27 



Boron concentrations for piezometer 71 2B were initially below 1.0 mg/L until an increasing trend 



began in 1987 with the concentration peaking in April 1992 at 26.6 mg/L. The boron concentration 

 then decreased and leveled off near 1 8 mg/L with an October 1 994 result of 1 7 mg/L. Chloride 

 concentrations trended down for 71 28 to 50 mg/L in 1988 from over 200 mg/L in 1984. Since 1988, 

 chloride levels have changed little with an October 1 994 result of 51 .8 mg/L. 



The total calculated seepage from the ash lagoons in 1994 was determined to be 1.67 litres per 

 second. This value is not significantly different from the 1993 calculated seepage of 1.62 litres per 

 second. 



The model used to determine seepage sums the calculated horizontal and vertical seepage 

 components from each ash lagoon. For Ash Lagoon 3 North the vertical seepage was detennined to 

 be zero due to pore pressure beneath the lagoon exceeding what the lagoon water level could 

 physically produce. This situation is believed to result from a compression of the soil beneath the 

 lagoon due to the weight of the ash and water in the lagoon. The effect is expected to dissipate over 

 time. 



Liner permeabilities for all lagoons except Ash Lagoon 3 North are about 10'^cm/sec. The line 

 permeability for Ash Lagoon 3 North could not be calculated because of the pore pressure situation 

 discussed above. 



3.3.3.2 Montana 



Samples were collected from monitoring wells 7, 16, and 24 during 1994. Well 7 is completed in Hart 

 Coal, well 16 is completed in the Fort Union Formation, and well 24 is completed in alluvium. No 

 significant changes in water quality were observed. Graphs of total dissolved solids for selected 

 wells are shown in Figure 3.12. 



