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| Titel |
Evaluation of Trichloroethylene vapour fluxes using measurements at the soil-air interface and in the atmosphere close to the soil surface |
| VerfasserIn |
Solenn Cotel, Vincent Nagel, Gerhard Schäfer, Salsabil Marzougui, Olivier Razakarisoa, Maurice Millet |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250081520
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| Zusammenfassung |
| Industrialization during the 19th and 20th century led to the use of chemical products such as
chlorinated solvents, e.g., trichloroethylene (TCE). At locations where volatile organic
compounds were accidentally spilled on the soil during transport or leaked from their storage
places, they could have migrated vertically through the unsaturated zone towards the
underlying groundwater. As a result of their high volatility a large vapour plume is
consequently formed. Understanding when, at which concentrations and how long, these
pollutants will be present in soil, groundwater, atmosphere or indoor air, still remains a
challenge up to date.
This study was conducted as part of a broader experiment of TCE multiphase mass
transfer in a large (25mÃ12mÃ3m) well-instrumented artificial basin. TCE was injected as
liquid phase in the vadose zone and experiments were conducted during several months.
Firstly, TCE vapour fluxes were experimentally determined in two different ways: (a) direct
measurements at the soil-air interface using a flux chamber and (b) evaluations based
on measurements of TCE concentrations in the air above the soil surface using a
modular experimental flume (5mÃ1mÃ1m) with a fixed air flow. Secondly, numerical
simulations were conducted to analyse the differences between these two types of
fluxes.
Several positions of the flume on the soil surface were tested. Based on the TCE
concentrations measured in the air, vapour fluxes were determined with the aerodynamic
method using the modified Thornthwaite-Holzmann equation. It assumes that the
concentrations and velocities are temporally and spatially constant in horizontal planes
and requires data on the gradients of concentration, horizontal wind velocity and
temperature.
TCE vapour fluxes measured at the soil-air interface decrease with distance
from the source zone. However, this decrease was either high, at the first stage of
experiment (120μg/(m2s) near the source zone compared to 1,1μg/(m2s) 2m away) or
low, 3 weeks later (38μg/(m2s) near the source zone compared to 29μg/(m2s) 2m
away) depending on the flume position on the basin. In the measuring sections
of the flume, a turbulent air velocity profile was established. TCE concentrations
were correctly measured despite their low values and the derived profiles were
close to logarithmic ones. In the case where the measured TCE vapour fluxes at the
soil-air interface did not vary significantly in air flow direction, the concentration
profiles obtained in the different modules were quite the same. Conversely, when
vapour fluxes were strongly decreasing in flow direction, the measured concentration
profiles were significantly different. Calculated vapour fluxes indicate the same
trend as the vapour fluxes measured at the soil-air interface: strongly decreasing
fluxes (31μg/(m2s) near the source zone compared to 9μg/(m2s) 2m away) or nearly
constant fluxes (2,3μg/(m2s) near the source zone compared to 2,2μg/(m2s) 2m away).
There is still one order of magnitude between the measured and calculated fluxes.
This can be due to concentration profiles which are not necessarily in equilibrium
conditions with TCE vapour fluxes from the vadose zone regardless of the flume
position on the basin. Numerical simulations were used to illustrate this phenomenon. |
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