10 



Five-year FWCs for TDS (Figure 3.5) remained below the long-term objective of 1 000 mg/L. An 



increase in the 5-year FWCs to approximately 955 mg/L was observed in 1992. A 200-mg/L 

 increase had previously occurred in early 1988 and had been maintained through 1989 and 1990. 

 The 1991 rise in FWCs corresponds to depressed spring flows in the East Poplar River A similar 

 increase in TDS was seen during mid-1987. Relatively low spring discharges have occurred since 

 1 984. If this trend continues, it is expected that FWCs will reach the 1 000 mg/L objective. The TDS 

 increase 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 TDS trend in the data. 



"I — ' — I — ' — I — ' — I — ' — r 



-1 — I — I- 



Long-Term Objective (1000 mg/L) 



J . I I I I L 



J I I ^ \ I L 



1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 



Year 



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

 Poplar River at International Boundary. 



The relation between TDS and specific conductance generated from data collected from 1 975 to 



1 992 is as follows: 



TDS = (0.632 X specific conductance) -f 1 9.97 

 (r2 = 0.86, n = 449) 



