Investigation of evaporation and biodegradation of fuel spills in Antarctica: II—Extent of natural attenuation at Casey Station

Chemosphere 63 (2006) 89–98 www.elsevier.com/locate/chemosphere

Investigation of evaporation and biodegradation of fuel spills in Antarctica: II—Extent of natural attenuation at Casey Station Ian Snape *, Susan H. Ferguson, Paul McA. Harvey, Martin J. Riddle Human Impacts Research, Australian Antarctic Division, Channel Highway, Kingston, Tasmania 7050, Australia Received 22 December 2004; received in revised form 5 July 2005; accepted 9 July 2005 Available online 16 September 2005

Abstract In many temperate regions, fuel and oil spills are sometimes managed simply by allowing natural degradation to occur, while monitoring soils and groundwater to ensure that there is no off-site migration or on-site impact. To critically assess whether this approach is suitable for coastal Antarctic sites, we investigated the extent of evaporation and biodegradation at three old fuel spills at Casey Station. Where the contaminants migrated across frozen ground, probably beneath snow, approximately half the fuel evaporated in the first few months prior to infiltration at the beginning of summer. Once in the ground, however, evaporation rates were negligible. In contrast, minor spills from fuel drums buried in an abandoned waste disposal site did not evaporate to the same extent. Biodegradation within all three spill sites is generally very minor. We conclude that natural attenuation is not a suitable management strategy for fuelcontaminated soils in Antarctic coastal regions. Ó 2005 Published by Elsevier Ltd. Keywords: GC-FID; Fingerprinting; Petroleum hydrocarbons; Special Antarctic blend; Diesel; Weathering

1. Introduction Antarctica is widely regarded as the EarthÕs last unspoilt wilderness; but decades of polar exploration, research and exploitation have left a significant legacy of on-land and nearshore-marine pollution (Snape et al., 2001). One of the most extensive and environmentally damaging pollution problems concerns petroleum spills. Although marine spills are often highly visible and are regarded as being ecologically most damaging because their

* Corresponding author. Tel.: +61 3 6232 3591; fax: +61 3 6232 3158. E-mail address: [email protected] (I. Snape).

0045-6535/$ - see front matter Ó 2005 Published by Elsevier Ltd. doi:10.1016/j.chemosphere.2005.07.040

impact is immediate at many trophic levels (Kennicutt II, 1990), many substantial spills also occur on-land where the immediate environmental impacts are often not so apparent. These spills typically percolated into porous soils or dispersed along impermeable frozen ground or ice before passing through catchments into the nearshore-marine environment. Of the 73 reported petroleum spill incidents recorded by COMNAP in 1999, 59 spills were on-land (COMNAP, 1999). Many of these petroleum plumes will continue to slowly migrate for decades after the initial accident, often releasing a pulse of contaminants each season with the summer snowmelt. In many temperate regions, fuel and oil spills are sometimes managed simply by allowing natural degradation to occur, while monitoring soils and groundwater

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to ensure that there is no off-site migration into adjacent areas. To critically assess whether this approach is suitable for coastal Antarctic sites, we have undertaken a multidisciplinary study into the origin, distribution and fate of terrestrial petroleum contaminants at Casey Station (see also Stark et al., 2003; Powell et al., 2005). This contribution documents the nature and distribution of petroleum contamination and extent of natural attenuation (through weathering and biodegradation) of three spills in the region.

2. Site characterisation and spill histories Casey Station is located in the Windmill Islands on the coast of east Antarctica at approximately 66°17 0 S, 110°32 0 E (Fig. 1). Like many stations in Antarctica, Casey is located on a coastal ice-free rock and gravel peninsula. The most commonly used product on the continent by Australia is Special Antarctic Blend (SAB) (2 million l yr1), which is used extensively for power generation and heavy vehicle transport. Aviation Turbine Kerosene (ATK/JetA1) and gasoline are used

to a much lesser extent for helicopters, light aircraft, boats, and vehicles. The oldest spill we chose to study, at the Old Casey workshop (Fig. 1), occurred in 1982 when 36 000 l of fuel leaked during fuel transfer. In 1994 Deprez et al. (1999) sampled several soils where petroleum concentrations were >5000 mg total petroleum hydrocarbons (TPH) kg1, with one sample contaminated with 47 631 mg TPH kg1. They also noted patches of soil that contained heavy fractions consistent with lubrication oil, and small patches likely to be caused by spills during vehicle refuelling. The youngest spill we examined in detail occurred at the New Casey Station main powerhouse (Fig. 1) in September 1999. Although it is likely that there were several small spills at the main powerhouse prior to 1999, a large spill of >2000 l leaked from a fuel storage tank during the winter. The plume, visually characterised by a red dye, appears to have migrated at the interface between snow and the frozen active layer during the winter months. As the summer thaw progressed, the plume infiltrated into the active layer, and then moved laterally by both surface and subsurface runoff.

Fig. 1. (a) Location of the Windmill Islands (WI) oasis in East Antarctica; (b) location of Casey Station in the Windmill Islands; (c) detailed map of Casey Station showing the location of petroleum spills at the main powerhouse, Thala Valley waste disposal site and Old Casey workshop.

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The third site we examined is the Old Casey waste disposal site in Thala Valley (Fig. 1), which was chosen because it has a different contamination history, and contrasting environmental conditions compared with the Old Casey workshop and New Casey main powerhouse spills. In particular the main spill mechanism was from leaking buried drums. Before a partial cleanup in 1995–96, the Thala Valley waste disposal site was known to be highly contaminated with mid- (n-C10–14) to heavy-range (n-C15–28) petroleum hydrocarbons (Deprez et al., 1999). Some sampling locations had concentrations up to 7524 mg TPH kg1.

3. Materials and methods 3.1. Soil sample collection Soil samples were collected in 1999 at a depth of 3– 5 cm from a variety of reference (non-impacted) and potentially contaminated sites. Reference sites did not contain extractable natural hydrocarbons above detection limits. Some of the contaminated sites at the Old Casey workshop correspond with locations sampled by Revill et al. (in press) in 1996, though they sampled soils closer to the surface. It was possible in 1999 to dig a soil pit into frozen ground to a depth of 1 m. Similarly it was possible to sample at depth (1 m) at the base of the Thala Valley waste disposal site in 2004 during site clean-up. For each sample, 400– 600 g of soil was collected using a metal trowel. Because soils were heterogeneous in grainsize, samples were screened to 2 mm through a clean brass sieve to provide better between-site comparability and to lessen the effects of dilution by omitting large pebbles. Experimental determination of the effects on the most volatile fractions of quickly screening the soil to this grainsize showed that losses were very minor and were not sufficiently large to be quantifiable by GC-FID analysis. Samples were then stored in clean glass jars with aluminium caps, and frozen at 18 °C until analysis. 3.2. Chemical analysis of samples The extraction and subsequent quantification of the aliphatic fraction is described in detail in Ferguson et al. (2003) and Snape et al. (2005). In brief, sub-samples of soil were extracted directly with either hexane, or a hexane: isopropanol mix. Analysis was undertaken on one of three cross-calibrated gas chromatographs (Varian 3410, 3800 and Agilent 6890) fitted with a flame ionisation detector (FID). Sample introduction methods included split, splitless and direct on-column injection and were cross-calibrated with an in-house SAB standard.

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3.3. Fuel composition Chemical compositions of reference fuels are summarized in Table 1 and are used for comparison with contaminated soil samples. SAB is primarily comprised of n-C9–14 alkanes with trace amounts of n-C15–23. The n-alkane series peaks at about n-C12 and fresh SAB has a relatively high proportion of resolved compounds of 70%. The pristane (pr)–phytane (ph) ratio is known to range from about 5 (recent fuel) to as high as 10 or 15 (cf. Revill et al., in press). Small variations in the feedstock and distillation process can have a significant influence on this ratio. Importantly, all the historic data confirm that the general profile of SAB is dominated by components in the range n-C9–14,and this main envelope has not changed substantially over time (cf. Kerry, 1993; Green and Nichols, 1995; Revill et al., in press). ATK/ Jet-A1 is similar to SAB with the same range of n-alkanes, but are slightly more volatile peaking at n-C11. The Arctic blend diesel also peaks at about n-C12, but contains higher concentrations of the alkanes from nC14–22 compared with SAB or ATK. Pristane/phytane is much lower (2.0) than SAB and in the fresh fuel the UCM is more dominant with only 65% resolved between n-C9–22. MGO also peaks at about n-C11, but it is the heaviest fuel used in Antarctica by Australia; it has the highest concentration of the heavier alkanes ranging up to about n-C24. Pristane/phytane is 4.5 and the resolved portion is similar to SAB (71%), but MGO has a greater proportion of compounds >n-C15.

4. Results: extent of natural attenuation at Casey To assess the extent of natural attenuation at Casey, we considered the concentration of fuel in the ground and indices that are known to change during biodegradation or evaporation. The indices include ratios of resolved well-characterised and easily identifiable nalkanes and isoprenoids, but also other resolved peaks and UCM of similar effective carbon number (ECN) and volatility relative to the alkanes or isoprenoids (see Snape et al., 2005). Quantitative measures of biodegradation loss are more complicated because fractionation depends more on chemical structure than volatility. However, our experimental data and other studies demonstrate that the general order of biodegradation is n-alkanes > branched and cyclic alkanes > aromatics and polar compounds, with alkyl branching and substitution increasing a compoundÕs resistance to biodegradation (e.g. Atlas, 1981). For minor biodegradation losses, the light n-alkanes such as n-C10 are amongst the most easily biodegraded, and an assessment of biodegradation can be made by monitoring ratios such as n-C10/(R + UCM)(ECN9.5–10.5). We found this

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Table 1 Petroleum hydrocarbon concentrations with chemical fuel indices for soils at Casey station (n.d. = not detected) Sample

Depth Fuel quantity Resolved Source Biodegradation Evaporation (cm) pr/ph C /R + C /i (mg kg1) (lg l1) % 12 12 13 C13/i14 C14/i15 C15/i16 C17/pr C18/ph C12/i14 i13/pr UCM

i14/pr

i15/pr i16/pr

75 74 71 73 71 70 65 71

4.4 8.7 8.6 8.1 4.4 5.0 1.9 4.5

0.20 0.18 0.22 0.24 0.19 0.21 0.13 0.21

4.28 3.88 4.26 4.49 3.74 3.52 3.02 4.98

2.97 2.84 4.35 4.05 2.98 3.00 2.83 1.73

4.06 4.33 3.69 4.73 3.87 4.35 3.53 4.27

1.20 1.35 1.73 1.77 1.27 1.09 1.86 1.99

1.47 1.55 1.79 1.70 1.27 1.12 0.99 1.48

2.64 6.41 6.74 5.41 4.80 4.60 1.27 6.27

4.67 3.87 4.12 3.95 3.83 2.95 3.19 2.50

Old Casey workshop soils (upper catchment) CW98S44 5–10 19 454 CW98S45 40–45 4702 CW98S46 80–85 4445 CW98S47 5–10 8434 CW98S48 40–45 4367 CW98S49 80–85 3737 CW98S50 5–10 13 650 CW98S51 40–45 16 062 CW98S06 5 6977 CW98S08 5 10 540 CW98S10 5 10 645 CW98S12 5 11 948 CW98S24 5 44 725 CW00S278 5 19 659 CW00S280 5 22 267 CW00S284 5 23 512 CW00S286 5 17 116

67 67 69 69 69 70 67 68 68 69 67 68 67 59 54 57 57

10.4 11.0 8.3 10.1 11.6 12.7 10.3 10.6 9.4 9.3 8.8 9.9 9.3 9.6 10.1 12.3 9.6

0.25 0.23 0.25 0.25 0.26 0.26 0.26 0.26 0.23 0.23 0.22 0.22 0.15 0.22 0.20 0.23 0.24

3.63 3.34 3.54 3.41 3.70 3.75 3.60 3.94 3.02 2.98 2.95 3.09 1.69 3.40 1.23 3.99 3.21

2.61 2.37 2.49 2.48 2.45 2.51 2.57 2.54 2.34 2.53 2.46 2.47 1.69 2.26 2.04 2.20 1.65

4.14 3.92 4.11 4.18 4.06 4.15 4.04 4.08 3.74 4.19 4.14 4.33 2.67 4.79 4.25 4.30 4.61

1.77 1.63 1.73 1.74 1.66 1.69 1.81 1.73 1.57 1.91 1.93 2.05 1.76 2.12 1.79 1.96 2.10

1.41 1.38 1.36 1.37 1.36 1.38 1.41 1.40 1.01 1.39 1.24 1.46 0.91 1.36 1.44 1.44 1.43

7.30 7.26 5.75 6.76 7.50 8.18 7.28 6.99 5.01 6.95 6.08 7.30 4.52 7.33 7.33 9.57 7.82

1.62 1.58 1.60 1.54 1.69 1.63 1.62 1.96 0.98 0.95 1.06 1.21 0.65 1.22 1.53 1.41 1.19

4.03 6.10 4.14 4.09 5.48 4.59 3.69 6.17 1.30 1.15 1.05 1.64 0.63 3.28 5.05 6.79 4.05

8.96 13.40 9.56 9.37 12.27 10.77 8.20 12.81 4.01 3.61 2.93 4.19 1.43 2.83 10.66 4.91 4.26

4.32 5.80 4.29 4.31 5.26 4.85 4.26 5.80 2.43 2.23 1.71 2.23 1.72 1.48 4.11 2.10 1.80

5.11 6.50 5.12 5.52 6.09 5.83 5.33 6.77 3.53 3.31 2.60 3.30 2.57 2.48 5.17 3.00 2.65

Old Casey workshop soils (middle catchment) CW98S26 5 3814 CW98S27 5 4238 CW98S31 5 343 CW98S33 5 3204 CW98S36 5 200

46 38 21 34 26

8.7 9.0 6.8 8.5 8.7

0.02 0.03 n.d. 0.06 0.02

0.46 1.54 1.28 1.69 1.24

0.19 0.32 0.59 1.17 0.53

0.29 0.11 0.79 1.67 0.79

0.23 0.02 0.42 0.76 0.46

0.13 0.02 0.36 0.69 0.38

0.70 1.10 1.08 2.68 1.54

0.05 0.10 n.d. 0.71 0.10

0.06 0.01 0.21 2.33 0.31

0.51 0.16 1.17 6.01 2.05

0.65 0.57 1.44 3.49 2.44

1.50 1.51 2.96 4.25 5.32

288 263 75.1 96.6 96.8 39.4 150 14.3 8.70 15.2 17.3 8.92 32.5 31.7 8.90 79.2 94.5 40.8 1.24 1.17 0.60 1.37 2.73 0.71

43.9 35.0 6.44 6.87 2.78 20.9 0.85 0.92

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Reference fuels Jet A1 ATK SAB 2001 SAB 2002 Batch 1 SAB 2002 Batch 2 SAB 2003 Arctic blend diesel MGO

39 27

11.1 8.1

0.01 0.02

0.34 0.75

0.22 0.32

0.49 0.37

0.32 0.23

0.42 0.20

2.34 0.86

0.07 0.14

1.74 0.55

6.91 2.94

4.14 2.32

4.96 3.17

Old Casey workshop waters (upper catchment) CW00W268 40 2032 CW00W269 40 16 418 CW00W272 40 12 673 CW00W274 40 29 734 CW00W276 40 91 976

64 65 64 61 57

5.0 8.7 8.9 10.0 10.1

0.10 0.22 0.22 0.22 0.21

1.75 2.80 2.69 3.13 2.89

1.53 2.19 2.13 2.29 2.27

4.03 4.16 4.54 4.47 4.65

1.64 1.72 1.64 1.72 1.63

1.01 1.23 0.36 1.19 1.17

2.91 5.69 5.26 6.03 6.05

0.59 1.02 1.04 1.15 1.20

1.68 2.43 2.08 2.46 2.63

5.67 6.74 5.33 6.41 5.87

2.26 3.50 2.75 3.30 2.99

3.32 4.61 3.98 4.40 4.38

Main powerhouse soils MPH1 MPH2 MPH3 MPH00S33 MPH00S34 MPH00S35 MPH00S36 MPH00S37 MPH00S38 MPH00S39 MPH00S40 MPH00S41 MPH00S42 MPH00S43 MPH00S44

66 67 58 63 64 62 64 72 64 64 66 63 66 64 61

1.8 1.8 11.2 1.7 1.8 1.8 1.7 1.8 2.1 1.8 1.7 1.8 1.7 1.9 1.6

0.21 0.15 0.25 0.17 0.17 0.54 0.19 0.18 0.33 0.26 0.22 0.38 0.18 0.16 0.07

3.16 3.55 3.05 3.51 4.08 3.25 3.88 4.15 3.78 3.49 3.86 3.41 3.51 3.93 1.70

3.21 2.42 2.13 3.12 2.96 3.74 3.36 2.77 3.07 3.43 3.41 3.57 2.98 3.03 1.43

3.11 2.80 3.68 2.72 2.78 3.01 2.94 3.22 3.22 3.05 2.86 3.14 2.50 3.11 1.65

1.83 1.66 1.56 1.75 1.69 1.75 1.84 1.76 1.75 1.84 1.78 1.89 1.54 1.75 0.82

1.00 0.95 0.36 1.00 1.01 0.98 0.98 1.00 1.07 1.06 0.97 1.05 0.84 1.02 0.37

1.27 1.21 2.40 1.35 1.33 1.32 1.28 1.33 1.65 1.49 1.22 1.47 1.16 2.03 0.95

1.86 3.18 1.23 2.90 3.15 1.52 3.37 3.35 2.09 2.55 3.07 1.96 2.84 3.29 1.00

0.37 1.40 0.49 0.61 0.79 0.16 0.44 1.92 0.57 0.49 0.44 0.23 0.51 1.33 0.22

0.64 1.56 1.21 0.73 1.00 0.33 0.51 2.40 1.04 0.67 0.56 0.40 0.61 1.54 0.38

0.57 0.80 0.65 0.51 0.65 0.41 0.41 0.88 0.64 0.50 0.46 0.39 0.49 0.69 0.36

0.86 0.99 1.04 0.72 0.91 0.74 0.68 1.02 0.93 0.76 0.74 0.68 0.73 0.85 0.56

65 61 58 70

1.9 1.8 1.8 1.8

0.16 0.16 0.03 0.15

2.92 2.88 0.87 2.70

2.27 1.71 0.43 1.43

3.64 2.96 0.85 2.17

1.58 1.40 0.38 1.15

0.94 0.71 0.29 0.61

1.24 0.84 0.24 0.72

2.38 0.97 0.29 1.50

0.92 0.26 0.13 0.47

1.12 0.77 0.40 0.85

0.50 0.41 0.31 0.42

0.80 0.67 0.54 0.62

4.4 n.d. 5.4 5.0 2.7 3.1 n.d. 3.2

0.10 n.d. 0.15 0.11 0.19 0.13 n.d. 0.15

2.53 n.d. 3.48 2.47 3.00 3.03 n.d. 1.27

1.17 n.d. 1.48 1.24 1.55 1.31 n.d. 1.10

1.61 n.d. 2.09 1.86 1.76 2.30 n.d. 1.81

0.84 n.d. 0.88 0.80 1.04 1.36 n.d. 0.95

0.73 n.d. 0.53 0.56 0.57 0.92 n.d. 0.24

1.36 n.d. 1.78 1.48 0.77 0.79 n.d. 0.66

1.25 n.d. 2.43 1.15 0.85 0.90 n.d. 0.33

2.01 n.d. 3.42 1.63 0.16 0.53 n.d. 0.14

4.08 n.d. 4.89 3.51 0.58 1.78 n.d. 0.52

2.37 n.d. 1.75 2.02 0.46 0.95 n.d. 0.51

2.71 n.d. 1.98 2.52 0.85 1.47 n.d. 0.96

5 5 5 5 5 5 5 5 5 5 5 5 5 5 5

20 872 84 540 76 26 073 92 604 54 159 27 199 1878 92 290 24 159 7091 39 149 6673 1960 1171

Main powerhouse waters MPH00W1 20 MPH00W7 20 MPH00W21 20 MPH00W24 20 Thala Valley waste disposal site TV1 100 TV2 100 TV3 100 TV4 100 BR99S09 5 BR99S10 5 BR99S13 5 BR99S14 5

4575 8908 1949 5445 61 <20 101 72 134 544 <20 202

64 n.d. 69 65 68 69 n.d. 55

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Old Casey workshop soils (lower catchment) CW98S38 5 4445 CW98S39 5 881

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ratio to be a highly robust measure of biodegradation at even intermediate amounts of evaporation, and this ratio should be assessed before considering ratios such as nC10/i-C16 as indices of evaporation. Where both biodegradation and evaporation have resulted in significant hydrocarbon loss, a cautious iterative approach is needed to apportion losses to the different processes, and relate changes in hydrocarbon chemistry back to concentrations in the environment and estimates of mass loss. 4.1. Old Casey workshop Soils from the Old Casey workshop area are highly contaminated with fuel (Table 1) and high concentrations persist to depths of at least 80 cm at concentrations of 4000 mg fuel kg1. Subsurface waters in this part of the catchment vary considerably in hydrocarbon concentration from 2000 to 92 000 lg fuel l1 (mean 30 000); such high concentrations are likely to be non-aqueous phases droplets and/or associated with

particulates. Near surface soil concentrations in the middle and lower catchment decrease to 4000 mg fuel kg1 or less, and obviously contaminated water discharges into Brown Bay in the summer months. The fuel from the upper catchment has an n-alkane distribution most similar to SAB. It has too much >n-C16 to be ATK, and not enough to be any of the other fuels. Compared with the reference fuels (Table 1), the soils at Old Casey are most similar to the 2001 SAB batch, but have slightly higher pr/ph at about 11.2 ± 0.7, which is consistent with documented changes in fuel composition over time (cf. Green and Nichols, 1995; Revill et al., in press). Because indices such as pr/ph that are known to vary significantly between batches are similar for all highly contaminated soils in the upper catchment, it is unlikely that the overall chemical signature represents two or more major fuel spills. This is also consistent with expeditioner records of one major spill in 1982. The most noticeable difference between the fuel in the soil and the reference SAB studied here is the obviously evaporated signature. Even the

Fig. 2. Selected gas chromatograms for reference fuels and soils (all chromatograms available on-line at ). CW98S06 is from the Old Casey workshop. CW98S27 is a highly evaporated and biodegraded sample from the middle catchment. CW98S38 is from the lower part of the middle catchment below the spring line; note that it is less degraded than CW98S27. MPH00S38 illustrates the double UCM hump characteristic of the Arctic blend diesel—ATK mix at the New Casey main powerhouse. TV04S3 is of a soil sample from the base of the Thala Valley waste disposal site—note that it still has light n-alkanes and isoprenoids, and that the sample is relatively unevaporated and undegraded compared with samples from the Old Casey workshop (samples prefixed CW).

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able change in n-C10/(R + UCM)(ECN9.5–10.5), C11/(R + UCM)(ECN10.5–11.5), C12/(R + UCM)(ECN11.5–12.5), parameters that readily fractionate by biodegradation (Snape et al., 2005). On this basis we conclude that samples taken from the soil pit in the upper catchment are not biodegraded, and that n-C10–12 have only been modified by evaporation. Fractionation of n-C10/i-C14 indicates that evaporation has certainly exceeded 25%; n-C11/i-C14 and n-C12/i-C14 are more diagnostic and indicate 40– 55% evaporative loss from the main plume. Our interpretation, therefore, is that evaporation indices normalised to i-C16 or heavier are not quantitative because of variability in reference fuels for the heavier compounds. Since there is very little difference in n-C10–12 relative to i-C14 (or i-C13/i-C16) between samples near the surface and at 40 or 80 cm depth (Fig. 3(d)–(f)), we further conclude that any substantial evaporative losses must have 0.35

0.25

n -C11/(R+UCM) 10.5-11.5

a

0.20 0.15 0.10 0.05

0.20 0.15 0.10 0.05

0.25

LC ~5

MC ~5

UC ~5

UC 80-85

UC 40-45

UC 5-10

REF. SAB

MC ~5

UC ~5

UC 80-85

UC 40-45

UC 5-10

LC ~5

2.50

c

d

2.00

LC ~5

MC ~5

REF. SAB

LC ~5

MC ~5

UC ~5

0.00 UC 80-85

0.00 UC 40-45

0.50 UC 5-10

0.05

UC ~5

1.00

UC 80-85

0.10

1.50

UC 40-45

0.15

UC 5-10

n -C10 /i-C14

0.20

REF. SAB

5.00

4.00

n -C12 /i-C14

e 3.00 2.00 1.00

f

4.00 3.00 2.00 1.00

LC ~5

MC ~5

UC ~5

UC 80-85

UC 40-45

LC ~5

MC ~5

UC ~5

UC 80-85

UC 40-45

UC 5-10

0.00 REF. SAB

0.00

UC 5-10

n -C12 /(R+UCM)11.5-12.5

0.30

n -C11/i -C14

0.25

0.00 REF. SAB

0.00

b

0.30

REF. SAB

n -C10 /(R+UCM) 9.5-10.5

least degraded soil samples (e.g. highest i-C13/i-C16 and n-C17/pr) peak at about n-C13 and there is a significantly greater proportion in the range n-C15–19. Estimates of mass loss in the upper catchment based on i-C13/i-C16 (the lightest isoprenoid ratio available for SAB) relative to recent reference fuels correspond to 55– 75% evaporation or more based on the fractionation model described in Snape et al. (2005). However, this estimate is inconsistent with the proportion of n-C11, n-C12 and (R + UCM)(ECN10.5–12.5) remaining in the soil. In soil samples from depths of 5–80 cm, n-C10/i-C14, nC11/i-C14 and C12/i-C14 are less than our best estimates of the reference fuels by 95%, 80% and 60% respectively (Fig. 3(d)–(f)) (uncertainly is about 10– 20% based on the variation in the reference fuels documented here and previous studies). These losses of n-C10–12 relative to i-C14 contrasts with no detect-

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Fig. 3. Chemical indices illustrating the extent of biodegradation (a)–(c) and evaporation (d)–(f) in the Old Casey workshop catchment. Note: samples in the upper catchment (UC) are not biodegraded relative to the SAB reference fuel; middle catchment (MC) and lower catchment (LC) is biodegraded; all locations are evaporated, even at depth. The lower catchement occurs beneath the spring line defined by Revill et al. (in press). Samples are averages from Table 1.

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occurred whilst pooled, probably under snow, and not after the spill penetrated into the active layer. This would be consistent with a scenario where the spill occurred in winter onto frozen highly impermeable soil, and then had sufficient time to evaporate before infiltration into the active layer as the summer thaw began. Such a lack of evaporation in the upper catchment at depths greater than a few centimetres is supported by the evaporation model of Kang and Oulman (1996). Soil samples from the middle catchment show evidence of further evaporative losses, typically of major extent (up to 75%), and evidence of biodegradation. In this region, where total concentrations are much lower, alkane/isoprenoid fractionation is much more apparent. Samples CW98S26, 27, 31, 33, and 36, for example, have obviously biodegraded signatures (Figs. 2 and 3(a)–(c)), and even n-C17/pr and n-C18/ph are clearly degraded (Table 1). Only two samples from the lower catchment were analysed in this study (Table 1; Fig. 3). Interestingly one sample has a mixed chemical signature (CW98S38; Fig. 2) where biodegradation is of more intermediate extent but with a less evaporated signature than the middle catchment. This soil sample CW98S38 is chemically more similar to the waters from the upper catchment (CWOOW268–276) than to the middle catchment soils (e.g. i-C13/pr), and is consistent with the observations by Revill et al. (in press) of subsurface transport through the middle catchment. 4.2. The main powerhouse (MPH) Soil and water samples indicate a very high level of contamination at the MPH, and there is an obvious plume of fuel that extends for tens of metres from the bunding, across the road, under the snow and into a nearby lake. Near-surface soils range up to 92 500 mg fuel kg1 (9 wt.% fuel), and waters range up to 8900 lg l1. Soil samples MPH1, MPH2 and MPH00S33-44 have much higher concentrations than a nearby control sample (MPH3). They also have a distinctive low pr/ph of 1.8 which confirms that some or all of the fuel in these samples is of Arctic blend origin. Engineering reports indicate that the Arctic blend fuel was mixed with about 20% ATK (to lower the waxing point) prior to the spill. Most soils have a hump between n-C10–12 that corresponds to ATK; the other hump that extends into the range n-C16–19 is clearly Arctic blend. Since neither evaporation nor biodegradation can produce such an effect, this double UCM hump is an unambiguous feature of the mixed source (e.g. Wang et al., 1999). MPH3 was initially sampled as a reference soil adjacent to the obvious spill, but it contains 76 mg fuel kg1 with a high pr/ph of 11.2. The contaminant in MPH3 is clearly SAB, indicating an earlier spill has also occurred at this site. The level of contam-

ination in the soil is highest in the melt-water stream (corresponding to the centre of the plume), with decreasing concentrations towards the edges of the flow path. Soil samples indicate that evaporation has been a significant mechanism of fuel loss; i-C13/pr has typically reduced by 60%, corresponding to an evaporative loss of fuel of 45–55% by mass. This reduction is consistent across the site and is not correlated with soil moisture or contaminant concentration. From this we conclude that most of the evaporation took place immediately following the spill, probably between September and December, before the ground began to thaw. Only one sample (MPH00S44) has significantly fractionated nalkane isoprenoid pairs indicative of biodegradation, with n-C15/i-C16, n-C17/pr, n-C18/ph 50%, 63% and 31% lower respectively. Such changes are consistent with minor stages of biodegradation and are probably most obvious because overall contaminant levels in this sample are sufficiently low (1200 mg fuel kg1). The pr/ph of subsurface water samples are clearly dominated by Arctic blend fuel (1.76 ± 0.07). Ratio pairs indicative of evaporation are fractionated to about the same extent as the main plume soil samples, or are slightly more advanced. Fuel residues in the waters, mostly comprising non-aqueous phase liquid (NAPL) droplets and particulate-sorbed hydrocarbons, are also more biodegraded than in the soil samples, with MPH00W21 in particular showing greater depletion of n-alkanes. In this case n-C15/i-C16, n-C17/pr, n-C18/ph have dropped by 76%, 72% and 83% respectively which approaches intermediate losses through biodegradation on a mass basis. 4.3. Thala Valley waste disposal site Soils in the waste disposal site have appreciably lower fuel concentrations than either the Old Casey workshop or the more recent spill at the main powerhouse (Table 1). Surprisingly there is little indication of degradation in these samples with much lower hydrocarbon concentrations. Based on the best preserved samples, the range of n-alkanes (n-C9–17) and the approximate extent of the UCM (centred at about n-C13) indicates a SAB source. The pr/ph of these samples range from 4.4 to 5.4, which is much higher than Arctic blend, but lower than the source signature from the Old Casey Workshop. Soil samples collected in 2004 from the base of the waste disposal site in Thala Valley (TV04S1, 2, 3 and 4) have largely undegraded signatures; in particular they have abundant light n-alkanes (n-C9–11). The high relative proportion of these light fractions indicates virtually no evaporative loss. Soil samples collected in the 1998– 1999 summer from near the surface of the waste disposal site also have relatively fresh undegraded signatures (e.g. samples BR99S09 and 10). These samples do not exhibit evidence for pervasive evaporative loss. One

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near-surface sample (BR99S14) is clearly evaporated to intermediate extent with losses of light n-alkanes up to about n-C14, and i-C13/pr of only 0.14. This sample is also biodegraded to a minor extent with clear reduction in n-alkanes relative to isoprenoids. Compared with the soils collected in 1994 (Deprez et al., 1999), the concentrations in the samples collected in 1998–1999 and 2004 were much lower. We attribute this decrease in concentration to mixing during the 1995–1996 and 2003–2004 clean-up operations. There is no evidence that this decrease was achieved through either evaporation or biodegradation. The absence of extensive evaporation in near-surface and at depth samples indicates a different mechanism of dispersal. It is likely that the fuel was spilled directly from the rusty fuel drums into the soil without extensive overland flow, and thus had very little opportunity for loss by evaporation.

5. Discussion and conclusions To assess the suitability of natural attenuation as a remediation option at these three sites we first considered a mass-balance for the Old Casey workshop plume. Expeditioner reports indicate that 36 000 l of fuel was spilled. We estimate that about half of that evaporated in the first few months, and the rest infiltrated into soil in the upper catchment as the summer thaw began. There is approximately 400 m3 of fine soil in the upper catchment contaminated to a concentration at 10 000– 20 000 mg fuel kg1, indicating somewhere between 8000 and 16 000 l remaining in the ground in this region. There is 300 m3 of contaminated soil in the middle catchment contaminated to a concentration of 2000 mg fuel residue kg1, but with chemical signatures indicating more extensive evaporative and biodegradation losses. On this basis we estimate 1600 l remaining in the ground, with losses through evaporation of perhaps 2000 l, and biodegradation of 800 l. Estimating the flux of NAPL in a sheen is notoriously difficult, but based on visual observations and estimated discharge of water via channel flow into Brown Bay, we estimate that discharge of free product is currently at the lower end of the range 50–500 l yr1. It is not possible to estimate past (or future) fluxes, but extrapolating this sort of flux back 20 years indicates a net loss from the catchment of between 1000–10 000 l, but most likely somewhere around 5000 l. These mass-balance estimates are consistent with our geochemical model for the evolution of the plume involving widespread initial evaporative losses. On this basis we estimate that present annual loss is mostly through off-site dispersal of free product into the nearby marine environment and evaporation of sheen in the lower catchment. Biodegradation rates are perhaps of the order 40–400 l yr1 at most. This would constrain natural attenuation of this plume to be dec-

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ades to centuries before concentrations were reduced to <500 mg fuel kg soil1. We draw similar conclusions for the MPH site. All samples in the main plume show evidence of intermediate levels of evaporative loss. The plume is currently widening each year and dispersing largely though NAPL transfer as sheen and subsurface droplets and/or particle-bound product, and current rates of evaporation and biodegradation are very slow. Our current estimate of the plume half-life is also decades to centuries. For the Thala Valley waste disposal site, where fuel originated from leaking drums, it is likely that the mode of contamination resulted in hotspots, and that these were observed by Deprez et al. (Deprez et al., 1999) in 1994. Subsequent re-sampling of approximately the same soils by us in 1998–1999 and 2004 indicates that perhaps dilution was the solution to pollution for these soils. Total concentrations are certainly much lower in the remaining homogenised stockpile of material, but the relatively fresh chemical signatures indicate that the total mass of hydrocarbon contaminant has probably not been reduced. In this case, it likely that lower concentrations were achieved by creating a greater volume of contaminated soil. In conclusion, analysis of chemical parameters that are sensitive to evaporation and biodegradation indicate that for winter spills, significant amounts of evaporation occurred in the first few months before the ground thawed and infiltration occurred. However, once in the soil, natural attenuation is not a sufficiently rapid process to prevent off-site migration to environmentally sensitive areas, and is not a suitable strategy to manage fuel spills in this coastal Antarctic region.

Acknowledgements This study was supported by the Australian Antarctic Science Grant 1163. We thank Shane Powell and Andrei Woinarski for field work and Andy Revill for numerous discussions.

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