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 F WC 

 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 from the five 

 year period preceding each plotted point. For example, the FWC for December 1999 is calculated from 

 data generated over the period December 1993 to December 1999. 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 International Boundary station. During periods of high 

 runoff such as spring freshet, TDS decreases 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 1999 are shown in Figure 

 3.4. The TDS ranged from 570 mg/L on March 30 to 1 040 mg/L on December 20. The proposed short- 

 term objective for TDS is 1500 mg/L. A time plot of the three-month moving FWC for TDS is 

 presented in Figure 3.5. 



