A Push–Pull Test To Measure Root Uptake of Volatile Chemicals from Wetland Soils

Abstract

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 (kv) 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 kv.

Dissolved gas dynamics in wetland soils: Root-mediated gas transfer kinetics determined via push-pull tracer tests

“…SF 6 and He were chosen because they are biochemically conservative and nonsorbing in organic sediments [ Wilson and Mackay , ; Busenberg and Plummer , ]. Due to their different sizes (Table ) we anticipated that they would exhibit gas exchange kinetics that bracket the range of values for common biogenic trace gases [ Reid and Jaffé , ].…”

“…Sampling was focused on the center of mass of the plume to minimize the effects of second‐order mixing processes at the edge of the tracer plume. Previous studies employing dissolved gas tracers in PPTs [ Harrison et al ., ; Reid and Jaffé , ] have observed sharp decreases in dissolved gas concentrations between the initial test solution concentration and the concentration in the first extracted sample. Since Br – is unaffected by this loss process, we attribute the difference to partitioning between water and trapped gas bubbles in the subsurface.…”

“…Modeling approaches are hindered by a lack of knowledge concerning the variation of root mass transfer kinetics as a function of gases' physiochemical properties. Reid and Jaffé [] addressed this uncertainty using experiments with multiple dissolved gas tracers in experimental wetland plant systems to evaluate the dependence of gas transfer rates on basic physiochemical properties. Gas transfer rates were found to vary inversely with the molecular diameter of the gases ( σ ), with gas transfer rate constants k v decreasing in the order He ethane > SF 6 > propane > dichlorodifluoromethane (CFC‐12).…”

“…This equation is analogous to the relationship describing air‐water gas transfer coefficients as the diffusivity ratio raised to an exponent n [ Jähne et al ., ], with σ providing a better correlation than diffusivity in this case of transfer across the root surface. The value of the scaling exponent n , −8.85±1, was determined experimentally (see supporting information Figure S1 for replotted data from Reid and Jaffé [] showing the determination of the scaling exponent). It is noteworthy that experiments with a single plant in hydroponic solution and with many plants in saturated soil mesocosms yielded roughly similar k v ratios, even though the k v value for the same gas in hydroponic versus soil mesocosm systems differed by an order of magnitude.…”

“…PPTs have been used in wetland soils to quantify rates of denitrification [ Addy et al ., ; Koop‐Jakobsen and Giblin , ; Harrison et al ., ; Johnson et al ., ], sulfate reduction [ Kneeshaw et al ., ; Bassein and Jaffé , ], and rhizosphere CH 4 oxidation [ Cho et al ., ]. Reid and Jaffé [] adapted the push‐pull approach to probe the root‐mediated gas transfer of five dissolved gas tracers, and developed an empirical relationship between the gases' physiochemical properties and gas exchange rate constants (see section 2.2 for further discussion). An advantage of this push‐pull approach is that measurements assimilate multiple soil‐plant variables, including soil porosity, root density distributions, and soil‐root mass transfer coefficients, into an empirical, overall transport parameter, k v , that does not require detailed information on root morphological characteristics.…”