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N aftz and Spangler . l\:) facility are sufficient; th,erefore, make-up water is not l'..) needed for injection op4rations. In contrast, varying <=> amounts of make-up water from the alluvial aquifer .....' have been used in the past as well as currently (1993), for injection operations at the Aneth, Rather- ford, White Mesa Unit, land McElmo Creek injection facilities. The mixing and continued recycling of the injection water at the i"iiection facilities has impart- ed a unique isotopic signature to these OFB waters that must be considered In the isotopic mixing modeL In order to consider, the effects of mixing, the isotopic composition of an injection water was approx- imated along the OFB mixing line in graph B (Fig- ure 9), mixing an equ41 volume of OFB from the Aneth, Ratherford, and McElmo Creek facilities with an equal volume of water from the alluvial aquifer. The isotopic compositio~ or this simulated injection water plots very close to' the mixing line constructed through the isotopically Ejnriched water-sample values from the Navajo aquifer'(graph C, Figure 9). On the basis of the isotopic composition of the simulated injection water (graph cj Figure 9), it is plausible to assume that this water could be the source of isotopi- cally enriched water in tJ-(e Navajo aquifer. The mixing line shoWl) in graph C (Figure 9) was used to estimate the proportion of isotopically enriched injection water ithat theoretically may have mixed with isotopically ijght water from the Navajo aquifer to produce the isbtopically enriched water in the Navajo aquifer. For illustration purposes, sample N40 (graph C, Figure 9)'was used as an example of water from the Navajo $quifer affected by isotopic enrichment, and sample :N1~ was used to represent isotopically light watet from the Navajo aquifer (graph C, Figure 9). On the basis of only the isotopic mixing model, sample N40 appears to be a mixture of 64 percent N12 water and 36 percent injectiCln water. The mixing proportions for sample N40 and other samples plotted along the mixing line in graph C (Fig, ure 9) are listed in Table 2. The mixing proportions calculated from the isotopic mixing model predict that significant volumes of injection water are mixing with water from the Nava- jo aquifer; however, the bromide-to-chloride ratio mix- ing model (Figure 6) and the iodide-to-chloride ratio trend line (Figure 7) indicate that even small volumes (less than one percent) of injection water have not mixed with water from the Navajo aquifer. Achloride mass balance was applied to the mixing proportions determined from the isotopic mixing model to resolve these conflicting results (Table 2). The chlodde c.on- centrations predicted by the isotopic mixing model are up to 13.4 times larger than the measured chloride concentrations in isotopically enriched samples from the Navajo aquifer (Table 2). Although the isotopic results indicate that water from an isot<ipically enriched end member is mixing with water from the Navajo aquifer, results of the chloride mass balance indicate that the injection water cannot be the source. IMPLICATIONS OF' RESULTS AND FUTURE STUDY NEEDS i Bromide, iodide, chloride, and stable isotope data collected to date (1993) consistently indicate that OFB . and injection waters are not causing the oQserved salinity increases in some parts of the Navajo aquifer. It is unclear at this time what the altemative~alinity source or the hydrologic mechanism causing the salin- ity increase might be. . -J , ~1 ,'1 :! j j :1 1 ! :1 + 'I , , I ~ ~ "M ~ ,~ ~ti ';: ,~ .~ ..~ 'fi . ...~ ,~ , .~ i :~ TABLE 2. ProportHm of Injection Waler Determined from the Isotopic Mixing Model Compared with Mea-sured and Calcul~ted Chloride Concentrations in Water from Selected Wells from the Navajo Aquifer {chloride concentration in milligrams per kilogram]. Calculated CWoride Percent Percent Measured Concentration Sample Injection Freshwater CWoride from Isotopic Site Water from N12 Concentration Mixing Model N40 36 64 1,700 8,300 N33 38 62 2,800 8,700 N19 47 53 2,600 11,000 N35 50 50 3,600 11,000 N32 53 47 3,500 12,000 N42 53 47 890 12,000 N46 58 42 1,100 13,000 N23 58 42 1,300 13,000 N36 59 41 4,200 13,000 N25 62 38 1,600 14,000 N2l 75 25 1,900 17,000 WATER RESOURCES BULI,.ETIN 1132 ~ ~ "! ~ ~ "1 ,:,)l' ,"' c..;;?1<i_~.J';, _d:' -~"