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Evaluation of VOC fluxes at the soil-air interface using different flux chambers and a quasi-analytical approach

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Abstract

Dense nonaqueous-phase liquids (DNAPLs) spilled on the soil migrate vertically depending upon gravity and capillary forces through the unsaturated zone of the porous aquifer, forming a vapour plume. These volatile organic compounds (VOCs) can be transferred by advection-diffusion to the groundwater or to the atmosphere. Evaluating DNAPL vapour fluxes at the soil-air interface is one of the key challenges in the remediation of contaminated sites. This work discusses the results of a large-scale vapour plume experiment with a well-defined trichloroethylene (TCE) spill, including a sequential raising and lowering of the water table, where the TCE vapour fluxes at the soil surface were experimentally quantified in two ways: (i) directly, with measurements at the soil-air interface using different flux chambers at various operational modes under both transient and steady-state conditions of the vapour plume, and (ii) indirectly, using a quasi-analytical approach based on soil gas measurements. It was shown that upward displacement of the water-air front during the controlled raising of the water table (approximately 10 cm h−1) increased the TCE vapour flux measured at the soil surface by factors of 4 to 10. Under steady-state transport conditions, TCE vapour fluxes measured using five types of flux chambers and three operational modes were similar. The effects of the flux chamber geometry, the accumulation of TCE vapours in the chamber headspace or the air recirculation at a low flow rate on the measured TCE vapour fluxes were low. At steady-state transport conditions, TCE vapour fluxes measured with the flux chambers and estimated using the quasi-analytical approach were of the same order of magnitude. However, under transient conditions of the vapour plume, the TCE vapour flux predicted by the quasi-analytical approach greatly underestimated or overestimated the real TCE vapour flux at the soil-air interface.

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Acknowledgments

The authors acknowledge the financial support of the French Agence Nationale de la Recherche (ANR) by way of project ANR-FLUXOBAT (PRECODD 2008).

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  1. Guillaume Gay

    Present address: present adress: Direction Régionale et Interdépartementale de l’Environnement et de l’Energie (DRIEE), 10 rue Crillon, 75194 Paris, France

Authors and Affiliations

  1. Laboratoire d’Hydrologie et de Géochimie de Strasbourg (LHyGeS), UMR 7517, Université de Strasbourg - Centre National de la Recherche Scientifique (CNRS), 1 rue Blessig, 67084 Strasbourg, France

    Solenn Cotel, Gerhard Schäfer, Salsabil Marzougui-Jaafar & Olivier Razakarisoa

  2. BURGEAP, 19 Rue de la Villette, 69003 Lyon, France

    Sylvie Traverse

  3. INERIS, Parc Technologique ALATA, 60550 Verneuil-en-Halatte, France

    Guillaume Gay

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  1. Solenn Cotel
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Appendices

Appendices

Appendix A Uncertainties in the experimental TCE mass fluxes measured without TCE vapour accumulation in the flux chamber (see Eq. (2))

Uncertainties in the TCE mass fluxes were determined using a total derivative expansion for correlated variables of Fsoil/atm:

$$ \frac{\varDelta {F}_{soil/ atm}}{F_{soil/ atm}}=\frac{\varDelta {m}_{ads}+{V}_{ch}\varDelta {C}_{ch}+{C}_{ch}\varDelta {V}_{ch}}{m_{ads}+{C}_{ch}{V}_{ch}}+\frac{\varDelta {A}_{ch}}{A_{ch}}+\frac{\varDelta \left(\varDelta t\right)}{\varDelta t} $$
(9)

Here, Δm ads and ΔC ch are the errors in the fixed TCE mass and residual vapour concentration in the chamber. In addition, ΔV ch , ΔA ch and Δ(Δt) are the net volume, measured area and time measuring interval errors, respectively. By neglecting errors in time recording and assuming relative uncertainties of 4 % in the sorbed TCE mass (gas chromatography analysis), 2.5 % in the TCE vapour concentration (INNOVA measurements) or 5 % (photo-ionisation detectors measurements), 4 % in the chamber section and chamber volume (CF2, CF3, CF4 and CF5) and 25 % in the net volume of very small flux chamber CF1, relative uncertainty in the pollutant vapour flux measured without TCE vapour accumulation in the flux chamber can be determined.

For Sect. 3.2.1, relative uncertainties in the flux were within the range of 8.8 and 10.5 %.

For Sect. 3.3, relative uncertainties in the flux were within the range of 9.2 and 23.1 %.

Appendix B Uncertainties in the calculated TCE mass fluxes using the quasi-analytical approach (see Eq. (3))

Uncertainties in the diffusive portion of the predicted TCE vapour fluxes were determined using the equation

$$ \frac{\varDelta {F}_{diff,z}}{F_{diff,z}}=\frac{10}{3}\frac{\varDelta {S}_a}{S_a}+\frac{4}{3}\frac{\varDelta \varepsilon }{\varepsilon }+\frac{\varDelta \left(d{C}_a/dz\right)}{d{C}_a/dz} $$
(10)

which follows from Eq. (3) using a total derivative expansion for correlated variables. Considering uncertainties of 3.4 % in the gas saturation measurements (ΔS a /S a ), 2 % in the sand porosity (Δε/ε) and 2.5 % in the vapour concentration gradients (Δ(dC a /dz)/(dC a /dz)) based on the INNOVA measurements and assuming no errors in the free air diffusion coefficient, a total uncertainty in the predicted diffusive vapour flux of approximately 16.5 % was obtained.

In a similar way, assuming no error in the estimation of the density of the uncontaminated soil air and in the dynamic gas viscosity and neglecting error in the relative gas pressure (manufacturer data yield a relative uncertainty of 0.1 %), uncertainties in the advective portion of the estimated TCE vapour fluxes were determined using the equation

$$ \frac{\varDelta {F}_{conv,z}}{F_{conv,z}}=\frac{\varDelta {C}_a}{C_a}+\frac{\varDelta {k}_{ra}}{k_{ra}}+\frac{\varDelta {k}^{\ast }}{k^{\ast }}+\frac{\varDelta {\rho}_a}{\rho_a}\kern0.5em \mathrm{with}\kern0.5em \frac{\varDelta {k}_{ra}}{k_{ra}}=\left({a}_a+\frac{2}{m}\right)\frac{\varDelta {S}_w}{S_w}\kern0.5em \mathrm{and}\kern0.5em \frac{\varDelta {\rho}_a}{\rho_a}=\frac{\varDelta {C}_a}{C_a} $$
(11)

Considering uncertainties of 2.5 % in the vapour concentrations (ΔCa/Ca) based on the INNOVA measurements, 3.4 % in the water saturation measurements (ΔS w /S w ) and 2 % in the sand permeability (Δk*/k*), a total uncertainty of 16.1 % in the predicted advective vapour flux was obtained.

Appendix C Uncertainties in the experimental TCE mass fluxes measured with TCE vapour accumulation in the flux chamber (see Eq. (1))

Uncertainties in the TCE mass fluxes were determined using a total derivative expansion for correlated variables of Fsoil/atm:

$$ \frac{\varDelta {F}_{soil/ atm}}{F_{soil/ atm}}=\frac{\varDelta \left(d{C}_{ch}/dt\right)}{d{C}_{ch}/dt}+\frac{\varDelta {V}_{ch}}{V_{ch}}+\frac{\varDelta {A}_{ch}}{A_{ch}} $$
(12)

Considering uncertainties of 2.5 % in the temporal TCE concentration variation (Δ(dC ch /dt)/(dC ch /dt)) (INNOVA measurements) or 5 % (photo-ionisation detectors measurements), uncertainties in the chamber section of 4 % and in the chamber volume of 4 % (CF2, CF3, CF4 and CF5) and uncertainties of 25 % in the net volume of very small flux chamber CF1, the total uncertainty in the measured fluxes with TCE vapour accumulation in the flux chamber ranged between 10.5 and 34 %.

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Cotel, S., Schäfer, G., Traverse, S. et al. Evaluation of VOC fluxes at the soil-air interface using different flux chambers and a quasi-analytical approach. Water Air Soil Pollut 226, 356 (2015). https://doi.org/10.1007/s11270-015-2596-y

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