188 oil & hydrocarbon spills, modelling, analysis & control · oil & hydrocarbon spills, modelling,...

Natural degradation of hydrocarbons in sandy

soils and its potential application to disposal of

oil-contaminated materials

R. E. Daniels, A. F. Harrison*, O.K. Lindley*, R. Scott*, G.

Hair & A. P. Rowland*

Institute of Terrestrial Ecology, Furzebrook Research Station

Wareham, Dorset, BH20 5AS. Institute of Terrestrial Ecology,

*Merlewood Research Station, Grange-over-Sands, Cumbria,

LA11 6JU. & * Institute of Freshwater Ecology, Windermere

Laboratory, Far Sawrey, Ambleside, Cumbria, LA22 OLP.

Abstract

Hydrocarbon-degrading micro-organisms are found in marine, freshwater andterrestrial environments. Investigations of possible cost-effective methods ofdisposal of oil-contaminated beach sand (OBS) relying on the enrichment andactivity of these micro-organisms in sandy coastal soils have been carried out. Arange of scales has been used; from small scale experiments to field trials, usingboth contaminated beach sand from an actual oil spill and artificially preparedOBS. Consistent results have been obtained, indicating rapid development ofsoil microbial populations, providing quick and effective breakdown of asignificant proportion of weathered oil without the application of culturedorganismsor the addition of fertiliser. Patterns of degradation of weathered oilsconsistently follow a power function under field conditions, without theapplication of special cultures, or the addition of fertiliser. Evidence ispresented to show that environmental risk from movement of hydrocarbons intosurrounding soil or groundwater are minimal. Advantages and limitationsassociated with this potential clean-up and disposal method are discussed.

Transactions on Ecology and the Environment vol 20, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541

188 Oil & Hydrocarbon Spills, Modelling, Analysis & Control

Introduction

Disposal of oil removed from the shoreline after a spill at sea, may pose anumber of problems. In highly sensitive habitats, mechanical or chemical

removal of oil or oil emulsion may be more ecologically damaging than

leaving it in place but oil is normally removed from beaches. Because of

legal and financial problems in final disposal of any collected material,there has been an increasing interest in in situ remediation; e.g. Bragg etal?, Venosa et al.™, Atlas*'*, Lunel & Swannell^, though the efficacy offertiliser application is open to question (Oudot et al"). Concentrations ofliquid oil may be pumped from the shore and recovered, but mixtures ofoil, water and sand removed pose a problem in final disposal. Asconsignment to landfill is becoming increasingly unacceptable, alternative

methods of disposal are required.Hydrocarbon-degrading micro-organisms occur as minor components

of unpolluted marine, freshwater and terrestrial microbial communities,(Austin et &//, Atlas^, Leahy & Col well*). These organisms respond to thechallenge of substrates present in oil and numbers can increase ten-foldfollowing exposure to oil (Atlaŝ ). The response of these hydrocarbon-

degrading populations to oil pollution is the basis for investigation of the

potential for on-shore bioremediation and of the work reported here.After the Christos Bitas incident in 1978, a mixture of oil and sand

was taken from Pendine Sands and dumped in a hollow in the nearby

dunes. Core samples taken from this deposit in 1992 still contained somehydrocarbons (450 mg kg"* compared with 25 mg kg"* for cores taken froman adjacent dune hollow). Assuming an initial oil concentration of morethan 10%, this represents a loss of some 99% of that oil. Althoughbacteria able to degrade aromatic ring structures could be detected, these

were a small proportion of the total bacterial population (0.1%),suggesting a return to pre-contamination population size.

The objectives of the study, undertaken on behalf of the MarinePollution Control Unit, were: first, to examine the efficiency of

decomposition by naturally-occurring soil micro-organisms in coastalsands and, second, to determine the environmental acceptability of buryingoil-contaminated beach sand (OBS) in such soils, as a possible alternativeto landfill disposal. The approach has involved studies at a range of scalesusing; lysimeters, field experiments at 15 sites around the British coast,field trials using artificial OBS and a full scale operation using beachedoil. This paper reports the findings from field trials in Cumbria and southWales, and relates the findings to those in the multi-site field experiment.


Oil & Hydrocarbon Spills, Modelling, Analysis & Control 189

Methods

Experiments at Pendine, south Wales

At the beginning of January 1994 patches of well-weathered fuel oil, froman unknown vessel, together with a skim of sand, was removed from thebeach and placed in the remaining part of the hollow containing the

material deposited in 1978. A total of some 20,000 tonnes of sand,

containing an estimated 2% oil, was removed from the beach. Systematic

sampling, using an Ejkelkamp bi-partite auger began four weeks later and

continued for two years. The cores were analysed for hydrocarbon content.

Experiments at Eskmeals, Cumbria

A series of field trials comparing incorporation of OBS in winter andsummer, was set up in a slightly sloping area of dune pasture at Eskmeals,Cumbria in January and June 1995. These trials used OBS produced from

a topped and washed heavy fuel oil (supplied by AEA Technology)

containing 38% water. The emulsion was further weathered for two days

on an artificial beach constructed in an excavated basin lined with a

polyethylene membrane and consisting of a 300mm gravel filter layer, a

300mm layer of coarse sand and an upper layer of local quarry sand, into

which the emulsion was rotovated in to give an OBS containing a nominal

10% oil.A landfarming trial used a split-split plot design laid out in three

blocks, each with 7m x 3.5m plots assigned one of two incorporation times(January or June), and to one of three ploughing regimes (at two week orfour week intervals, or no ploughing). A layer of OBS, 100mm. thick,was applied to experimental plots and ploughed into the top 100mm. ofsoil to give a layer 200mm thick containing 5% oil. Control plots received100mm of quarry sand. Ploughing was carried 2-weekly or 4-weekly for a

year by rotovating the upper 200mm of soil with its incorporated OBS.Adjacent to the landfarming trial a burial experiment, comprising 144

plots arranged in three blocks, was set up. Within each block, six plotswere assigned to either winter burial, summer burial, winter control orsummer control. On each occasion, as in the case of the landfarmingexperiment, control plots were set up before any OBS was moved from theartificial beach, in order to avoid cross-contamination. Soil was excavated

to a depth of 700mm, heavy duty plywood boxes (1m square) werepositioned in the excavation and the areas around were backfilled. The



boxes were then filled with a 500mm deep layer of either quarry sand

(controls) or prepared OBS which had previously been thoroughly mixedwith an equal quantity of quarry sand to give a nominal 5% concentration

of oil. The boxes were then removed and the plots covered with 200mm of

topsoil to restore the ground surface to its former level.Piezometer tubes, extending to beneath the water table, were placed in

the centre and adjacent to each side of each landfarming and burial plot.

Core samples from the plots were analysed for hydrocarbon content.

Extensive site study

At fifteen dune pasture locations around the coast of Great Britain, nylonbags, each containing approximately 1kg of OBS, were buried andretrieved at intervals over two years. The OBS was made on site from the

same emulsion as that incorporated into the Eskmeals trials, but using a

standard weight of sand from a nearby beach to prepare an OBS with a

nominal 5% oil content. Six bags were buried 30cm deep in three replicate

blocks at each site in February 1996, together with blanks containing only

beach sand, A further four bags were buried in each block at each site in

September-October 1996. One bag from each block was retrieved at

regular intervals during the trial period and analysed for hydrocarbons.Some between-site variation was found in the exact starting concentrationbecause of differing moisture content of the sand used and differentweather conditions at the time of mixing, which influenced viscosity. Siteswere chosen to reflect differences in climate and sand type, in order toexamine the importance of local factors in controlling hydrocarbondegradation. The sands used for preparing the OBS were characterisedchemically and baseline respiratory activity (COz production) of themicrobial populations was determined.

Sample analysis

Non-volatile hydrocarbon content of sand samples was determinedgravimetrically. A 50g fresh weight sample was extracted for an hour withlOg anhydrous sodium sulphate and 50ml hexane (carbon tetrachloride ininitial samples), and filtered under slight suction using a Buchner funnel.The residue in the funnel was washed with a further 3 x 25ml aliquots ofhexane, and both extract and washings were evaporated to dryness in apre-weighed evaporating dish. Weight of residual material was determinedby difference and corrected to dry weight following determination ofmoisture content on a separate sub-sample of the starting material. Water



samples were analysed using smell, an oil-in water monitor and, for some

samples, GC-MS.

Results and Discussion

Hydrocarbon decomposition

Early samples taken from the 1994 Pendine deposit showed differential

distribution of hydrocarbons between the sand matrix, and small and largelumps, which were aggregates of small tar balls and sand. The largestlumps contained 4.5% hydrocarbons by weight, the small lumps contained1.17% and the sand matrix contained 0.03%. Estimates of total

hydrocarbon content of were made on the basis of the relative proportions

of each component found in any one sample. The original estimate of 2-

2.5% oil in the OBS may have been an over-estimate but, as there was adelay before systematic sampling began, an exact starting concentration

cannot be defined. Figure 1 assumes a starting concentration of 2.5% andshows the concentration of hydrocarbon remaining in core samples taken

over a two year period from the beginning of the experiment. Followingearly rapid loss of hydrocarbons, the rate of degradation slowed after a

few months. The pattern of hydrocarbon loss is best described by a powercurve with the general formula,

Y = A . X" (1)

where Y = concentration of hydrocarbons remaining, X = time sinceincorporation of the OBS, A = initial hydrocarbon content of the OBS andn = the rate function.

Because of the presence of a range of compounds from simplealiphatics to more complex cycloalkanes and aromatics within OBS, sucha slowing of hydrocarbon loss would be expected as the more readilydegradable compounds are removed and the recalcitrant forms persistIntractable resins and asphaltenes remain unaffected by microbial actionand so persist almost indefinitely.

Samples taken at intervals from the Eskmeals trials showed a similarpattern of rapid early loss of hydrocarbons followed by a progressivedecrease in the rate of decomposition. However, the basic pattern ofdecline was modified by both treatment applied and by block position.



Time (months)

Figure 1: Decline in hydrocarbon content of core samples from Pendine

(filled squares) and fitted power curve, Y = 2A6X*'™*. R* = 0.66.

Table 1: Hydrocarbon content (%) of samples taken from Eskmealslandfarming plots at different times after trial establishment.

Winter incorporation plots

Ploughing treatment2-weekly4-weeklyno plough

Summer incorporation

Ploughing treatment2-weekly4-weeklyno plough

Time

04.84.8

4.8

plots

Time

0

from st

16n.d.n.d.2.48

from st

85.106.376.30

art

3111

art

234

5

ofti

2

.29

.17

.38

ofti

4.97.65.89

rial

5:i.i.

i.

rial

3(3.2.

6.

(we

;

1176

88

(we

20

9815

eks)

75

0.871.36

2.42

eks)

562.003.255.57

1

01

1

8225

04.82

.37

.93

3.37.72.23

1

01

1

1225

29

.96

.32

.97

09.48.97.23

Table 1 shows that, although differences between treatments werefound towards the end of the trial, these were not consistently significant.Control plots consistently gave values of


plots, the no plough treatment had significantly more oil residues than both

the 2-weekly ploughing and the 4-weekly ploughing treatments. In the

summer plots, significant treatment effects were also found and, similarly,

the no-plough treatment samples retained significantly higher

concentrations of hydrocarbons than those ploughed. Differences between

ploughing frequency were not significant. There was a significant

difference in hydrocarbon degradation between the winter and summer

incorporation plots, but it is not clear whether this is a real effect or a

result of incomplete dispersal of OBS throughout the soil matrix due to

particularly dry conditions when the summer OBS was prepared. Even

distribution of oil in plots was not readily achieved and spatial variation in

oil concentration created difficulties for interpretation of results.

Nevertheless, a consistent treatment effect was found, with more frequentploughing giving more rapid and more complete hydrocarbon breakdown.

Table 2: Hydrocarbon content (%) of samples taken from Eskmeals dunepasture burial plots at different times after trial establishment.

Winter

Tim<

0

32

5382102

129

Summe

Tinm

08

24365582109

burial plots

5 since burial

(weeks)

;r burial plots

; since burial(weeks)

4

1

2111

42

21111

Block 1

.77

.88

.12

.87

.77

.53

Block 1

.90

.27

.50

.97

.60

.93

.33

4

2

2121

4

2

22211

Block 2

.77

.95

.65

.98

.61

.66

Block 2

.90

.75

.63

.87

.10

.90

.78

4

3

322

2

4

332222

Block 3

.77

.90

.45

.37

.76

.43

Block 3

.90

.40

.03

.02

.50

.40

.25

4

2

2221

4.2,

22,2,2.1.

Block

mean

.77

.91

.74

.07

.38

.87

Blockmean

.90

.81

.72

.62

.0708

,79

Patterns of hydrocarbon decline in the dune pasture burial plots overtime are shown in Table 2. There was no significant difference between



winter and summer burial, but significant block affects were found. Block

1 was located higher up the slope and Block 3 at a lower point, so thatwater table and soil moisture levels were highest in Block 3 and lowest inBlock 1. This difference in soil moisture regime was reflected in the rate of

oil degradation, which was progressively retarded by increasing wetness.As in the Pendine trial, power curves could be fitted to all the data sets

from Eskmeals, although the rate function differed between them. The

differences may be explained, in part, by the influence of water table depth

and soil moisture regime. The extensive site survey showed that, although

at some sites (where flooding is known to occur), water relations wereimportant, at others, additional variables needed to be considered.

Table 3: Estimated loss of hexane-extractable hydrocarbon from OBS

buried at different sites after two years. For sites marked *, development

of a statistically significant power curve was possible.

Site Estimated initialconcentration % HC

Estimated %HC

degraded

Shetland

OrkneyCromartyHebrides

W ScotlandW Scotland

FifeGallowayCumbriaNE EnglandLincolnshireNorfolkW Wales

N Devon

Sussex

winter

burial (WB)4.31

4.295.014.74

4.154.544.364.82

4.114.043.813.803.98

5.084.47

summer

burial (SB)11.87

10.503.755.61

4.57

6.095.44

6.792.815.885.295.957.57

6.467.07

WB after

19 months19.28

12.4211.2910.30

7.6015.8014.0710.64*21.24

14.27*2.44

4.795.91

9.480

SB

1 1 mor

18.45*

25.39*013.01

10.7218.5622.0632.84*

0

26.91*15.3112.44

13.34

11.47

23.76*

after

iths

Table 3 shows the degradation with time at each site. Although therewas an apparent decline in hydrocarbon concentration, variability among

samples was high. Not all sites showed a statistically significant reduction- only a declining trend, but the final sample set has yet to be analysed.There is no clear geographical pattern, indicating that climate may be less



important than had been thought initially, though the higher effectiveness

of summer degradation suggests that temperature, and possibly soil

moisture balance at the time the bags were buried may have been

important determining variables. Given that the fastest rate of degradation

occurs in the early stages of the burial, starting conditions (including

presence of suitable micro-organisms) in the OBS and adjacent soil maybe considered critical in promoting decomposition.

Table 4: Microbial respiratory activity in starting components at differentsites and changes in this over time (jimoles CC>2 per g. dry wt per day).

site

Shetland

Orkney

Cromarty

HebridesW Scotland

W ScotlandFife

Galloway

Cumbria

ME England

Lincolnshire

NorfolkW Wales

N DevonSussex

native

soil

0.112

0.075

0.134

0.1790.066

0.241ns

0.032

0.054

0.1760.285

0.0850.333

0.3060.800

beach

sand

0.032

0.177

0.279

0.3410.022

0.0430.141

0.107

0.024

0.013

0.129

0.0170.038

0.0230.037

starting

OBS

0.116

0.246

1.165

0.9280.032

0.117

0.591

0.148

0.120

0.0590.054

0.0300.147

0.1010.170

OEM at 3

months

0.324

0.320

1.329

0.3540.044

0.124

0.499

0.225

0.173

0.1700.052

0.1640.2590.1100.685

OEM at 6

months

0.187

0.229

0.983

0.6630.256

0.2610.681

0.317

0.317

0.147

0.061

0.2530.3150.1720.314

Microbial populations

Studies of culturable counts of bacteria in beach sand and OBS showedlittle ability in the populations to degrade aromatic ring structures.However, organisms with this ability rapidly enriched at some sites but notat others. The reasons for these differences are not yet clear. Patterns ofchange in CO? emission by bacteria cultured at 20°C from sand and OBSfrom different extensive site survey locations are given in Table 4.Differences were found in the activity of the starting materials and the

OBS, at the start of the trial and during its course. Data obtained in thelaboratory indicated the hydrocarbon-degrading potential of the local



microbial populations, though under field conditions this potential may not

be fulfilled. Differences between laboratory and field conditions cast somedoubt on the value of bioremediation using cultured micro-organisms.

Soil conditions

The importance of soil conditions in influencing decomposition of

hydrocarbons has been noted by Srivastava & Cutright^ and Apitz &

Meyers-Schulte*, whilst Loehr & Webster^ and Smith et al.™ have stressed

that increased contact time with soil increases resistance of hydrocarbonsto both physico/chemical and microbiological action. The optimisation of

soil nutrient, water and aeration conditions may be critical in effecting

efficient decomposition of OBS. It is well known, for example, thatcalcium is important in controlling nutrient cycling (especially P and, to a

lesser extent, N) and, via pH, the breakdown of organic matter in soils.The importance of nitrogen in promoting oil breakdown has been shown

by Atlas^. More effective breakdown of OBS occurred at Pendine than atEskmeals in the field trials and this may be attributable to the low calciumcontent of quarry sand compared with beach sand and its included shell

fragments at Pendine. This observation appears to be borne out by results

from the extensive site study where those sites with a high calcium contentshowed significant declines of hydrocarbons and persistence of aromaticring-degrading populations of bacteria. Investigations of the relationshipbetween nutrient conditions, in combination with variations in soilmoisture are part of our continuing programme of work.

Mobility of hydrocarbons

Although water samples collected from the piezometer tubes installed

around dune pasture burial plots and landfarming plots contained somesuspended matter or were cloudy (and produced readings on the oil-in -water meter), only four of 144 collected on one occasion smelted of oil.The lack of penetration of hydrocarbons to groundwater has beenconfirmed subsequently in a further field experiment in a dune hollow atPendine. This used 23 tonnes of medium fuel oil, emulsified with seawater,and mixed with sand to give an OBS with 12% hydrocarbons.Groundwater samples taken from piezometer tubes in and around thehollow during the first eight months of the trial, and analysed by u/vfluorescence spectrometry at the EA laboratory, Llanelli, have shown thathydrocarbon concentrations did not rise significantly above backgroundlevels recorded before setting up the experiment (Harrison et al?).



Conclusions

Breakdown of hydrocarbons by adaptation of naturally-occurringmicrobial communities occurs in coastal sands and sandy soils. The rate at

which such breakdown occurs is, however, highly dependent on local

conditions. Where soil moisture levels or the water table are high, aerobic

degradation is inhibited. A well-drained soil accompanied by adequate

buffering (usually through the presence of calcium) would seem to provide

the best conditions for degradation. Degradation is not accompanied by

movement of hydrocarbons to groundwater and, the longer undecomposedcomponents remain, the less mobile and biologically active they probably

become. Our trials suggest that burial in sandy soil is a suitable disposal

method for OBS with moderately low concentrations of weathered oil.

Acknowledgements

We wish to thank the Marine Pollution Control Unit of the Coastguard

Agency for funding this work and the various landowners, especially

DERA, for permission to carry out experiments on their land.

References

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[12] Smith, M.J., Lethbridge, G. & Burns, R.G. Bioavailability andbiodegradation of polycyclic aromatic hydrocarbons in soils, FEMSMicrobiology Letters, 152, pp. 141-147, 1997.

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[14] Venosa, A.D., Suidan, M.T., Wrenn, B.A., Strohmeier, K.L., Haines,J.R., Eberhart, B.L., King, D. & Holder, E. Bioremediation of an

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