Boron isotopes as intrinsic and artificial hydrologic tracers

Abstract

This work investigates the conservative behavior of boron isotopes and their potential use as an artificial hydrologic tracer. The usefulness of boron isotopes as intrinsic hydrologic tracers and the assumption that boron isotopes are conservative in many natural environments led to the hypothesis that they can be easily adapted as artificial hydrologic tracers. The results are demonstrated in three field studies and a laboratory study carried out under the Soil Aquifer Treatment project. Non-conservative behavior of intrinsic boron isotope ratios was found in vadose zone and groundwater samples. Although this behavior was recognized, its overall importance is relatively small when compared to the range of values found in the vicinity of the field site that ranged from 0 to 39‰. Fractionation of boron isotopes from its source value, domestic effluent with a range of 0 to 6‰; and an average of 3‰, is on the order of 2‰ and is confirmed by a laboratory study. The fractionation is in part related to the humic acid found in organic matter collected from recharge basin surface soils. Boron isotopes, specifically boric acid enriched in 10B, were successfully applied as an artificial tracer. Xenon and oxygen/hydrogen isotopes, and sulfate support interpretation of boron tracer results. However, non-conservative behavior of boron isotopes is also identified in this artificial tracer study. The non-conservative behavior is demonstrated by the late breakthrough of the boron isotope tracer, two days later, relative to that of xenon isotopes added by researchers from Lawrence Livermore Laboratory.