- 26 - 



conditions (Figure 10) . The highest boron concentrations occurred in 

 1981, 1986, and 1988 and are likely the product of low flow conditions. 

 The seasonal boron pattern observed in Figure 19 closely resembles that 

 of TDS (Figure 11) ; the lowest boron concentrations occur from March to 

 July coinciding with high flow conditions. 



3.5 

 3.0 

 2.5 



2.0 



c§ i.o 



0.5 - 



0.0 





JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC 



Month 



Figure 19. --Box plots of monthly boron concentrations for East Poplar 

 River at International Boundary for period 1981-90. 



The histogram and summary statistics for boron are shown in Figure 20. As 

 with TDS, boron data are not normally distributed (90% confidence level) , 

 showing a significant negative skew. As shown in Figure 21, boron levels 

 undergo a statistically significant increase from 1981 to 1990 (90% con- 

 fidence level) . The boron increase, as calculated by the Seasonal Ken- 

 dall Sen Slope Estimate, is approximately 0.0285 mg/L/year. As with TDS, 

 this trend may be explained by the drought conditions in addition to con- 

 sumptive use of water for cooling and natural evaporation from Cookson 

 Reservoir . 



