Dye tracer experiments and modeling of infiltration patterns in a contaminated water repellent sandy soil

Water repellency of soils near the surface leads to preferential flow and transport, because it prevents water from entering hydrophobic areas and concentrates water fluxes in hydrophilic areas. Heterogeneity in the flow field is the main problem for the prediction of contaminant transport. To investigate the flow patterns in a sandy soil, which was contaminated with hydrophobic tar oils and heavy metals, tracer experiments were conducted. Brilliant Blue FCF tracer was irrigated at different irrigation heights at three dates. Dye coverage maps were produced using image analysis. The variability of dye coverage was calculated. In all profiles infiltrating water was heterogeneously distributed in the first 10 cm. Areas with high water repellency coincided with very low water contents and were not stained. Water infiltrated where the top layer was less water repellent staining these areas. In the subsoil preferential flow was also found, although it was throughout not water repellent. A two-dimensional numerical transport model was used to analyze the influence of the heterogeneous water flux in the soil on the leaching and transport of contaminants. It is found that, in the early phase, the contaminant flux towards the groundwater is enhanced as compared to the homogeneous case. For solutes applied with the precipitation the differences in leaching masses between preferential flow paths and other areas increased constantly. For the contaminants situated in the soil, the overall transport of contaminants is quickly greatly reduced. Firstly much of the contamination is effectively preserved because in the bypassed area it is protected from leaching. Secondly, in preferential flow paths the contaminant concentration is already strongly reduced by leaching. For low adsorbing contaminants complete leaching from the soil is much faster under homogeneous flow conditions. The simulation further shows that sampling of soil solution by suction cups at certain points in the profile or sampling of sub regions of the soil for predicting the risk of ground water contamination can yield extremely different results, depending on whether the sampling position is situated in a preferential flow region or not. Introduction The risk of ground water pollution from contamination of the soil is difficult to predict. This is mainly due to the heterogeneity of the subsurface, causing spatial and temporal variance of contaminant transport in the vadose zone (Johnson et al., 2003; Hillel, 1998). Durner et al. (2004) found that volumes extracted with suction devices were orders of magnitude below those expected for homogeneous infiltration volumes and were highly variable in space and time. Typical preferential flow fields can largely increase danger for the ground water. Preferential flow must not necessarily lead to preferential transport. A decreased risk for the ground water is achieved, if leaching from contaminated areas is prevented. This is often found, since the concentrations in areas of preferential leaching decline rapidly (Kung et al., 2000; Allaire-Leung et al., 2000). However, when the dissolved contaminants are preferentially transported in flow paths, large parts of the unsaturated zone are by-passed and therefore not filtering contaminants on their transport through the soil (Flühler et al., 1996). A possible cause for heterogeneous infiltration is water repellency. Evidence of unstable flow in field soils caused by water repellency has been reported by numerous researchers, including Ghodrati and Jury (1990), Dekker and Ritsema (1994), Ritsema et al. (1998), Doerr et al. (1998), and DeBano (2000). Anthropogenic contamination with hydrophobic substances such as wood preservatives can also cause severe water repellency (Bauters et al., 2000; Quium et al., 2002). The leachate of wood preservatives and its effect on the flow patterns in the soil has not been investigated (Hingston et al., 2001; Poppe et al., 2002). In order to describe solute transport in the field, experimental data is needed. However extraction of samples from the soil and chemical analysis is costly. Various researchers have therefore used dye tracers to visualize the flow patterns in the soil (Flury et al. 1994; Aeby et al., 1997; Mooney et al., 1999; Flury and Wai, 2003). Images of stained soil profiles can easily be classified into stained and unstained areas using image analysis. This allows a semi-quantitative measure of the flow patterns, the so-called dye coverage (Flury et al. 1994; Petersen et al., 1997; Reichenberger et al., 2002). This paper deals with the dye tracer experiments conducted on a contaminated sandy soil. The dye coverage is identified and analyzed. The flow patterns are simulated with two-dimensional solute transport models. By simulation of the structured heterogeneity as well as stochastic heterogeneous profiles the leaching amounts and concentrations are compared between homogenous and heterogeneous soils. Material and Methods Site Description The soil on the site of the Pfleiderer company in Neumarkt in der Oberpfalz (N: 49°15 ́13 ́ ́, E: 11°28 ́46 ́ ́), is contaminated by storage of freshly impregnated wood. Precipitation leached wood preservatives containing heavy metals and tar oils into the top soil, rendering the soil water repellent. Soil texture is homogeneous sand, with less than 1% clay and gravel respectively. Soil type is an anthropogenically modified podsol. Dye Tracer Experiments To investigate the solute transport on the Pfleiderer Site, several dye tracer experiments have been conducted. To identify the general flow patters, the causes of the preferential flow and the ideal method for the main experiments on the site where intensive sampling devices were installed (Durner et al., 2004) two preliminary experiments were conducted. The results served to optimize the design of the final experiment at the intensive sampling site. Brilliant Blue FCF was applied manually with watering cans (Fig. 1) on 8 m in the preliminary studies and on 63 m in the main experiment. Irrigation height was 50 mm. Degree of water repellency was measured with the Water Drop Penetration Time (WDPT) Test, which based on the gradual break down of hydrophobicity under the influence of water (Dekker und Ritsema, 1994; Doerr, 1998). Three water drops are place on the soil surface and time until complete infiltration is measured. To analyze the dye experiments the pictures of the stained profiles were separated into stained and unstained areas yielding dye coverage maps.. The image analysis steps are shown in Fig 2. The original image was divided into binary images for single channels. For each pixel it was tested if the value was above or below a threshold. The threshold was set as selective as possible. In the final dye coverage maps only those areas are defined as stained, for which all channels are classified as stained (Fig. 2). Fig. 1: Pictures of Tracer application and profile preparation Fig. 2: Image analysis steps to identify the dye coverage exemplarily shown for a 1 m wide profile (a) shows the original picture (b) the binary images for each of the 5 used channels and the respective threshold values and (c) the resulting dye coverage map, with the dyed area in black. Modeling Two-dimensional numerical modeling was used to compare the leaching of contaminants for homogeneous and heterogeneous flow fields. Flow and Transport parameters were determined in preliminary column experiments. The flow patterns identified in the dye tracer experiments were reconstructed. This is shown for one profile in Fig. 3. In a second group heterogeneity was caused by stochastic distribution of Miller-Miller similar scaling factors (Miller and Miller, 1956) to systematically investigate the impact of heterogeneous flow patterns on contaminant leaching. The correlation length and coefficient of variation of variance of the scaling factors was varied. One of the scenarios is shown in Fig 4. All simulation scenarios were compared to a homogeneous scenario, that otherwise had the same initial and boundary conditions. The water and solute transport modeling was conducted with Hydrus2D Version 2.05. (Simúnek et al., 1999). Water transport is solved with the Richards equation (Richard, 1931) and solute transport with the convection dispersion equation (e.g. Hillel, 1998). Adsorption is modeled with Blue>0.01