A push-pull test to measure root uptake of volatile chemicals from wetland soils

This paper introduces a novel modification of the single-well "push-pull" test that uses nonvolatile and multiple volatile tracers to investigate the transport and root uptake kinetics of volatile chemicals in saturated soils. This technique provides an estimate of potential volatilization fluxes without relying on enclosure-based measurements. The new push-pull methodology was validated with mesocosm experiments, and bench-scale hydroponic measurements were performed to develop an empirical relationship for scaling root uptake rates between chemicals. A new modeling approach to interpret data using sulfur hexafluoride and helium as dual volatile tracers was developed and shown to decrease errors relative to existing analytical techniques that utilize bromide as a conservative tracer. Root uptake of the volatile tracers was diffusion-limited, and uptake rate constants (k_v) in vegetated experimental mesocosms ranged from 0.021 ± 9.0 × 10^-4 h^-1 for CFC-12 to 2.41 ± 0.98 h^-1 for helium. Hydroponic and mesocosm experiments demonstrate that the molecular diameter is a robust empirical predictor of k_v.