions, indicates that a very small
proportion of the overburden is likely to produce acid. The acid that is produced will be
quickly neutralized.
Geochemical Modeling.
Introduction. The geochemical speciation code PHREEQE was used to investigate
whether precipitation and/or dissolution of minerals influences water chemistry. Analyses
of samples from a variety of environments in the Nucla and Nucla East areas were chosen
for study. Results are presented in Table 17-10. The water analyses are given in the
17-44 Revised 04/11/88