- 42 - 

 Montana 



Ground water quality during 1986 was essentially unchanged in 19 of 

 the 21 monitoring wells. This condition is believed to be the 

 representative condition for the area being monitored. Figure 10 

 shows the field specific conductance values for the same wells 

 discussed in the water level section. These data show considerable 

 scatter; the scatter is attributed in part to the difficulty in 

 developing wells after installation, in part to differing quantities 

 of water pumped from the wells prior to measuring the specific 

 conductance, and in part to some natural variability within the 

 aquifer being sampled. However, data from some wells show so much 

 variability that the well construction is questioned. The Fort 

 Union well (no. 9) fits into this category. This well is reported 

 to be open to the formation from 53 to 203 feet (16.2 to 61.9 m) 

 below the land surface; measured depths Include 49 and 140 feet 

 (14.9 and 42.7 m) as well as 208 feet (63.4 m) suggesting that the 

 size of slots in the casing may be too large within parts of the 

 unit. 



In 1986, wells 4 and 22 had substantial changes in water chemistry. 

 Well 4, located east of the Poplar River, is reported to be completed 

 in the Fox Hills-Hell Creek aquifer. Figure 10 shows the specific 

 conductance data from this well. The specific conductance, which 

 has been averaging 2,540 PS/ cm fell to 1,206 WS/cm in the 1986 sample 

 (Figure 10). The lower conductance was accompanied by at least a 50 

 percent decrease in the major constituents sodium, bicarbonate, 

 chloride, and sulfate. The calcium content in 1986 (13.6 mg/L) is 

 essentially unchanged; however, the strontium content more than 

 tripled to 0.92 mg/L. This indicates that shallow alluvial water or 

 surface runoff has entered the well, and that ion exchange reactions 

 have occurred to decrease the concentration of alkaline earths in 

 the water. The dilution, compared with previous samples, is also 

 supported by the decrease in boron content from a previous average 

 value of 1,760 to 760 Ug/L. 



