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South San Juan River ~ Navajo aquifer N ..... CO (II Upper Paleozoic aquifer (potential source of saline water) Middle Paleozoic aquifer North Upper Mesozoic aquifer Middle Mesozoic aquifers and confining units Lower Mesozoic confining units Upper Paleozoic aquifers and confining units (source of oil.field and non~oil-field brines) Figure 3. Aquifers and confining units within the Greater Aneth Oil Field, Utah. Arrows indicate potential for vertical ground~water movement due to hydraulic~head differences. Bromide and Chloride Data Indicate Oil-Field Brines Not the Salinity Source Bromide and chloride concentrations in ground-water samples are sometimes useful for differentiating salinity sources because neither element generally participates in chemical reactions except in very saline systems and because bromide is enriched in organic materials, which provide considerable enrichment in bromide concentrations in oil-field brine samples. Results Ii'om the Aneth study indicate that bromide-ta-chloride ratio values are greater in oil-field brine samples than in saline water samples from the upper Paleozoic aquifer and in non-oil-field brine (fig. 4). The bromide-to-chloride ratio in water samples from the Navajo aquifer generally decreases as the chloride concentration increases (fig. 4). 'TWo mixing lines were constructed by using the mean bromide and chloride concentrations from water samples of the oil-field brine and upper Paleozoic aquifer saline end members. A mixing line is the estimated chemical composition of a water derived from mixing two waters of different chemical composition in various proportions. The mcan chloride and bromide concentration of water samples from the Navajo aquifer (chloride concentration less than 60 milligrams per liter) was used as the chemical composition of the non-saline water in the mixing model. The trend of decreasing bromide-to-chloride ratio values with increasing salinily in Navajo aquifer water samples generally does not follow the oil-field brine mix line (fig. 4), which indicates that oil~field brines are not the source of salinity to the Navajo aquifer, The mix line that represents the chemical composition of water from the upper Paleozoic aquifer presents a more realistic fit to the Navajo aquifer data, which indicates that this aquifer is a geochemically feasible source of saline water. 70 60 0 OUl OUl oW 50 ~..J ><l:: wZ 0::;) 40 a:--' oQ ..J!cc iJa: 30 u]l- oJ: -CI ::E- OW 20 a::!: , m- 10 0 10 Strontium Isotopic Data Indicate Oil-Field Brines Not the Salinity Source In addition to the chemical elements of bromide and chloride, natural variations in the isotopes of the chemical element strontium (Sr) can be used to verify or eliminate sources of saline ground water in the Aneth area. An isotope of any clement, such as Sr, has the same number of protons (a positively charged particle with a mass equal to one atomic mass unit) in the atomic nucleus, but a different number of neutrons (a non-charged particle with a mass equal to one atomic mass unit). Relative ratios between two different isotopes of Sr (Sr- 87 and Sr-86) in water samples from the study area were measured with a very sensitive analytical instrument called a mass spectrometer. Each atom of Sr-87 has one more neutron than an atom of Sr-86, and the relative mass difference between the two isotopes can be used asa natural hydrologic tracer. Small differences in the Sr-87 to Sr-86 values can be emphasized by comparing the sample ratio with the ratio in modem seawater by using the following formula: 8"5r = ((("5rl"5r),,/("5rl"5r)..)- t)1 ,000, W~t~r sa:mp19$Jtdm thC'Oltifield brlOElS piHield btine mix line ~...~~~~p..-.-.~-_._---~.. ."~... ..' . W~t(tf~ll!.PI&S frQ91.tM NavaJo . :}flQu<<er wjthAtiio~ride corv::entraliorl .weater,than'60 milligrams per titer. A~ . :&', :&, .:6. ~.,.; ;'.'" :"" .' .~ " ,,,.,,. .' :&" ;tJpper~ale6~Olc ~aquifer:\ln. Mean value for water sOlmplss.froffl.tha.'u'ppar PaleQtoic aqUifer' ../" :&,~ :;i. &~.,~, ,>it ;A:0A'.4'"' . ~, Ai ~-).M~'\A' ..~'l:.. .:<+... ;.-&'M. ...,.. 'A. .. I, :.':~&i, .~. ._,1. I.......M. . 100 1,000 10,000 100,000 CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER Water samr>Jes from the'non.oit-ft'(Jld prines "" \; 01 Figure 4, Mixing lines constructed between potenlial saline end-member water samples compared with bromide~to~chloride weight ratios of water samples from the Navajo aquifer.