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 three-month and five-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 samples). 



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 2003 is calculated from data 

 generated over the period December 1998 to December 2003. 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 East Poplar River 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. 



Concerns over an upward frend in the concentrations of TDS in the East Poplar River between the late 

 1980's and 1995 were investigated in 2002. The results of the investigation indicated that the temporal 

 changes in TDS concentrations were most likely primarily linked to natural drought events. Statistical 

 analyses of discrete time periods showed no significant difference in TDS concentrations between the 

 period 1976 to 1985, in which no trend was noted, and the period 1986 to 1995, in which an apparent 



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