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

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Nucla East Water Quality Data. Two monitoring wells, both completed in the lower Dakota coal, yield water samples with pH's lower than 5.0. GW-N14 has produced samples with pH's of 5.05, 6.47, 5.80, 4.90 and, 4.91. The sample with a pH of 4.90 had a cadmium concentration of 10 ug/l (the Drinking Water Standard). No other Primary Standards were met or exceeded. The cadmium concentration may not be a function of pH, however. Well GW-N15, which samples the overburden at this site, has a neutral pH and higher cadmium concentrations. The arsenic concentration in the same sample was 8 ug/l, and the arsenic concentration in the 4.91 pH sample was 1 ug/l (the detection limit). The Primary Drinking Water Standard is 50 ug/l . No other samples had detectable arsenic. The other well with low pH water is GW-N20, also completed in the coal . Through time, the pH of its water has been 4.68, 4.95, 5.522 5.82, and 5.65. Only the first sample had detectable arsenic (4.0 ug/1) and lead (70 ug/1). The lead data are very suspect (Wells GW-N13, GW-N14, GW-N18, GW-N20, GW-N21, and GW-N22 had reported lead concentrations ranging from 40 to 80 ug/l for samples collected in July 1986). No other samples from any of these wells have had detectable lead. Sample contamination or laboratory problems may have occurred. Of interest in the GW-N20 samples is the increase in sample pH through time. This well has a higher yield than GW-N14 does. The increase in pH (and coincidental decrease in sulfate and many other parameters) suggest that the low pH water in the coal has been removed during sampling and has been replaced by other nearby water. The rate of acid production is not rapid enough to maintain pH's less than 5.0. 'cable 17-9 summarizes the available data on pH of water samples from the Nucla East area, as a function of stratigraphy. Twenty percent of the measurements were less than pH 6. All of the low pH readings are for samples collected from the Dakota coal; presumably any problems associated with the coal will decrease when it is mined. Monitoring wells which sample the interburden also sample the overburden so that it is not possible to determine the quality of the interburden water, especially from the upper and lower interbeds. However, no samples with pH less than 6.0 have been collected from these wells. Presuming that acidic water is present, mixing easily neutralizes the acid. Mixing of acidic water from GW-N20 (coal) and neutral water from GW-N21 (overburden) was investigated using PHREEQE. The results can be easily summarized. If no minerals (such as calcite) are (theoretically) allowed to dissolve or precipitate, a mixture of 80 17-42 Revised 04/11/88