14 

 example, the FWC for December 1 991 refers to the FWC of the period December 1 986 



to December 1991. It should be emphasized that the calculations have been based on 



the results of all samples collected for the three-month and five-year periods, and not 



restricted to samples collected during March to October. 



3.5.1 Total Dissolved Solids 



There is an inverse relationship between IDS and stream flow at the International 

 Boundary station. During periods of high runoff, such as spring freshet, IDS drops as 

 the proportion of stream flow derived ultimately from groundwater decreases. Conversely, 

 during times of low stream flow (late summer, winter) the contribution of groundwater to 

 stream flow is proportionally greater. Because the natural groundwater has a higher ionic 

 strength than the surface water entering the river, the TDS of the stream increases 

 mari<edly during low flow conditions. 



TDS grab sample data collected by Environment Canada and the USGS in 1991 are 

 shown in Figure 3.3. TDS ranged from 866 mg/L (March 14) to 1144 mg/L (May 16). 

 The proposed short-term objective for TDS is 1500 mg/L. A time plot of the three-month 

 moving FWCs for TDS is presented in Figure 3.4. No exceedences of the objective have 

 been observed during any three month period since 1975. The three-month FWCs 

 remained confined within a narrow range centred around a mean of approximately 900 

 mg/L during 1991. 



