Permeable Environmental Leaching Capsules (PELCAPs) for in Situ Evaluation of Contaminant Immobilization in Soil

We encapsulated radioisotope-spiked soil within a water-permeable polyacrylamide matrix cast in a small cylindrical geometry (≈5 cm³) to measure the persistence of immobilized soil contaminants. As a proof-of-principle, soils contained within these permeable environmental leaching capsules (PELCAPs) were labeled with either ⁸⁵Sr or ¹³⁴Cs and were leached in both laboratory tests and continuously in situ with ground and streamwaters at two field sites on the Oak Ridge reservation. Groups of PELCAPs were retrieved, assayed nondestructively for radioisotopes via γ spectroscopy, and then replaced in ground and surface water repeatedly over a 6-month period. PELCAPs that contained no soil readily and quantitatively leached either ⁸⁵Sr or ¹³⁴Cs into laboratory extractants or ground or surface water with effective diffusion coefficients (D_eff) of (1.14 ± 0.06) and (4.8 ± 0.2) × 10^-6 cm²/s, respectively. PELCAPs containing untreated soil readily leached >90% of ⁸⁵Sr but <1% of ¹³⁴Cs during field leaching at both sites, whereas thermally treated soils quantitatively retained both isotopes under all conditions. Permeable polymer encapsulation methods, such as PELCAPs, offer the potential capability to conveniently test large numbers of soils and soil treatments for contaminant release and uptake under actual field environmental conditions.