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, N ~ co "f 140 8 ii ~^ s3~ ~~ ~~ ~g ~~ .120 100 80 60 '40 20 ,-' N aftz and Spangler ... _... NON-OIL-FIELD BRINE MIX LINE -OIL-FIELD BRINE MIX LINE -.__.- INJECTION W A lliR MIX LINE Do WA1ERSAMPLESFROMNAVAJO AQUIFER WI1H CL > 60 MG/KG .//.-.- \ // BRlCL RATIO PREDICTED AFIER Do / MIXING WITH ONE PERCENT OF / / OFB END-MEMBER WAlliR / . / / / ., ~ '~f ............4 ~ Do .........~Do Do~~..~.... o 10 100 1,000 10,000 100,000 CHLORIDE CONCENTRATION, IN MILLIGRAMS PER KILOGRAM constructed with the m,'ean non-OFB end-member water composition (Figu~e 6). If the salinity source was OFB or injection water, bromide-to-chloride ratios in water from the iN avajo aquifer wells should follow the upper mixing lines as the chloride concen- tration increases (Figu!,\e 6). This is not the trend shown and indicates OFB!is not the source of salinity. Because of the large difference in bromide concen- trations between the upg-qadient Navajo aquifer water and the OFB and injection water (Table 1), only a small volume of OFB is .required to produce a very large increase in the brorrlide-to-chloride weight ratio. For example, a mixture of one percent OFB with 99 percent Navajo aquifer :end-member water would increase the bromide-to"chloride weight ratio from less than 40 to larger tha~ 90 in the resulting mixture (Figure 6). The sensitivity of the bromide-to-chloride ratio in the Navajo aquifer to small volumes of OFB or injection water provid~s additional evidence that even small volumes of OF/3 or injection water salinity sources probably are not/responsible for the salinity increases in the Navajo 'aquifer. Furthermore, the sensitivity of the bromideLto-chloride ratio in Navajo aquifer waters to OFB ar.d injection-water salinity Figure 6. ~xing Lines Constructed Between ,Mean Concentrations of Potential End-~embe~ Water Comp~red to Bromide.to.Chloride Weight Ratios of Water Samples from the NavajO AquIfer. WATER RESOURCES BUGLETIN sources provides a useful tool for future monitoring in the study area. . Iodide.To-Chloride Ratios Similar to bromide, iodide can be enriched in organic material, providing a useful geochemical tool to differentiate between OFB and non-OFB salinity sources (Richter and Kreitler, 1991). Iodide is also l"el- atively conservative in ground-water systems;howev- er, it can be affected by sorption with iron and aluminum (Maida, 1989). Because of the lack of iodide analyses for non-OFB, . iodide data are not sufficient to construct end-member mixing models; however, a plot of the iodide-to-chlo- ride weight ratios compared with increasing chloride concentrations also indicates that OFB is riot the source of salinity in the Navajo aquifer (Figure 7). The five OFB samples in the study area have enriched iodide-to-chloride weight ratios that are sig- nifican tly larger than the ratios reported for' halite solution brines (Richter and Kreitler, 1991) (Figure 7). 1128 -~ ';~ ., ., d.