Transport of hydrocarbons from an emplaced fuel source experiment in the vadose zone at Airbase Værløse, Denmark

Journal of Contaminant Hydrology 81 (2005) 1 – 33 www.elsevier.com/locate/jconhyd

Transport of hydrocarbons from an emplaced fuel source experiment in the vadose zone at Airbase Værløse, Denmark Mette Christophersen a, Mette M. Broholm a, Hans Mosbæk a, Hrissi K. Karapanagioti b, Vasilis N. Burganos c, Peter Kjeldsen a,* a

c

Institute of Environment and Resources, Technical University of Denmark, Building 115, DK-2800 Kgs. Lyngby, Denmark b Marine Sciences Department, University of the Aegean, Lofos Panepistimiou, 81100 Mytilene, Greece Institute of Chemical Engineering and High Temperature Chemical Processes-Foundation for Research and Technology, Hellas (ICE/HT-FORTH), P.O. Box 1414, 26500 Patras, Greece Received 19 August 2004; received in revised form 14 June 2005; accepted 20 June 2005 Available online 18 August 2005

Abstract An emplaced hydrocarbon source field experiment was conducted in the relatively homogeneous sandy geology of the vadose zone at Airbase Værløse, Denmark. The source (10.2 l of NAPL) consisted of 13 hydrocarbons (n-, iso- and cyclo-alkanes and aromates) and CFC-113 as a tracer. Monitoring in the 107 soil gas probes placed out to 20 m from the centre of the source showed spreading of all the compounds in the pore air and all compounds were measured in the pore air within a few hours after source emplacement. Seven of the fourteen compounds were depleted from the source within the 1 year of monitoring. The organic vapours in the pore air migrated radially from the source. The CFC-113 concentrations seemed to be higher in the deeper soil gas probes compared with the hydrocarbons, indicating a high loss of CFC-113 to the atmosphere and the lack of degradation of CFC-113. For the first days after source emplacement, the transport of CFC-113, hexane and toluene was successfully simulated using a radial gas-phase diffusion model for the unsaturated zone. Groundwater pollution caused by the vadose zone hydrocarbon vapours was only detected in the upper 30 cm of the underlying groundwater and only during the first 3 months of the experiment. Only the most water-soluble compounds were detected in the groundwater and

* Corresponding author. Tel.: +45 45 25 15 61; fax: +45 45 93 28 50. E-mail address: [email protected] (P. Kjeldsen). 0169-7722/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jconhyd.2005.06.011

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concentrations decreased sharply with depth (approximately one order of magnitude within 10 cm depth) to non-detect at 30 cm depth. The groundwater table varied more than 1 m within the measurement period. However that did not influence the direction of the groundwater flow. Approximately 7 months after source emplacement the groundwater table rose more than 1 m within 1 month. That did not cause additional pollution of the groundwater. D 2005 Elsevier B.V. All rights reserved. Keywords: Oil spill; Groundwater contamination; Vapour diffusion; Field experiment

1. Introduction Above and underground storage tanks are often used for storing gasoline and other oil products in residential housing, gasoline stations, and industry. It is assumed that in the USA about 2.5 million storage tanks exist (Bedient et al., 1999). Leaks in the storage tanks have led to smaller or larger spills into the subsurface. Besides, pipeline or truck accidents may have led to spills on ground with subsequent leaching of oil products into the ground. Larger spills may have led to large volumes of free product floating at the groundwater table with resulting direct dissolution of pollutants into the groundwater. However, many spills are of a size where the free product is retained as residual free phase in the unsaturated zone. The extremely high numbers of such spills do not allow for a case-by-case investigation due to economical reasons. A way to go could be to perform scenario specific risk assessment, where the risk of such spills is assessed by performing transport modelling on different scenarios in respect to geology, oil type, and meteorological factors taking into account all the processes governing the transport and fate of the oil/gasoline constituents in the subsurface including the unsaturated zone and the underlying aquifer. A conceptual model of the transport and fate of pollutants following a spill of gasoline or another oil product in the unsaturated zone reveals a complicated pattern of interrelated physical, chemical, and microbial processes (Mendoza et al., 1995). Following the actual spill the NAPL may be transported due to gravity leaving a residual NAPL phase behind in the unsaturated zone (Abdul, 1988). Here a four-phase environment (free phase, soil, water and air) is created with subsequent volatilisation especially of the most volatile compounds of the multicomponent hydrocarbon mixture as governed by Raoult’s law (Mendoza et al., 1995). The volatilized mass is transported away from the source especially by diffusion in the pore air governed by Fick’s law through the effective diffusion coefficient and concentration gradient (Jury et al., 1983). The largest gradient is observed for the most volatile compounds in the mixture with the highest pore air concentrations (Raoult’s law). The effective gaseous diffusion coefficient is especially governed by the temperature and the tortuosity of the soil (Millington and Quirk, 1961; Moldrup et al., 2001), implying low diffusion coefficient at high water content. Pollutants may be further retarded by dissolution into pore water of the soil and by sorption to the solids (Jury et al., 1983; Mendoza et al., 1995). The high diffusional flux of pollutants away from the source may lead to a later depletion in the source, which affects the source air concentrations of undepleted substances (Gioia et al., 1998).

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The transport in the pore air may also be affected by forces leading to advective gas transport such as gas density differences (Altevogt et al., 2003) and changes in barometric pressure (especially under special geological circumstances (Auer et al., 1996; Elberling et al., 1998; Ostendorf et al., 2000)). Density differences can be introduced by spatial temperature differences in the subsurface (Nassar et al., 1999) or by high vapour concentration of the pollutants in the source air phase (Mendoza and Frind, 1990; Lenhard et al., 1995). Density effects have been observed in a field experiment for heavy chlorinated compounds in sandy material by Conant et al. (1996). They conducted the field experiment to provide detailed analysis of the transport behaviour of solvent vapours (TCE) from residual NAPL within the unsaturated zone and found that temperature, organic matter content, and soil moisture content were the important parameters controlling contaminant migration. The transport was diffusion-dominated, but densityinduced advection was an appreciable component of net transport under summer conditions where the vapour concentrations were high. Pollutants may also leak out directly from the residual NAPL by recharging rainwater. Besides, the pollutants transported away from the source and dissolved into the pore water of the soil can also be leached out by infiltrating rainwater. In this way, the pollutants are transferred across the capillary fringe and into the underlying aquifer (Mendoza et al., 1995). Another process transferring pollutants to the aquifer is the diffusive/dispersive transport of volatile pollutants from pore gas across the capillary fringe (Klenk and Grathwohl, 2002), which can be affected by water table fluctuations (Werner and Hohener, 2002). During the whole transport process, degradation processes may occur. It has previously been shown that many hydrocarbons are readily degradable in soil environments by natural microorganisms (Aelion and Bradley, 1991; Hohener et al., 2003b). This is especially true for top soils, but has also been observed in deeper soils, sometimes with an initial lag phase, especially for pristine soil or groundwater environments which never have experienced the presence of hydrocarbons (Aamand et al., 1989; Lahvis and Baehr, 1996). The degradation processes may lead to spreading of the hydrocarbons to a much lesser degree and a lower hydrocarbon recharge to the groundwater. Due to the complexity of the transport and fate processes following an oil/gasoline spill in the unsaturated zone, the use of advanced models is needed to obtain realistic results of scenario specific modelling. Such models exist but have not been validated towards field data of high complexity. Detailed information on the source history, soil physics, hydrogeology, pollutant degradation and parameters controlling the transport in the vadose zone and the groundwater are necessary. Such high quality data never exist for real spills. However, performing a controlled field experiment with an emplaced source containing a mixture of oil constituents would increase the understanding of the complex spreading of degradable pollutants from an oil spill in the unsaturated zone, and would create a basis for model validation. The objectives of this study are (1) to obtain a high quality data set which may be used for validation of advanced 3D transport simulation models for scenario specific risk assessment and (2) to evaluate the governing factors for the physical transport of pollutants released from residual free phase of an oil spill situated in the unsaturated zone. The objectives are met by performing a long-term field experiment using an emplaced source

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of an artificial oil product with known initial composition, including monitoring of pore air concentrations in space and time, and pollutants transfer to the underlying groundwater zone. The controlled field-scale hydrocarbon emplaced source experiment was conducted at the Airbase Værløse, Denmark. To our knowledge this is the first controlled field experiment evaluating transport and fate of vapours from a multicomponent NAPL source in the unsaturated zone. This article focuses on the physical transport processes, and gives all the details of methods and materials needed for the performance of such a long-term extensive field experiment. Due to brevity, the article contains not a full interpretation of the obtained data. Instead extensive interpretation of the results is presented in related articles. The important aspects of source compositional changes are presented in Broholm et al., submitted for publication. The importance of biodegradation to the fate of the hydrocarbons is discussed by Kaufmann et al. (2004), which focus on the microbial ecology of the field site, and Hohener et al. (in preparation), which present comparison of laboratory degradation experiments and field observation using a 3D model. The experiences of interpreting the transport and fate processes by mathematical models are further given by Gaganis et al. (2004) and Maier (2005) using two different modelling approaches. Methods for simultaneous measurement of nonaqueous phase liquid (NAPL) saturation and diffusive fluxes at the field site are presented by Werner et al. (2005).

2. Geology and hydrogeology The field experiment was conducted at Airbase Værløse, located 20 km northwest of Copenhagen in Denmark, see Fig. 1a. The area chosen for the field experiment was a flat grass field, which had not been used for any other purpose for many years. The grass has occasionally been cut. The experiment took place within the fenced area in Fig. 1b. The boreholes named A and D were existing boreholes, established in 1991 and 1994 in connection with environmental investigations at the Airbase. They were used as piezometers during the experiment. The local geology at the selected area of the airbase was described based on soil samples from 11 boreholes to 2.5–7 m below ground surface (b.g.s.) and the existing 13 boreholes in the area. In general, the site was covered with 20 cm of mouldy (rich in organic content) top soil followed by 30–50 cm of brown mouldy sand overlying a layer of 2–3 m of relatively homogeneous glacial melt water sand, followed by a thin layer of sand till (0.5–1 m)—see Fig. 2. The melt water sand at the site contained small lenses of silty sand and lenses of coarser sand or gravel. At the southern part of the site the melt water sand contained more gravel and the sand till appeared at 2.5 m b.g.s. The upper secondary groundwater was in the lower part of the melt water sand or in the sandy till, at approximately 3 m b.g.s. Grain size distributions were performed on 15 soil samples from boreholes B1 to B3 (see Fig. 1b for placement). One hundred gram of soil was dried at 105 8C for 24 h and then sieved through 8, 4, 2, 1, 0.5, 0.25, 0.125 and 0.075 mm sieves. The soil samples, which were described as melt water sand, contained 1.8% F 1.6% gravel (N2 mm), 95% F 2% sand (0.075–2 mm) and 2.8% F 1.3% silt and clay (b0.075 mm). The topsoil contained a little more gravel, a little less sand and a little more silt and clay (6%, 89% and

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Fig. 1. (a) The geographical location of Airbase Værløse in Denmark shown with asterisk. (b) Map showing the placement of the source and the boreholes. Two sets of hydraulic head are shown: 5/9/01 (solid line) and 20/3/02 (bold line). The location of the source is shown. (c) Plan view of the sampling network showing the placement of the source, the pore gas samplers, and the multilevel samplers. Unit of scales is metres.

Fig. 2. Cross-section of the sampling network in the south radial (along the A–AV section shown in Fig. 1c) showing the geology at the site and the placement of the source, the pore gas samplers, the multilevel samplers and the porous cups.

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5%, respectively). The sand till was more heterogeneous with 3–54% gravel, 42–82% sand, and 4–14% silt and clay. Total organic carbon (TOC) associated with the soil was determined by dry combustion after treatment with sulphurous acid for removal of carbonates (Heron et al., 1997). TOC was measured on the soil samples from B2 and for every 10 cm in an intact soil core taken 1.5 m from the centre of the source. TOC was decreasing from 1.5% in 0–10 cm to 0.3% at 0.5 m b.g.s., 0.1% at 0.8 m b.g.s (top of source) and stable around 0.02–0.04% from 1 to 4 m b.g.s. Four undisturbed soil samples were taken out from a test hole located 30 m from the source zone close to well B2 (see Fig. 1c). The samples were taken out in depths of 30–40 cm, 80–90 cm, 100–110 cm, and 120–130 cm in sample cylinders. Bulk density, water suction, and saturated conductivity were measured according to Schlichting et al. (1995). Based on the water retention curves, the empirical parameters for the van Genuchten water retention equation were calculated using the method of non-linear regression. Results of the analysis are given in Table 1. In order to get a more detailed description of the geology and thereby be able to choose the area with the most homogeneous geology, the geophysical methods, Ground Penetration Radar and Electric Resistivity, were performed at the site (the fenced area). Ground Penetration Radar was performed using a GSSI (Geophysical Survey Systems, Inc., North Salem, NH) radar unit type SIR-2 for every 10 m in the north–south direction and for every 5 m in the east–west direction. Electric Resistivity measurements were performed in the north–south direction at the edges and in the centre of the fenced area using an ABEM (ABEM Instrument AB, Sundbyberg, Sweden) system type terrameter SAS 300. The results of the geophysical investigations showed that the most homogeneous geology was found in the central and eastern part of the site. Groundwater table fluctuations were monitored in 16 piezometers placed within 60 m from the source (see Fig. 1b). One borehole (B2) near the source was equipped with a data logger. The groundwater table was measured 37 times (approximately every second week) during the 1-year field experiment. The variation in the groundwater table is illustrated in Fig. 3. Over the monitoring period (1 1/2 year) the groundwater table elevation varied 1.2– 1.6 m. Examples of potential maps are given in Fig. 1b. The direction of the groundwater flow was southeast towards a small creek. The direction of the groundwater flow (Fig. 1b) did not vary significantly during the monitoring period, not even in the early spring 2002 in respond to snow melt-off and heavy precipitation (Fig. 3).

Table 1 Soil physical parameters as determined on undisturbed samples taken from the Værløse field site Horizon

30–40 cm 80–130 cm a b c

Bulk density (g/cm3) 1.61 1.59

Saturated conductivity (10
5 m/s) b

3.7 F 1.5 8.8 F 1.5c

Van Genuchten parametersa a

n

hr

hs

0.03 0.03

1.84 2.97

0.04 0.03

0.39 0.34

Parameters as defined in Van Genuchten (1980). Average and standard deviation based on measurement on 9 different samples. Average and standard deviation based on measurement on 7 different samples from 80 to 130 cm.

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Fig. 3. Variation in groundwater level in selected boreholes during the 1 1/2 years of monitoring.

In order to determine the hydraulic conductivity of the shallow aquifer, several slug tests were performed using the mini slug test method (Hinsby et al., 1992), which was performed in a small-diameter (1 in.) driven well with a 10 cm screen just above the drive point. The head data was recorded with a pressure transducer, and the time/head relationships were transferred to a portable computer. The hydraulic conductivity was calculated from the relationship by the Hvorslev method assuming isotropic conditions (Butler, 1998). Slug tests were performed at eight different locations in the vicinity of the source location at depths very close to the groundwater table (8–38 cm below GWT). Table 2 gives the result of the slug tests. The table shows that the average conductivity in the shallow aquifer consisting of sand till (confer Fig. 2) is in the same order as the conductivities measured in the laboratory of the unsaturated melt water sand at 0.8–1.3 m depth. There were certain requirements to the source location: the geology should be homogeneous sand, the secondary groundwater should be placed 3–4 m b.g.s. and it Table 2 Results from the measurements of hydraulic conductivity using slug tests Slug testa

GWT elevation (m above sea level)

Screen locationb (cm below GWT)

Screen locationb (m below surface)

Hydraulicc conductivity (10
5 m/s)

ST-1-1 ST-1-2 ST-2-1 ST-2-2 ST-2-3 ST-5-1 ST-5-2 ST-5-3 Average

15.31 15.31 15.30 15.30 15.30 15.27 15.27 15.27

8 18 10 22 34 14 25 38

3.6 3.7 3.6 3.7 3.8 3.7 3.8 3.9

8.7 14 14 13 7.5 5.6 9.1 8.7 10 F 3.2

a

The first digit refers to the distance in meter from the slug test to the mid-point of the source. All slug tests were carried out along the southern transect. b Location of the middle of the 92 mm long screen. kr2 s c 2kL ffii ; where s is slope of the Determined from the equation: K ¼ Fc ; F ¼ h qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ln 2rLw þ 1þðlnð 2rLw ÞÞ ln(h/h 0) versus time curve, r c = 0.00907 m, L = 0.092 m, r w = 0.0155 m.

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should be possible to remediate the secondary groundwater. The results of the geophysical measurements, boreholes, and the initial monitoring of groundwater flow direction (6 months) were used to decide the exact location for the source. The most homogeneous geology was found in the central and western part of the fenced area. In order to facilitate subsequent remediation of possible polluted secondary groundwater, it was decided to place the source upstream B3—see Fig. 1b, which also gave the opportunity to monitor the groundwater in the down stream radial.

3. Materials and methods of the field experiment 3.1. The hydrocarbon source An artificial NAPL phase with a limited number of compounds was chosen in order to be able to analyse and quantify all the compounds. The NAPL phase was an artificial hydrocarbon mixture consisting of volatile and semi-volatile compounds (BTX, n-, isoand cyclo-alkanes) similar to jet fuel. The halocarbon CFC-113 (C2Cl3F3) was used as a conservative tracer. The primary groups of compounds in jet fuel are represented by two or more compounds. The NAPL phase represents a broad spectrum of compounds regarding volatility, solubility, etc. The composition of the source was designed taking into account that the source should last more than 1 year, and that at least one compound should be depleted. The physico-chemical properties of the source compounds and the source composition can be seen in Table 3. The numbers in Table 3 have been chosen after a thorough investigation of the literature concerning physico-chemical properties of the relevant compounds. The values which appeared most precise, and which were based on experimental determination were chosen. A circular source consisting of sand from the site mixed with the NAPL was placed 0.8–1.3 m below ground surface at the selected site. A total of 10.2 l of NAPL was emplaced resulting in a residual NAPL saturation of 14.3%. Sudan IV (2 g/l), which is a hydrophobic dye, was added to the NAPL phase for visual assessment of the source placement. Bromide salt (100 g LiBr) was mixed with the 2 cm sand just beneath the source for detection of any groundwater pollution caused by direct infiltration of water through the source. The NAPL and sand from the site were mixed in a concrete mixer, equipped with a lid and a closed slide to prevent volatilisation, in five shifts and packed between 0.8 and 1.3 m below ground surface in a hole with a diameter of 0.70 m to form a source of hydrocarbons. The source emplacement was conducted within 30 min. The hole above the source was filled with the original soil. A large circular tube (O.D. 124 cm) was placed on 30 cm pillars over the source area to prevent rainwater from infiltrating the source. The source was emplaced July 3rd 2001. 3.2. Model description The code R-UNSAT (Lahvis and Baehr, 1997) was used in this study to simulate attenuation of volatile pollutants in the unsaturated zone. The model was used prior to the experiment as a guide for the sample network design and the sampling strategy, and after

Aromatic

n-Alkanes

Cycloalkanes

Isoalkanes

Freon 1

Compound

Density [g/ml]

Molecular weight [g/mol]

Boiling point [8C]

Vapour pressure of the pure compound 25 8C [kPa]

H [–], 25 8C

Log K ow

Water solubility [mg/l], 25 8C

Source composition [wt.%]

Benzene Toluene m-Xylene 1,2,4 Trimethyl-benzene Hexane Octane Decane Dodecane Methyl-cyclo-pentane Cyclo-pentane Methyl-cyclohexane 2,2,4-Trimethyl-pentane = Isooctane 3-Methylpentane 1,1,2-Trichloro-1,2,2-trifluoro-ethane = CFC-113

0.8765 0.8667 0.864 0.8761 0.660 0.703 0.730 0.749 0.7486 0.7457 0.7694 0.692

78.11 92.142 106.17 120.19 86.17 114.23 148.28 170.34 84.16 70.13 98.19 114.2

80.1 110.63 139.1 169.3 69.0 126.0 174.1 216.3 71.8 49.2 100.9 99.2

12.6725 3.8055 1.1065 0.2715 20.209 1.889 0.1759 0.01579 18.42,15 42.413 6.1313 6.5611

0.2173 0.2443 0.267 0.288 68.589 121.09 197.8510 296.7710 14.812,14 7.64 17.615 123.611

2.131 2.731 3.201 3.781 4.1111 5.1511 6.2511 7.2411 3.371,16 3.001,16 3.611,16 4.0921

17906 5565 1586 574 9.511 0.6611 0.05211 0.003711 4217 15617,18 1417,18 2.4411

1.02 2.93 4.57 10.99 7.26 7.16 15.99 9.50 5.79 1.59 10.23 15.36

0.664 1.575

84.16 187.38

64 47.6

68.611 14.2020

3.6011 3.168

25.311 44.6719

12.811 17022

7.45 0.16

Hansch et al. (1995); 2USEPA (1994); 3Peng and Wan (1997); 4Verschueren (1983); 5Shiou and Ma (2000); 6Montgomery and Welcom (1990); 7Dewulf et al. (1995); Hansen et al. (1995); 9Mackay and Shiu (1981); 10Yaws and Yang (1992); 11Mackay et al. (1993); 12Boublik et al. (1984); 13Daubert and Danner (1989); 14VP/WSOLSRC PhysProb Database; 15Hine and Mookerjee (1975); 16Biobyte Corp. (1994); 17McAuliffe (1966); 18Yalkowsky and Dannenfelser (1992); 19Yaws (1999); 20Bu and Warner (1995); 21chemfinder; 22Horvath et al. (1999). 8

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Table 3 Physico-chemical properties of the compounds used for the emplaced source experiment

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the experiment to simulate the first week of vapour migration following emplacement of the source. Cylindrical coordinates were used in the model formulation, which involved gas and aqueous phase diffusion and sorption. Equilibrium was assumed for the partitioning between phases: gas-aqueous phase (Henry’s law), gas-NAPL (Raoult’s law), and aqueous phase-solid (sorption). Biodegradation was described as a first-order process, taking place in the aqueous phase, and was introduced as a sink term. More details of the model setup are given elsewhere (Karapanagioti et al., 2003, 2004). In order to keep the number of simulated components within the limit of the aforementioned unsaturated zone code (but also, of most currently available unsaturated zone codes; see Karapanagioti et al., 2003), the mixture compounds were organized into 7 effective pseudo-species, that is, groups of actual species sharing similar property values. Based on the analyses presented in previous studies (Gaganis et al., 2002; Karapanagioti et al., 2004), the compounds of this particular mixture are optimally organized in groups that share similar values of solubility or Henry’s law constant. Using either one of these criteria, the following groups are obtained. Group 1: benzene; Group 2: toluene; Group 3: CFC-113; Group 4: m-xylene, cyclopentane; Group 5: 1,2,4-tri-methyl-benzene; methylcyclopentane, methylcyclohexane, 3-methylpentane hexane; Group 6: isooctane, octane; and Group 7: decane, dodecane. For comparison with the actual data during the first days after the source emplacement, another grouping scheme is used in order to provide simulation data for individual compounds (toluene, CFC-113 and hexane). This grouping scheme is also formed using the solubility as the grouping criteria and the following groups are obtained: Group 1: benzene; Group 2: toluene; Group 3: CFC-113, m-xylene, cyclopentane; Group 4: 1,2,4-tri-methyl-benzene; methylcyclopentane, methylcyclohexane, 3-methylpentane; Group 5: hexane; Group 6: isooctane, octane; and Group 7: decane, dodecane. Outputs for the groups containing the same compounds in both schemes were identical in both tests. The group properties were calculated based on the weighted averaged properties of the species included in each group. The sorption distribution coefficient of the group was that of the least sorbed individual compound of the group as proposed in Gaganis et al. (2002). The details of the calculations along with and the use of the numerical predictions in the sample network design and sampling strategy are presented in the next section. 3.3. Experimental setup 3.3.1. Sample network design and sampling strategy The success of the field experiment is dependent on the magnitude and spatial distribution of the hydrocarbon concentrations in the pore air during the controlled field experiment. The hydrocarbon concentrations should not only be measurable by the technology available over reasonable length and time scales, but they should also show spatial gradients and temporal derivatives that will provide valuable information regarding the dynamics of the spreading and attenuation of the hydrocarbons. Model predictions, using R-UNSAT, were carried out to estimate a reasonable amount of NAPL for the source to last for the duration of the experiment, to adjust selected positions for the multilevel samplers and the sampling frequency for the experiment. Mainly, two different cases were investigated: (a) attenuation including biodegradation and

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(b) attenuation without biodegradation. The latter provided a conservative estimate of the vapour plume migration. All the physical parameters of the field site used in the model are detailed in Table 4. Some of the parameters were not yet available at the time of the modelling, and were instead based on best estimates. The field site was represented as a sandy unsaturated zone with an estimated, constant f oc value of 0.0032 and five homogeneous horizontal layers. Various cases were investigated, including notably (a) the presence or absence of biodegradation, (b) the use of a cover on the soil surface, (c) variable source volumes, and (d) variable sorption coefficients. The effect of biodegradation was quantified using biodegradation rates from Pasteris et al. (2002) for the mixture components and compared to computations that completely neglected biodegradation. The use of a surface cover was incorporated in the model by Table 4 Physical representation of the source and site and the parameters used for the unsaturated zone model R-UNSAT Model geometry Axial symmetry, cylindrical coordinates. r-direction: length: 10 m; number of nodes: 101; nodal spacing: 10 cm z-direction: length: 3.5 m; number of nodes: 36; nodal spacing: 10 cm Upper boundary: the atmosphere Lower boundary: the water table (located 3.9 m below surface) Outer side boundary: at 10 m from the source centre Model meteorological conditions Temperature: 10 8C (16 8C) Pressure: 1 atm Recharge rate: 15 cm/year (0 cm/year) Source zone Total volume: 98 l (223 l); radius: 20 cm (37.5 cm); lower source boundary: at 2.5 m (2.6 m) above the water table; upper source boundary: at 3 m (3.1m) above the water table Pore space volume: 34 l (71 l) NAPL volume: 1.7, 5, 6.7, 10, 20 l (10.2 l) Porous medium Porosity: 0.35 (0.32) Bulk density: 1.74 (g/cm3) Layer Depth (m) Moisture content Gas phase tortuosity Fraction of organic C, f oc (%)

1 0.3 0.20 (0.10) 0.092 0.32 (0.9)

2 1.1 0.11 (0.07) 0.30 0.32 (0.06)

3 1.9 0.025 (0.08) 0.59 0.32 (0.03)

4 2.5 0.036 (0.08) 0.55 0.32 (0.03)

5 3.5 0.014 (0.20) 0.64 0.32 (0.03)

Code parameters Constant time step: 0.1 h Ending times: 1 and 3 days; 1, 2, 4 and 12 weeks (0.5, 1, 2, 3 and 6 days) Maximum number of iterations per time step: 500 Maximum relative error accepted: 0.001 Values given in italics and in parentheses were used for the final simulation of the initial spreading of compounds from the source.

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changing the upper boundary condition, over the entire model area, from Dirichlet (open to the atmosphere, C g = 0) to reflective. Five different volumes of kerosene were tested (1.7, 5.0, 6.7, 10, 20 l). In all cases, it was assumed that the NAPL source was immobile. Sorption coefficients were calculated from literature values of the contaminant organic carbon partition coefficients (K oc) and the organic carbon content ( f oc) of the field. The possible overestimation of the f oc value was addressed through a sensitivity analysis, which involved decreasing the sorption coefficient value by one order of magnitude and repeating the calculations. The results from the modelling show that in order to get a successful design and monitoring of the field experiment, the following recommendations should be fulfilled. (a) The NAPL volume should be of the order of 10–20 l. At lower NAPL volumes the highly and moderately volatile compounds would be depleted quickly. (b) The multilevel sampling network around the source zone should be dense. The model runs compared profiles of CFC-113 (highly volatile and not biodegradable) and Group 5 (moderately volatile and moderately biodegradable) for a scenario including sorption but no biodegradation. A continuous increase of the spreading of the non-biodegradable compound, CFC-113, was noted over the entire 12-week period. CFC-113 reached 2 m already at day 1 and after 12 weeks the CFC-113 plume extended 10 m from the centre of the source. For Group 5, which is moderately volatile and moderately biodegradable, the model predicted the spreading was limited and after 12 weeks the compounds will not yet have reached 2 m. This showed that the majority of the mixture components would not be expected to migrate far from the source. (c) The sampling rate should be relatively high immediately after the source emplacement in order to capture the dynamics of the attenuation of the more volatile compounds. The sampling network was adjusted to these recommendations (Figs. 1b and 2). In total 107 soil gas probes were installed—the main part in the south direction and with 8 soil gas probes (2 and 3 m from the centre of the source) in each of the north, east and west directions to monitor the radial distribution of the vapour migration in different directions. Close to the source the sampling network was very dense, and further away fewer sampling points were installed. During the experiment sample frequency was adjusted based on the measured vapour migration. The first week samples were taken everyday, then twice a week. After 3 weeks samples were taken once a week and after 3 months the sampling frequency was lowered to once every second week. After 7 months samples were taken once a month. 3.3.2. Soil gas probes One soil gas probe was constructed for each sampling point. The lower part of the probe consisted of 40 cm stainless steel pipe (OD 10 mm and ID 7 mm), which was closed at the bottom and provided with 6 slits (1 mm wide) at the lower 3 cm. The upper part of the soil gas probe consisted of stainless steel pipe (OD 6 mm and ID 4 mm), which was welded on to the lower part of the probe. A hole (D 54 mm) was driven to 40 cm above the

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13

requested soil gas sampling depth with a Geoprobe (Salina, Kansas) drill rig. Then the soil gas probe was pressed manually into the ground, to ensure that the slits were placed in undisturbed soil. Bentonite–concrete slurry was placed in the bottom (10–20 cm) of the driven hole. Then the hole was refilled with filter sand (0.13–0.50 mm, which was similar to the dominating grain size in the melt water sand) and the upper 10 cm at soil surface was filled with the bentonite–concrete slurry to prevent intrusion of atmospheric air along the sides of the probes. The shallow (down to 1 m b.g.s.) soil gas probes were made entirely of 10 mm stainless steel pipe and gently hammered directly into the ground. Bentonite–concrete slurry was added at the surface. To avoid perforating the source area several times all the soil gas probes located beneath the source were placed inside a single pipe. In a 1-in. stainless steel pipe a 6.3 mm wide slit was made for each sampling depth. The bottom parts of copper pipes (OD 6 mm and ID 4.4 mm, one for each sampling depth) were welded into the slits and small slits (1 mm) were cut in the copper pipe at the sampling depth. Within the source area four soil gas probes made entirely of 10 mm stainless steel was installed at 1.05 m b.g.s.—one in each direction. Additionally two stainless steel capillaries (1/16 in.) were installed near the centre of the source at 0.95 and 1.15 m b.g.s. They were attached to the 1-in. stainless steel pipe. All gas probes were sealed with butyl rubber stoppers between sampling events to prevent intrusion of atmospheric air and venting of organic vapours. The installation technique used did not allow placement of adjacent probes closer than 20 cm apart. Therefore soil gas probes of different depths were placed on the perimeter of a circle with the desired distance from the centre of the source. The probes placed at the same depth as the source were placed in the planned transect direction. 3.3.3. Groundwater multilevel samplers Seven multilevel samplers (MLSs) were installed at the site: one beneath the source and at 1, 5 and 10 m from the source in the south direction and 5 m from the source in north, east and west directions. The purpose of the MLSs was to yield groundwater samples with a high vertical resolution in the vicinity of the potentially fluctuating groundwater table below 3.3 m b.g.s. The MLSs consisted of a solid PVC rod (D 10 mm) with 9 hard Nylon (PA 11) sampling tubes (OD 4 mm, ID 2 mm) attached on the outside with a vertical spacing of 10 cm. Each tube was screened over 5 cm. The MLSs were installed using a Geoprobe drilling rig. A galvanized iron pipe equipped with a stainless steel tip was driven to a depth 15 cm deeper than the planned depth for the lowest sampling point on the MLS. The iron pipe was filled with groundwater from a nearby well to prevent influx of sand, and the tip was knocked out and left in the ground. The MLSs were then lowered inside the iron pipe and levelled. The iron pipe was withdrawn and the sediment was allowed to collapse around the MLSs. 3.3.4. Soil moisture measurements The volumetric water content was measured in-situ by Time Domain Reflectometry (TDR) using a Tektronix 1502C metallic TDR cable tester (Beaverton, Oregon, USA), which measures the dielectric constant. The average volumetric water content over the depth of the probes was then calculated using an empirical relationship between the

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dielectric constant and the volumetric water content developed by Topp et al. (1980) using the data acquisition and analysis programme AutoTDR (Prenart, Copenhagen). The TDR probes consisted of two parallel 20 cm steel bars (D 6 mm, 26 mm apart), which were attached to a PVC tube (OD 41 mm). The soil moisture content was measured at 7 depths: 0.4–0.6, 0.65–0.85, 0.9–1.1, 1.15–1.35, 1.4–1.6, 1.9–2.1 and 2.4–2.6 m b.g.s. A hole (D 54 mm) to the top of the requested depth was driven with a Geoprobe drill rig and the TDR probes were pressed manually into the undisturbed soil at the bottom of the hole. The hole was refilled with filter sand (0.13–0.50 mm), and the upper 10 cm at soil surface was filled with bentonite–concrete slurry to prevent infiltration of rain along the sides of the probes. The TDR probes were installed on the perimeter of a circle with radius 1 m in an angle of 208 towards the centre of the circle to prevent rainwater infiltration along the TDR probes. Comparison of soil moisture content measured with the TDR probes and gravimetrically on soil samples taken close to the TDR probes (using bulk density = 1.6 kg/l to calculate the volumetric soil moisture content) shows that at low water content (b 10 vol.%) the TDR probes had a tendency to overestimate the soil moisture content by 1–4 vol.%. At higher soil moisture content (10–15 vol.%) consistency was much better especially for the deeper probes. Above the source and close to the source two sets of regular TDR probes (70 cm long) were installed to control the influence of the covering of the source area on the soil moisture content. A few times during the experiment the source areas was wetted to prevent the source area from being dryer than the surrounding soil. 3.3.5. Soil temperature and meteorological data A soil temperature probe was installed at the site to measure the soil temperature at 0.5, 0.75, 1, 1.25, 1.5, 2 and 2.5 m b.g.s. The temperature probes were made of NiCr–Ni thermo-element wire (Gravquick, Glostrup, Denmark). The temperature was measured with a precalibrated digital thermometer (Gravquick S80000, Glostrup, Denmark) using an adapter. Soil temperature was logged every hour in the centre of the source (1.05 m b.g.s.) with a temperature data logger (Tinytag Plus, TG12-0020, Germini Data Loggers, UK) during the field experiment. Meteorological data were obtained from the Danish Meteorological Institute from the weather station at Airbase Værløse located a few hundred meters from the field site. Air temperature (2 m above terrain), precipitation, barometric pressure, relativity humidity and wind speed and direction monitored every 3 h were available. 3.4. Sampling The migration of the hydrocarbons and tracer in soil gas, pore water, and groundwater was monitored intensively for 1 year (July 3rd 2001 to July 3rd 2002). At each sampling campaign, approximately 100 pore gas samples were collected in 1 day. Monitoring of hydrocarbon and tracer migration in soil gas has been conducted in 32 measuring campaigns. Determination of hydrocarbons in pore water and groundwater has been conducted in 26 sampling campaigns for pore water (6 porous cups) and 13 sampling campaigns for groundwater (7 MLS with 9 levels each). The groundwater samples have

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15

also been analysed for anions—particularly bromide. Oxygen and pH have been measured 3 times during the experiment by electrodes in a flow cell placed in line between the multilevel tube and a peristaltic pump. Soil moisture content and soil temperature were measured at each sampling campaign. Before sampling for organic compounds in the pore air the probes were purged to remove the air standing in the probe. The soil gas was sampled using a stainless steel fitting with Viton o-rings, which connected the sorbing tubes directly to the soil gas probes. The pump was a SKC Personal Air Sampler (series 222-3, SKC Ltd., UK). The sorbing tubes were packed with 50 mg Tenax TA 60/80 mesh and 200 mg Carbotrap 20/40 mesh (Sigma-Aldrich, Denmark). The air volume sucked onto the sorbing tubes varied between 5 and 200 ml depending on the expected hydrocarbon concentration level. The sorbing tubes were stored cold (4 8C) until analysed. The pore air samples for CH4, CO2 and O2 analyses were sampled after purging of air standing in the probe and then 3 ml of sample was withdrawn with a syringe. The gas samples were stored in evacuated blood collection tubes (Venoject tubes, Terumo, Leuven, Belgium), and were stored cold until analysed in the laboratory using a portable Chrompack Micro GC (Middelburg, The Netherlands). The water samples for organic compounds were collected with a gas-tight glass syringe using vacuum. The first 20 ml sampled from each tube, which corresponds to a minimum of twice the tube volume, was discarded to ensure that no stagnant water was sampled. Thereafter samples for the organic compounds and anions composing a total of 25 ml were collected. The groundwater samples were extracted in the field with 1 ml and 2 ml pentane containing undecane as internal standard, respectively. The samples were kept at 4 8C until analysed in the laboratory. The water samples for anion analysis were frozen until analysed. 3.5. Analytical methods Analysis of hydrocarbons in pore air was performed by thermal desorption followed by GC analysis. Thermal desorption of the sorbing tubes was achieved with a Perkin-Elmer Turbomatrix Automated Thermal Desorber (Perkin Elmer, CT, USA) as a two-stage thermal desorption with electrical trap cooling. The unit is capable of processing 50 tubes sequentially in an unattended mode of operation. The tubes were thermally desorbed in the reverse direction to sample flow. The desorption conditions were as follows: purge for 2 min at valve temperature 200 8C, desorption for 7 min at 320 8C, desorption flow 50 ml/ min, trap hold for 1 min, cold trap low temperature
30 8C, cold trap high temperature 225 8C, transfer line 200 8C, outlet split 1:21. The cold trap was packed with 50% Tenax and 50% Carbotrap. The thermal desorber was connected to a Perkin-Elmer Autosystem GC with a Zebron capillary GC column, ZB-1, 30 m  0.25 mm ID, 0.10 Am FT. Temperature programme: 35 8C for 4 min, 20 8C/min to 300 8C, 300 8C for 2 min, cycle time 38 min, FID temperature 250 8C and ECD temperature 350 8C, carrier gas nitrogen. Calibration curves were made from dilutions of headspace (10 different) of a stock solution containing the most volatile compounds (CFC-113 to toluene). Variable volumes from the diluted

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headspace were sucked onto the sorbing tubes. The less volatile compounds (octane to dodecane) were dissolved in methanol and 10 different dilutions were made. Two microliters of the methanol dilution was injected into clean sorbing tubes. The detection limits were between 0.002 Ag/l pore air (CFC113) and 0.096 Ag/l pore air (isooctane) at 200 ml pore air sampling volume. The pentane extract from the groundwater samples was analysed on a Shimadzu gas chromatograph (GC-14A) equipped with FID detector. The column was a Wcot fused silica 30 m  0.32 mm, CP sil CB DF = 1.0 Am. Temperature programme: 45 8C for 2 min, 20 8C/min to 200 8C, 200 8C for 2 min, cycle time 20 min, temperature FID 250 8C, carrier gas nitrogen. Calibration curves were made from 10 different dilutions in pentane of a stock solution containing 11 of the 14 compounds. CFC-113, cyclopentane and 3-methylpentane could not be quantified as they elute concomitantly with the pentane extract. The detection limits were between 0.05 Ag/l (decane) and 3 Ag/l (methylcyclopentane). Bromide and the other anions were analysed by a Dionex ion chromatography system (DX120, Dionex Corp., CA, USA). An Ion Pac 144 mm (10-32) Column (P/N 46124) was used in combination with an anion suppresser (ASR II 4 mm, self-regenerating) and 3.5 mM Na2CO2/1 mM NaHCO3 buffer solution as eluent at a flow velocity of about 1.3 ml/ min. The analysis also provided data for nitrate, nitrite, sulphate and chloride. 3.6. Data interpretation Contour plots were made by gridding the measured concentrations. Kriging was used as gridding method for all the sampling campaigns. The contour plots were made using Surfer 7 (Golden Software Inc, USA). The plots were adjusted manually to ensure that the contours always obey the measured concentrations. The measured pore air concentrations in the south direction were used to calculate the mass of each compound in the pore air for each sampling campaign. The field site was divided into parts of cylinders each with a representative sampling point. The volumes of the cylinders were then multiplied with the measured concentrations and summed up. Not all sampling points were sampled in all sampling campaigns. Concentrations for the points that were not sampled were estimated based on interpolation of concentrations in the nearby sampling points. An average air filled porosity of 20% was used in the calculations. The calculated mass in the pore air was normalised using the initial mass of the compound in the source. This gives a fraction of the initial mass found in the pore air for each sampling campaign. The distribution of the organic compounds between the different phases (solid, air, and water) was estimated using simple equilibrium partitioning equations (Schwarzenbach et al., 1993). The calculations have been performed for soil outside the source area, where no free phase was occurring. The solid–water distribution ratio K d has been estimated in laboratory batch experiments for benzene, methylcyclohexane, toluene and m-xylene. The experiments were performed at 10 8C with sediment from 1.2 m b.g.s. at the field site and with a water content of 12 vol.%. The results are shown in Table 5. The organic carbon content was low ( f oc b 0.1%) for the source depth and deeper. Therefore the solid–water distribution ratio

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Table 5 Measured solid–water distribution ratio (K d) for selected compounds compared with theoretical calculated K d Compound

K d,measured (l/kg)

K d,Abdula (l/kg)

K d,P&Bb (l/kg)

K d,Abdul/ K d,measured

K d,P&B/ K d,measured

Benzene Methylcyclohexane Toluene m-Xylene

0.02 F 0.01 0.09 F 0.04 0.04 F 0.02 0.07 F 0.02

0.005 0.16 0.02 0.060

0.049 1.5 0.18 0.59

0.25 1.8 0.50 0.86

2.1 17 4.8 8.7

a b

logK d,Abdul = 1.04d logK ow + logf oc
0.84 (Abdul et al., 1987). logK d,P&B = 1.01 logK ow
3.46 (Piwoni and Banerjee, 1989).

K d was estimated using the equation from Piwoni and Banerjee (1989) which was developed as an estimate for K d for soils having f oc below 0.1%. For soils with f oc N 0.1%, K d can be estimated using the equation from Abdul et al. (1987). The calculated theoretical K d values are shown in Table 5. By comparison to measured values for selected compounds on the Værløse sand (Table 5) it is evident that the Piwoni and Banerjee equation strongly overestimates the K d and thereby the sorption. The table shows that the Abdul equation yields a much better estimate. Therefore the equation from Abdul et al. (1987) has been used to estimate K d values for all the compounds.

4. Results and discussion 4.1. Migration of the tracer in the pore air CFC-113 (CCl2FCClF2) was used as a conservative tracer in the field experiment. CFC113 is an anthropogenic compound with no known natural source. It is mainly used as cooling and cleaning agent, especially in the electronics industry, from where it is released to the atmosphere. CFC-113 has not been reported biodegradable under aerobic conditions. Under anaerobic conditions CFC-113 can undergo reductive dechlorination (Hohener et al., 2003a). Aerobic conditions with oxygen concentrations continuously N 17% were observed throughout the field experiment (Kjeldsen et al., 2003). Background pore air samples were collected prior to installation of the source from all the soil gas probes outside the source area. Background CFC-113 concentrations in the pore air samples were relatively uniform between 5 d 10
5 and 9 d 10
5 mg/l down to 2 m b.g.s. and then increased with depth to 6 d 10
4 mg/l at 3.3 m b.g.s. At the end of the experiment when the CFC-113 had been depleted from the source, the concentration in the soil gas probes closest to the groundwater (2.3 m b.g.s.) was 6–30 d 10
5 mg/l. The concentration of CFC-113 in the atmospheric air at Airbase Værløse was measured to be 5–9 d 10
7 mg/l. That is close to the 82 pptv (equals 6.7 d 10
7 mg/l at the annual average air temperature in Denmark (8 8C)), reported as ambient atmospheric concentration of CFC-113 (IPCC, 1995; Montzka et al., 1999; Walker et al., 2000). The atmospheric concentration can be transformed to a water concentration using Henry’s law with the constant from Table 3 and the Henry’s law constant temperature dependency from Bu and Warner (1995). At 10 8C this gives a water concentration of 1.0 d 10
7 mg/l. The soil gas concentrations measured just above the groundwater before

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and at the end of the experiment equal a groundwater concentration of 9–90 d 10
6 mg/l. These findings show the presence of a weak pollution with CFC-113 in the secondary groundwater prior to the start of the field experiment. It probably originates from prior activities at the airbase. Concentration contours for CFC-113 in the south transect on selected days are illustrated in Fig. 4 for the first week and in Fig. 5 for the following year. Initial CFC-113 vapour migration from the source was very fast. A few hours after source emplacement CFC-113 concentrations in the pore air exceeded the background level as far as 1.25 m from the centre of the source (Fig. 5). The maximum migration distance of CFC-113 was 12 m at days 70–100 (Table 6). Hereafter, the CFC-113 plume decreased, and at day 260

Fig. 4. Measured (top) and simulated (bottom) concentration contours for CFC-113, toluene and hexane in the south radial for selected times during the first week. Values are in mg (l pore air)
1.

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Fig. 5. Concentration contours of CFC-113 in the pore air in the south radial at days 9, 17, 30, 63, 113 and 329. Values are in mg (l pore air)
1.

CFC-113 concentrations at the site were below the background level. However, due to the very low detection limit of 2 d 10
6 mg/l at 200 ml pore air sample, CFC-113 was measured throughout the experiment. The initial spreading of CFC-113 was simulated by the model R-UNSAT with input data based on measured parameters. The input data for the simulation is given in Table 4. The results of the simulation are given in Fig. 4 in comparison with measured data. In

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Table 6 Summary of the migration behaviour for the source compounds in the pore air Compound

Maximum migration (m)

Day for maximum migration

Depletion day

Detection limita (Ag/l)

Benzene Toluene m-Xylene 1,2,4 Trimethylbenzene Hexane Octane Decane Dodecane Methylcyclopentane Cyclopentane Methylcyclohexane Isooctane 3-Methylpentane CFC-113

5 4 3 3 20 5 3 1 10 10 20 10 20 12

3–4 6 6–184 37–154 17 3–30 37–113 6–350 13 13–17 17 13 17 70–100

205 364 n.d. n.d. 260 n.d. n.d. n.d. 237 93 n.d. n.d. 237 260

0.093 0.095 0.030 0.051 0.047 0.026 0.027 0.15 0.051 0.040 0.048 0.096 0.095 0.002

n.d.: not depleted. a At the maximum migration distance on the day/period of maximum migration distance equal to 200 ml pore air sampling volume for all compounds except for dodecane (100 ml pore air sampling volume).

general the measured and the simulated contours show similar concentrations and distances. This indicates that the initial spreading is mostly governed by diffusion processes. A thorough discussion on the diffusional behaviour for the full experimental period is included in Gaganis et al. (2004) and Maier (2005). The concentration of CFC-113 in the pore air as a function of time is shown at Fig. 6a for selected sampling points. The initial vapour concentrations were higher and the migration was faster than expected compared with the modelling carried out in the planning phase. This resulted in overloading of some of the sorbing tubes at the beginning of the experiment whereby part of the most volatile compounds (CFC-113 was the most volatile compound—see Table 3) were lost because they were desorbed from the tube due to the overloading. That is the reason for the lack of data points at the beginning of the experiment. When the source was removed it was evident by observation of present Sudan IV dye that the NAPL phase had migrated downward into the location of sampling point S-0-7, which was placed 1.4 m b.g.s. in the centre of the source (Broholm et al., submitted for publication). The sampled pore gas from this sampling point is therefore taken from within the source area. The fraction of CFC-113 in the source was low and that combined with a high volatility (see Table 3) resulted in a fast decrease in the concentration of CFC-113 in the source as can be seen at Fig. 6a especially for S-0-7. However, the concentration of CFC-113 in the source was high and relatively stable for approximately 1 month. The general trend was considering the scatter in the data points as expected. The concentrations were highest in the source. For the sampling points a little further away (S0-12 and S-2-5), a few samples of low concentrations of CFC-113 were found before the concentrations increased. The highest concentrations were measured within the first month of the experiment.

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Fig. 6. Time series of (a) CFC-113 concentrations (log scale) and (b) toluene concentrations in the pore air at selected measuring points together with (c) the average soil moisture content and (d) temperature in the centre of the source and sum of precipitation. Sampling point S-0-7 was placed 1.4 m b.g.s. in the source area, S-0-12 was placed 2 m below the lower perimeter of the source, and S-1-5 and S-2-5 were placed 1 and 2 m horizontally from the centre of the source in the south radial, respectively, at source depth (1.05 m b.g.s.).

To evaluate the radial distribution of the organic vapours in the pore air, the concentrations of the organic compounds were measured in three other directions at 2 and 3 m from the centre of the source at almost each measuring campaign. Fig. 7a shows an example of the measured concentrations of CFC-113 in the other three directions compared with the measured concentrations in the south direction. The concentrations were quite similar in all directions, which was the general observation from these types of plots. That indicates that the radial migration of the vapours in the relatively homogeneous sand deposit was uniform as also observed in the experiment reported by Conant et al. (1996). In Fig. 6a there appear to be a tendency for higher concentrations in the deeper probes, which was the case for all CFC plots. This tendency is also slightly evident in the contour plots (Fig. 5), especially after 1 month. As previous mentioned, background

Fig. 7. The concentration of (a) CFC-113 and (b) hexane in the pore air in all four directions at 2 (the first 4 points) and 3 m (the last 4 points) from the centre of the source at day 30. The sampling points are placed 0.55, 1.05, 2.3 and 3.3 m b.g.s, respectively.

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Fig. 8. Calculated mass of CFC-113 in the pore air as a function of time normalised to the initial mass of CFC-113 in the source area.

pollution in the groundwater only accounts for pore air concentrations in the 10
4 mg/ l range. This is believed to be due to significant loss to the atmosphere of the nondegradable tracer, CFC-113. The total mass of CFC-113 in the pore air is shown in Fig. 8 normalised by the 9.7 g of CFC-113 initially present in the source. The maximum amount of CFC-113 in the pore air was measured on day 17 with 5.5 g of CFC-113 in the pore air. Hereafter, there was a profound decrease in the amount until day 45, where the CFC-113 amount stabilised for a short period (7 weeks). From day 260 the amount of CFC-113 was around 0.005 g at the field site, similar to the background level. Phase distribution calculations (Table 7) performed for the low organic carbon content soil at the source depth and deeper showed that at 10 8C, 90% of the CFC-113 would be in the air phase, 5% in the water phase, and 5% sorbed to the soil showing that small amounts of the CFC-113 would be retained in the pore water and the sandy soils. 4.2. Migration of hydrocarbons in the pore air The distributions of selected hydrocarbons in the pore air on selected sampling days are shown in Fig. 4 for toluene and hexane covering the first week and in Fig. 9 for the following year (hexane, isooctane, toluene and decane). The migration of the compounds in the pore air occurred very fast and all the compounds were detected in the pore air within 3 h after source installation (Fig. 4). As expected the highest concentrations were found within the source area, and the concentrations decreased with distance from the source. Not all sampling points were sampled in each sampling campaign at the beginning Table 7 Distribution of the source compounds outside the source area between air, water and solid phases at 10 8C and f oc = 0.0002 using Abdul et al. (1987) to calculate K d f air f water f solid

CFC-113

CP

3-MP

Hex

MCP

Ben

Iso

MCH

Tol

Oct

Xyl

TMB

Dec

Dod

0.90 0.05 0.05

0.85 0.09 0.06

0.98 0.01 0.01

0.74 0.03 0.23

0.94 0.03 0.04

0.22 0.73 0.06

0.97 0.00 0.03

0.39 0.17 0.45

0.20 0.62 0.18

0.08 0.01 0.91

0.15 0.43 0.42

0.06 0.19 0.75

0.25 0.00 0.75

0.05 0.00 0.95

CP: cyclopentane, 3-MP: 3-methylpentane, Hex: hexane, MCP: methylcyclopentane, Ben: benzene, Iso: isooctane, MCH: methylcyclohexane, Tol: toluene, Oct: octane, Xyl: m-xylene, TMB: 1,2,4 trimethylbenzene, Dec: decane and Dod: dodecane.

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of the experiment. The initial concentrations were higher and migration of the hydrocarbons was faster than expected. Therefore, the vapour plume was not entirely delineated by the points sampled initially. The higher concentrations and faster migration were probably predominantly caused by higher soil temperature and lower soil moisture

Fig. 9. Concentration contours of (a) hexane, (b) isooctane, (c) toluene and (d) decane in the pore air in the south radial at days 9, 17, 30, 63, 113 and 329. Values are in mg (l pore air)
1.

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Fig. 9 (continued).

content than applied in the preliminary modelling, used to estimate the initial pore air concentrations and thereby sampling volumes. Another consequence was overloading of some of the sorbing tubes. These problems occurred during the first 1–2 weeks after source emplacement.

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25

The initial spreading of toluene and hexane was simulated by the model R-UNSAT with input data based on measured parameters. The input data for the simulation is given in Table 4. Biodegradation was neglected due to the very short modelling period. The results of the simulation are given in Fig. 4 in comparison with measured data. In general the simulated contours are good predictions of the measured contours both in respect to concentration ranges and front distances. It indicates that the initial spreading is mostly governed by diffusion and sorption processes. The migration behaviour in the pore air of all compounds over the 1-year period is summarised in Table 6. The maximum migration varied from 1 m for dodecane to at least 20 m for hexane, methylcyclohexane and 3-methylpentane. The detection limits for the pore air sampling volume used at the maximum migration distance on the day/period of maximum migration distance are shown in Table 6. The volatile compounds migrated very fast to their maximal migration distance and then immediately their plume extend decreased, whereas the less volatile compounds had a relatively stable plume at the maximal migration distance for a longer period. Seven compounds were depleted from the source within 1 year—the first compound, cyclopentane, after 3 months. The calculated phase distributions for each of the compounds in the melt water sand at 10 8C are given in Table 7. The compound 3-methylpentane was the source compound with the highest fraction in the pore air followed by isooctane and CFC-113. The alkanes have very low water solubility but they sorb to the soil. The aromatic compounds have a quite large fraction in the pore water due to their high water solubility. In the following the behaviour of the 13 hydrocarbons will be described with special focus on four selected representative compounds. These compounds were selected based on their properties to ensure that a large range of volatility, water solubility and sorption capacity was represented. Hexane is a very volatile hydrocarbon with low water solubility and a relatively high sorption capacity. Selected concentration contours for hexane in pore air are shown in Fig. 9a. The migration of hexane was fast. At day 17 hexane together with 3-methylpentane and methylcyclohexane had reached 20 m (see Table 6), in concentrations of 0.024, 0.021 and 0.002 mg/l, respectively, all well above the detection limit of 5 d 10
5 mg/l for these compounds. The size of the hexane plume then decreased and stabilised at approximately 4 m from the centre of the source from around days 30–100 probably as a result of degradation (se also Hohener et al., in preparation). Hereafter, the hexane plume decreased fast. Hexane was depleted from the source at day 260. Toluene is less volatile compared with hexane and has considerably higher water solubility. A considerable amount of toluene was expected to partition to the water and sorbs to the soil (Table 7) resulting in retardation of the vapour migration of toluene. Consistent with that, the concentration contours at Fig. 9b show much less spreading of the toluene plume compared with hexane. The toluene plume does not as the hexane plume increase and then decrease within the first weeks of the experiment. A very small amount of toluene was still present by day 329, but at day 364 no toluene was left in the pore air. Isooctane is a branched alkane with vapour pressure similar to toluene and water solubility similar to hexane. Concentration contours are shown in Fig. 9c. The fraction of isooctane in the source was high (15.36%) compared with toluene (2.93%) resulting in

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significantly higher pore air concentrations of isooctane. Isooctane had a maximal migration distance of 10 m at day 13 and the plume extension stabilised at 6 m from day 17 and until day 125. At the end of the experiment isooctane was still present in the pore air. The amount of decane in the source was very similar to isooctane. However, the vapour pressure of decane is approximately 40 times smaller than for isooctane so the pore air concentrations of decane were limited. The water solubility of decane is very low (0.052 mg/l) but it has a high sorption potential (logK ow = 6.25). These physical properties resulted in a very limited migration of decane in the pore air. As can be seen in Fig. 9d the concentration of decane increased at the end of the experiment as a result of the depletion of other source compounds. This is discussed in details in Broholm et al. (submitted for publication), which focus on the evolution of the source composition in the field experiment. Time series for toluene in four measuring points are shown in Fig. 6b. Point S-0-7 was placed just below the emplaced source, at the point where the NAPL had migrated (Broholm et al., submitted for publication), point S-0-12 is placed 2 m below the lower boundary of the source, and S-1-5 and S-2-5 1 and 2 m horizontally from the centre of the source, respectively, at source depth. For S-0-7 the sampling problems (overloading of the sorbing tubes at the beginning of the experiment) described earlier caused low measured concentrations during the first days. The average soil moisture content (down to 2.6 m b.g.s.), the accumulated sum of precipitation, and the temperature in the centre of the source are also shown in Fig. 6. Since benzene was depleted early on (see Table 6), toluene constituted the most water-soluble compound. For a rain event of 10 mm precipitation an amount of toluene corresponding to 13% of the amount in the pore air could be dissolved. Hence, heavy rainfall may result in a temporary decrease of the concentration of toluene. This indicates that dissolution of toluene in infiltrating rainwater could be of some significance. In Fig. 6 there appears to be a slight tendency for an inverse relationship between soil moisture content and toluene concentrations at S-0-7 and S-1-5 at days 40–100, which was a period with heavy rain. However, the temperature may also influence the concentration level due to the temperature dependency of the vapour pressure. From Fig. 6d it can be seen that the temperature in the source was almost constant until day 70. As can be seen from Fig. 9a, the concentrations of hexane had already decreased significantly at day 70, so for hexane the decreasing concentration was not a result of decreasing temperature but more probably a result of decreasing mole fraction in the source as a consequence of volatilisation (confer Broholm et al., submitted for publication for more details). The migration of hexane vapours in the pore air in all four directions is shown in Fig. 7b at day 30. As for the tracer CFC-113 (Fig. 7a) there was no general trend with higher concentrations in one direction compared with the other directions. The concentrations were quite similar in all directions for all the measuring campaigns. Hence the distribution of the vapours was quite uniform in the relatively homogeneous sand. For the hydrocarbon vapours there was a tendency for higher concentrations at the same depth or a little deeper than the source, probably due to evaporative losses to the atmosphere or retardation to the more organic rich sediments close to the ground surface.

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27

4.3. Hydrocarbons in the groundwater As described earlier the southern radial of the monitoring network design was placed in the direction of the general groundwater flow as determined prior to the experiment from groundwater level measurements in installed wells. After the experiment was initiated more piezometers were installed, which gave a more detailed picture of groundwater potentials. Table 8 shows the determined gradient of the ground water table in the vicinity of the source together with the actual direction of groundwater flow in that area. Data representing the first 100 days of the experiment is given. On the basis of the measured average hydraulic conductivity in the area, 1 d 10
4 m/s (Table 1), the average gradient of the groundwater table, 1% (Table 8), and an average saturated porosity of 0.32, an average pore water velocity in the aquifer of 102 m year
1 is obtained. The flow direction of the upper secondary groundwater was towards southeast with an average deviation from the main monitoring radial of 158 and did not change significantly during the 1.5-year monitoring period despite of 1.2–1.6 m fluctuations in the groundwater level (see Figs. 1 and 3). Prior to source emplacement background water samples were sampled and analysed from all points at the multilevel samplers and piezometers and no hydrocarbons were found. Therefore, it can be concluded that the hydrocarbons found in the secondary groundwater after source emplacement were derived from the source in the vadose zone. Oxygen and pH were measured three times during the field experiment. The secondary groundwater was at all times aerobic with oxygen content above 6 mg/l. pH was neutral around 7.1. The bromide analysis showed very low level or below detection limit of bromide indicating that insignificant amounts of rainwater had percolated the source area. In Fig. 10 time series for the hydrocarbons with the highest concentrations found in groundwater are shown for the multilevel sampler placed beneath the source. Results are shown for the two sampling points closest to the soil surface from which it was possible to sample water during the first part of the experiment. The water height above the sampling point is shown at the right axis of the figures and this shows that for MLS-0-5 (3.7 m.b.s.) the water sampled at days 31 and 42 actually represented water from the capillary fringe. At day 64 it was not possible to sample water from MLS-0-5 but from MLS-0-6, which was placed 10 cm lower. In general the water table varied 20 cm within the 100-day period where organic compounds were detected in the groundwater. The water samples taken from MLS-0-5 until day 185 were taken very close to the groundwater table (Fig. 10). The compounds with the highest vapour pressures were observed first in the upper groundwater (methylcyclopentane followed by methylcyclohexane and isooctane). The Table 8 Local gradient and the local deviation of the stream line from the experimental centre line in the first 3 months of the experiment Day

3

7

13

22

32

37

45

55

64

71

87

93

Xa

Local gradient (%) 0.9 1.0 1.0 0.9 0.9 1.0 0.9 1.0 1.0 1.1 1.3 1.3 1.0 F 0.1 Stream line deviation Degrees 16 15 16 15 13 14 15 15 17 14 13 14 15 F 1 from centre line a

Average and standard deviation.

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Fig. 10. Time series of the concentration of selected compounds in the groundwater at (a) MLS-0-5 and (b) MLS0-6 (3.7 m b.g.s. and 3.8 m b.g.s. at the source area, respectively), and (c) MLS-1-7 (1 m from the source centre and 3.9 m b.g.s). See Table 7 for compound name abbreviations.

aromatic compounds toluene, xylene and trimethylbenzene arrived a little later due to their retardation in the pore water. Toluene has the highest water solubility (besides benzene, which was only found at low concentrations and disappeared very fast from the groundwater) and was found in the highest concentrations. The organic compounds disappeared from the groundwater quite suddenly around day 100 and were not detected again—not even at sampling points closer to the soil surface during a rise in groundwater in the early spring. Table 7 shows phase distribution calculations at 10 8C for the low organic carbon content in the melt water sand ( f oc = 0.0002). The aromatic compounds have the largest fraction in the water phase and the alkanes have the smallest fraction in the water phase. That is also indicated in Fig. 10. The trends of the time series for the two sampling points closest to the water table (Fig. 10a and b) are very similar. The same compounds are measured and in the same order. However, when the concentration levels in the points are compared it is remarkable that within the 10 cm distance between them, the concentration drops approximately one order

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29

Table 9 Concentration of toluene (in Ag/l) in the groundwater beneath the source as a function of depth (in meters below ground surface (m b.g.s.) and sampling day Day

2 9 16 31 42 64 98 126 155

Sampling depth (m.b.s) 3.7

3.8

3.9

4

4.1

2.35 59.5 62.3 218 606 59.5 ~0 ~0 ~0

1.82 1.86 2.9 ~0 55 2.99 ~0 4 ~0

1.91 1.9 4.45 ~0 3.9 ~0 ~0 ~0 ~0

1.86 1.98 2.28 ~0 0.5 ~0 ~0 ~0 ~0

n.a. 1.97 3.39 ~0 2 ~0 ~0 ~0 ~0

of magnitude. This is also clearly illustrated by the depth profile for toluene in the groundwater beneath the source at all measuring dates until day 155 (Table 9). The concentrations generally decreased very significantly with depth, and significant levels of the compounds were only found in the two uppermost points sampled. The groundwater table rose 1.15 m from day 185 to day 262 and from day 239 and forward all MLS sampling points could be sampled (3.3–4.1 m b.g.s.). At that time several of the compounds had already been depleted from the source (see Table 6) resulting in low vapour concentrations, and only very low concentrations for a few compounds were found in the groundwater as a result of the rise in groundwater table. The multilevel sampler, MLS-1 (located 1 m from the source), is within the expected hydrocarbon plume stretching from the area beneath the source (in despite of the deviated groundwater flow direction from the experimental centre line). Results from the screen closest to the groundwater table are shown in Table 9. It is observed that the concentrations are close to detection limit at nearly all times. The travel time from a location beneath the source and close to MLS-0 where significantly elevated concentrations were observed would be around 3 days for the estimated velocity of 100 m/year. There appears to be very effective natural attenuation of the hydrocarbons in the shallow aquifer. Only traces of the compounds were found at the multilevel samplers 5 and 10 m from the centre of the source during the experiment. Overall, only a minor effect of the emplaced hydrocarbon source on the underlying groundwater was observed.

5. Conclusions An emplaced hydrocarbon source field experiment was successfully conducted in the relatively homogeneous sandy geology of the vadose zone at Airbase Værløse, Denmark. A data set was obtained which can be used for validating mathematical models of the transport and attenuation of volatile organic compounds in the unsaturated zone. The experiment showed fast spreading of all the compounds in the pore air and all compounds were measured in the pore air a few hours after source emplacement. A 2D radial mathematical model of the diffusion of the hydrocarbons in the pore air and sorption

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to the soil successfully simulated the measured results for the first 6 days of the experiment. This shows that diffusion is the most important governing process of the compound migration in the initial stage. The 2D radial model did also prove helpful for designing such type of field experiment. Half of the 14 compounds initially present in the source were depleted from the source within the 1 year of monitoring leading to very reduced pollutant release at the end of the experiment. Groundwater input of volatile compounds occurs even though direct leaching from the source is avoided, showing that the compounds are transferred to the groundwater zone through vapour transport. The transversal vertical dispersion at the groundwater table must be very low, preventing the formation of a deep plume. Besides, the natural attenuation in the groundwater and capillary fringe is very effective, preventing the formation of a long plume. Overall the 10 l of hydrocarbons emplaced about 2 m above the groundwater table only gave rise to a minor influence on the underlying aquifer. Acknowledgements The authors wish to thank the Danish Defence Construction Service (Mogens Ohmsen and Ida Nielsen) for assistance in locating a suitable field site, Airbase Værløse (J. Mathiesen) for assistance in preparing the field site and Jens Schaarup Sørensen, Rita Kallesøe Hansen and Bent Skov for conducting field setup, sampling and laboratory work. The Danish Meteorological Institute provided us with meteorological data, Torben Dolin and Birte Brejl made the illustrations, Francois Baumgartner, Department of civil engineering, Technical University of Denmark performed the geophysical measurements, and Frank-Michael Lange, Universita¨t Hohenheim determined the physical properties of the soil samples. All these contributions are gratefully acknowledged. The project was financed by EU (Fifth Framework Programme) and was part of the European project Groundwater risk assessment at contaminated sites GRACOS, EVK1-CT-1999-00029. References Aamand, J., Jørgensen, C., Arvin, E., Jensen, B.K., 1989. Microbial adaptation to degradation of hydrocarbons in polluted and unpolluted groundwater. J. Contam. Hydrol. 4, 299 – 312. Abdul, A.S., 1988. Migration of petroleum products through sandy hydrogeological systems. Ground Water Monit. Rev. 8, 73 – 81. Abdul, A.S., Gibson, T.L., Rai, D.N., 1987. Statistical correlations for predicting the partition-coefficient for nonpolar organic contaminants between aquifer organic-carbon and water. Hazard. Waste Hazard. Mater. 4 (3), 211 – 222. Aelion, C.M., Bradley, P.M., 1991. Aerobic biodegradation potential of subsurface microorganisms from a jet fuel-contaminated aquifer. Appl. Environ. Microb. 57 (1), 57 – 63. Altevogt, A.S., Rolston, D.E., Venterea, R.T., 2003. Density and pressure effects on the transport of gas phase chemicals in unsaturated porous media. Water Resour. Res. 39 (3), 1061. Auer, L.H., Rosenberg, N.D., Birdsell, K.H., Whitney, E.M., 1996. The effects of barometric pumping on contaminant transport. J. Contam. Hydrol. 24 (2), 145 – 166.

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