8 



For the period prior to 1982, the three-month moving flow- weighted concentration (FWC) for 

 boron and TDS was calculated solely from monthly monitoring results. Since the begiiming of 

 1982, the USGS has monitored specific conductance daily in the East Poplar River at the 

 International boundary, making it possible to derive boron and TDS concentration using a linear 

 regression relationship with specific conductance. Thus, the three-month FWC for boron and TDS 

 for the period 1982 to 1996 was calculated from both the results of monthly monitoring and the 

 daily concentration estimates. 



The Bilateral Monitoring Committed adopted the approach that for the purpose of comparison with 

 the proposed IJC long-term objectives, the boron and TDS data are best plotted as a five-year 

 moving FWC which is advanced one month at a time. Prior to 1988, long-term averages were 

 calculated for a five-year period in which 2.5 years preceded and 2.5 years followed each plotted 

 point. Begiiming in 1988, the FWC was calculated from the five year period preceding each 

 plotted point. For example, the FWC for December 1996 is calculated from data generated over 

 the period December 1991 to December 1996. The calculations are based on the results of samples 

 collected throughout the year, and are not restricted to only those collected during the months 

 bracketing the period of irrigation (March to October) each year. 



3.2.5.1 Total Dissolved Solids 



There is an inverse relationship between TDS and streamflow at the International Boundary 

 station. During periods of high runoff such as spring freshet, TDS drops as the proportion of 

 streamflow derived from ground water decreases. Conversely, during times of low streamflow 

 (late summer, winter) the contribution of ground water to streamflow is proportionally greater. 

 Because ground water has a higher ionic strength than the surface water entering the river, the 

 TDS of the stream increases markedly during low flow conditions. 



TDS grab sample data collected by Environment Canada and the USGS in 1996 are shown in 

 Figure 3.3. TDS ranged from 731 mg/L on February 14 to 1100 mg/L on December 19. The 

 proposed short-term objective for TDS is 15(X) mg/L. A time plot of the three-month moving 

 FWC for TDS is presented in Figure 3.4. The objective has not been exceeded during the period 

 of record. On inspection of the plot however, it is apparent that the three-month FWC has been 

 increasing gradually, year by year. 



