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Transport and Degradation of Toluene and o-Xylene in an Unsaturated Soil with Dipping Sedimentary Layers

^a Dep. of Geology, P.O. Box 1047, Blindern, 0316 Oslo, Norway
^b Dep. of Soil and Water Science, Agricultural University of Norway, P.O. Box 5028, 1432 Ås, Norway
^c NVE (The Norwegian Water and Energy Directorate), P.O. Box 5091, Majorstuen, 0301 Oslo, Norway

View larger version (30K): [in a new window]   Fig. 1. The Moreppen field site. Ground-penetrating radar (GPR) profiles from both the work of Langsholt et al. (1998) and this study are indicated on the map. Universal Transversal Mercator (UTM) is a map projection, where the last two digits in the coordinates represents meter.

View larger version (59K): [in a new window]   Fig. 2. The vertical soil profile at GP2, where the suction cups and gas samplers were located (• and , respectively). The figure indicates the location of the water and gas samplers in relation to the soil structure. The numbers of the suction cups are indicated on the figure. TFS, topset unit, fine sand; TCS, topset unit, coarse sand; FFS, foreset unit, fine sand; FCS, foreset unit, coarse sand; FSS, foreset bed, silty sand. The location of the TDR probes (thick, vertical lines), the walls of the trench (thin, vertical lines), the application line, and the width of the trench are indicated on the figure. Only the segments of the TDR probes that were working are shown. The local coordinate system uses the southeastern corner of the trench as the point of origin.

View larger version (28K): [in a new window]   Fig. 3. The relation between the silt content and the volumetric water content for TDR1 and TDR2. The background values of the volumetric water content (from summer 1997) are also shown.

View larger version (35K): [in a new window]   Fig. 4. The volumetric water content as a function of time for selected segments of the TDR probes (a) TDR1 and (b) TDR2. The silty sand layer for TDR1 is situated at the depth of 190 to 212 cm, while for TDR2 it is situated at the depth of 237 to 250 cm. The time spans when bromide and hydrocarbons were applied are indicated on the figure.

View larger version (39K): [in a new window]   Fig. 5. Measured concentrations of bromide in the suction cups after 2 h, and 4, 8, and 16 d (the profile is the same as in Fig. 2). Injection continued until Day 3 as indicated by the half circle at the top of the graphs. The size of the big circles is proportional to the percentage of the input concentration measured at each cup. The dots indicate the suction cups. The local coordinate system uses the southeastern corner of the trench as the point of origin.

View larger version (40K): [in a new window]   Fig. 6. The horizontal and vertical centers of mass of the bromide plume versus time (a and b). The centers of mass are related to the southeastern corner of the lysimeter trench. The horizontal and vertical variances of the bromide plume versus time (c and d) and versus vertical center of mass (e and f).

View larger version (42K): [in a new window]   Fig. 7. Breakthrough curves for bromide (a), toluene (b), and o-xylene (c) in selected suction cups (s.c.). The numbers refer to the numbers of the suction cups (see Fig. 2). Concentrations are plotted against time, where x labels represent the days at 0900 h. The vertical lines indicate the cease of application of bromide and hydrocarbons.

View larger version (40K): [in a new window]   Fig. 8. The plume of toluene after 1, 7, 10, and 15 d (the profile is the same as in Fig. 2). Application continued until Day 14 as indicated by the half circle at the top of the graphs. The size of this half circle is proportional to 16 mg L-1 toluene. The sizes of the other circles are proportional to the percentage of this concentration measured in each suction cup. The dots represent suction cups. The local coordinate system uses the southeastern corner of the lysimeter trench as the point of origin.

View larger version (40K): [in a new window]   Fig. 9. The plume of o-xylene after 1, 7, 10, and 15 d (the profile is the same as in Fig. 2). Application continued until Day 14 as indicated by the half circle at the top of the graphs. The size of this half circle is proportional to 16 mg L-1 o-xylene. The sizes of the other circles are proportional to the percentage of this concentration measured in each suction cup. The dots represent suction cups. The local coordinate system uses the southeastern corner of the lysimeter trench as the point of origin.

View larger version (43K): [in a new window]   Fig. 10. Spatial moments of the hydrocarbon plumes plotted against time: (a) horizontal centers of mass, (b) vertical centers of mass, (c) horizontal variances, and (d) vertical variances. The centers of mass are related to the southeastern corner of the lysimeter trench.

View larger version (56K): [in a new window]   Fig. 11. Breakthrough curves (BTC) for toluene and o-xylene in (a) Cup 20 and (b) Cup 13 compared with the BTC for bromide. Bromide was applied during 3 d, thus only the first 3 d of this experiment was directly comparable with the hydrocarbon experiments. That is why only the first part of the BTC of bromide is shown. The concentration of CO2 and O2 as functions of time are shown for Cup 20 in (c) and (e), respectively, and for Cup 13 in (d) and (f), respectively. The vertical line indicates the cease of hydrocarbon application.

View larger version (30K): [in a new window]   Fig. 12. Graphs showing the steps in the calculation of the first-order degradation coefficient for the separate suction cups, exemplified with Cup 22. The total mass of solute recovered in a suction cup (a and b) was normalized with the total mass injected. The natural logarithm of the fraction of reactive solute in solution, ln[F(x,z)'], was related to the time value where 50% of the total mass had passed the suction cup (c). Plotting ln[F(x,z)'] against this time value gave a graph with just two points (d), and the slope of this regression line gave a estimate of the first-order degradation coefficient.