10 



conductance readings collected by the USGS. During some years, estimated monthly water-quality 

 sample data may be used in order to complete regression calculations of 3-month and 5-year flow- 

 weighted concentrations. Estimated monthly water-quality samples are based upon concentration data 

 from neighboring water-quality samples and specific-conductance data on the date of the missing water- 

 quality sample (or date midway between neighboring water-quality sample). 



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 from the five-year 

 period preceding each plotted point. For example, the FWC for December 2002 is calculated from data 

 generated over the period December 1995 to December 2002. 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 water-quality sample data collected by Environment Canada and the USGS in 2002 are shown in 

 Figure 3.4. There were seven water-quality samples obtained this year with TDS concentrations ranging 

 from 702 mg/L on April 18 to 1.080 mg/L on June 13. The proposed short-term objective for TDS is 

 1 .500 mg/L. 



The three-month moving FWC for TDS for the period of record is presented in Figure 3.5. The TDS 



