Measurement of distribution coefficients using a radial injection dual‐tracer test

Abstract

The dispersive and adsorptive properties of a sandy aquifer were evaluated by using a radial injection dual‐tracer test with 131 I as the nonreactive tracer and 85 Sr as the reactive tracer. The tracer migration was monitored by using multilevel point‐sampling devices located at various radial distances and depths. Nonequilibrium physical and chemical adsorption effects for 85 Sr were treated as a spreading or dispersion mechanism in the breakthrough curve analysis. The resulting effective dispersivity values for 85 Sr (range of 0.7–3.3 cm; mean of 1.9 cm) were typically a factor of 2–5 larger than those obtained for 131 I (range of 0.4–1.5 cm; mean of 0.8). The distribution coefficient values obtained from analysis of the breakthrough curves at three depths and two radial distances ranged from 2.6 to 4.5 ml/g. These compare favorably with values obtained by separation of fluids from solids in sediment cores (4.3–11.7 ml/g), by batch experiments on core sediments (2.8–10.8 ml/g), and by analysis of a 25‐year‐old radioactive waste plume (mean of ≃10 ml/g) in another part of the same aquifer. Correlations of adsorbed 85 Sr radioactivity with grain size fractions demonstrated preferential adsorption to the coarsest fraction (associated with micaceous minerals) and to the finest fraction. The relative amounts of electrostatically and specifically adsorbed 85 Sr on the aquifer sediments were determined with desorption experiments on core sediments using selective chemical extradants. The withdrawal phase breakthrough curves for the well, obtained immediately following the injection phase, showed essentially full tracer recoveries for both 131 I and 85 Sr. Relatively slow desorption Of 85 Sr, indicated by extreme tailing of the return break‐through curve and analysis of residual radioactivity on sediment cores, provided further indication of the nonequilibrium nature of the adsorption‐desorption phenomena.