1

RIKZ-99.018

Commissioned by the Ministry of Transport, Public Works and Water Management, NorthSea Directorate and NOGEPA

OBM Drill Cuttings Discharges:

Assessment Criteria

August 1999

Dr. J.W. Dulfer

Ministry of Transport, Public Works and Water Management

Directorate-General of Public Works and Water Management

National Institute for Coastal and Marine Management/RIKZ

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Preface

During the Fourth International Conference on the Protection of the North Sea, held on 8 and9 June 1995 at Esbjerg in Denmark, Ministers agreed to invite OSPAR to investigate the needfor oily cuttings, to study their environmental impact, and to consider possible means ofcleaning up seabeds contaminated by them. OSPAR's SEBA working party subsequentlyasked the Contracting Parties to evaluate national legislation and technical feasibility in thisrespect. National and international concern about oil discharges from offshore activitiesprompted the conduct of an environmental impact assessment and, in 1993, a total ban ondischarges of OBM cuttings. British and Norwegian oil industry associations have recentlylaunched an initiative to tackle the problem of accumulated oily (OBM) drill cuttings and mudtraces from the past. The options being studied include removing the cuttings completely ortreating them in situ on the seabed. In the Netherlands, however, definitive policy conclusionshave yet to be reached concerning the presence of OBM residues in the seabed. Suchconclusions will need to be based on a proper ecotoxicological risk assessment revealingwhich parameters fail to meet selected criteria of environmental quality, and therefore whatenvironmental risks are involved.

The parties to the environmental covenant between the Dutch authorities and the local oil andgas industry have now launched a project aimed at identifying the environmental risks posedby present residues of drill cuttings in the seabed of the Netherlands Continental Shelf (NCS),and considering what action may be required to alleviate any confirmed risks. During the firststage of the project, relevant criteria will be developed for the study of field data which havebeen sampled previously. During the second stage, field data will be examined on the basisof the selected criteria and an assessment will be made of the environmental risks posed bycuttings residues from the wells involved. In addition, an extrapolation will be made toestimate the degree of contamination of the seabed of the NCS by past discharges of OBMdrill cuttings. The technical and legal aspects of any action required will then be analysedduring the third and fourth stages of the project.

This report contains the results of the study of relevant parameters and criteria. No attempthas yet been made to determine the actual degree of contamination of the NCS, to assessthe risks it poses to the environment, or to evaluate the possible consequences of thisassessment. Where the criteria discussed in this paper are exceeded, this will notnecessarily mean that the polluted sites have to be cleaned up, though it may suggest theneed for closer study of the problem.

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Contents

PREFACE..................................................................................................................................1

SUMMARY..................................................................................................................................4

1. INTRODUCTION....................................................................................................................5

2. SEDIMENT QUALITY MANAGEMENT ...................................................................................8

2.1 SEDIMENT PROTECTION .....................................................................................................82.2 SEDIMENT MANAGEMENT....................................................................................................8

3. OBM EMISSIONS AND EFFECTS OF MINERAL OIL ON BENTHIC FAUNA.....................10

3.1 EMMISSIONS FROM DRILLING OPERATIONS .......................................................................103.2 BIOACCUMULATION ..........................................................................................................113.3 SEDIMENT TOXICITY .........................................................................................................123.4 SMOTHERING...................................................................................................................12

4. PARAMETERS.....................................................................................................................13

4.1 POTENTIAL EFFECTS ........................................................................................................134.2 ACTUAL EFFECTS .............................................................................................................154.3 RECOVERY ......................................................................................................................16

5. CRITERIA.............................................................................................................................18

5.1 POTENTIAL RISK...............................................................................................................185.2 ACTUAL RISK ...................................................................................................................245.3. (ECO)SYSTEM RECOVERY................................................................................................27

6. CONCLUSIONS...................................................................................................................29

6.1 SEDIMENT........................................................................................................................296.2 DUMPING SITE..................................................................................................................31ACKNOWLEDGEMENTS ...........................................................................................................32

7. REFERENCES....................................................................................................................33

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Summary

Until 1993 discharges of oil based drilling muds (OBM) from the offshore oil and gasindustries were a considerable source of seabed oil contamination in the NetherlandsContinental Shelf (NCS) area of the North Sea. Studies conducted on behalf of the Dutchauthorities and the local oil and gas industry concerning the occurrence and environmentaleffects of OBM cuttings piles on the NCS have shown that they have significant impacts onbenthic fauna.

This paper provides background information on OBM emissions and the effects of mineral oilpollution of the seabed. It also discusses relevant parameters which can be used to assessthe pollution of the seabed in the NCS area by residues from past discharges of OBMcuttings. Criteria for the acceptability of actual and potential risk levels are discussed inrelation to each individual parameter.

Relevant parameters can be divided into those relating to the specific location and toemissions into the sea, and those relating to the effects of the discharges on the marineenvironment. Parameters can be functional, chemical, physical, or (eco)toxicological.

For every assessment parameter, criteria have been selected both to identify pollution anddetermine the degree of contamination by OBM and to evaluate the potential and actual riskposed to the environment and to human health. The acceptability of the potential and actualrisk posed by OBM pollution in a particular locality can then be determined, together with anyaction required. It is also possible to assess by means of extrapolation the expected overalleffect of OBM discharges on the NCS.

Some criteria are considered more appropriate or important than others. For this reason, ahierarchy of criteria has been drawn up. In accordance with general Dutch environmentalpolicy, the actual risk to the environment is regarded as decisive in estimating theconsequences of OBM pollution. The risk that OBM cuttings piles and oil residues pose to theenvironment also depends on parameters and criteria relating to the significance of thepossible impact of such pollution on the environment. These criteria are qualitative ratherthan numerical and indicate the consequences of specific situations.

The use of selected parameters and criteria and the analysis of technical and legal aspectsof required clean-up operations will be undertaken in future phases of the project.

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1. Introduction

Up to 1993, discharges of drill cuttings contaminated with oil based drilling muds (OBM) fromthe offshore oil and gas industries constituted a considerable source of seabed oilcontamination in the Netherlands Continental Shelf (NCS) area of the North Sea (Evers et al.,1997). Between 1981 and 1992, more than 200 drilling operations were conducted at 113locations in the area (Evers et al., 1997). As a result, piles of cuttings polluted with OBM arespread over the North Sea floor. Oil and OBM may spread from these piles to pollute the seabottom over a much wider area. In the Northern and Central North Sea, cuttings piles can beup to 20 metres high (UKOOA, 1996). In the Southern North Sea, the piles are much smallerand have probably been completely dispersed by wave and tidal action (Daan and Mulder,1996). On the NCS, residues of contaminated discharges are scattered patchily aroundformer drilling sites.

In 1990, the Netherlands Ministry of Economic Affairs (EZ) published an environmental impactstatement evaluating the effects of discharges of oil components from oil exploration andproduction installations on the environmental and economic functions of the North Sea (EZ,1990). This resulted in a total ban on discharges of OBM drill cuttings, introduced in 1993. Inthe Ministry's EIS, oil concentrations in water and sediments and assessments of effects andrisks to populations on the NCS were computed from data on oil pollution discharge loadsfrom the installations (mainly discharges of cuttings). However, the model used at that timehad still to be finalised and validated.

Subsequently, the North Sea Department of the Ministry of Transport, Public Works, andWater Management, the Ministry of Economic Affairs, and the Netherlands Oil and GasExploration and Production Association (NOGEPA) commissioned extensive studies tomonitor the effects of discharges of OBM drill cuttings on the seabed of the NCS sector.These were conducted between 1985 and 1995 by the TNO Technology for Society Division(TNO/MEP) and the Netherlands Institute for Sea Research (NIOZ). The results showed thatOBM drill cuttings degrade slowly on the seabed and that oil concentrations present in thesediment can have significant impacts on benthic fauna (Daan and Mulder, 1996). Fig. 1shows the distribution of discharges of OBM drill cuttings from wells in the NCS areabetween 1981 and 1991 (source: RWS North Sea Department, The Hague, 1997). Data onconcentrations of OBM-related compounds in sediment and biota were collected around anumber of drilling sites. Analysis of mineral oil was performed using a single GC-FID method,avoiding non-compatibility of results between different analytical techniques. The effects ofdischarges were determined on population densities of various species, including theindicator species Echinocardium cordatum. Local fauna was sampled and effects on thestructure of the affected ecosystem analysed at most of the relevant locations. Finally,experiments were also performed with active biological monitoring and boxcosms.

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Fig.1. Discharges of OBM drill cuttings on the Dutch Continental Shelf in the period 1981-1991

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As a result of these studies, a large amount of data is now available on the effects ofdischarges of OBM drill cuttings from offshore platforms in the NCS area. The availableinformation includes figures for discharged volumes, chemical analyses of sediment andbiota, toxicity information in the form of no observed effect concentration (NOEC) figuresfrom laboratory experiments, and the results of field studies on the actual effects on the localecosystem. Taken in isolation, each individual type of data may not be conclusive. Recordeddischarges of OBM drill cuttings may have been dispersed over time, identified sedimentpollution may produce no actual effects on organisms, and observed effects may be theresult of factors other than oil contamination. By totalling up the results of each parametergroup, however, it is possible to clarify the relationship between source and effects. ThisTRIADE-like approach can be used to rank contaminated sites in order of priority (Smaal etal.,1994).

The purpose of this paper is to provide background information on discharges of OBM drillcuttings and the effects of mineral oil pollution of the seabed. It will also discuss relevantparameters which can be used to assess the pollution of the seabed in the NCS area byresidues from past discharges of OBM cuttings. Criteria for the acceptability of actual andpotential risk levels will be discussed in relation to each individual parameter. In order tocompare and rank different types of contamination and effects, a hierarchical series ofparameters and corresponding criteria will be given, based primarily on current policystatements on oil pollution management. The paper will also elucidate the principles whichform the basis of current Dutch North Sea pollution policies and management practices, andwill include expertise provided by the offshore industries.

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2. Sediment quality management

2.1 Sediment protection

The primary aim of sediment protection policy is to prevent sediment pollution. To achievethis, both international policy within OSPAR and national policy under the first NationalEnvironmental Policy Plan (NEPP, 1989) strive to reduce pollution at source by imposingemission requirements and licensing procedures. The basic principle underlying thisapproach is that unnecessary pollution and further damage to the environment should beavoided, and that abatement at the source is preferable to treatment at a later stage.

However, the source-oriented approach cannot guarantee that no further damage will bedone even if all requirements are met. Pollution may still result from emissions in the past, orresidual emissions may continue in the future despite measures to prevent them. For thisreason, the policy also includes an impact-oriented approach which relies on establishingthreshold levels beyond which adverse effects on humans or populations of organisms maybe expected to occur. If these levels are exceeded, even more stringent requirements maybe imposed at source, going beyond the requirements of the source-oriented track (VROM,1994).

In order to allow the North Sea ecosystem to regain its natural balance, the Water SystemPlan for the North Sea 1991-1995 (WSP, 1993) laid down strategies designed to produce arealistic prospect of sustainable development in 2010. These allow additional protectivemeasures to be taken in the “environmental zone”: i.e. the coastal zone below the 20 misobath, and the Frisian Front - Cleaver Banks (see also Fig.1). As laid down in the North SeaAction Plan (1990), total loads of compounds discharged directly into the North Sea are to bereduced to a level presenting a negligible risk to the marine environment. At the Third NorthSea Conference, Ministers agreed that no more contaminated cuttings from offshoreinstallations should be discharged into the sea. Dutch regulations made consequent to theWSP banned the discharge of OBM cuttings from July 1993.

2.2 Sediment management

In the Netherlands, quality criteria for water and sediments are based on a risk approachwhich provides a framework for these quality objectives (Tweede Kamer, 1989). Sedimentsanitation and management measures are directed at an existing situation. If an actual orpotential new source of sediment contamination is located, the first step is to assess the riskto the environment and public health. The degree of pollution and consequent potential risk isevaluated on the basis of potential risk objectives. If these are exceeded, the actual risk isassessed in the light of local circumstances (Denneman, 1998). Potential risk levels areestablished on the basis of an assumption of maximum exposure to the contaminant. Thismeans that toxicological effects can occur whenever the potential risk concentration isexceeded. Toxicological effects resulting from exposure to pollutants are considered at theindividual level where human health is concerned and at the population level in the case ofother organisms.

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The next step is to decide how urgent it is to undertake remedial measures at the particularsite. This allows different contaminated sites to be ranked in order of priority and the startingdate for clean-up work to be determined for each (Tweede Kamer, 1994; Staatscourant,1997). The main prioritisation criterion is whether the pollution is actually affecting publichealth or the environment. Having determined this, the costs of clean-up measures then haveto be compared with the expected environmental benefits. The location of the pollution is animportant factor in this respect, as is the size of the polluted site compared with thesurrounding area. The mobility of the pollution is also an important consideration (VROM,1991).

A Severe Risk level ("Ernstig Risico" / ER; VROM, 1994) is one where over 50% of thespecies in the ecosystem are likely to be affected by the pollutants concerned. The MaximumPermissible Risk ("Maximaal Toelaatbaar Risico" / MTR) is defined as a risk level whichguarantees protection of 95% of the species in the ecosystem. Pollution exceeding the MTR,but still below the "severe risk" (ER) level is to be considered as a high risk, requiring theapplication of emission reduction measures. Where public health is concerned, the ER levelis defined as “exceeding the MTR”. Below the MTR level, environmental risk is consideredunclear, but further risk reduction is recommended in order to achieve the Negligible Risk("Verwaarloosbaar Risico" / VR) level. In principle, the Negligible Risk concentration is set atone-hundredth of the MTR concentration (VROM, 1994). This is the level at which damagecan be avoided in the long term and sustainable development can therefore be achieved(NEPP2, 1993).

Mineral oil has been placed on the list of priority substances and chemicals or substancesrequiring special attention (Tweede Kamer, 1988 and Staatscourant, 1992). The normalcriteria for listing are that the current risk is thought to exceed the negligible risk and that thesubstance is a national policy priority. The aim with regard to such priority substances is todevelop well-founded, integrated ER, MTR and VR levels and environmental quality objectivesbased on them (VROM, 1994). However, no conclusive risk evaluation has yet beenconducted with regard to mineral oil and no ER, MTR or VR levels have so far been agreed.

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3. OBM emissions and effects of mineral oil on benthic fauna

3.1 Emmissions from drilling operations

Oil based drilling muds are made from a mixture of oil, ground rock and clays, and have afluid consistency. The mud is circulated in the well to build up pressure and prevent oil andgas from gushing upwards in a blow out. During drilling operations the mud lubricates andcools the drill. As the drill cuts through rock, small cuttings of rock are produced which arecarried up out of the drill by the mud. If the cuttings are brought up to the platform, they areseparated from the muds, which can then be re-used. When cuttings with attached residuesof OBM are discharged to the seabed, their exact dispersion depends on depth, currentvelocity and particle sizes. In any case, they become spread over the sea-floor with the largercuttings closest to the rig. OBM will generally stick to the cuttings that sink closest to the rig(Lie and Langfeldt, 1998). OBM pollution can therefore be regarded as a two-stage process,with a dump of OBM contaminated cuttings occurring close to the rig and sediments furtheraway being contaminated by resuspension and redistribution of OBM from the dump.

In the NCS area, oil based drilling muds were mainly used during the 1980s and began to begradually replaced by water based muds from 1987 onwards. In the early eighties, diesel oilwas usually employed, chiefly due to its ready availability and low cost. Because of its hightoxicity, diesel was later replaced by low aromatic base oils (Davies, 1992). Since thebeginning of 1992, restrictive regulations have put an end to discharges of OBM on the NCS.Emissions of oil from the use of OBM are summarised by Asjes et al. (1995).

Total annual oil emissions from discharges of OBM drill cuttings on the NCS increased fromless than 50 t/year in 1981 to approximately 4500 t/year in 1986, after which they againdeclined to less than 50 t/year in 1991 (see Fig. 2). A total of 11,818 tonnes of oil was emittedin the NCS area between 1981 and 1992. In the 1993 Quality Status Report on the North Sea(QSR, 1993), the contribution of offshore activities to the total input of oil into the entire NorthSea was estimated at between approximately 14 and 33%. In the NCS area, the contributionof the offshore industry to total emissions in the more recent period between 1990 and 1995has been estimated at 1 - 2% (Evers et al., 1997). In a recent environmental scenario study(WSV, 1996) future emissions of oil from the offshore industry in the NCS area are estimatedat 133 t/year in 2000 and 95 t/year in 2015, mainly due to drain water and the discharge ofproduction water.

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0

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t/y

Incidents

Drilling muds + cuttings

Production water

Fig 2. Operational discharges of oil from offshore oil and gas production in the Dutchsector of the North Sea 1980-1994 (Evers et al., 1997).

3.2 Bioaccumulation

OBM cuttings can affect biota by raising local carbon loads, by producing toxic effects, andby smothering. Initial effects of oil contamination by OBM cuttings include severe reductionsin populations of organisms which are key components of local benthic communities. Thesereductions may persist for a number of years after discharges of drill cuttings cease (Daan etal., 1994; Olsgard and Gray, 1995). No differences can be found in bioavailability,accumulation and adverse effects of oil between marine and freshwater organisms (ENW,1994; Evers et al, 1997).

Marine molluscs, crustaceans and fish readily accumulate hydrocarbons, especiallyaromatics, both in solution and in dispersion (Neff et al., 1976). In the case of mussels,Schobben & Scholten (1993) have calculated a bioconcentration factor (BCF) of 0.99 ± 0.07(L/g fresh weight). Given an ash-free dry weight of 11% for mussels, this results in a BCF of9 (L/g ash free dry weight). On the NCS, field concentrations of 25-125 ?g oil per kg (freshweight) have been measured in mussels in the vicinity of platforms (Groenewoud & Scholten,1992). The accumulation of aromatics is no different from that of other hydrocarbons, buttheir elimination rate is lower (Schobben & Scholten, 1993). In the North Sea, a naphthaleneconcentration of approx. 1500 mg/kg dry weight has been found in the livers of haddock andcod feeding near platforms discharging OBM, compared to a “background” concentration ofaround 220 mg/kg dry weight in fish from the North Sea generally (Vogt et al., 1988). Giventhe efficiency of the elimination route and the relatively low BCF values for oil constituents,food chain effects can be regarded as fairly negligible. This means that the risk ofbiomagnification of mineral oil can be disregarded.

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3.3 Sediment toxicity

Fresh oil with a high aromatic content is particularly toxic to benthic organisms. The onlypublished standard sediment test using oil yielded an NOEC of 120 mg/kg dry sediment forthe sandlance, Ammodytus hexapterus (Pinto et al., 1984). In bioassays using oil basedmuds, NOECs have been established at concentrations of 10-20 mg fresh oil per kg of drysediment (Groenewoud & Scholten, 1992). This NOEC concentration for fresh oil has beenconfirmed in a Corophium bioassay (Foekema et al., 1996), where a lowest effectconcentration of 15 mg/kg dry sediment was measured. However, the observed toxicity of“weathered” oil is lower than that of fresh oil (Scholten et al, 1997). In experiments usingsediments spiked with different types of oil and subsequently aged for 28 weeks, an acutetoxicity value (LC50) of 700-800 mg/kg dry weight was observed for DMA gasoil. Using OBMtreated thermally in order to evaporate volatile constituents, Groenewoud & Scholten (1992)found an NOEC of 130 mg oil per kg dry sediment. The reduced toxicity of weathered oil maybe attributed to higher viscosity and the degradation or evaporation of toxic compounds.However, it should be remembered that little mineral oil degradation is found in the anoxicenvironment that exists below the top 10 mm of OBM cuttings piles (UKOOA, 1996). Largerbenthic organisms, such as bivalves and larger polychaete species, burrow considerablydeeper into the sediment, where the toxic effects of OBM cuttings may be similar to those offresh oil.

3.4 Smothering

Besides directly toxic effects, discharges of OBM drill cuttings are expected to causephysical and indirect effects on the local ecosystem. The deposition of large quantities ofcuttings with attached OBM will certainly affect the texture of the sediment. Adjacent toplatforms, the natural sediment is mixed with fine particles from the drilling mud andsediment particles may stick together as a result of the smothering effect of oily muds. Inpiles of cuttings examined in sedimentation areas of the British sector, the consequentlyreduced oxygen diffusion and pore water exchange had created anoxic conditions just a fewmillimetres below the surface (UKOOA, 1996). Similar asphyxiation has been shown to occurin organisms as a result of oil droplets sticking gills and mandibles together. A bioassay usingCorophium volutator has suggested that detrimental effects on biota may be caused primarilyby direct physical contact with oil, rather than by the uptake of dissolved constituents(Foekema et al., 1996).

Research on piles of OBM cuttings in the deeper Central North Sea showed a significantincrease in the total carbon content of the sediment, due to the presence of hydrocarbonsand other organic compounds in the discharged drill cuttings (UKOOA, 1996). This organicenrichment may be responsible for a substantial proportion of the effects of OBM on benthicfauna. In general, organic enrichment will reduce biological diversity and increase individualabundance (Davies, 1992). Under organically enriched anaerobic conditions, the microbialcommunity converts to an aggregation of sulphate-reducing bacteria, greatly adding to thedirectly toxic effects of drill cuttings by producing highly toxic H2S. Under thesecircumstances, hydrocarbon degradation will be limited by sulphate depletion of the porewater (UKOOA, 1996).

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4. Parameters

Assessment parameters have been defined to assess the degree of existing contaminationand risks of further contamination of the NCS seabed by discharges of OBM drill cuttings.Relevant parameters can be divided into those relating to the specific location and toemissions into the sea, and those relating to the effects of the discharges on the marineenvironment. Parameters can be functional, chemical, physical, medical or biological. Thissection describes potentially relevant parameters.

Table 1 shows parameters and criteria which can be used to determine whether residues ofOBM drill cuttings piles are present in sediment in specific locations. An oil concentration ofless than 10 mg/kg dry weight is regarded as less than the natural background concentration.Results of studies in the Netherlands Continental Shelf area (Groenewoud 1991) and in theNorwegian sector (Olsgard and Gray, 1995) show total hydrocarbon concentrations in pilesof OBM drill cuttings considerably in excess of 1000 mg/kg dry weight: the concentrationchosen as a positive criterion to identify residues of OBM drill cuttings piles. Between 10 and1000 mg/kg dry weight, patchy oil residues and resuspended OBM drill cuttings may bepresent. Criteria concerning pile height and depth penetration of oil are based on indicationsof biodegradation in English and Dutch studies respectively (UKOOA, 1996; Groenewoud,1996). Positive identification criteria for changes in grain size distribution and reduction innumber of functions seem to be most relevant, but have still to be discussed andestablished. Final decisions on the presence or absence of residues should be based on anintegrated appraisal of the parameters listed in Table 1.

Table 1. Parameters and criteria selected to identify the presence of residues of OBM drill cuttingspiles and to determine the volume of the pile

Parameter criterion Pile absent Pile present

Oil concentration in pile < 10 mg/kg > 1000 mg/kg

Pile height oxidation < 10 mm > 10 mm

Depth of oil penetration < 10 cm > 10 cm

Grain size ? distribution < 20% ?

Seabed functions reduction none ?

4.1 Potential effects

• Location The stability of OBM drill cuttings piles and contaminated sediments depends oncurrent velocity and wave action in the area. In erosion areas, OBM can be resuspendedand oil pollution can become a hazard to the surrounding environment. In areas where rigsor platforms have been abandoned and there is intense fishing, piles can be redistributedby heavy beam trawling and so pollute the surrounding sea-floor. It should also beremembered that the North Sea bottom is not a homogeneous sand floor: the ecosystemis more diverse and vulnerable to pollution in some places than in others.

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• Emissions of OBM drill cuttings are associated with the sinking of wells. For this reason,emissions are expressed in tonnes/well. Oil emissions from OBM contaminated drillcuttings in the Dutch sector of the continental shelf have been summarised by Asjes et al.(1995). The volume of emissions is expected to show a direct relationship to that of thepiles. Unfortunately, studies in the British area have shown that current and wave actionmake it hard to determine a direct relationship between the magnitude of discharge ofOBM drill cuttings and the radius of OBM distribution, especially in erosion and transitionareas (UKOOA, 1996). Emission volumes may nevertheless be used to assess by meansof extrapolation the area of seabed that may be affected by OBM discharges.

• Type of oil may be related to the expected toxicity of oil contaminated drill cuttings. In theearly eighties, diesel oil was usually employed, but this was later replaced by low aromaticbased oils (Davies, 1992).

• Oil concentration in sediment is expressed in mg oil/kg standardised sediment (dryweight, 10% organic matter, 25% lutum). Most aspects of national policy and legislation onpublic health and the environment are based on concentrations of chemicals in sediment(NW3, 1989; VROM, 1994). Oil concentration in OBM drill cuttings can best be expressedon a dry weight basis.

• Cuttings pile height, expressed in cm above the sea-floor, is a measure of the thicknessof the layer of OBM drill cuttings spread over the sea-floor. Related to the radius of the pilefoot, it provides a measure of the volume of cuttings still present in the pile. If oildegradation is assumed to occur only at the top of the cuttings pile (UKOOA, 1996), theheight of the pile may be a relevant factor in estimating the time when pollution will declinedue to the disappearance of the pile.

• Depth of oil penetration in sediment is expressed in cm. Oil may leak into thesediments directly underneath the pile. Related to the contaminated area, the depth of oilpenetration is a measure of the amount of contaminated sediment present under anexisting or former OBM cuttings pile.

• Depth at which oil is found in sediment is also expressed in cm. Oil may becomeredistributed in the sediment surrounding an OBM drill cuttings pile. As a result ofsedimentation and bioturbation, it may be buried under layers of unpolluted sediments.The depth at which oil is found may be a relevant factor in estimating the risk posed to thebenthic community and the risk of resuspension by waves and tidal currents. Related tothe time which has elapsed since the OBM discharges, it may be a measure of the speedat which pollution is likely to disappear as a result of contaminants being buried.

• Oil concentration in biota is expressed in mg oil/kg fresh weight. Chemicalconcentrations in benthic fauna are assumed to be in steady state or equilibrium withchemical concentrations in the sediment. Based on general toxic mechanisms such asnarcosis by neutral organics, oil concentrations in biota can predict possible effects inorganisms. In addition, oil concentrations in biota reflect contamination levels in thesediment, even where these are too low to measure.

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4.2 Actual effects

• Change (∆∆ ) in grain size distribution as a result of deposition of OBM drill cuttings isexpressed as change in grain size (in mm) and organic carbon content. This can beregarded as relevant in two ways. Firstly, a change in grain size distribution can indicatethe presence of an OBM drill cuttings pile or remnants of one. Secondly, fine-grained OBMcan induce changes in the original structure of the local sediment. This can have majorimpacts on the benthic community and, according to studies in British waters, mayproduce anaerobic conditions (UKOOA, 1996).

• Change (∆∆ ) in microbial activity is related to the organic enrichment of sediment byhydrocarbons from OBM drill cuttings. The presence of mineral oil in OBM drill cuttings willincrease the organic carbon level of the sediment. Organic enrichment stimulatesmicrobial activity (Davies, 1992). Microbial hydrocarbon degradation may be measured interms of carbon turn-over, oxygen depletion, carbon dioxide production, production ofsulphur hydroxide, etc. Since the choice of parameter will usually depend on the particularexperimental conditions, no selection can be made at this stage.

• Change (∆∆ ) in organism physiology Organisms which are exposed to contaminantsshow reduced activity, modifications in larval development, enhanced enzyme induction,etc. These parameters are usually monitored in bioassays and are part of the usualTRIADE approach (Smaal et al., 1994). Little is known about sublethal physiologicalresponses in organisms exposed to oil in sediments, but immune responses have beenobserved in mussels (Coles et al., 1994) and carcinogenesis in flounder and dab(Vethaak, 1992, 1996) as a result of exposure to PAHs, which are present in all types ofmineral oil. Physiological damage to organisms can also be the result of smothering by oil(Foekema, 1996).

• Change (∆∆ ) in organism morphology Organisms which are exposed to contaminantsshow modifications in various aspects of their morphology. In the North Sea, exposure toOBM contamination has been shown to produce loss of ventral spines in Echinocardiumcordatum, resulting in reduced burrowing behaviour (Daan et al.,1990).

• Ecosystem effects can be expressed in terms of reduced density in populations ofmarine species or alteration of ecosystem structures. Population density is expressed innumbers per unit surface area. Biodiversity and similarity are structural parametersdescribing the complexity of the ecosystem. Indices used to quantify this structuralcomplexity are relative parameters expressed as percentages (Heimbach and Ratte,1997).

• Change (∆∆ ) in seabed functions can be regarded as a parameter relevant to future riskassessment. The North Sea serves many different purposes, including transport,fisheries, nature, recreation and offshore mining (RWS, 1997). If one or more of thesefunctions proves impossible following abandonment of an oil rig, system functionality hasbeen reduced.

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4.3 Recovery

• Abiotic recovery is the degree to which the abiotic compartment of the polluted systemhas returned to its original pre-pollution state. Recovery is a process which takes placeover time and is expressed as an annual percentage (% recovery/year). The time requiredfor the system to recover can be assessed by extrapolation (e.g. number of years torecover 50%). Abiotic recovery can relate to oil concentrations in sediments, the height ofOBM drill cuttings piles, and grain size distribution. Recovery can therefore be regarded asthe reversal of the effects of pollution on the system. Recovery is complete once theremaining effects on the system have declined to the point of insignificance. Simultaneoussamples from reference stations outside the deposition area can be used to represent thesituation prior to OBM discharges.

• Biological recovery can be expressed as the difference or similarity between two ormore samples taken at different points in time. Recovery of the ecosystem is a processwhich takes place over time and is expressed as an annual percentage (% recovery/year).The time required for the system to recover can be assessed by extrapolation (e.g. yearsto recover 50%). The problem of pollution can be regarded as ended once the marineenvironment has recovered from the damage caused by it (Warwick and Clark, 1995).Biological recovery can therefore be regarded as the reversal of the effects of pollution onthe system. Recovery is complete once the remaining effects on the ecosystem havedeclined to the point of insignificance. Simultaneous samples from reference stationsoutside the deposition area can be used to represent the situation prior to OBMdischarges.

Table 2. List of parameters studied and their suitability for use in assessing the effect of OBM drillcuttings piles and contaminated sediments

Parameter Suitable for OBM drill cuttingspile

Suitable for contaminated sediment

Location + +Emissions + +Type of oil + +Oil concentration insediment

+ +

Pile height + -Depth of oilpenetration

+ -

Depth of oil present - +Oil concentration inbiota

- +

∆ Grain sizedistribution

+ +

∆ Microbial activity - +∆ Physiology - +∆ Morphology - +Ecosystem effects - +System functionality - +

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The cuttings pile and the contaminated sediments around the pile are considered separately.Table 2 shows the suitability of each parameter for use in establishing the presence ofcuttings piles and pollution in surrounding sediments. In general, parameters relating toemissions of OBM cuttings are suitable for use in establishing criteria for the pile, andparameters relating to effects for use in establishing criteria for the surrounding sediments.

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5. Criteria

For every assessment parameter listed above, criteria can be selected to identify pollution byOBM drill cuttings, determine the degree of contamination, and assess the actual andpotential risk to human health and the environment. The first step is to use these criteria todetermine the presence of OBM drill cuttings piles or residues of former piles. The second isto assess the actual and potential risk posed by local OBM pollution and to determinenecessary action. Finally, the expected total effect of discharges of OBM drill cuttings on theNCS can be assessed by means of extrapolation. This section defines criteria and discussestheir basic concept and current function in Dutch policy.

5.1 Potential risk

5.1.A. Location

OBM drill cuttings and polluted sediments are exposed to resuspension and dispersion bycurrents and waves. In the turbulent southern part of the North Sea, it is likely that piles havebeen completely dispersed. Where piles are located in erosion zones, there is a significantchance that OBM drill cuttings will be resuspended and pollution spread over large areas.Location in an erosion zone is therefore a risk-enhancing criterion. Beam trawling takes placein all areas, producing a widespread risk of redistribution of OBM drill cuttings over theseabed.

In addition, the ecosystem in the environmental zone - i.e. the coastal zone below the 20misobath, and the Frisian Front -Cleaver Banks (see Fig. 1) - and in estuaries is regarded asmore valuable and vulnerable than that elsewhere. Location in the environmental zone or inan estuary is therefore an effect-enhancing criterion.

5.1.B. Emissions

OBM drill cuttings consist mainly of rock chippings brought up from the seabed, mixed withoil and clays (UKOOA, 1998). The discharge of such cuttings can be regarded in twodifferent ways. Firstly, since they consist of local rock cuttings and mineral clay mixed withorganic compounds, they can be regarded as dredgings contaminated with mineral oil andtreated accordingly. On the other hand, piles of OBM drill cuttings can be regarded as dumpsof waste material produced during the industrial process of drilling and production and likelyto form a secondary source of mineral oil pollution. Contaminated materials dumped indesignated landfills and aquatic waste disposal sites are supposed to be isolated, controlledand monitored (ICM criteria; VROM, 1989). Although the places where OBM drill cuttings aredischarged are not designated as ICM sites, secondary pollution from poorly isolated,controlled and monitored OBM drill cuttings piles should be avoided. After all, the policy is thatno activity posing a more than negligible risk to the North Sea environment should beaccepted (WSP, 1993). However, in their study of the long-term effects of OBM dischargeson the North Sea environment, Daan and Mulder (1996) found no relationship between the

19

volume of past emissions of OBM drill cuttings and detected effects on the environment. Thismeans that no relevant emission criteria can be defined.

5.1.C. Type of oil

In the early eighties, diesel oil was used for OBM, but because of its toxicity it was laterreplaced by aromatic free oils (Davies, 1992). At most discharge sites in the NCS area, thetype of oil emitted is known (Asjes, 1995). Since diesel oil is regarded as more toxic thanaromatic free oils, it is likely to make a greater contribution to possible effects on the NorthSea environment.

5.1.D. Oil concentration in sediment

Oil concentration in sediment is a very effective parameter for the assessment of theecological and health risks of OBM contamination. A series of effective assessment criteriaand valid quality objectives are available and this parameter can be regarded as thebackbone of most aspects of Dutch water management and policy in this respect. As a resultof the redistribution of OBM drill cuttings piles by currents, wave action and beam-trawling,patchy lumps of OBM can often be observed spread about in the sediment. Reportedconcentrations of oil in sediment should therefore be regarded as averages. The OBMpatches will contain much higher oil concentrations and the sediment between them will becleaner (Groenewoud, 1991; 1996).

5.1.D.1 Water management

Since mineral oil has a high sorption tendency to organics and mineral oil pollution is a majorproblem in Dutch aquatic sediments, water management quality objectives have beenestablished for oil in suspended matter and sediments. In theory, water management criteriashould be based on levels indicating the potential toxicological risk of oil concentrations insediment. However, the lack of adequate toxicological data on mineral oil means that thepresent criteria for water management still have a purely pragmatic basis. The current targetvalue for mineral oil in sediments is equal to the former Soil Protection Guideline referencevalue for good soil quality, which was based on the opinion of experts (VROM, 1990). Theeffective limit value for mineral oil is based on concentrations currently found in sedimentfrom waters with low contamination levels, such as the Markermeer and Eastern Scheldt; thetest value is based on sediments from moderately polluted waters, such as the IJsselmeer;and the intervention value is based on present oil levels in sediments found in heavily pollutedwaters, such as the Port of Rotterdam (Stortelder et al, 1989).

20

Target value

• The target value indicates the level of negligible risk and is the ultimate objective forenvironmental quality, though no time-limit has been set for its achievement. With regardto sediment quality, the target values given in Dutch Soil Protection Guidelines indicate thegoal to be achieved, if possible within one generation (VROM, 1994).

• The current target value for mineral oil in sediment is set at 50 mg/kg dry weight and is acontinuation of the former “general environmental quality of sediment” reference value. Noecotoxicological evaluation of mineral oil has yet been performed. However, Dutchenvironmental policy does not distinguish between the status and effect of target valuesbased on risk evaluations and those of other target values (ENW, 1994).

• Target values apply both to inland waters and to the NCS (ENW, 1994). No significantdistinction can be drawn between ecotoxicological effects on freshwater organisms andthose on marine organisms (Evers et al., 1997).

Natural background level

• In Dutch water management policy, natural background levels of mineral oil prevail overtarget values, especially in the North Sea and estuaries (ENW, 1994).

• Data on the natural occurrence of mineral oil in North Sea sediments are scarce. Ingeneral, sediment concentrations exhibit large spatial heterogeneity which covaries withorganic carbon content and grain size distributions. Studies from baseline surveys in theNorwegian sector show background levels of 0.2-1.0 mg/kg dry weight (Olsgard and Gray,1995). Based on data summarised in Zevenboom et al. (1992), background levels in theNCS area appear to range from 0.2 to 7 mg/kg dry weight. The natural backgroundconcentration in NCS sediments can therefore be assumed to lie within the range 1-10mg/kg dry weight.

Limit value

• The ultimate objective of Dutch policy is to achieve a sediment quality at least equal to thetarget value. However, since this process will take time, intermediate objectives have beenset in the form of limit values to be met within certain periods of time (VROM, 1994).

• Current limit values are regarded as requirements to be met in the year 2000. For oil insediment, the limit value is a continuation of the value referred to in NW3 as “qualityobjective 2000” and set at 1000 mg/kg dry weight (ENW, 1994).

• Limit values are determined with due regard to environmental, economic and socialinterests and technical options and are set as low as reasonably achievable (ALARA).Therefore, limit values are not directly related to the risk levels described above, and willbe revised over time until the target values are finally achieved (VROM, 1994). Water

21

managers have a duty to make certain efforts to meet the limit values by the date set(ENW, 1994).

• The quality of the top 5 to 10 cm of sediment is directly dependent on fluctuations in thequality of the surface water. Limit values are designed to monitor the effectiveness ofcurrent environmental policy and hence are effective only for this newly formed sediment,and not for the quality of the sediment as a whole, since that improves much more slowly(VROM, 1994).

• The limit value has been eliminated in the Fourth Policy Document on Water Management(NW4, 1998)

Test value

• Test values were defined in NW3 (1989) for purposes of dredging policy. Test values areused to decide whether the loads of contaminants in dredged sediments are acceptablefor disposal onshore and in inland surface waters. The principle is that quality of therecipient sediments should not be reduced by the disposal of dredgings.

• The test value for mineral oil in sediments is 3000 mg/kg dry weight. Dredgings containinggreater concentrations than this are not regarded as acceptable for disposal in theenvironment, and have to be stored in isolation under controlled conditions (ENW, 1994).

• Since the test values for sediment refer only to the disposal of dredgings in inland surfacewaters and not their discharge into marine waters (ENW, 1994), the test value is irrelevantto discharges of OBM cuttings on the seabed.

Standard Content Test (SCT) value

• The SCT value (or “Target value level 2”) is a numerical value for the distribution ofdredgings in salt waters (ENW, 1994). The quality of sediments in the North Sea isregarded as critical because a large proportion of pollutants generated on land accumulatehere and become available to the marine ecosystem. NW3 and WSP state that anongoing decline in the quality of North Sea surface water is unacceptable. In order toprotect the quality of the North Sea ecosystem, an SCT value has been defined to test theacceptability of dredgings disposal at sea (ENW, 1994). This is more rigorous than thetest value for inland waters.

• The SCT value for oil in dredgings discharged into the North Sea is 1250 mg/kg dryweight: this is considerably lower than the common test value. Since the publication of theFourth Policy Document on Water Management (NW4, 1998), the former transitional valueof 1500 mg/kg dry weight, which was in temporary operation in the Port of Rotterdam areato minimise discontinuities in current policy trends, has been eliminated.

• In accordance with the stand-still principle, maximum allowable pollution loads indredgings dumped in the North Sea are defined in comparison with the situation in 1988

22

(ENW, 1994). However, discharges of OBM cuttings have steadily reduced since 1986and ceased entirely in 1993 (see Fig. 2). The stand-still principle is therefore not beingviolated.

Intervention value

• The intervention value is the concentration beyond which a serious reduction in thefunctional properties of the sediment can be expected (VROM, 1991). Above this level,more than 50% of the species potentially present will experience adverse effects.

• In principle, if the intervention value for oil (5000 mg/kg dry weight) is exceeded,emergency measures must be taken. The urgency of sediment remediation must bedetermined on the basis of the actual risk posed by the contamination.

• Dredged marine sediments exceeding the intervention value (i.e. those from harbourentrances) are collected onshore in the Slufter disposal site (ENW, 1994).

5.1.D.2 Toxicological risk levels

The health risks to humans from discharges of OBM cuttings are thought to be very slight.Current human exposure to oil and oil constituents from OBM drilling cuttings in the seabedcan be regarded as insignificant. Under normal conditions, individuals do not readily comeinto contact with sediments from the vicinity of present and former drilling platforms. Humanuptake of mineral oil by consumption of sea food probably provides only a very smallexposure pathway. No toxicological risk levels have yet been determined for mineral oil.Indicative Serious Risk (ER), Maximum Permissible Risk (MTR) and Negligible Risk (VR)levels have been established on the basis of experimental results concerningecotoxicological NOEC levels (e.g. Scholten et al., 1993 and 1997).

Ecotoxicological risk values

• The NW4 sets the MTR value for mineral oil as equal to the former limit value of 1000mg/kg dry weight, but with the proviso that it can be redefined if ecotoxicological databecome available (NW4, 1998). No ecotoxicologically based MTR value or VR value formineral oil has so far been determined.

• Environmental risk is to be reduced to the MTR value by the year 2000 (VROM, 1994).However, the NW4 no longer sets a specific date for this, but simply states that the MTRlevel should be achieved in the short term, which is not further specified (NW4, 1998).

• Scholten et al. (1993) calculated an indicative MTR value for oil in sediments of 12 mg/kgdry weight. This indicative MTR value is based on a single experimental No ObservedEffect Concentration (NOEC), taking account of experimental results from two otherstudies. Computed MTR values using a much larger set of experimentally determined

23

aqueous NOEC data and octanol-water partitioning coefficients vary by two orders ofmagnitude: between approximately 0.1 and 100 mg/kg (Scholten et al., 1993).

• Recent experiments conducted by TNO and RIKZ using marine species (Scholten et al.,1997) suggest that a future ER value might be set somewhere around an oil in sedimentconcentration of 100-1000 mg/kg dry weight, and that the future MTR value may be in theorder of 10-100 mg/kg dry weight. This means that the future MTR value could be 1-2orders of magnitude below the current limit/MTR value as proposed in NW4.Consequently, an indicative VR value to be achieved in the long term might be equal to thebackground value, somewhere in the range of 1-10 mg/kg (VROM, 1994).

• An ecotoxicological risk exceeding the ER value is certain to be unacceptable and willrequire immediate action to assess the urgency of remediation. Between the ER and MTRlevels, the risk is likewise regarded as unacceptably high, requiring action by means ofsource reduction to reduce the potential hazard. Below the VR value, oil concentrationsare assumed to cause zero effects, and between the VR and MTR values effects aredeemed to be present but acceptable.

5.1.E. Cuttings pile height

Assuming that oil degradation occurs only in the top few millimetres of the cuttings pile(UKOOA, 1996), the height of a pile may be relevant to the estimation of recovery time oncedischarges of OBM drill cuttings have ceased. In this respect, layers of OBM drill cuttingsthicker than 10 mm cannot be accepted as sediment and should be regarded as piles.Results from the British study suggest that present piles of OBM drill cuttings in deep waterare not degradable, but on the Netherlands Continental Shelf piles seem to be dispersed as aresult of currents and wave action and the remnants are found spread in patches around thesites of former discharges (Groenewoud, 1996). All locations investigated in the NCS areahave been sampled by means of a Van Veen grab to a maximum depth of 10 cm (Daan andMulder, 1996). In the deep waters of the UK sector, a weak correlation has been foundbetween pile height and pile foot radius (UKOOA, 1996).

5.1.F. Depth of oil penetration

Oil can penetrate into the sediment under piles of OBM drill cuttings on the sea-floor. Afterthese have eroded away, their former presence can be deduced from OBM residues in thesediment. Following the reasoning adopted in relation to the limit value that mineral oil in thetop 5-10 cm of sediment is likely to be quickly washed away, residues of oil leaked fromformer OBM drill cuttings piles are expected to be found only in sediments deeper than 10cm. However, all the locations investigated in the NCS area have been sampled by means ofa Van Veen grab to a maximum depth of only 10 cm (Daan and Mulder, 1996).

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5.1.G. Depth at which oil is found in sediment

As a result of sedimentation and bioturbation, concentrations of OBM residues are likely tohave been buried under layers of unpolluted sediments. Depending on the stability of theseabed, mineral oil buried deeper than 50-100 cm is thought to be no longer subject toresuspension, and probably out of reach of benthic organisms. The depth at which oil isburied in sediment is therefore an effect-hampering criterion, although it is one which is hardto define in the case of a stable sediment cover and which is not currently accepted in Dutchpolicies on the environment and water management.

5.1.H. Oil concentration in biota

Bioaccumulation experiments have been conducted on bivalves in the vicinity of former andpresent platforms (Groenewoud, 1991). Concentrations of mineral oil in benthic fauna areassumed to be in steady state or equilibrium with those in the sediment. The fact thatbenthos living in the vicinity of OBM drill cuttings piles display higher mineral oilconcentrations than those found at reference stations is therefore an indication that mineraloil from the cuttings is bioavailable and that concentrations in the sediment are above thenatural background level. In other words, they are considered to be the result of pollution.High concentrations in biota may also pose risks to consumers by means ofbiomagnification, although in the case of mineral oil this pathway is thought to be fairly small.

No information is available about toxic body burdens of mineral oil in organisms. The internalmolar concentration in the organism at death is regarded as the lethal body concentration.Based on the assumption that most neutral organics show a lethal body burden of about 5mmol/kg wet weight caused by narcotic effects, and that an average mole weight of mineraloil is 212 (pentadecane), the lethal body burden for mineral oil can be estimated atapproximately 1000 mg per kg wet weight (MacCarthy et al., 1992).

5.2 Actual risk

5.2.A. Sediment structure

The mean particle size of used drilling muds varies between 6 and 10 µm (Lie and Langfeldt,1998). Depending on the characteristics of the original sediment, residues of OBM drillcuttings can have a dramatic effect on its structure. Severe increases in the organic contentsof the sediment and decreases in grain size will have major impacts on the redox situationand community structure of the sediment, tending to create anaerobic conditions (Lehman etal., 1995). However, quantitative relations between particle size distribution, organic carboncontent and effects on marine ecosystems are not known. Given the anoxic conditions inOBM (UKOOA, 1996), no diverse benthic macro-fauna is expected in the pile.

Particle size distribution and organic carbon content can be used as tracers to identify OBMdrill cuttings piles. Taking due account of the natural variability of North Sea sediments

25

(Smaal, 1994), a lutum and organic carbon content of over 20% in a sample may mean that itcomes from a pile, rather than from natural sediment. Likewise, if more than 50% of thesample consists of very coarse particles (1-10 mm), the sample is also likely to come from apile (Lie and Langfeldt, 1998).

5.2.B. Microbial activity

Microbial hydrocarbon degradation can be measured by carbon turn-over, oxygen depletion,carbon dioxide production, production of sulphur hydroxide, etc. Experiments with OBM drillcuttings have shown increasing numbers of heterotrophic bacteria and hydrocarbon-utilisingbacteria and fungi (Chaineau et al., 1995), but less than 5% biodegradation of mineral oilcuttings in semi-natural sea bottom conditions (Bake and Lake, 1991). This is probably due tolow oxygen concentrations and the low porosity of the OBM. Given this low oil degradationrate, a significant decrease of microbial activity is expected in OBM cuttings. Following thereasoning adopted by Aldenberg and Slob to deduce ecotoxicological risk levels andmanagement standards (Aldenberg and Slob, 1993), a reduction of 0-1% compared with areference sediment can be regarded as a zero effect, and a reduction of 1-5% as acceptable.A reduction of 5-50% is regarded as unacceptable and requiring measures to reducepollution at source. A reduction of microbial activity of more than 50% means that the effect isso severe that immediate action must be taken.

5.2.C. Organism physiology

Modifications in larval development, enzyme induction and different aspects of morphologyare usually monitored in bioassays and are part of the usual TRIADE approach (Smaal et al.,1994). Studies using mineral oil have shown various effects in marine fish and crustaceans(Evers et al., 1997). According to Dutch risk evaluation, following the reasoning adopted byAldenberg and Slob to deduce ecotoxicological risk levels and management standards(Aldenberg and Slob, 1993), an increase of 0-1% in the rate of defects in organism physiologycompared with those in organisms in a reference sediment can be regarded as a zero effect,and an increase of 1-5% as acceptable. An increase of 5-50% is regarded as a unacceptableand requiring measures to reduce pollution at source. Over 50% means that the effect is sosevere that immediate action must be taken.

5.2.D. Organism morphology

Modifications in different aspects of morphology are usually monitored in bioassays and arepart of the usual TRIADE approach (Smaal et al., 1994). According to Dutch risk evaluationpractice, following the reasoning adopted by Aldenberg and Slob to deduce ecotoxicologicalrisk levels and management standards (Aldenberg and Slob, 1993), an increase of 0-1% inthe rate of deformations in organisms compared with those in a reference sediment can beregarded as a zero effect, and an increase of 1-5% as acceptable. An increase of 5-50% isregarded as an unacceptable effect, requiring measures to reduce pollution at source. Over50% means that the effect is so severe that immediate action must be taken.

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5.2.E. Changes in the ecosystem

The risk posed by contaminated sediments can be assessed not only through the use ofsediment quality criteria developed for purposes of environmental and water management,but also by direct ecological research (Smaal et al., 1994). The results of exposure to toxiclevels of oil can be determined by studying adverse effects in biota present in the vicinity ofthe platforms, or in biota that are exposed under experimental conditions (i.e. experimentswith caged organisms or with boxcosms). The effects may be either structural or functional,and may occur in individuals, populations and at the level of the ecosystem. In the case ofcontaminated sediments adjacent to present and former platforms on the NCS, analyses ofthe present macro-zoobenthic fauna have been conducted at many stations around differentplatforms, in relation to the distance of the sediments from the drilling well (i.e. Groenewoud1991; Daan et al. 1996).

Accumulation of oil and effects on organisms, populations and the structure of the benthiccommunity need to be assessed in comparison to the reference condition that existed beforethe start of drill cuttings discharges (base line surveys). If no data is available on this,present-day reference populations and communities can be employed, taking due account ofsuch environmental boundary conditions as depth, grain size and current.

5.2.E.1. Population density

The ENW and WSP clearly state that discharges of contaminants must pose no more than anegligible risk to the marine environment, meaning that no reduction in population densitybeyond natural variation will be accepted. This makes population density an importantassessment parameter. Data on this are collected in sediments contaminated with OBM onthe NCS.

Extensive research has been conducted in the vicinity of oil drilling operations and productionplatforms to find the most suitable benthic species to monitor effects of oil contaminationfrom OBM drill cuttings. Echinocardium has eventually been selected on the grounds that it isa common species in the NCS area and extremely sensitive to oil contamination (i.e. Daan etal., 1996). In view of the fact that the WSP states that even the most sensitive species in theNorth sea system should be protected from the effects of pollution, and following thereasoning adopted by Aldenberg and Slob to deduce ecotoxicological risk levels andmanagement standards (Aldenberg and Slob, 1993), a reduction of 0-1% in the density of theEchinocardium population compared with that of a reference population can be regarded as azero effect and a reduction of 1-5% as acceptable. A reduction of 5-50% of the population isregarded as unacceptable and requiring measures to reduce pollution at source. A reductionof more than 50% means that the effect is so severe that immediate action is required.

5.2.E.2. Biodiversity and similarity

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Indices used to quantify structural community complexity describe the complexity of theecosystem in terms of percentages. Biodiversity and similarity are assessment parametersof equal importance to population density. Significant adverse effects caused by dischargesof OBM drill cuttings from offshore platforms to the structure of the benthic fauna aretherefore regarded as damage, especially in the environmental zone (WSP, 1993). However,as with changes in population density, it is important to distinguish contamination effects fromnatural fluctuations and care should be taken to identify reliable reference conditions (Olsgardand Gray, 1995).

No conclusion has yet been reached concerning the best indices of effects on the ecosystemand the most appropriate analytical techniques for the detection of shifts in communitystructure linked to specific stressors (i.e. Bloom, 1981; Boyle et al., 1990 and Daan et al.,1994). Indices need to be sensitive, stable and consistent with different input conditions(Boyle, 1990). In general, similarity indices are considered to be more appropriate thandiversity indices, since the latter may produce very misleading biological interpretations of theinitial structure of the community and the manner in which it has changed (Bloom, 1981;Boyle et al., 1990, and Heimbach and Ratte, 1996). However, Warwick's recently presenteddiversity index of taxonomic distinctness may prove a highly sensitive univariate measure ofchanging community structure (Warwick and Clarke, 1995). As the outcome may depend onthe methodology employed, it is advisable to use two different tools before drawing any firmconclusion on the pattern of the data analysed. If discharges of OBM drill cuttings are havinga critical effect on the ecosystem of the seabed in the NCS area, the results of the similarityindex should confirm those of Warwick's diversity index.

According to Dutch risk evaluation, following the reasoning adopted by Aldenberg and Slob todeduce ecotoxicological risk levels and management standards (Aldenberg and Slob, 1993),a 0-1% change in species richness compared with reference communities can be regardedas a zero effect, and a change of 1-5% as acceptable. A change of 5-50% in communitystructure is regarded as unacceptable and requiring measures to reduce pollution at source.A change of more than 50% in the species richness of a contaminated sediment means thatthe effect is so severe that immediate action must be taken.

5.2.F. Seabed functions

Under the area-specific water management policy, stricter environmental criteria can beestablished for the environmental zone (WSP, 1993). The North Sea serves many differentpurposes, including transport, fisheries, nature, recreation and offshore mining (RWS, 1997).The current function of oil and gas exploitation can be regarded as temporary. After theabandonment of the wells, the seabed is expected to regain its previous functions (VROM,1991). The main functions of the North Sea are nature and fisheries, both of which require anunpolluted seabed (RWS, 1997). Closure of fisheries around former OBM dumping sites inorder to prevent redistribution of residues would represent a significant reduction in potentialfunctions.

5.3. (Eco)system recovery

One of the main goals of North Sea environmental policy is to protect marine life fromcontamination (NAP, 1990). The damaging effects of contamination can be regarded as

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ended once the polluted system has returned to its original pre-pollution state. Since therecovery of a system from pollution damage is a process which takes place over time,samples from two or more points in time need to be compared in order to measure(eco)system recovery. Results of investigations in the UK and Norwegian sectors showsome recovery in the most heavily contaminated core areas of drilling sites, but the progressobserved was counteracted by the greater spread of adverse effects at the circumference ofthe contaminated area (Olsgard and Gray, 1995). In the NCS area, sampling has taken placeat three stations at a series of points in time (Daan and Mulder, 1996).

The speed of (eco)system recovery can be regarded as a factor helping to determine thesignificance of the expected effects of pollution from OBM drill cuttings residues. Forexample, contamination of a particular sediment may be serious, but a fast recovery rate willmean a rapid restoration of the ecosystem. In the case of OBM drill cuttings, action requiredto reduce pollution at source is not expected to be taken before the next environmentalcompany plan period (2002-2007) agreed in the covenant between the Dutch authorities andthe oil and gas industries (Anonymous, 1995). If the observed recovery rate is fast enough toproduce in the interim an autonomous (eco)system recovery of more than 50% incomparison to the reference situation, the effectiveness and significance of proposed actionswill be seen as less decisive. If recovery is expected to take much longer, the effects on theNorth Sea environment will be regarded as more serious.

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6. Conclusions

To assess the risk to human health and the marine environment posed by oil contaminationfrom present OBM drill cuttings residues in the NCS area, specific parameters and criteriacan be defined using a TRIADE-like approach. The TRIADE-like approach used herecomprises data from chemical analysis, toxicity data and data on the structural effects ofOBM contamination. Together with criteria used in current Dutch policies on environmentaland water management, these groups of data will provide a comprehensive picture of thedegree of contamination and ecological risks posed by contamination of the North Sea floorfrom OBM drill cuttings.

6.1 Sediment

The criteria discussed in section 5 can be used to assess the degree of pollution fromdischarges of OBM cuttings on the NCS seabed. Some criteria are regarded as moreappropriate or important than others. For this reason, a hierarchy of criteria has been drawnup. In accordance with general Dutch environmental policy (VROM, 1994), the actual riskposed to the environment is regarded as decisive in estimating the consequences of OBMpollution. The actual risk of pollution can be assessed by studying ecological effects, effectson organism physiology and morphology, and microbial activity following contamination bydischarges of OBM drill cuttings. In addition, the potential risk of OBM cuttings pollution canbe assessed on the basis of criteria relating to oil concentrations in biota and sediments.

Table 3 lists for all parameters the criteria and objectives concerning sediment pollution in thevicinity of OBM cuttings discharges, together with the zero effect, acceptable effect andunacceptable effect levels. The table also shows when each policy objective is due to beachieved. Some are already in operation, while others are set for the future (VROM, 1994).

In line with Dutch policy practice (VROM, 1994), mineral oil concentrations in sedimentsbelow the VR value are assumed to cause a zero effect, while between the VR and MTRvalues the effects are regarded as significant but acceptable. Between the MTR and ERlevels, the risk is regarded as unacceptably high and requiring measures to reduce pollutionat source. Above the ER value, the effects are deemed unacceptable and requiringimmediate action to study the urgency of sediment remediation.

Criteria for oil concentrations in sediments are relevant both to water management and totoxicological risk levels (see 4.2.1. and 4.2.2.). In theory, water management criteria arebased on toxicological risk levels, but these levels have yet to be established for mineral oil.Table 3 reveals a gap between the current water management criteria and the indicativetoxicological risk values for weathered oil from pilot experiments conducted by TNO-MEP(Scholten et al., 1997). In the NW4, the limit value is replaced by the MTR of the sameamount, but with the proviso that the value can be redefined if toxicological data becomeavailable (NW4, 1998).

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Table 3. Zero effect, acceptable effect and unacceptable effect data for assessment parameters andcriteria concerning sediments contaminated by oil from OBM drill cuttings discharges andthe time at which they come into effect according to statements in WSP, 1993; VROM,1994; and NW4, 1998. Mineral oil content is expressed as mg/kg wet weight for biota andmg/kg dry weight for sediment

Parameter criterion no-effect acceptable

effect

notacceptable,

sourcereduction

notacceptable,immediate

action

Effective

in

Actual risk

Ecosystemeffect

∆ Populationdensity

0-1% 1-5% 5-50% >50% currently

∆Biodiversity/ similarity

0-1% 1-5% 5-50% >50% currently

∆ Physiology e.g. imposex 0-1% 1-5% 5-50% >50% currently

∆ Morphology e.g. ventralspines

0-1% 1-5% 5-50% >50% currently

Microbialactivity

∆ turn-overrate

0-1% 1-5% 5-50% >50% --

Potential risk (oil content)

organism Lethal bodyburden

>1000 -

sedimentmanagement

Naturalbackground

<1-10 long term

Target value <50 long term

Limit value/MTR1

<1000 short term

Interventionvalue

>5000 currently

toxicologicalrisk(sediment)

indicative-VR <1-10 long term

indicative-MTR

<10-100 short term

indicative-ER >100-1000 currently

1 In the preliminary NW4, the limit value is replaced by the MTR, see text for details

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6.2 Dumping site

It is also possible to consider the OBM drill cuttings pile and the surrounding sedimentsseparately, regarding the pile as a dump of waste substances surrounded by sedimentswhich may be polluted by the dump. In that case, the surrounding sediment can be studiedas described above, but the pile itself will be regarded as a (secondary) source of oilpollution. A risk evaluation of environmental effects in the pile itself is then inappropriate, sinceno environmental processes will be expected to be present in the source of the emissionsitself.

The significance of the environmental risk posed by OBM drill cuttings piles and oil residuesdepends on their location and the type of oil discharged. Piles and polluted sediments locatedin erosion areas are more susceptible to resuspension than those in sedimentation zones.The ecosystem in the environmental zone is regarded as more vulnerable to pollutiondamage than that elsewhere in the North sea, and diesel oil is more dangerous than aromaticfree oils. Table 4 lists parameters and criteria which can be used to assess the significanceof possible environmental effects of OBM cuttings pollution. Unlike the criteria listed in Tables2 and 3, those in Table 4 are not expressed numerically, but rather as indications of theconsequences of specific situations.

Table 4. Parameters and criteria which can be used to assess the significance of possible effects ofOBM cuttings piles residues on the environment

Parameter Criterion Significance

Location resuspension sedimentation < transition < erosion

vulnerability North Sea < environmental zone

Type of oil aromatic free < diesel

Oil buried insediment

depth deep < surface

(Eco)systemrecovery

50% recovery before 2007< within one generation<long term

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Acknowledgements

This study was conducted on behalf of the North Sea Department of the Netherlands Ministryof Transport, Public Works and Water Management (RWS, North Sea Department), theNetherlands Ministry of Economic Affairs (EZ), and the Netherlands Oil and Gas Explorationand Production Association (NOGEPA). The project was coordinated by the ‘MonitoringOffshore Installations; M4’ working group, the membership of which was as follows:

Dr. T. van Brummelen (RWS, North Sea Directorate); chairmanDr. K.W. Mess (NOGEPA); secretary till October 1998Ing. C.W. van Oosterom (NOGEPA); secretary since October 1998Ir. R. van de Heuvel (RWS, North Sea Directorate)Dr. S. de Bie (NOGEPA)Ing. C.J. Dix (NOGEPA)Drs. J. Nijdam (EZ)Ir. L. Henriquez (EZ, State Supervision of Mines)Dr. K. Meijer (VROM)Dr. D. Vethaak (RWS, RIKZ)Ir. H. van het Groenewoud (TNO)Dr. R. Daan (NIOZ)Ir. M. Mulder (NIOZ)Dr. J.W. Dulfer (RWS, RIKZ)

33

7. References

Aldenberg, T., and W. Slob (1993). Confidence limits for hazardous concentrations based on logisticallydistributed NOEC toxicity data. Ecotox. Environ. Saf. 25, 48-63.

Anonymous (1995). Intentieverklaring uitvoering milieubeleid olie- en gaswinningsindustrie. ConvenantNOGEPA, Min. EZ, Min. VROM, MIN. VW, Den Haag, juni 1995.

Asjes, J., K.M.H. Schrader en R.G.O. Orth (1995). Emissies van olie in oliehoudend boorgruis op hetNederlands Continentaal Plat 1981-1992. Rapport Nr. NZ-95.10. Rijkswaterstaat Directie Noordzee,Rijswijk

Bedborough, D.R., R.A.A. Blackman and R.J. Law (1987). A survey of inputs to the North Sea resultingfrom oil and gas developments. Phil. Trans. R. Soc. Lond. B 316, p. 495-509.

Bloom, S.A. (1981). Similarity indices in community studies: Potential pitfalls. Mar. Ecol. Prog. Ser., 5:125-128.

Boyle, T.P., G.M. Smillie, J.C. Anderson and D.R. Beeson (1990). A sensitivity analysis of nine diversityand seven similarity indices. Res. J. Water Pollut. Control Fed., 62: 749-762.

Daan, R. and M. Mulder (1996). Long-term effects of OBM cuttings discharges at 12 locations on theDutch continental shelf. NIOZ-report 1996-6.

Daan, R., M. Mulder and A. van Leeuwen (1994). Differential sensitivity of Macrozoobenthic species todischarges of oil-contaminated drill cuttings in the North Sea. Neth. J. Sea Res. 33: 113-127.

Davies, J.M. and P.F. Kingston. Sources of environmental disturbance associated with offshore oil andgas developments. In: North sea oil and the environment: developing oil and gas resources,environmental impacts, and responses, W.J. Cairns ed. Elsevier Science Publishers LTD: London,England, 1992; pp. 417-440.

Denneman, C.A.J. (1998). Aanpassing van bodemnormen? Biobeschikbaarheid als onderdeel van eennorm of als flexibel element in een risicobeoordeling? Bodem (8) 1:23-25.

ENW (1994). Ministry of Transport, Public Works and Water Management. Evaluation Report Water.Tweede Kamer 1993-94. 21 250. nrs. 27-28. The Hague, The Netherlands, 1994.

Evers, E.H.G., J.W. Dulfer et al. (1997) Oil and oil constituents. Aquatic Outlook Project 1997. Ananalysis of problems associated with oil in the aquatic environment. Ministry of Transport, PublicWorks and Water Management. The Hague, The Netherlands. Report RIKZ-97.032/RIZA Report97.046.

EZ (1990). Milieu-effectrapport. Lozing oliehoudende mengsels vanaf mijnbouwinstallaties op zee.Ministry of Economic Affairs, The Hague, The Netherlands.

EZ (1991). Oil and gas in the Netherlands exploration and production 1990. Ministry of Economic Affairs,The Hague, The Netherlands.

Farrington, J.W., A.C. Davis, N.M. Frew & K.S. Rabin (1982): No. 2 fuel oil compounds in Mytilus edulis.Retention and release after an oil spill. Mar. Biol. 66:15-26.

Foekema, E.M., H.P.M. Schobben, J. Marquenie & M.C.Th. Scholten (1996): A comparison betweenthe toxicity of produced oil and gas condensate using a new sediment bioassay for deposited oil. In:Proc. 1995 International Seminar on Produced Water. Trondheim.

Fossato, V.U. & W.J. Canzonier (1976): Hydrocarbon uptake and loss by the mussel Mytilus edulis.Mar. Biol. 36:243-250.

Groenewoud, H. van het (1991). Monitoring offshore installations on the Dutch continental shelf. TNO-report R 90/380.

Groenewoud, H. van het & M.C.Th. Scholten (1992): The assessment of a no-effect concentration of oilin thermally treated OBM drill cuttings for marine benthos: Sediment analysis and bioaccumulation.TNO report R92/147.

Groenewoud, H. van het (1996). Inventory of long-term effects of discharges of oil based drilling muds(OBM) on the Dutch Continental Shelf (1994-1995). TNO-MEP 96/259.

34

Heimbach, F. and H.T. Ratte (1997). Use of a similarity index to compare the species composition ofaquatic communities in order to investigate ecosystem recovery. Seventh Annual Meeting ofSETAC-Europe, Amsterdam The Netherlands 1997, Abstractbook: pp 96.

Krebs, C.T. & K.A. Burns (1977): Long-term effects of an oil spill on populations of the salt-marsh crabUca pugnax. Science 197:484-487.

Kuiper, J., H. van het Groenewoud & G. Hoornsman (1983): A study of marine oil pollution in outdoormodel ecosystems representing a tidal flat (OPEX) - final report - TNO report R83/014.

Kuiper, J., H. van het Groenewoud, G. Hoornsman & Marijke van der Meer (1986): The fate of oil and/ordispersant on the development of plankton, phytobenthos and physico chemical parameters duringOPEX 1984 - 1986. TNO report R86/182e.

Lee, R.F., R. Sauerheber & A.A. Benson (1972): Petroleum hydrocarbons: Uptake and discharge by themarine mussel Mytilus edulis. Science 177:344-346.

Lie, H.N. & J.N. Langfeldt (1998): A spreading model for drilling mud and cuttings on the sea-floor.Second International Marine Environmental Modelling Seminar ’98, Lillehammer, Norway 1998.

McCarthy, L.S., D. Mackay, A.D. Smith, G.W. Ozbum & D.G. Dixon (1992): Residue-basedinterpretation of toxicity and bioconcentration QSARs from aquatic bioassays: neutral narcoticorganics. Environ. Tox. Chem. 11: 917-930.

Neff, J.M., B.A. Cox, D. Dixit & J.W. Anderson (1976): Accumulation and release of petroleum-derivedaromatic hydrocarbons by four species of marine animals. Mar. Biol. 38:279-289.

NW3 (1989) Ministry of Transport, Public Works and Water Management. Third National PolicyDocument on Water Management. Water in the Netherlands: a time for action. Tweede Kamer,session 1988-1989, 21 250, no. 1-2, The Hague, The Netherlands, 1989.

NW4 (1998) Ministry of Transport, Public Works and Water Management. Fourth National PolicyDocument on Water Management. The Hague, The Netherlands

Ogata, M., K. Fuijsawa, Y. Ogino & E. Mano (1984): Partition coefficients as a measure ofbioconcentration potential of crude oil compounds in fish and shellfish. Bull. Environ. Contam.Toxicol. 33:561-567

Olsgard, F. and J.S. Gray (1995). A comprehensive analysis of the effects of offshore oil and gasexploration and production on the benthic communities of the Norwegian continental shelf. Mar.Ecol. Prog. Ser., 122: 277-306.

Pinto, J.M., W.H. Pearson & J.W. Anderson (1984): Sediment preferences and oil contamination in thePacific sand-lance Ammodytes hexapterus. Mar. Biol. 83:193-204.

Pruell, P.J. (1986): Polycyclic aromatic compounds in marine sediments and organisms.ACS Symp. Ser. 192:ENVR87.

QSR (1993). North Sea Quality Status Report 1993. North Sea task Force OSPARCOM, ICES, London.Schobben, H.P.M. & M.C.Th. Scholten (1993): M&M simulation system. Biological effect module.

Report TNO Den Helder (NL) & Oceanor A/S (N). TNO report DH93/018.Scholten, M.C.Th., T. Bowmer & H. van het Groenewoud (1987): Effects of a light crude oil and of two

selected oil combat methods in experimental tidal flat ecosystems. Final report Oil PollutionExperiments 1986. TNO report R87/348.NEPP (1989). National Environmental Policy Plan. Ministryof Housing, Spatial Planning and Environment VROM, Den Haag.

Scholten, M.C.Th., H.P.M. Schobben et al. (1993). De berekening van het Maximaal Toelaatbaar Risiconiveau van olie en oliecomponenten in water en sediment. TNO-report IMW-R 93/187.

Scholten, M.C.Th., S. Huwer, E.M. Foekema, H.P. van Dokkum, C.C. Karman, R.J.B. Peters (1997).Slurp*Olie: Pilot study on the dose-effect responses of petroleum hydrocarbons in sediments. TNOreport TNO-MEP-R 97/420.

Smaal, A.C. P. van den Hurk, and F. Kerkum (1994). Karakterisering en beoordeling vanzoutwaterbodems met behulp van de TRIADE benadering. Ministry of Transport, Public Works andWater Management. The Hague, The Netherlands. Report RIKZ-94.051.

Staatscourant. Regulations on chemicals requiring special attention under the Chemical SubstancesAct, no. 138. 1992.

35

Staatscourant. Bepaling saneringstijdstip voor gevallen van ernstige verontreiniging waarvoor saneringurgent is. Nummer 47, 1997.

Stortelder, P.B.M., M.A. van der Gaag and L.A. van der Kooij (1989). Kansen voor waterorganismen:Een ecotoxicologische onderbouwing voor kwaliteitsdoelstellingen voor water en waterbodem. Deel1: Resultaten en berekeningen. Ministry of Transport, Public Works and Water Management. TheHague, The Netherlands. RIZA Report 89.016a.

Teal, J.M., K. Burns & J. Farrington (1978): Analyses of aromatic hydrocarbons in intertidal sedimentsresulting from two spills of no. 2 fuel oil in Buzzards Bay. Massachusetts. J. Fish. Res. Bd. Can.35:510-520.

Tweede Kamer (1988). ‘List of priority substances and integrated criteria documents’, EnvironmentalProgress report 1989-1992, session 1988-1989, 20 803, nos. 1-2, page 32.

Tweede Kamer (1989). Omgaan met Risico’s, sessie 1988-1989, nr 21 137, nr 5.Tweede Kamer (1994) Memorandum Soil Clean-up Intervention Values. session 1994, 22 727, no. 5.UKOOA (1996). Review of drill cuttings piles. ALTRA Safety & Environment Limited. Job No: UKO.872,

Rev-1.UKOOA (1998). Key facts: Drilling, Muds, and Cuttings. UKOOA website: http://www.ukooa.co.uk.Vogt, N.B., N.B. Davids & C.E. Sjoegren (1988): Di- and triaromatic hydrocarbons in fish liver from the

North Sea: Multivariate and statistical analysis. Oil Chem. Pollut. 4:217-242.VROM (1990) Soil Protection Guidelines part II (6), Ministry of Housing, Spatial Planning, and

Environment, The Hague, The Netherlands.VROM (1994) Environmental quality objectives in the Netherlands: A review of environmental quality

objectives and their policy framework in the Netherlands. Ministry of Housing, Spatial Planning, andEnvironment, Samsom-Sijthoff grafische bedrijven B.V., Alphen a/d Rijn, The Netherlands, 1994.

Warwick, R.M. and K.R. Clark (1995). New ‘biodiversity’ measures reveal a decrease in taxonomicdistinctness with increasing stress. Mar. Ecol. Prog. Ser., 129: 301-305.

WSP (1993). Ministry of Transport, Public Works and Water Management: Watersysteemplan Noordzee1991-1995. Ministerie van Verkeer & Waterstaat. Directie Noordzee, Rijswijk

WSV (1996). Ministry of Transport, Public Works and Water Management. Aquatic Outlook ProjectWater Systeem Verkenningen, WSV Rijkswaterstaat, Den Haag

Zevenboom, W., M. Robson, L. Massie and L.O. Reiersen (1992). Environmental effects of dischargesfrom the offshore oil and gas industry in the North Sea. PARCOM-SOP, Rijkswaterstaat North SeaDirectorate, Rijswijk.