11 



Five-year FWCs forTDS (Figure 3.5) remained below the long-term objective of 1 000 mg/L. A slight 

 decrease in the 5-year FWCs for TDS from approximately 975 mg/L to 966 mg/L was observed in 

 1994. This decrease was a result of spring runoff in 1994. A 200-mg/L increase had previously 

 occurred in early 1988 and had been maintained through 1989 and 1990. The 1991 rise In FWCs for 

 TDS corresponds to depressed spring flows in the East Poplar River. A similar increase in FWCs for 

 TDS was seen during mid-1987. Relatively low spring discharges have occurred since 1984. Even 

 though there was a slight decrease in the FWCs for TDS for 1994, the upward trend in FWCs for 

 TDS will probably continue if the surface-water runoff continues to be below average. The Increase 

 in FWCs for TDS can be explained in part by salt build-up in Cookson Reservoir as a result of water 

 being used for cooling. Forced evaporation causes salts to concentrate within the reservoir. This 

 process is further driven by drought conditions which prevailed over the last half of the data record 

 (Lang and Jones, 1988). In addition, low-flow conditions (when flows are derived largely from 

 ground-water sources) likely increase TDS concentrations and yields a positive trend in FWCs for 

 TDS. 



Q 



1300 

 1200 

 1 100 - 

 1000 



900 



800 



700 - 



600 - 



500 



400 



300 



200 



100 



1 — ' — i — ' — r 



J I I I \ i \ I I I I . 1,1 ,1,1,1,1 



1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 



Year 



Figure 3.5 Five- Year Moving, Flow-Weighted TDS Concentration for East Poplar 

 River at International Boundary. 



