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_ Nucla East Water Ouality Data. Two monitoring wells, both completed in the lower <br />• Dakota coal, yield water samples with pH's lower than 5.0. GW-N74 has produced samples <br />with pH's of 5.05, 6.47, 5.80, 4.90 and, 4.91. <br />The sample with a pH of 4.90 had a cadmium concentration of 10 ug/1 (the Drinking Water <br />Standard). No other Primary Standards were met or exceeded. The cadmium concentration <br />may not be a function of pH, however. Well CW-N15, which samples the overburden at this <br />site, has a neutral pH and higher cadmium concentrations. The arsenic concentration in <br />the same sample was B ug/1, and the arsenic concentration in the 4.91 pH sample was 1 ug/1 <br />(the detection limit). The Primary Drinking Water Standard is 50 ug/1. No other samples <br />had detectable arsenic. <br />The other well with low pH water is CW-N20, also completed in the coal. Through time, the <br />pH of its water has been 4.68, 4.95, 5.52, 5.82, and 5.65. Only the first sample had <br />detectable arsenic (4.0 ug/1) and lead (70 ug/1). The lead data are very suspect (Wells <br />GW-N13, CW-N14, CW-N18, CW-N20, GW-N21, and CW-N22 had reported lead concentrations <br />ranging from 40 to 80 ug/1 for samples collected in July 1986). No other samples from any <br />of these wells have had detectable lead. Sample contamination or laboratory problems may <br />have occurred. Of interest in the GW -N20 samples is the increase in sample pH through <br />time. This well has a higher yield than GW-N74 does. The increase in pH (and <br />coincidental decrease in sulfate and many other parameters) suggest that the low pH water <br />in the coal has been removed during sampling and has been replaced by other nearby water. <br />The rate of acid production is not rapid enough to maintain pH's less than 5.0. <br />Table 17-9 summarizes the available data on pH of water samples from the Nucla East area, <br />as a function of stratigraphy. Twenty percent of the measurements were less than pH 6. <br />All of the low pH readings are for samples collected from the Dakota coal; presumably any <br />problems associated with the coal will decrease when it is mined. Monitoring wells which <br />sample the interburden also sample the overburden so that it is not possible to determine <br />the quality of the interburden water, especially from the upper and lower interbeds. <br />However, no samples with pH less than 6.0 have been collected from these wells. Presuming <br />that acidic water is present, mixing easily neutralizes the acid. <br />Mixing of acidic water from GW-N20 (coal) and neutral water from CW-N21 (overburden) was <br />• investigated using PHREEpE. The results can be easily summarized. If no minerals (such <br />as calcite) are (theoretically) allowed to dissolve or precipitate, a mixture of 80 <br />17-42 Revised 04/71/88 <br />