GROUND WATER QUALITY 



Montana 



The ten observation wells on the United States side of the International 

 Boundary have been sampled during October on an annual basis since I98O. 

 Wells 2, 3, and h have a casing diameter of 3-8 centimeters and this small 

 diameter has precluded adequate pumping for obtaining a reliable sample. Thus, 

 analytical results from these wells are considered unreal iable and have not 

 been included in characterization of the water quality. Wells 5-11 have larger 

 diameter casings and sufficient pumping prior to sample collection was possible 

 to provide reliable water-quality results. 



Throughout the sampling period the wells had little change in their major ion 

 water chemistry (Figure 5). Wells 6 and 7, from the Hart coal seam, were 

 similar in major ion chemistry with the cations dominated by calcium and 

 magnesium with sodium contributing less than 30 percent of the total. The 

 anions were strongly dominated by bicarbonate. Total dissolved solids concen- 

 trations were near 500 mg/L and boron concentrations were near 0.8 mg/L. Water 

 from alluvial wells 5, 7, and 8 had similar chemical compositions as those 

 from the Hart coal seam, especially with respect to anion ratios and TDS concen- 

 trations. The cation ratios and boron concentrations were slightly more 

 diverse. Waters from wells that penetrate the Fort Union and Fox Hills-Hell 

 Creek aquifers (wells 9 and 11, respectively) had a sodium bicarbonate type 

 water. The total dissolved solids concentrations from both wells was near 

 900 mg/L and the boron concentrations were slightly higher than 1.8 mg/L. 



Concentrations of nitrate nitrogen were usually low except for alluvial well 

 number 5 where the 3 samples (1980, I98I , and I982) ranged from I.8I to 

 7.58 mg/L. The maximum concentration was below 10 mg/L which is the recommended 

 upper limit for domestic water supplies. Fluoride concentrations were always 

 less than 1 mg/L except for well number 11 where concentrations up to 3.1 mg/L 

 were measured. 



Trace element concentrations from all samples met drinking-water standards. 

 But, the following elements showed slightly higher concentrations in the wells 

 listed as compared to the average of all wells. 



19. 



