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3. Well GW-N3 -- Underburden <br />• Well GW-N3 is a part of the present ground water quality monitoring <br />program and has been sampled semiannually since October, 1979. The <br />TDS concentrations of the water samples collected from this monitor <br />well range from 2050 to 3250 mg/1 with a mean concentration of 2761 <br />mg/1. The pH values range from 6.4 to 7.5 units with a mean value <br />of 6.66 units. The water is very hard, with a mean hardness of <br />1932 mg/1. Figure 20 shows that the TDS concentration decreases <br />with the corresponding rise in water level caused by recharge of <br />lower TDS irrigation water. <br />4. Well GW-N5 -- Underburden <br />Well GW-N5 is located next to the mine shop and was intended to <br />be used as the office and shop supply welt. Extremely poor <br />water quality (primarily taste) precluded its intended use <br />except for use as a monitor well. This well was sampled <br />semiannually for water .quality from October, 1979 until <br />December, 1982. The TDS concentrations of the water samples <br />• range from 580 to 855 mg/1 with a mean concentration of 679 <br />mg/1. The pH values range from 7.7 to 9.5 units with a mean <br />value of 8.34 units. The water is hard with a mean hardness of <br />343 mg/1. However, it is by far the softest water sampled at <br />the Nucla Mine. There does not appear to be a correlation <br />between depth to water and TDS at this well (see Figure 22). <br />5. Well GW-N6 -- Underburden <br />Well GW-N6 has been sampled semiannually for water quality from <br />October, 1979 until December, 1982. The TDS concentrations of <br />the water samples range from 3667 to 4292 mg/1 with a mean <br />concentration of 4025 mg/1. The pH values range from 6.5 to <br />6.6 units with a mean value of 6.56 units. The water is very <br />hard with a hardness of 2770 mg/1. There is a very subdued <br />correlation between TDS and depth to water (Figure 23). <br />6. Well GW-N9 -- Overburden and Dakota Coal <br />• Well GW-N9 is part of the present ground water quality monitoring <br />103 <br />