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Physical Appearance of Oil in Oil-Contaminated Sediment
Spill Science & Technology Bulletin, Vol. 8, No. 1, pp. 55–63, 2002 2003 Elsevier Science Ltd. All rights reserved Printed in Great Britain 1353-2561/03 $ – see front matter
doi:10.1016/S1353-2561(02)00121-4
Physical Appearance of Oil in Oil-Contaminated Sediment G.A.L. DELVIGNE* WL/Delft Hydraulics, P.O. Box 177, 2600 MH Delft, The Netherlands
The physical appearance of oil was investigated in three types of oil-contaminated sediment: (1) natural harbour sediment, (2) laboratory samples of sediment prepared by simulating the natural oil–sediment interaction processes and (3) laboratory samples of spiked sediment prepared by direct mixing of large volumes of oil and sediment. Microscopic observations show the possible presence of three oil phases in the sediment: (1) distinct oil droplets, (2) oil coating on sediment particles and (3) Ôoil patchesÕ. Through quantitative investigations, it was concluded that the division of oil in the different phases is affected by the oil–sediment interaction processes, oil type, and oil concentration. As far as the droplet phase is concerned, these processes and parameters also influence the droplet size distribution. 2003 Elsevier Science Ltd. All rights reserved. Keywords: Oil, oil-contaminated sediment, oil–sediment interaction
Introduction The interaction between oil and mineral fine sediments in natural marine conditions has important effects on the contamination of the environment as well as on the removal of oil contamination. It has been well known for a long time that oil contamination of sediment on the seabed or in a harbour is often due to the interaction of oil droplets and suspended sediment in the water column. Oil droplets are usually generated by the break-up of a surface oil layer by breaking waves or other sources of turbulence and plunging. During the last decade, it has been recognized that the removal of oil from contaminated shorelines increases due to the interaction of mineral particles and oil. The oil–mineral interactions and the influence on biodegradation were investigated by, e.g. Bragg and Owens (1994), Wood et al. (1998) and Weise et al. (1999).
*Tel.: +31-15-285-8585; fax: +31-15-285-8582. E-mail address: [email protected] (G.A.L. Delvigne).
The present paper deals with the physical appearance of oil in sediment. (Investigations on the oil phase of oil-contaminated sediment may include chemical and physical analysis.) The physical characteristics of oil in sediment could be important with regard to the buoyancy of the oil–sediment agglomerates, the weathering behaviour and last but not least the ecotoxicological characteristics of the contaminated sediment. It is very likely that the physical characteristics of oil in oil-contaminated sediment depend on the following processes and parameters: • interaction mechanisms of oil and sediment (including natural and artificial interaction mechanisms), • oil type and sediment type, • oil concentration, • weathering of the sample (degradation state). The author took part in research programs on oil dispersion and oil-contaminated sediment. Research on natural oil dispersion and the formation of oil– sediment flocs was carried out mainly in the years 1986–1987 and described by Delvigne et al. (1987) and Delvigne and Sweeney (1988). Recent research on ecotoxicological effects of oil-contaminated sediment 55
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has been described by Brils et al. (2002). The research programs included investigations on the physical appearance of oil in sediment, which will be described in the present paper. The following types of oil-contaminated sediment were investigated: • Natural sediment: oil-contaminated sediment samples formed under natural conditions were taken from the bottom of a Rotterdam petroleum harbour. This investigation was carried out to compare artificially spiked sediments with naturally formed oil-contaminated sediment and among others to select the most appropriate spiking method. • Simulated natural sediment: oil-contaminated sediment resulted from the interaction of dispersed oil droplets and suspended sediment in laboratory experiments. The laboratory experiments were part of a research program to investigate the formation of oil-contaminated sediment in conditions similar to those in the natural (sea) environment after an oil spill. The simulations started with the introduction of an oil layer on the water surface, followed by the formation of oil droplets by a turbulence source (e.g. dispersion by breaking waves) and finally the formation of oil–sediment agglomerates (flocs). • Artificially spiked sediment: oil-contaminated sediment resulted from the addition and mixing (spiking) of oil and wet sediment. Spiked sediments were prepared and used in a research program on ecotoxicological quality criteria for oil-contaminated sediments and dredged material. Spiking methods were investigated in order to obtain large quantities of oil-contaminated sediment with wellknown parameters (oil type, oil concentration, weathering state), resembling as closely as possible contaminated harbour sediments. (Artificial spiking produces large amounts of material, which is not possible using the preparation method of simulating natural interaction conditions as described in the paragraph above.)
Observation and Analytical Methods The observation of oil and the analytical methods dealing with the physical appearance of oil were developed in the 1986 research program and were applied as well in the more recent program. Observation equipment and methods An optical microscope was used to observe the physical appearance of oil in oil–sediment samples, as 56
well as to observe separate oil droplets and sediment particles before the interaction takes place. The following observation methods were applied: • Samples of oil droplets in water and oil–sediment agglomerates taken from experiments in the laboratory vessels were observed inside the (small) sampling device. The samplers were put directly in the object space of the microscope. (See the description of the sampling equipment in Delvigne and Sweeney (1988) and Delvigne (1989).) • Small volumes (still wet) of the artificially spiked samples and the harbour samples were put on the observation table and a surface area of about 10– 100 mm2 was investigated. The samples were observed with visible light and with UV light, in order to distinguish the sediment from oil phases (and air bubbles). The oil shows fluorescent behaviour in the UV light (with a wavelength ). Usually, the observations were of about 3560 A carried out with 100 amplification. Examples of how images of separate oil droplets and different oil–sediment agglomerates look like under the microscope are given in Fig. 1. The photographs in Fig. 1(a) and (b) were taken from laboratory experiments on natural dispersion of oil in water without sediment, and Fig. 1(c) and (d) from dispersion experiments with dispersed sediment. The experiments have been described in Delvigne et al. (1987). Visible oil phase Observed and analysed physical oil phases in the samples: • Oil droplets. In particular the oil droplet size distribution was determined. The resolution of the microscope limits the visibility of oil droplets (as well as sediment particles) to 1–2 lm or greater. • Oil-coated sediment particles. Tests indicated that an oil layer thickness of about 1 lm or smaller cannot be recognised as oil because the fluorescent behaviour in UV light is no longer visible. If an oil droplet with a diameter of do ¼ 10 lm covers the entire surface of a sediment particle of 25 lm, then the oil layer thickness is about 0.3 lm. This could mean that many oil-coated sediment particles are not visible by the observation method. Consequently, Ôlost oilÕ in the observed mass balance could be present as invisible coating on sediment particles. • Oil ‘‘patches’’. Oil patches were observed only in sediments with high oil concentration. In contrast to oil droplets and coated sediment particles, the coherent oil patches have no well-defined shape in the sediment mass. The patches usually cover several Spill Science & Technology Bulletin 8(1)
PHYSICAL APPEARANCE OF OIL IN OIL-CONTAMINATED SEDIMENT
Fig. 1 (a) Macrograph of droplets in oil dispersion experiment. (b) Micrograph of droplets in oil dispersion experiment. (c) Oil droplets in sediment flocs. (d) Oil droplets covered with sediment particles.
sediment particles and change shape when the sample is stirred. Typical dimensions of oil patches on a sample surface in the present investigations were a thickness of the order of micrometers and horizontal dimensions of tens of micrometers. Oil concentrations in the different phases in the sediment mass were estimated from the observations (see Section ‘‘Determination of oil concentration’’). Visible oil in the different phases are droplets larger than 1–2 lm and oil coatings thicker than 1 lm; oil patches are always visible. The difference between the concentration of the visible oil and the real oil concentration derived from chemical analysis or the known total oil concentration in the spiked samples is assumed to be mainly in the phase of sediment coating. Note: based on previous investigations, it was concluded that the volume of oil in the invisible Spill Science & Technology Bulletin 8(1)
droplet size range, smaller than 1 lm, is very small relative to the volume of the larger oil droplets. Determination of oil concentration The droplet size distribution in a sample, observed on the sample surface, has been identified generally by the number of droplets within size classes. Defined size classes are, for instance, classes with droplet diameter do < 5 lm, do ¼ 5–10 lm, do ¼ 10–20 lm, do ¼ 20–40 lm, do ¼ 40–80 lm, etc. and ÔpatchesÕ. In order to identify the oil concentration in a sample, an oil volume is derived from the number of droplets in a size class by estimating a weighted mean diameter in each size class. The oil concentration is calculated by relating the oil volume (on the sample ÔsurfaceÕ) to the volume of the observed surface layer. For the present samples, the thickness of the 57
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observation layer is estimated to be about 50 lm based on the following rationale: • The focus depth of the microscope is in the order of 25 lm. • The grain size of the coarse part of the sediment in the present samples was in the order of 10–50 lm. Then the number of sediment particles, usually somewhat transparent, in the upper profile of 50 lm is one or only a few. • The observed largest oil droplets in the present samples are in the range of 20–40 lm. The oil concentration determined from the microscopic observation of oil could be compared with the oil concentration determined from chemical analysis. In the cases of the spiked samples, the observed oil concentration is compared with the known oil concentration applied in the mixing process. With regard to observed oil droplets in the samples, the following parameters are usually determined in the experiments (per unit of observed sample volume/ surface): do is droplet diameter; Nd is number of droplets per size class; Vd is volume of droplets per size class; Vt is total volume of droplets over all size classes; Ct is total oil concentration over all size classes. Other parameters dealing with the oil mass are: Qos is oil concentration in spiked sediment as known from the applied mixture ratio oil/sediment; Qoc is oil concentration in sample measured by chemical analysis.
troducing oil slick in three different laboratory facilities. A grid column was used generating homogeneous turbulence with different energy levels by an oscillating grid. A relatively small oil flume (15 m long, 0.5 m water depth) and a large flume (200 m long, 5 m water depth) were used, generating breaking waves in a surface oil layer. The wave heights were up to 0.2 and 2.0 m respectively. These experiments were described by Delvigne and Sweeney (1988). Also, small-scale experiments were carried out on oil droplet generation just by pouring an oil slick and water into a small water tank, simulating the turbulent energy of natural conditions (see Delvigne and Hulsen, 1994). The following main results refer to oil droplet generation: • The droplet size distribution in dispersed oil is described by Nd do2:3 , where Nd is the number of droplets with diameter do . This relationship describes the droplet distribution immediately after the break-up of a coherent oil mass by a severe turbulence source. The relationship leads to droplet size distributions as illustrated in Fig. 2. • The slope of the curve in Fig. 2 is always the same, independent of the break-up source, oil type and oil concentration (or thickness of the surface layer). • The intercept of the curve in Fig. 2 depends on the following parameters: amount of (surface) oil per unit area (linear dependence),
Results and Discussion Oil and sediment in natural conditions As stated before, oil contamination of sediment on the seabed or in a harbour is often due to the interaction of oil droplets and suspended sediment in the water column. The oil droplets are usually generated by the break-up of a surface oil layer by breaking waves or other sources of turbulence and plunging. The simulated natural processes of oil dispersion and oil–sediment interactions were investigated by the author in the mid-eighties. Recently, the physical appearance of oil in natural oil-contaminated sediment was investigated from sediment samples taken in a harbour. Simulated natural oil dispersion. Laboratory experiments on oil droplet generation in simulated marine and inland water conditions were carried out by in58
Fig. 2 Oil droplet size distributions in dispersion experiments with different sources of plunging and turbulence (breaking waves and pouring, see text). Spill Science & Technology Bulletin 8(1)
PHYSICAL APPEARANCE OF OIL IN OIL-CONTAMINATED SEDIMENT
oil type (the main parameter is the oil viscosity, which is also dependent on the oil weathering state and temperature) and energy intensity of the break-up source. • The extent (size range) of the curve depends on the following: the curve always exists in the smaller size classes, and is assumed to continue even in the smaller size classes than those presented in Fig. 2 where the curve of observed oil droplets is limited by the microscopic resolution to about 2 lm; the presence of the largest droplet sizes depends on the amount of available oil. These conclusions mean, for instance, that the decreased dispersion with increased viscosity or decreased energy intensity will be represented in Fig. 2 by a lower curve, whilst the curve continues for larger droplet size classes. As mentioned before, these results are valid for the oil droplet distribution immediately after the break-up occurrence. After the break-up, the droplet distribution in a specific water volume strongly depends on the resurfacing of (mainly) large oil droplets.
Natural oil-contaminated sediment: laboratory simulation. Laboratory experiments were carried out on the interaction of dispersed oil droplets and suspended sediment particles simulating as closely as possible natural conditions. The experiments were carried out in the grid column, see Section ‘‘Simulated natural oil dispersion’’, while adding sediment additionally to the oil slicks in the column. In particular this interaction is characterised by low concentrations of droplets and suspended sediment, and turbulence conditions similar to (different) sea states. The applied oil types were Prudhoe Bay Crude and Ekofisk Crude, with different weathering states. The sediment was taken from an intertidal zone of the North Sea. The resulting oil droplets, sediment flocs and oil–sediment agglomerates were analysed by microscopic observations. (See Delvigne et al., 1987.) The purpose of those oil–sediment interaction experiments was to obtain data on the oil–sediment agglomerates, in particular with respect to buoyancy and floc size. These parameters are of great influence on the vertical motion of oil-contaminated particles in a turbulent ambience and, ultimately, on the sedimentation on the seabed. With respect to the physical behaviour of oil in the delicate structures of oil–sediment agglomerates, the following conclusions were drawn from optical microscopy: Spill Science & Technology Bulletin 8(1)
• Oil–sediment agglomerates ranged from Ôoil droplets coated with sediment particlesÕ to Ôoil droplets incorporated in sediment flocsÕ (see Fig. 1(c) and (d)). The buoyant, neutral or negatively buoyant behaviour of an agglomerate is dependent on the oil– sediment ratio in the agglomerate. • All visible oil in the agglomerates consisted of discrete droplets. The droplets observed in negatively buoyant agglomerates were always in the size range from 1 lm (limit of observation) to about 60 lm. Neither the size distribution of oil droplets in the agglomerates or the sediment coating of buoyant oil droplets were significantly variable in experiments with different oil types, sediment types and turbulence level. Natural oil-contaminated sediment: Rotterdam harbour. Oil-contaminated sediment samples were taken from seven different locations from the bottom of Rotterdam harbour, in order to compare the characteristics between these natural samples and artificial samples (see Section ‘‘Introduction to spiked sediment for ecotoxicological research’’). Of course, a problem arises with harbour samples of unknown history with interaction with different oil types and subjection to different weathering and turbulence conditions. Contrary to the delicate structures of the laboratory oil– sediment agglomerates discussed in Section ‘‘Natural oil-contaminated sediment: laboratory simulation’’, the harbour samples were subjected to phases of settling and consolidation of the material. Data on the harbour samples are given in Tables 1 and 2. Chemical analysis of the harbour samples shows oil concentrations from 275 to 1047 ppm. Microscopic observation of the oil droplets in the samples indicated droplets up to the size class of 20–40 lm and, as far as the accuracy of the observation method permits, about 70% of the oil exists as droplets. Also, some oil-coated sediment particles were observed (see Table 1); the size of the coated particles was in the range 30–50 lm. As far as the accuracy of the observation method permits, the droplet size distributions in the harbour samples seem similar to the size distributions in the dispersion experiments in Fig. 2.
Oil in artificially spiked sediments Introduction to spiked sediment for ecotoxicological research. Artificial oil-contaminated sediments were applied in the recent ecotoxicological research program. The spiked sediments were prepared by mixing large amounts of wet sediment with known quantities of oil. With the standard method A, the wet (50–80% moisture content on dry weight basis) sediment was 59
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Table 1 Observed oil droplets and oil-coated sediment particles in Rotterdam harbour samples and in samples with different artificial spiking methods Artificially spiked samples Qon ¼ 1000 ppm Droplet size classes, do (lm) Dropletsa Oil type and spiking method
a
Rotterdam harbour samples (taken at seven different locations) Oil concentration Qoc (ppm)
Dropletsa
Coatingsa
Only ÔcleanÕ sediment
600
1000 400 50 10
4.0
<5 5–10 10–20 20–40
50 10 – –
<5 5–10 10–20 20–40
160 120 10 5
DMA oil. Standard method
980
500 250 35 20
2.0
<5 5–10 10–20 20–40
100 100 32 20
DMA oil. Reduced mixing energy
450
750 300 20 8
2.5
<5 5–10 10–20 20–40
120 32 – –
DMA oil. Reduced water
560
500 300 30 5
4.0
<5 5–10 10–20
500 50 5
DMA oil. Batchwise oil addition
520
500 150 20
4.0
<5 5–10 10–20
275
400 200 10
2.0
<5 5–10 10–20
1047
600 200 15
4.0
Number of droplets/coatings per 10 mm2 of sample surface area.
homogenised for 1 min using a mixer at a speed of 720 rpm. The necessary amount of oil was then added by using a pipette and the spiked sediment was homogenised, as above, for 2 min. The physical appearance of oil has been investigated for the spiked oil–sediment samples using the following parameters: • Different spiking methods: method B deviated from standard method A by a reduced mixing speed of 240 rpm, method C deviated from A by a reduced water content and in method D the oil was added in five batches.
• Different oil types: the selected mineral oil types in this particular research program were Gulf OilÕs DMA gasoil (ships fuel with the chemical characteristics composing primarily of the C10–C22 fractions) and HV46 hydraulic oil (composing primarily the C22–C40 fractions). • Oil concentrations varying from 40 to 40,000 ppm (and clean sediment, 0 ppm). • Sediment: the (clean) sediment was taken from a tidal flat in the Eastern Scheldt, very similar to the Rotterdam harbour sediment. Two sieve fractions were used: sediment passing a 0.5 mm sieve and passing a 5 mm sieve.
Table 2 Percentage of visible oil in droplet phase in Rotterdam harbour samples and in samples with different spiking methods Sample oil type, spiking method
Oil concentration, Qoc or Qos (ppm)
Percentage concentration of visible oil, Ct =Qon;oa 100%
Rotterdam harbour, averaged
1000 (av)
70.0 (av)
DMA, standard DMA, reduced mixing energy DMA batchwise addition
1000 1000 1000
18.0 54.0 5.4
DMA, reduced mixing energy DMA, reduced mixing energy
40 4000
86.0 38.0
HV46, reduced mixing energy HV46, reduced mixing energy
40 4000
48.0 25.0
60
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PHYSICAL APPEARANCE OF OIL IN OIL-CONTAMINATED SEDIMENT
• Weathering periods of 0 (ÔfreshÕ), 3 and 12 months after preparation of the samples, by storing of samples under a layer of seawater in closed containers in a dark room at 18 C. Probably, an essential difference in the interaction processes between natural conditions and the artificial spiking conditions is that in the natural conditions the coherent oil has been broken up into droplets prior to the collision with sediment particles. In the artificial spiking methods the first interaction takes place between coherent oil and the sediment mass and in a second phase the oil mass is broken up into smaller pieces together with the sediment. Prior to the investigation of the different spiking samples, the following extreme tests were carried out in view of different oil–sediment interactions: • Test 1. An oil–sediment mass was prepared by first mixing dry sediment with oil and subsequent mixing of the oil–sediment mass with water. By microscopic observation, the resulting sample did not show any oil droplets attached to the sediment particles, but only oil-coated sediment particles. • Test 2. An oil–sediment sample prepared by synchronous mixing of water, oil and sediment showed that the visible oil consists almost entirely of oil droplets attached to the sediment particles. Different spiking methods have been tested and analysed, in order to find a reproducible method of preparation of sediments for ecotoxicological experiments, resembling as much as possible the field (harbour) characteristics of the physical appearance of oil. Artificially spiked sediments versus harbour sediment. As indicated in Section ‘‘Introduction to spiked sediment for ecotoxicological research’’, the spiking methods A, B, C and D were investigated for DMA oil and HV46 oil, using one type of sediment. (For the preparation of the samples according to the different methods, see Brils et al., 2002.) The selection of the best spiking method was based on the observed oil droplet phase in the spiked sediment samples that showed the best match with the Rotterdam harbour sediment samples. For the selection procedure, the spiked sediments were prepared with oil concentrations of Qos ¼ 1000 ppm, in the same order as the natural harbour samples. Microscopic observation of the harbour samples concluded that up to about 70% of the oil are visible droplets, with some oil-coated sediment particles being observed (see Table 1). For the cases with DMA oil, the visible oil droplet distributions in the spiked samples are given in Table 1, Spill Science & Technology Bulletin 8(1)
and the fractions of visible oil relative to the added oil amount Qon ¼ 1000 ppm are given in Table 2. Obviously, the relative distribution of droplets over the size classes are not significantly different for the spiking methods. At the same time a reasonable similarity occurs with the harbour samples (thus also similar with the droplet size distributions of dispersed oil, as in Fig. 2). Significant differences occur for the percentage of oil in the droplet phase, whilst for all spiking methods the percentage of oil in the droplets phase is smaller than in the harbour samples (see Table 2). Mainly on the bases of the oil droplet percentage, the spiking method B has been selected for the preparation of oil-contaminated sediment for ecotoxicological research. Because the artificial spiking methods lead to a lower percentage of oil in the droplet phase than for natural oil-contaminated sediment, it is concluded indirectly that a higher percentage will be in the coating phase. The following conclusions can be drawn: • The observation of a mean value of about 70% of oil in the droplet phase in the Rotterdam harbour sediment, and taking into account the limited accuracy of the observations, justifies the conclusion that a large fraction or even almost all oil is present as oil droplets; • The DMA oil and the HV46 oil in the spiked sediments are present at significant but limited percentages in the droplet phase. Oil concentration. Results of the droplet observations in samples with different oil concentrations are shown as examples for DMA oil in Fig. 3 (see also Table 1), and in Fig. 4 for DMA oil and HV46 oil. Data are given in terms of the observed number of droplets per size class, Nd , the derived oil volumes per size class, Vd , and the total volume of the observed oil droplets, Vt . The numbers and volumes of droplets are based on observation areas of 10 mm2 . Influence of oil concentration on percentage of oil in droplet phase. All observations show that the concentration of observed oil in the droplet phase is linearly dependent on the oil concentration in the sediment: the percentage of oil in the droplet phase stays equal (see the Vt -curve in Fig. 3). Oil patches were observed with HV46 oil (almost entirely), when Qos P 4000 ppm. Influence of oil concentration on droplet size distribution. Almost all observations found that the number of oil droplets in the different size classes was dependent on the total oil concentration: Nd Qpon with 0 < p < 1 (see the curves in Fig. 4). The linear dependence of Qos 61
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ance of larger size classes with increasing oil concentration.
Fig. 3 Oil droplets by number Nd and volume Vd in size classes versus oil concentration Qos in seven sediment samples spiked with DMA oil.
Oil type, sediment type and ageing. DMA oil and HV46 oil showed significant differences in the percentages of observed oil droplets. The fraction was on the average about 30% for DMA oil and on the average about 11% for HV46 oil. The latter lower fraction of oil in the droplet phase may indicate that more HV46 oil was present as coating on sediment particles. This could be due to the lower surface tension of HV46 oil. Another difference between DMA and HV46 oil is the presence of oil patches with HV46 oil; oil patches were hardly present in sediment contaminated with DMA oil. Oil patches were present in a limited number of samples, in the investigated samples with HV46 oil only when the oil concentration exceeded 4000 ppm. The two applied sediment types, sediment sieved over 0.5 and 5 mm, did not show any difference in the physical appearance of the observed oil. The effect of weathering on the oil appearance was investigated after weathering periods of 0 (fresh samples), 3 and 10 months (Fig. 5). Neither DMA oil nor HV46 oil in sediment with different weathering periods show significant differences in the physical appearance of observed oil for the relative droplet size distribution and the fraction of oil in the droplet phase.
Fig. 4 Oil droplet distributions of Fig. 3, adding seven samples spiked with HV46 oil and one sample from Rotterdam harbour.
on the total oil volume Vt in the droplet phase, Qos Vt as concluded above, is not in contradiction with the present conclusion because of a shift of the droplets to larger sizes and (therefore) the appear62
Fig. 5 Oil droplets by Nd and total volume Vt for 2 6 samples spiked with DMA oil, fresh (T0 ) and after three months (T1 ) weathering period. Spill Science & Technology Bulletin 8(1)
PHYSICAL APPEARANCE OF OIL IN OIL-CONTAMINATED SEDIMENT
Conclusions Microscopic observations of oil-contaminated sediment samples lead to the following conclusions: (1) There are three possible phases for the presence of oil: oil droplets, oil-coated sediment particles and Ôoil patchesÕ. The phase of the distinct oil droplets ranged from oil droplets covered by sediment particles, to oil droplets encapsulated by the sediment mass or sediment flocs. (2) The fractions of oil in the different phases in the sediment depend on the investigated parameters dealing with the interaction processes and the type of oil. (3) The physical appearance of oil depends on the interaction processes. This was concluded from investigations on three types of oil-contaminated sediment: (1) natural harbour sediment, (2) laboratory samples of oil–sediment agglomerates (flocs) after simulating the interaction processes and (3) laboratory samples of spiked sediment obtained from direct mixing of large volumes of oil and sediment. Because of the limited accuracy, the observations do not really prove, but certainly suggest, that the oil in the delicate structures of the laboratory oil–sediment flocs consists only of distinct droplets (no coating of sediment particles was observed), apparently due to the weak interaction of oil droplets and suspended sediment particles in simulated turbulent sea conditions. The sediment samples from the harbour bottom show mainly oil in the droplet phase and a limited fraction as coating of sediment particles, the latter probably formed in the severe interaction in the settling/consolidation processes. The spiked sediments seem to include a significant fraction of oil in the coating phase due to the severe interaction in the oil–sediment mixing process. (4) The fraction of oil in the droplet phase in the sediment was independent of the oil concentration in the sediment, but the droplet size distribution shifts to larger droplets with increasing oil concentration. (5) The various parameters influencing the oilÕs physical appearance in oil-contaminated sediment do
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not show any influence on the relative distribution of oil droplets in the different size classes. This conclusion, with the restriction of the limited accuracy of the observations, includes the droplet size distributions in natural sediments and artificially prepared sediments. Finally, the observed droplet size distributions are similar to the distributions found in the experiments on natural dispersion of oil, dealing with the break-up of droplets from a coherent surface oil layer. (6) The present limited investigations did not show any differences in the physical appearance of oil in sediment for the applied different sediment and weathering states of the samples. References Bragg, J.R., Owens, E.H., 1994. Clay–oil flocculation as a natural cleaning process following oil spills: Part 1. Studies of shoreline sediments and residues from past spills. Proceedings of the 17th Arctic and Marine Oil Spill Program (AMOP) Technical Seminar, Environment Canada, vol. 1. pp. 1–3. Brils, J.M., Huwer, S.L., Scholten, M.C.Th., Kater, B.J., Schout, P.G., Harmsen, J., Delvigne, G.A.L., 2002. Oil effect in freshly spiked marine sediment on Vibrio fischeri, Corophium volutator and Echinocardium cordatum. Environmental Toxicology and Chemistry 21 (10), 2242–2251. Delvigne, G.A.L., 1989. A sampler for the collection of dispersed oil droplets. Proceedings of the 1989 Oil Spill Conference (Am. Petroleum Inst.), San Antonio, USA, pp. 567–568. Delvigne, G.A.L., Sweeney, C.E., 1988. Natural dispersion of oil. Oil and Chemical Pollution 4 (4), 281–310. Delvigne, G.A.L., Hulsen, L.J.M., 1994. Simplified laboratory measurement of oil dispersion coefficient––application in computations of natural oil dispersion. Proceedings of the 17th Arctic and Marine Oil Spill Program (AMOP) Technical Seminar, Environment Canada, vol. 1. pp. 173–187. Delvigne, G.A.L., VanderStel, J.A., Sweeney, C.E., 1987. Measurements of vertical turbulent dispersion and diffusion of oil droplets and oiled particles. Report to US Department of the Interior, Minerals Management Service, Alaska. WL/Delft Hydraulics, Report Z75, Delft, The Netherlands. 1987p. Weise, A.M., Nalewajko, C., Lee, K., 1999. Oil–mineral fine interactions facilitate oil biodegradation in seawater. Environmental Technology 20, 811–824. Wood, P.A., Lunel, T., Daniel, F., Swannell, R., Lee, K., StoffynEgli, E., 1998. Influence of oil and mineral characteristics on oil– mineral interaction. Proceedings of the 21th Arctic and Marine Oil Spill Program (AMOP) Technical Seminar, Environment Canada, vol. 1. pp. 51–77.
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doi:10.1016/S1353-2561(02)00121-4
Physical Appearance of Oil in Oil-Contaminated Sediment G.A.L. DELVIGNE* WL/Delft Hydraulics, P.O. Box 177, 2600 MH Delft, The Netherlands
The physical appearance of oil was investigated in three types of oil-contaminated sediment: (1) natural harbour sediment, (2) laboratory samples of sediment prepared by simulating the natural oil–sediment interaction processes and (3) laboratory samples of spiked sediment prepared by direct mixing of large volumes of oil and sediment. Microscopic observations show the possible presence of three oil phases in the sediment: (1) distinct oil droplets, (2) oil coating on sediment particles and (3) Ôoil patchesÕ. Through quantitative investigations, it was concluded that the division of oil in the different phases is affected by the oil–sediment interaction processes, oil type, and oil concentration. As far as the droplet phase is concerned, these processes and parameters also influence the droplet size distribution. 2003 Elsevier Science Ltd. All rights reserved. Keywords: Oil, oil-contaminated sediment, oil–sediment interaction
Introduction The interaction between oil and mineral fine sediments in natural marine conditions has important effects on the contamination of the environment as well as on the removal of oil contamination. It has been well known for a long time that oil contamination of sediment on the seabed or in a harbour is often due to the interaction of oil droplets and suspended sediment in the water column. Oil droplets are usually generated by the break-up of a surface oil layer by breaking waves or other sources of turbulence and plunging. During the last decade, it has been recognized that the removal of oil from contaminated shorelines increases due to the interaction of mineral particles and oil. The oil–mineral interactions and the influence on biodegradation were investigated by, e.g. Bragg and Owens (1994), Wood et al. (1998) and Weise et al. (1999).
*Tel.: +31-15-285-8585; fax: +31-15-285-8582. E-mail address: [email protected] (G.A.L. Delvigne).
The present paper deals with the physical appearance of oil in sediment. (Investigations on the oil phase of oil-contaminated sediment may include chemical and physical analysis.) The physical characteristics of oil in sediment could be important with regard to the buoyancy of the oil–sediment agglomerates, the weathering behaviour and last but not least the ecotoxicological characteristics of the contaminated sediment. It is very likely that the physical characteristics of oil in oil-contaminated sediment depend on the following processes and parameters: • interaction mechanisms of oil and sediment (including natural and artificial interaction mechanisms), • oil type and sediment type, • oil concentration, • weathering of the sample (degradation state). The author took part in research programs on oil dispersion and oil-contaminated sediment. Research on natural oil dispersion and the formation of oil– sediment flocs was carried out mainly in the years 1986–1987 and described by Delvigne et al. (1987) and Delvigne and Sweeney (1988). Recent research on ecotoxicological effects of oil-contaminated sediment 55
G.A.L. DELVIGNE
has been described by Brils et al. (2002). The research programs included investigations on the physical appearance of oil in sediment, which will be described in the present paper. The following types of oil-contaminated sediment were investigated: • Natural sediment: oil-contaminated sediment samples formed under natural conditions were taken from the bottom of a Rotterdam petroleum harbour. This investigation was carried out to compare artificially spiked sediments with naturally formed oil-contaminated sediment and among others to select the most appropriate spiking method. • Simulated natural sediment: oil-contaminated sediment resulted from the interaction of dispersed oil droplets and suspended sediment in laboratory experiments. The laboratory experiments were part of a research program to investigate the formation of oil-contaminated sediment in conditions similar to those in the natural (sea) environment after an oil spill. The simulations started with the introduction of an oil layer on the water surface, followed by the formation of oil droplets by a turbulence source (e.g. dispersion by breaking waves) and finally the formation of oil–sediment agglomerates (flocs). • Artificially spiked sediment: oil-contaminated sediment resulted from the addition and mixing (spiking) of oil and wet sediment. Spiked sediments were prepared and used in a research program on ecotoxicological quality criteria for oil-contaminated sediments and dredged material. Spiking methods were investigated in order to obtain large quantities of oil-contaminated sediment with wellknown parameters (oil type, oil concentration, weathering state), resembling as closely as possible contaminated harbour sediments. (Artificial spiking produces large amounts of material, which is not possible using the preparation method of simulating natural interaction conditions as described in the paragraph above.)
Observation and Analytical Methods The observation of oil and the analytical methods dealing with the physical appearance of oil were developed in the 1986 research program and were applied as well in the more recent program. Observation equipment and methods An optical microscope was used to observe the physical appearance of oil in oil–sediment samples, as 56
well as to observe separate oil droplets and sediment particles before the interaction takes place. The following observation methods were applied: • Samples of oil droplets in water and oil–sediment agglomerates taken from experiments in the laboratory vessels were observed inside the (small) sampling device. The samplers were put directly in the object space of the microscope. (See the description of the sampling equipment in Delvigne and Sweeney (1988) and Delvigne (1989).) • Small volumes (still wet) of the artificially spiked samples and the harbour samples were put on the observation table and a surface area of about 10– 100 mm2 was investigated. The samples were observed with visible light and with UV light, in order to distinguish the sediment from oil phases (and air bubbles). The oil shows fluorescent behaviour in the UV light (with a wavelength ). Usually, the observations were of about 3560 A carried out with 100 amplification. Examples of how images of separate oil droplets and different oil–sediment agglomerates look like under the microscope are given in Fig. 1. The photographs in Fig. 1(a) and (b) were taken from laboratory experiments on natural dispersion of oil in water without sediment, and Fig. 1(c) and (d) from dispersion experiments with dispersed sediment. The experiments have been described in Delvigne et al. (1987). Visible oil phase Observed and analysed physical oil phases in the samples: • Oil droplets. In particular the oil droplet size distribution was determined. The resolution of the microscope limits the visibility of oil droplets (as well as sediment particles) to 1–2 lm or greater. • Oil-coated sediment particles. Tests indicated that an oil layer thickness of about 1 lm or smaller cannot be recognised as oil because the fluorescent behaviour in UV light is no longer visible. If an oil droplet with a diameter of do ¼ 10 lm covers the entire surface of a sediment particle of 25 lm, then the oil layer thickness is about 0.3 lm. This could mean that many oil-coated sediment particles are not visible by the observation method. Consequently, Ôlost oilÕ in the observed mass balance could be present as invisible coating on sediment particles. • Oil ‘‘patches’’. Oil patches were observed only in sediments with high oil concentration. In contrast to oil droplets and coated sediment particles, the coherent oil patches have no well-defined shape in the sediment mass. The patches usually cover several Spill Science & Technology Bulletin 8(1)
PHYSICAL APPEARANCE OF OIL IN OIL-CONTAMINATED SEDIMENT
Fig. 1 (a) Macrograph of droplets in oil dispersion experiment. (b) Micrograph of droplets in oil dispersion experiment. (c) Oil droplets in sediment flocs. (d) Oil droplets covered with sediment particles.
sediment particles and change shape when the sample is stirred. Typical dimensions of oil patches on a sample surface in the present investigations were a thickness of the order of micrometers and horizontal dimensions of tens of micrometers. Oil concentrations in the different phases in the sediment mass were estimated from the observations (see Section ‘‘Determination of oil concentration’’). Visible oil in the different phases are droplets larger than 1–2 lm and oil coatings thicker than 1 lm; oil patches are always visible. The difference between the concentration of the visible oil and the real oil concentration derived from chemical analysis or the known total oil concentration in the spiked samples is assumed to be mainly in the phase of sediment coating. Note: based on previous investigations, it was concluded that the volume of oil in the invisible Spill Science & Technology Bulletin 8(1)
droplet size range, smaller than 1 lm, is very small relative to the volume of the larger oil droplets. Determination of oil concentration The droplet size distribution in a sample, observed on the sample surface, has been identified generally by the number of droplets within size classes. Defined size classes are, for instance, classes with droplet diameter do < 5 lm, do ¼ 5–10 lm, do ¼ 10–20 lm, do ¼ 20–40 lm, do ¼ 40–80 lm, etc. and ÔpatchesÕ. In order to identify the oil concentration in a sample, an oil volume is derived from the number of droplets in a size class by estimating a weighted mean diameter in each size class. The oil concentration is calculated by relating the oil volume (on the sample ÔsurfaceÕ) to the volume of the observed surface layer. For the present samples, the thickness of the 57
G.A.L. DELVIGNE
observation layer is estimated to be about 50 lm based on the following rationale: • The focus depth of the microscope is in the order of 25 lm. • The grain size of the coarse part of the sediment in the present samples was in the order of 10–50 lm. Then the number of sediment particles, usually somewhat transparent, in the upper profile of 50 lm is one or only a few. • The observed largest oil droplets in the present samples are in the range of 20–40 lm. The oil concentration determined from the microscopic observation of oil could be compared with the oil concentration determined from chemical analysis. In the cases of the spiked samples, the observed oil concentration is compared with the known oil concentration applied in the mixing process. With regard to observed oil droplets in the samples, the following parameters are usually determined in the experiments (per unit of observed sample volume/ surface): do is droplet diameter; Nd is number of droplets per size class; Vd is volume of droplets per size class; Vt is total volume of droplets over all size classes; Ct is total oil concentration over all size classes. Other parameters dealing with the oil mass are: Qos is oil concentration in spiked sediment as known from the applied mixture ratio oil/sediment; Qoc is oil concentration in sample measured by chemical analysis.
troducing oil slick in three different laboratory facilities. A grid column was used generating homogeneous turbulence with different energy levels by an oscillating grid. A relatively small oil flume (15 m long, 0.5 m water depth) and a large flume (200 m long, 5 m water depth) were used, generating breaking waves in a surface oil layer. The wave heights were up to 0.2 and 2.0 m respectively. These experiments were described by Delvigne and Sweeney (1988). Also, small-scale experiments were carried out on oil droplet generation just by pouring an oil slick and water into a small water tank, simulating the turbulent energy of natural conditions (see Delvigne and Hulsen, 1994). The following main results refer to oil droplet generation: • The droplet size distribution in dispersed oil is described by Nd do2:3 , where Nd is the number of droplets with diameter do . This relationship describes the droplet distribution immediately after the break-up of a coherent oil mass by a severe turbulence source. The relationship leads to droplet size distributions as illustrated in Fig. 2. • The slope of the curve in Fig. 2 is always the same, independent of the break-up source, oil type and oil concentration (or thickness of the surface layer). • The intercept of the curve in Fig. 2 depends on the following parameters: amount of (surface) oil per unit area (linear dependence),
Results and Discussion Oil and sediment in natural conditions As stated before, oil contamination of sediment on the seabed or in a harbour is often due to the interaction of oil droplets and suspended sediment in the water column. The oil droplets are usually generated by the break-up of a surface oil layer by breaking waves or other sources of turbulence and plunging. The simulated natural processes of oil dispersion and oil–sediment interactions were investigated by the author in the mid-eighties. Recently, the physical appearance of oil in natural oil-contaminated sediment was investigated from sediment samples taken in a harbour. Simulated natural oil dispersion. Laboratory experiments on oil droplet generation in simulated marine and inland water conditions were carried out by in58
Fig. 2 Oil droplet size distributions in dispersion experiments with different sources of plunging and turbulence (breaking waves and pouring, see text). Spill Science & Technology Bulletin 8(1)
PHYSICAL APPEARANCE OF OIL IN OIL-CONTAMINATED SEDIMENT
oil type (the main parameter is the oil viscosity, which is also dependent on the oil weathering state and temperature) and energy intensity of the break-up source. • The extent (size range) of the curve depends on the following: the curve always exists in the smaller size classes, and is assumed to continue even in the smaller size classes than those presented in Fig. 2 where the curve of observed oil droplets is limited by the microscopic resolution to about 2 lm; the presence of the largest droplet sizes depends on the amount of available oil. These conclusions mean, for instance, that the decreased dispersion with increased viscosity or decreased energy intensity will be represented in Fig. 2 by a lower curve, whilst the curve continues for larger droplet size classes. As mentioned before, these results are valid for the oil droplet distribution immediately after the break-up occurrence. After the break-up, the droplet distribution in a specific water volume strongly depends on the resurfacing of (mainly) large oil droplets.
Natural oil-contaminated sediment: laboratory simulation. Laboratory experiments were carried out on the interaction of dispersed oil droplets and suspended sediment particles simulating as closely as possible natural conditions. The experiments were carried out in the grid column, see Section ‘‘Simulated natural oil dispersion’’, while adding sediment additionally to the oil slicks in the column. In particular this interaction is characterised by low concentrations of droplets and suspended sediment, and turbulence conditions similar to (different) sea states. The applied oil types were Prudhoe Bay Crude and Ekofisk Crude, with different weathering states. The sediment was taken from an intertidal zone of the North Sea. The resulting oil droplets, sediment flocs and oil–sediment agglomerates were analysed by microscopic observations. (See Delvigne et al., 1987.) The purpose of those oil–sediment interaction experiments was to obtain data on the oil–sediment agglomerates, in particular with respect to buoyancy and floc size. These parameters are of great influence on the vertical motion of oil-contaminated particles in a turbulent ambience and, ultimately, on the sedimentation on the seabed. With respect to the physical behaviour of oil in the delicate structures of oil–sediment agglomerates, the following conclusions were drawn from optical microscopy: Spill Science & Technology Bulletin 8(1)
• Oil–sediment agglomerates ranged from Ôoil droplets coated with sediment particlesÕ to Ôoil droplets incorporated in sediment flocsÕ (see Fig. 1(c) and (d)). The buoyant, neutral or negatively buoyant behaviour of an agglomerate is dependent on the oil– sediment ratio in the agglomerate. • All visible oil in the agglomerates consisted of discrete droplets. The droplets observed in negatively buoyant agglomerates were always in the size range from 1 lm (limit of observation) to about 60 lm. Neither the size distribution of oil droplets in the agglomerates or the sediment coating of buoyant oil droplets were significantly variable in experiments with different oil types, sediment types and turbulence level. Natural oil-contaminated sediment: Rotterdam harbour. Oil-contaminated sediment samples were taken from seven different locations from the bottom of Rotterdam harbour, in order to compare the characteristics between these natural samples and artificial samples (see Section ‘‘Introduction to spiked sediment for ecotoxicological research’’). Of course, a problem arises with harbour samples of unknown history with interaction with different oil types and subjection to different weathering and turbulence conditions. Contrary to the delicate structures of the laboratory oil– sediment agglomerates discussed in Section ‘‘Natural oil-contaminated sediment: laboratory simulation’’, the harbour samples were subjected to phases of settling and consolidation of the material. Data on the harbour samples are given in Tables 1 and 2. Chemical analysis of the harbour samples shows oil concentrations from 275 to 1047 ppm. Microscopic observation of the oil droplets in the samples indicated droplets up to the size class of 20–40 lm and, as far as the accuracy of the observation method permits, about 70% of the oil exists as droplets. Also, some oil-coated sediment particles were observed (see Table 1); the size of the coated particles was in the range 30–50 lm. As far as the accuracy of the observation method permits, the droplet size distributions in the harbour samples seem similar to the size distributions in the dispersion experiments in Fig. 2.
Oil in artificially spiked sediments Introduction to spiked sediment for ecotoxicological research. Artificial oil-contaminated sediments were applied in the recent ecotoxicological research program. The spiked sediments were prepared by mixing large amounts of wet sediment with known quantities of oil. With the standard method A, the wet (50–80% moisture content on dry weight basis) sediment was 59
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Table 1 Observed oil droplets and oil-coated sediment particles in Rotterdam harbour samples and in samples with different artificial spiking methods Artificially spiked samples Qon ¼ 1000 ppm Droplet size classes, do (lm) Dropletsa Oil type and spiking method
a
Rotterdam harbour samples (taken at seven different locations) Oil concentration Qoc (ppm)
Dropletsa
Coatingsa
Only ÔcleanÕ sediment
600
1000 400 50 10
4.0
<5 5–10 10–20 20–40
50 10 – –
<5 5–10 10–20 20–40
160 120 10 5
DMA oil. Standard method
980
500 250 35 20
2.0
<5 5–10 10–20 20–40
100 100 32 20
DMA oil. Reduced mixing energy
450
750 300 20 8
2.5
<5 5–10 10–20 20–40
120 32 – –
DMA oil. Reduced water
560
500 300 30 5
4.0
<5 5–10 10–20
500 50 5
DMA oil. Batchwise oil addition
520
500 150 20
4.0
<5 5–10 10–20
275
400 200 10
2.0
<5 5–10 10–20
1047
600 200 15
4.0
Number of droplets/coatings per 10 mm2 of sample surface area.
homogenised for 1 min using a mixer at a speed of 720 rpm. The necessary amount of oil was then added by using a pipette and the spiked sediment was homogenised, as above, for 2 min. The physical appearance of oil has been investigated for the spiked oil–sediment samples using the following parameters: • Different spiking methods: method B deviated from standard method A by a reduced mixing speed of 240 rpm, method C deviated from A by a reduced water content and in method D the oil was added in five batches.
• Different oil types: the selected mineral oil types in this particular research program were Gulf OilÕs DMA gasoil (ships fuel with the chemical characteristics composing primarily of the C10–C22 fractions) and HV46 hydraulic oil (composing primarily the C22–C40 fractions). • Oil concentrations varying from 40 to 40,000 ppm (and clean sediment, 0 ppm). • Sediment: the (clean) sediment was taken from a tidal flat in the Eastern Scheldt, very similar to the Rotterdam harbour sediment. Two sieve fractions were used: sediment passing a 0.5 mm sieve and passing a 5 mm sieve.
Table 2 Percentage of visible oil in droplet phase in Rotterdam harbour samples and in samples with different spiking methods Sample oil type, spiking method
Oil concentration, Qoc or Qos (ppm)
Percentage concentration of visible oil, Ct =Qon;oa 100%
Rotterdam harbour, averaged
1000 (av)
70.0 (av)
DMA, standard DMA, reduced mixing energy DMA batchwise addition
1000 1000 1000
18.0 54.0 5.4
DMA, reduced mixing energy DMA, reduced mixing energy
40 4000
86.0 38.0
HV46, reduced mixing energy HV46, reduced mixing energy
40 4000
48.0 25.0
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PHYSICAL APPEARANCE OF OIL IN OIL-CONTAMINATED SEDIMENT
• Weathering periods of 0 (ÔfreshÕ), 3 and 12 months after preparation of the samples, by storing of samples under a layer of seawater in closed containers in a dark room at 18 C. Probably, an essential difference in the interaction processes between natural conditions and the artificial spiking conditions is that in the natural conditions the coherent oil has been broken up into droplets prior to the collision with sediment particles. In the artificial spiking methods the first interaction takes place between coherent oil and the sediment mass and in a second phase the oil mass is broken up into smaller pieces together with the sediment. Prior to the investigation of the different spiking samples, the following extreme tests were carried out in view of different oil–sediment interactions: • Test 1. An oil–sediment mass was prepared by first mixing dry sediment with oil and subsequent mixing of the oil–sediment mass with water. By microscopic observation, the resulting sample did not show any oil droplets attached to the sediment particles, but only oil-coated sediment particles. • Test 2. An oil–sediment sample prepared by synchronous mixing of water, oil and sediment showed that the visible oil consists almost entirely of oil droplets attached to the sediment particles. Different spiking methods have been tested and analysed, in order to find a reproducible method of preparation of sediments for ecotoxicological experiments, resembling as much as possible the field (harbour) characteristics of the physical appearance of oil. Artificially spiked sediments versus harbour sediment. As indicated in Section ‘‘Introduction to spiked sediment for ecotoxicological research’’, the spiking methods A, B, C and D were investigated for DMA oil and HV46 oil, using one type of sediment. (For the preparation of the samples according to the different methods, see Brils et al., 2002.) The selection of the best spiking method was based on the observed oil droplet phase in the spiked sediment samples that showed the best match with the Rotterdam harbour sediment samples. For the selection procedure, the spiked sediments were prepared with oil concentrations of Qos ¼ 1000 ppm, in the same order as the natural harbour samples. Microscopic observation of the harbour samples concluded that up to about 70% of the oil are visible droplets, with some oil-coated sediment particles being observed (see Table 1). For the cases with DMA oil, the visible oil droplet distributions in the spiked samples are given in Table 1, Spill Science & Technology Bulletin 8(1)
and the fractions of visible oil relative to the added oil amount Qon ¼ 1000 ppm are given in Table 2. Obviously, the relative distribution of droplets over the size classes are not significantly different for the spiking methods. At the same time a reasonable similarity occurs with the harbour samples (thus also similar with the droplet size distributions of dispersed oil, as in Fig. 2). Significant differences occur for the percentage of oil in the droplet phase, whilst for all spiking methods the percentage of oil in the droplets phase is smaller than in the harbour samples (see Table 2). Mainly on the bases of the oil droplet percentage, the spiking method B has been selected for the preparation of oil-contaminated sediment for ecotoxicological research. Because the artificial spiking methods lead to a lower percentage of oil in the droplet phase than for natural oil-contaminated sediment, it is concluded indirectly that a higher percentage will be in the coating phase. The following conclusions can be drawn: • The observation of a mean value of about 70% of oil in the droplet phase in the Rotterdam harbour sediment, and taking into account the limited accuracy of the observations, justifies the conclusion that a large fraction or even almost all oil is present as oil droplets; • The DMA oil and the HV46 oil in the spiked sediments are present at significant but limited percentages in the droplet phase. Oil concentration. Results of the droplet observations in samples with different oil concentrations are shown as examples for DMA oil in Fig. 3 (see also Table 1), and in Fig. 4 for DMA oil and HV46 oil. Data are given in terms of the observed number of droplets per size class, Nd , the derived oil volumes per size class, Vd , and the total volume of the observed oil droplets, Vt . The numbers and volumes of droplets are based on observation areas of 10 mm2 . Influence of oil concentration on percentage of oil in droplet phase. All observations show that the concentration of observed oil in the droplet phase is linearly dependent on the oil concentration in the sediment: the percentage of oil in the droplet phase stays equal (see the Vt -curve in Fig. 3). Oil patches were observed with HV46 oil (almost entirely), when Qos P 4000 ppm. Influence of oil concentration on droplet size distribution. Almost all observations found that the number of oil droplets in the different size classes was dependent on the total oil concentration: Nd Qpon with 0 < p < 1 (see the curves in Fig. 4). The linear dependence of Qos 61
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ance of larger size classes with increasing oil concentration.
Fig. 3 Oil droplets by number Nd and volume Vd in size classes versus oil concentration Qos in seven sediment samples spiked with DMA oil.
Oil type, sediment type and ageing. DMA oil and HV46 oil showed significant differences in the percentages of observed oil droplets. The fraction was on the average about 30% for DMA oil and on the average about 11% for HV46 oil. The latter lower fraction of oil in the droplet phase may indicate that more HV46 oil was present as coating on sediment particles. This could be due to the lower surface tension of HV46 oil. Another difference between DMA and HV46 oil is the presence of oil patches with HV46 oil; oil patches were hardly present in sediment contaminated with DMA oil. Oil patches were present in a limited number of samples, in the investigated samples with HV46 oil only when the oil concentration exceeded 4000 ppm. The two applied sediment types, sediment sieved over 0.5 and 5 mm, did not show any difference in the physical appearance of the observed oil. The effect of weathering on the oil appearance was investigated after weathering periods of 0 (fresh samples), 3 and 10 months (Fig. 5). Neither DMA oil nor HV46 oil in sediment with different weathering periods show significant differences in the physical appearance of observed oil for the relative droplet size distribution and the fraction of oil in the droplet phase.
Fig. 4 Oil droplet distributions of Fig. 3, adding seven samples spiked with HV46 oil and one sample from Rotterdam harbour.
on the total oil volume Vt in the droplet phase, Qos Vt as concluded above, is not in contradiction with the present conclusion because of a shift of the droplets to larger sizes and (therefore) the appear62
Fig. 5 Oil droplets by Nd and total volume Vt for 2 6 samples spiked with DMA oil, fresh (T0 ) and after three months (T1 ) weathering period. Spill Science & Technology Bulletin 8(1)
PHYSICAL APPEARANCE OF OIL IN OIL-CONTAMINATED SEDIMENT
Conclusions Microscopic observations of oil-contaminated sediment samples lead to the following conclusions: (1) There are three possible phases for the presence of oil: oil droplets, oil-coated sediment particles and Ôoil patchesÕ. The phase of the distinct oil droplets ranged from oil droplets covered by sediment particles, to oil droplets encapsulated by the sediment mass or sediment flocs. (2) The fractions of oil in the different phases in the sediment depend on the investigated parameters dealing with the interaction processes and the type of oil. (3) The physical appearance of oil depends on the interaction processes. This was concluded from investigations on three types of oil-contaminated sediment: (1) natural harbour sediment, (2) laboratory samples of oil–sediment agglomerates (flocs) after simulating the interaction processes and (3) laboratory samples of spiked sediment obtained from direct mixing of large volumes of oil and sediment. Because of the limited accuracy, the observations do not really prove, but certainly suggest, that the oil in the delicate structures of the laboratory oil–sediment flocs consists only of distinct droplets (no coating of sediment particles was observed), apparently due to the weak interaction of oil droplets and suspended sediment particles in simulated turbulent sea conditions. The sediment samples from the harbour bottom show mainly oil in the droplet phase and a limited fraction as coating of sediment particles, the latter probably formed in the severe interaction in the settling/consolidation processes. The spiked sediments seem to include a significant fraction of oil in the coating phase due to the severe interaction in the oil–sediment mixing process. (4) The fraction of oil in the droplet phase in the sediment was independent of the oil concentration in the sediment, but the droplet size distribution shifts to larger droplets with increasing oil concentration. (5) The various parameters influencing the oilÕs physical appearance in oil-contaminated sediment do
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not show any influence on the relative distribution of oil droplets in the different size classes. This conclusion, with the restriction of the limited accuracy of the observations, includes the droplet size distributions in natural sediments and artificially prepared sediments. Finally, the observed droplet size distributions are similar to the distributions found in the experiments on natural dispersion of oil, dealing with the break-up of droplets from a coherent surface oil layer. (6) The present limited investigations did not show any differences in the physical appearance of oil in sediment for the applied different sediment and weathering states of the samples. References Bragg, J.R., Owens, E.H., 1994. Clay–oil flocculation as a natural cleaning process following oil spills: Part 1. Studies of shoreline sediments and residues from past spills. Proceedings of the 17th Arctic and Marine Oil Spill Program (AMOP) Technical Seminar, Environment Canada, vol. 1. pp. 1–3. Brils, J.M., Huwer, S.L., Scholten, M.C.Th., Kater, B.J., Schout, P.G., Harmsen, J., Delvigne, G.A.L., 2002. Oil effect in freshly spiked marine sediment on Vibrio fischeri, Corophium volutator and Echinocardium cordatum. Environmental Toxicology and Chemistry 21 (10), 2242–2251. Delvigne, G.A.L., 1989. A sampler for the collection of dispersed oil droplets. Proceedings of the 1989 Oil Spill Conference (Am. Petroleum Inst.), San Antonio, USA, pp. 567–568. Delvigne, G.A.L., Sweeney, C.E., 1988. Natural dispersion of oil. Oil and Chemical Pollution 4 (4), 281–310. Delvigne, G.A.L., Hulsen, L.J.M., 1994. Simplified laboratory measurement of oil dispersion coefficient––application in computations of natural oil dispersion. Proceedings of the 17th Arctic and Marine Oil Spill Program (AMOP) Technical Seminar, Environment Canada, vol. 1. pp. 173–187. Delvigne, G.A.L., VanderStel, J.A., Sweeney, C.E., 1987. Measurements of vertical turbulent dispersion and diffusion of oil droplets and oiled particles. Report to US Department of the Interior, Minerals Management Service, Alaska. WL/Delft Hydraulics, Report Z75, Delft, The Netherlands. 1987p. Weise, A.M., Nalewajko, C., Lee, K., 1999. Oil–mineral fine interactions facilitate oil biodegradation in seawater. Environmental Technology 20, 811–824. Wood, P.A., Lunel, T., Daniel, F., Swannell, R., Lee, K., StoffynEgli, E., 1998. Influence of oil and mineral characteristics on oil– mineral interaction. Proceedings of the 21th Arctic and Marine Oil Spill Program (AMOP) Technical Seminar, Environment Canada, vol. 1. pp. 51–77.
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