- 30 - 



The second group of monitoring wells, completed in coal and located 

 essentially to the west of the first group, has distinctly higher 

 sodium levels in four of the five wells as evidenced by the September 

 1988 sample results: 



The results provide an interesting contrast to the results of the 

 first group. The sodium content does not seem to decrease with 

 depth in a pattern similar to that of the first group. This could 

 be the result of inadequate equilibration time to remove the effects 

 of sealing the zone above the sand-packed screens with a sodium- 

 bentonite sealant; however, the effect is essentially restricted to 

 completion in coal beds. 



An alternative interpretation, which has fewer inconsistencies, is 

 that the coal horizons are separated by sands of tributary channels 

 and by crevasse-splay sands. The wells completed in coal beds, which 

 are characterized by higher sodium concentrations and SAR values, are 

 in the western half of the study area. They probably are not hydro- 

 logically continuous with the coal-completion wells in the eastern 

 half of the area. The stratigraphic relationships are changing 

 rapidly in this area, with less than eight feet (net) of Hart Coal 

 present at the International Boundary (Irvine and others, 1978, Part 

 1, p. 71; Part II, pi. 30, 31). Plate 33 of that report also shows 

 that the Cornach Coal thins (net coal of four feet or less) at the 

 International Boundary, and the southern lobe of this deposit has 

 been separated from the more extensive northern lobe by erosion. 



