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

The dispersive and adsorptive properties of a sandy aquifer were evaluated by using a radial injection dual‐tracer test with 131I as the nonreactive tracer and 85Sr 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 85Sr were treated as a spreading or dispersion mechanism in the breakthrough curve analysis. The resulting effective dispersivity values for 85Sr (range of 0.7–3.3 cm; mean of 1.9 cm) were typically a factor of 2–5 larger than those obtained for 131I (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 85Sr 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 85Sr 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 131I and 85Sr. Relatively slow desorption Of 85Sr, 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.