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The relatively low SAR values and sodium concentrations of less than 

 100 mg/L associated with the first group of coal wells is in distinct 

 contrast with coal well data from other eastern coal fields in Mon- 

 tana and the second group of wells at this site. These samples may 

 represent waters in the recharge portion of the flow system, alluvial 

 recharge, or a separate flow system. Because the coals are thin, and 

 the monitoring wells south of the border are deep, sealing materials 

 possibly did not isolate permeable units adjacent to the coal bed 

 that was the monitoring target during drilling and well installation. 

 Alternatively, the chemical differences in the analytical results may 

 be the result of ground water interaction with the overburden mate- 

 rials (Fort Union or Ravenscrag Formations). The value of the moni- 

 toring wells is not decreased so long as consideration is given to 

 the above conditions. Using the first group of wells completed in 

 the coal as a control group, the September 1988 samples show the 

 following results: 



The hardness represents substantial calcium and magnesium concentra- 

 tions in the water. These indicate that the water chemistry data 

 reflect shallow overburden effects or even alluvial recharge. This 

 hypothesis is supported by the one Fort Union or Ravenscrag comple- 

 tion (well 9) in the older group. The well is 208 feet deep and its 

 water analyses have a model SAR value of 9.5; even if an arithmetic 

 average were used which would include two (spurious) low values, 

 the SAR from these samples is 8.0, a value distinctly greater than 

 that associated with the coal bed completions. 



