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classification diagram (Figure 18) classifies they ground water as having a high to very high salinity hazard and a low sodium hazard. Generally, the ground water is not suitable for irrigation under ordinary conditions. The water quality data presented in this section represents up to seven years of ground water monitoring. IC is apparent after close inspection of the data that no noticeable riegredation to the ground water quality has occurred. The following section contains a brief description of the site specific ground water quality. 1. Well GW-N1 -- Underburden Well GW-N1 was sampled semiannually 1`or water quality from October 1979 until December 1982. They TDS concentrations of the water samples range between 2868 and 3446 mg/1 with a mean concentration of 3137 mg/1. The pH values range from 6.5 to 7.0 units with a mean value of 6.7 units. The water is slightly acidic, and very hard with a mean hardness of 2096 mg/1. Figure 19 shows that the TDS concentration decreases with • the shallowing of the water level. The influx of lower TDS recharge water from the irrigation system temporarily lowers the overall TDS of the ground water monitored at this well. 2. Well GW-N4 -- Dakota Coal Well GW-N4 is located down gradient from mining and below the outfall of NPDES discharge point 001. this well is part of the present ground water quality monitoring proyram and has been sampled on a semiannual schedule since October, 1979. This well has the highest TDS concentrations observed in any of the monitor wells at the Nucla Mine, ranging from 5438 to 7100 mg/1 with a mean concentration of 6723 mg/1. The pH of the water ranges from 6.6 to 7.3 units with a mean value of 6.8 units. The water is very hard with a mean hardness of 4829 mg/1. Figure 21 shows that there is a slight lowering of the TDS concentration corresponding to the shallowing of the water level caused by recharge from the irrigation system. , 102