10 



The Bilateral Monitoring Committee 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. Beginning in 1988, the FWC was calculated fi-om the five-year 

 period preceding each plotted point. For example, the FWC for December 2000 is calculated fi-om data 

 generated over the period December 1995 to December 2000. 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 



TDS is inversely related to streamflow at the Intemational Boundary station. During periods of high 

 runoff such as spring fi-eshet, TDS decreases as the proportion of streamflow derived fi"om groimd 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 2000 are shown in Figure 

 3.4. The TDS ranged fi-om 720 mg/L on May 26 to 1,050 mg/L on December 20. The proposed short- 

 term objective for TDS is 1,500 mg/L. A time plot of the three-month moving FWC for TDS is 

 presented in Figure 3.5. Note that grab-sample-derived data are now plotted with dot symbols. 



