MEASUREMENT OF BROMIDE ION USED AS A SOLUTE-TRANSPORT MONITOR VIA EPITHERMAL NEUTRON-ACTIVATION ANALYSIS

In soil science (ca. 1970), bromide ion (Br-) in various forms (e.g., KBr, NaBr, SrBr2) was introduced as a non-reactive stable tracer in so lute transport studies normally moving freely with the flux of water w ithout substantial chemical or physical interactions with the soil; Ty pically, Br is extracted from soil and quantified using either a bromi de selective electrode (sensitivity is similar to 10 mu g/ml) or by hi gh-performance liquid chromatography (sensitivity is similar to 0.010 mu g/ml). Where the sensitivity is adequate, the selective conductivit y method, which is simple, affordable and fast, is preferred. More rec ently (ca. 1990), workers have reported that 20% of Br tracers, at low groundwater pH, may be adsorbed by iron oxides and kaolinite when pre sent in the alluvial aquifer. We investigated the use of Epithermal Ne utron Activation Analysis (ENAA) as a means of measuring Br directly i n soil samples without an extraction. ENAA was chosen because of its h igh theoretical advantage factor over aluminum (i.e. similar to 20), t he principal interfering soil constituent calculated for the Br-79(n,g amma)Br-80 reaction compared to Al-27(n,gamma)Al-28. Br- was measured (sensitivity is similar to 0.050 mu g/g) in one gram soil samples from a 5 s irradiation (phi(epi) = 2.5.10(12) n.cm(-2).s(-1)) using a BN c apsule.