Transport and degradation of contaminants in the vadose zone

Schotanus, D.


Leaching of contaminants from the vadose zone to the groundwater depends on the soil properties and the infiltration rate. In this thesis, organic degradable contaminants were studied, such as de-icing chemicals (consisting of propylene glycol, PG) and pesticides. Heterogeneous soil properties lead to spatial variability in leaching, which is particularly important for degradable contaminants. The infiltration rate determines the travel time in the vadose zone, and thus the time available for degradation.

Two field experiments were performed with a multi-compartment sampler (consisting of 10 x 10 cells of 3.15 x 3.15 cm2each) to examine the dependence of spatial variability in contaminant leaching on the infiltration rate. The first experiment was carried out during the snowmelt period, characterized by high infiltration rates from snowmelt. The second experiment was carried out with irrigation to mimic homogeneous rainfall. The preferential flow paths were similar for both experiments. With a high infiltration rate during the snowmelt experiment, the leaching was distributed more homogeneous than during the irrigation experiment. Therefore, it is concluded that the soil heterogeneity is mainly caused by spatial differences in the soil hydraulic properties, and not by macropores. The leached masses of the degradable PG and a nondegradable tracer were highly correlated. At the scale of the experiment, heterogeneous infiltration resulting from spatial differences in snowmelt did not have much influence on the flow and solute paths.

The results from the field experiment were used to parameterize a random field for the scaling factor of the retention curve. As a criterion to compare the results from simulations and observations, the sorted and cumulative total drainage in a cell was used. The effect of the ratio of the infiltration rate over the degradation rate on leaching of degradable solutes was investigated. Furthermore, the spatial distribution of the leaching of degradable and non-degradable solutes was compared. The infiltration rate influences contaminant leaching in two ways. Firstly, the travel time of the contaminant in the vadose zone depends on the infiltration rate. Secondly, the fraction of the soil which is active in transport is influenced by the infiltration rate. As a result, the spatial distribution of contaminant leaching, and therefore the leached fraction, depends on the infiltration rate.

The leached fraction of a degradable contaminant is often estimated from average soil properties and stationary weather series. For contaminants that degrade in both the adsorbed and aqueous phase, it is known how these averaged properties should be derived from heterogeneous properties. However, for contaminants that only degrade in the aqueous phase, this is not well known. In soils that are layered with respect to the adsorption constant, the propagation of the contaminant plume, and thus the travel time in the vadose zone, depends on the adsorption constant, degradation rate, and dispersivity. Regarding variable weather series, seasonal fluctuations in precipitation lead to large differences in travel times in a dry climate, and thus large differences in the leached fraction, especially for contaminants with little adsorption. In a wet climate, the effect of such seasonal fluctuations is diminished.

In the vadose zone, PG can be degraded by micro-organisms, for which electron-acceptors are needed. A field experiment showed that aerobic as well as anaerobic degradation occurs in the vadose zone. For anaerobic degradation, manganese-oxides (which are present in the soil) or nitrate (applied to enhance biodegradation) can be used as electron-acceptors. Reduced forms of manganese can be transported to the groundwater, and thus the soil could be depleted from manganese-oxides. A model was developed in which both types of degradation were included. The application of nitrate did not lead to a lower PG leaching, or in a slower depletion of manganese-oxides. The leached fraction is higher with a thick snowcover and high meltrate, as then PG is transported rapidly in the soil. Snowmelt did not result in anaerobic soil, despite the high soil moisture content, and thus low oxygen diffusion.