Contaminant Leaching from Drill Cuttings Piles of the Northern and Central North Sea: a Review

Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

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Contaminant Leaching from Drill Cuttings Piles of the Northern and Central North Sea: A Review

E. Breuer1, J. A. Howe2, G. B. Shimmield 1, 2, D. Cummings1 & J. Carroll3

1. Centre for Coastal and Marine Science, Dunstaffnage Marine Laboratory, PO Box 3, Oban, Argyll, PA34 4AD, Scotland, United Kingdom. 2. Scottish Association for Marine Science, Dunstaffnage Marine Laboratory, PO Box 3, Oban, Argyll, PA34 4AD, Scotland, United Kingdom. 3. Akvaplan-niva AS , The Polar Environmental Centre, Hjalmar Johansensgt 14, 9296 Tromso, Norway. Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

The Brae platform, approximately 120 miles NE of Peterhead, northern North Sea (Courtesy of Marathon Oil Ltd.)

ii Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

Contents Page Glossary of Terms v

Executive Summary 1 1 Introduction & Overview 2 1.1 Scope of Work 2 1.2 Objectives of Study 2 1.3 Regional Setting 3 1.3.1 Physiography 3 1.3.2 Physical Oceanography 6 1.3.3 Geological history & structure 8 1.3.4 Seabed sediments and processes 11 2. History of Cuttings in the central and northern North Sea 15 2.1 Origin of Cuttings 15 2.2 Physio-chemical description of Cuttings Piles 16 2.3 Composition of Cuttings 18 2.3.1 Drill mud types 19 2.3.2 Metals 20 2.3.3 Hydrocarbons 21 2.3.4 Specialty chemicals 21 2.3.5 Radioisotopes 23 2.3.6 Contaminants from similar environments 27 3. Status of Existing Cuttings 29 3.1 Biological Effects 29 3.2 Contaminant behavior 32 3.2.1 Hydrocarbons 32 3.2.2 Metal concentrations 33 3.2.3 Leaching 35 4. Conclusions 39 Acknowledgements 40 References 41

iii Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

List of Figures Page

Figure 1: UK continental shelf with the 17th round of offshore licensing. 4 Figure 2: Bathymetry of the north and central North Sea. 5 Figure 3: General oceanography of the north and central North Sea. 7 Figure 4: General pre-Quaternary geology of the north and central North Sea. 9 Figure 5: Seabed sediments of the north and central North Sea. 12 Figure 6: Erosion and deposition in the north and central North Sea. 14 Figure 7: Total UK and Norwegian wells drilled between 1963-1993. 17 Figure 8: Radioactive decay series for 238U and 232Th. 24 Figure 9: Relationship between faunal effect and distance from platform 30 Figure 10: Captured video image from ROV of cuttings pile. 31 Figure 11: Sulphide measured 100m from the NW Hutton platform. 36 Figure 12: The loss of hydrocarbons from surface sediment 38

List of Tables

Table 1: Total metal concentrations from central and northern North Sea. 21 Table 2: Chemical use and discharge data. 22 Table 3: Estimated benthic copper fluxes (from Pedersen, 1985) 28 Table 4: Diagenetic scheme for the remineralization of organic matter. 34

iv Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

Glossary of Terms

Alkanes – Hydrocarbons of general formula CnH2n+2, E.g. CH4 Anthropogenic – Of human origin. Benthic – The bottom of the sea. Benthic biomass – The total sum of living matter at the bottom of the sea. Benthic colonisation – The establishment of a living group on the bottom of the sea. Bentonite – A special assemblage of clay minerals, usually formed through weathering. Bioaccumulation – An accumulation of biological material. Biogeochemical – The distribution and movement of elements in both living and dead organisms and in relation to their geological environment. Bioturbation – The physical mixing of sediment by benthic organisms. Caledonides – A geologic period of mountain building (see Orogeny) during the middle Ordovician-middle Devonian. Produced during the closure of the Iapteus Ocean. Chemoautotrophic – The process by which an organism that derives energy from the oxidation of inorganic compounds. Clastic – Sediments built up of fragments of pre-existing but weathered rocks. Complexation - The ability of a contaminant to combine with another phase usually under aqueous conditions. Dalradian – The youngest division of the oldest period of geological time (the PreCambrian). Deglaciation – The stage of retreat from widespread glaciation. Depocentre- An area of gross sediment accumulation. Desorption – The release of an absorbed substance. Diagenesis – The processes of mineral and rock formation. Diamicts – Very poorly sorted sediments with grain sizes ranging from clay-pebble (and coarser). Fluviodeltaic – Sediments originating from both a mixture of rivers (fluvial) and deltas (deltaic). Glaciomarine – Sediments produced as a result of sea ice, either floating, grounded or as pack. Halokinesis – The movement (usually upward) of salt or evaporite deposits within a rock, as result of their lower density. Iapetus Ocean – Ancient ocean present between southern Scotland and northern England from the Cambrian to the Ordovician periods, the closure of which produced the Caledonian Orogeny. Infauna – Organisms living within the sediment, e.g. burrowing polychaete worms. Kimmeridge Clay – The main hydrocarbon source rock in the North Sea. Jurassic age. Lacustrine – Sediments deposited in a lake. Lithologies – Rock or sediment type. Lithostratigraphy – The use of informal dating of a succession of rocks based on their type. Mid-North Sea-Rinkøbing-Fyn High – The major geological structural high extending across the central UK North Sea to the Danish / Norwegian basin sector. Munsell Chart – A chart of standard sediment and soil colours. Orogeny – A geologic term for the episode of mountain building. Outlier – A geologic term for an isolated area of younger rocks surrounded by older rocks.

v Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

Oxyhydroxides – Non-stoichiometric hydrated oxides, often associated with iron and manganese. Palaeozoic – A geologic era of time extending from the Cambrian – Permian (600 – 230 Ma). Peneplaned – The end product of erosion in humid climates. Physiography – The description of the seafloor. Porewater – The aqueous fluid between mineral or lithic grains in a rock or sediment. Redox – Oxidation and reduction reactions within the sediment. Remineralization – Secondary diagenetic mineral formation. Sulphides –Sulphur bearing minerals. Thermocline – A density boundary within the sea due to temperature differences. Viscosifiers –Additive to drilling mud used to increase viscosity.

vi Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

Executive Summary

This review concentrates on contaminant leaching from drill cuttings piles that have accumulated on the seabed of the northern and central North Sea (56°N-62°N) since the first exploratory wells were drilled in 1961. Drill cuttings and drill cuttings piles refer to the accumulated mixture of drilling mud, fluids and solids, rock fragments, sediment and speciality chemicals resulting from the drilling of exploration, appraisal and production wells. Higher concentrations of certain metals (Cr, Cu, Ni, Pb, Zn and especially Ba) and hydrocarbons are observed in association with the cuttings piles above that seen in natural North Sea sediments.

Once on the seafloor microbial reactions begin to consume available oxidants and the resulting diagenesis may result in the flux of metals and contaminants into and out of (leaching) the cuttings piles. The fate of metal contaminants within the cuttings piles is influenced by: particle size, organic matter content, the type of benthic fauna and the local sedimentation rate along with biogeochemical pathways such as adsorption and desorption from oxyhydroxides of iron and manganese, adsorption into organic matter or the assimilation into the gut of benthic infauna). Hydrocarbons within the cuttings piles remain relatively unchanged with time as a result of the piles depleted oxygen, low ambient temperature, the type of drilling fluids used and the lack of significant bioturbation.

The noticeable effect of drill cuttings piles on nearby benthic communities (~1-2 km) has been well reported. The influence ranges from physical smothering, organic enrichment and chemical contamination (by hydrocarbons, heavy metals, speciality chemicals and sulphides).

Generally drill cuttings piles contain significant amounts of contaminants, the levels of which decrease rapidly from source. No drill cuttings pile is the same, each represents a unique combination of sediment signature, contaminants and benthic community and each is affected by the local hydrodynamic regime.

1 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

1. Introduction and Overview

1.1 Scope of Work

This literature review provides an overview of the current understanding of the behaviour of contaminants within drill cuttings piles. Information has been extracted and collated from all available published sources. These include archived data on the seabed held by the field operators and background oceanographic and chemical environment held at the British Oceanographic Data Centre (BODC) and the British Geological Survey (BGS). The review concentrates on metals, hydrocarbons, speciality chemicals and biota in the cuttings piles, sediments and pore waters. Other case histories pertaining to similar environments, such as mine tailings are also described.

1.2 Objectives of Study

The fate of North Sea drill cuttings piles resulting from the exploration and production of hydrocarbons is of concern to both the industry and to Non - Governmental Organisations (NGO's). Drill cuttings have been accumulating under and around offshore oil and gas installations in the North Sea since the first exploratory wells were drilled in 1961. De Groot (1996) estimates that there have been 7 million m3 of drill cuttings dumped onto the North Sea seabed between 1964 to1993. The environmental impact of drill cuttings on benthic communities has been well documented (Hartley and Ferbrache, 1983; Davies et al., 1984; Gray et al., 1990; Kroncke et al., 1992; Plante-Cuny et al., 1993; Daan et al., 1992,1994; Daan and Mulder, 1996). These studies have been largely devoted to describing the temporal effects of contaminants on benthic communities, or to the effects of distance from the point source in relation to cuttings deposition and volume (ERT 97/285, Gardline 1999, Daan and Mulder 1996, Steinhauer et al., 1994 and Mair, et al., 1987).

In contrast, the study of the leaching of contaminants from the cuttings has received less attention. This review aims to: · Evaluate the literature pertaining to contaminant leaching from drill cuttings piles.

2 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

· Assess the contaminants’ relationship to the drill cuttings, to the concentration in the piles, and any possible effects on the surrounding environment. · Provide an overview of geochemical parameters influencing drill cutttings mounds.

1.3 Regional Setting

The North Sea is a dynamic shelf sea with oceanic waters entering from the south (Strait of Dover) and from the north, and exiting via the Norwegian Trench (Laane et al., 1996). Within the North Sea (as in many other of the world’s hydrocarbon provinces), there exists a range of subsurface geological formations. The chemistry of the rocks, and in particular the fine-grained shales such as the Kimmeridge Clay (Jurassic age, and principal source rock in the North Sea) have a range of heavy metal and radioactivity contents. The metals are often incorporated as sulphides, whilst radionuclides of the natural uranium and thorium decay series, along with 40K, are enriched in the more organic-rich fractions of the sedimentary column. The chemistry of the reservoir and cap rocks are also related to the chemistry of the formation fluids (or “brines”). Different provinces within the North Sea have markedly different reservoir brine chemistries (Helgeson et al., 1993). Production of the field therefore requires drilling of the sedimentary column and reservoir rock (and sometimes source rock) being mixed with the drilling mud and accumulating on the seafloor during drilling.

1.3.1 Physiography

The southern North Sea has little cuttings accumulation due to shallow water depth and increased sediment redistribution. This report is concerned with the northern and central North Sea, encompassing an area of between 56°N and about 62°N where active oil and gas extraction is taking place within the United Kingdom (UK) sector (Figure 1). The general physiography of this large (>100,000 km2) area is complex, extending from the relatively smooth coastline of the eastern UK mainland to the fjordic inlets and islands of Orkney and east of the Shetland Isles. Generally the bathymetry of the central and northern North Sea is smooth with depths averaging no more than 100-120 m deep (Figure 2). There are, however, a number of depressions, such as the Devil’s Hole and the Witch Ground Basin, where depths can exceed 150 m (Johnson et al., 1993). The largest

3 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

4 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

5 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles. and deepest feature in the northern North Sea is the Southern Trench, a 70 km long, 300 m deep, E-W depression on the southern side of the Moray Firth. This deep is predominantly fault-related, running E-W to ENE-WSW, downthrown to the north and may form the southern boundary of the Moray Firth Mesozoic Basin (Chesher and Lawson, 1983). As well as the depressions, also present across the region are highs or banks such as Pobie Bank, east of Shetland, linear features extending for tens of kilometres with minimum depths of 40 m or less. These features are thought to be the surface expression of underlying resistant basement rocks (Pantin, 1991). The largest high present in the northern North Sea is the Tampen Ridge, which trends NW-SE for a distance of 70 km, and is the result of an outlier of Quaternary lateral moraine. The central North Sea is dominated by the bathymetric highs of the Aberdeen and Marr Banks, and to the east, the deeps of the Devil’s Hole area, which are thought to be the result of glacial scouring (Wingfield, 1990).

1.3.2 Physical Oceanography

The waters of the northern and central North Sea are dominated by an extension of the northeasterly flowing North Atlantic Current (NAC) or North Atlantic Surface Water (Figure 3). This is an oceanic, relatively warm and saline body of water extending from the surface to bathe the outer shelf and upper slope. The NAC flows north of the Shetland Isles before passing southwards along the Norwegian Trench to enter the North Sea, east of the Shetland Isles where it is termed the Dooley Current. A smaller body of the NAC flows southwest of the Shetland Isles to enter the northern North Sea between Fair Isle and the Orkney Isles, termed the Fair Isles Current (FIC). This flow is an admixture of coastal and oceanic Atlantic water, once within the confines of the North Sea (Stevenson et al., 1995; North Sea Task Force, 1993; Reid et al., 1988, ICES, 1983).

The northern North Sea is well mixed vertically in winter by the effects of storms, but by spring a thermocline is established with divides an upper and lower layer. Thermal expansion of the upper (surface) layer produces less dense, stable surface waters. Seasonal cooling and increased storms destroys this layer in the autumn and mixes the upper and lower layers. Both the central and northern North Sea are characterised by the presence of thermal fronts, the boundaries between well-mixed and stratified water 6 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

7 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles. masses. These are particularly important in coastal regions as they restrict lateral dispersion of solutes and fines but have no affect on bedload transport. Figure 3 shows the distribution of fronts across the northern and central North Sea. The East Shetland Front and the Buchan Fronts, are areas of mixed water, whilst the region of the Dogger Bank is dominated by a complex pattern of fronts and upwelling (Stevenson et al., 1995).

The North Sea water outflow originates from the Skaggerak, with additions from the Baltic Sea and Norwegian coastal water, leaving the North Sea as the Norwegian Coastal Current.

8 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

1.3.3 Geological history and structure

The composition of cuttings (see Section 3, below) comprises material from the underlying formations drilled in the quest for hydrocarbons. Knowledge of the subsurface lithologies is therefore very important. An overview of north and central North Sea geology is presented below. Drill cuttings mounds will naturally be composed of material derived from the underlying lithologies peculiar to individual well sites. This is not discussed here. The pre-Quaternary geology of the central and northern North Sea is summarised in Figure 4.

The oldest rocks in the region are of Dalradian age (approximately 550 My BP) and occur in the northern North Sea, north of the Highland Boundary Fault. The Shetland Isles represent exposed deformed and metamorphosed sediments of Lower Palaeozoic age, altered during the Caledonian Orogeny as the Iapetus Ocean suture closed during the Silurian. The Caledonian Fold Belt is formed of this metamorphic basement linking Norway and mainland Scotland, across the northern North Sea. Cocks and Fortey, (1982) suggest that another metamorphic belt, the German-Polish Caledonides, lie under the Central Graben and represent the closure of the Tornquist Ocean during the late Ordovician.

The Devonian produced varied sedimentation across the region. In the northern North Sea, thick sequences of Early to Mid-Devonian-age sediments are displayed on Fair Isle. These are thought to represent the erosion of the Caledonian mountain chain. The Midland Valley of Scotland was a major depocentre throughout Mid-Silurian to Early Devonian times, although a high produced by bouyant Caledonide granites, may have existed in the south central North Sea (Zeigler, 1982). In the Moray Firth region,

9 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles. extensive Mid-Devonian lacustrine sediments and marine limestones were deposited during the first phase of post-Caledonian rifting. Clastic terrestrial redbed sedimentation occurred in the central North Sea during the Late Devonian, continuing into the Early Carboniferous.

During the Carboniferous the Shetland Platform may have been a source of clastic sedimentation with the deposition of coal-measure deltas to the south. Early Carboniferous marine transgressions, as a result of regional crustal extension, resulted in fluviodeltaic and shallow-marine deposition in the offshore extensions of the Midland Valley and Forth Approaches Basin and to the basins within the Mid North Sea High. Within these sediments, coal-bearing strata is abundant being found in offshore wells throughout the central North Sea (Gatliff et al., 1994).

The Early Permian saw the deposition of arid, terrestrial sediments, as part of the Northern Permian Basin, located to the north of the Mid-North Sea-Rinkøbing-Fyn High. Glennie (1986) suggested that the initial rifting phase of the Central Graben also occurred at this time. Marine transgression led to the formation of evaporites and carbonates. Subsequent sedimentation was affected by continued halokinesis from the Late Permian onwards. In the northern North Sea, localised marine Permian sediments were deposited in the southern region of the Viking Graben. Extensional stresses produced the fault- bounded basins as the North Sea grabens opened, leading to the concentration of Triassic continental deposition (Johnson et al., 1993).

Early Jurassic marine transgressions led to the deposition of mudstones and incursions of Mid-Jurassic deltas spreading from the south or southwest in the northern North Sea. Vulcanism became established in the Central Graben of the central North Sea, with fluviodeltaic sediments confined to the graben during the Mid-Jurassic (Gatliff et al., 1994). The Late Jurassic was dominated by episodes of major rifting of the Viking Graben and East Shetland Basin and the establishment of the main elements of the graben system throughout the North Sea. A major marine transgression (Kimmeridge Clay) led to the deposition of organic-rich shales, minor limestones and thin turbidites across the region. Alternating transgressions across the basin led to deposition of shales, often anaerobic, termed maximum flooding surfaces. 10 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

Cretaceous sedimentation was characterised by the deposition of thick marine mudstones and sands and variable calcareous muds associated with sea level rise and the subsidence of fault-bounded blocks at basin margins. The Viking Graben saw thick accumulations of sediments compared to the peneplaned East Shetland Platform in the northern North Sea (Johnson et al., 1993). Late Cretaceous sea level rise continued and led to the blanketing of chalk across the region.

The opening of the Atlantic Ocean during the Early Tertiary and resultant thermal-sag centred over the Viking Graben, led to the deposition of over 2000 m of sediments across the graben and over the East Shetland Platform. In the Central Graben, deep-water muds dominated, but the sands of deltaic systems extending from Orkney and the Scottish mainland, provided the main hydrocarbon reservoirs for what is now the Forties and Montrose fields (Johnson et al., 1993). Subsidence and therefore sedimentation diminished into the Late Tertiary, with deposition concentrated in the grabens.

Quaternary glaciation increased sediment supply across the North Sea, with Lower Pleistocene shallow-water sediments overlain by glaciomarine and glacial deposits. Local glacial erosion created deep seabed incisions and, with the deposition of moraines, seabed highs across the region. Sea level rise with deglaciation produced a drape of Holocene superficial sediments, including offshore carbonate banks.

The geological history and structure represents the situation underlying the platforms of the north and central North Sea. With regard to drill cuttings mounds the drilled geological sequences will be a substantial component and thus affect the permeability, compaction and porosity of the mound and therefore the subsequent leaching of contaminants. This situation may change from mound to mound in the north and central North Sea with changes in the local geological structure.

1.3.4 Seabed sediments and processes

Throughout the central and northern North Sea, sand is the predominant type of sediment encountered on the seabed, although coarser and finer sediments are also encountered (Figure 5). 11 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

Coarse gravely sand and sandy gravel occur in localised patches, although the main areas are: (i) 100 km east of the coast of Aberdeen, (ii) in the central outer Moray Firth, and near the coast of the north-western Moray Firth, (iii) extensive gravely patches across Dogger Bank and (iv) east of Orkney and Shetland, and around Fair Isle (Pantin, 1991). These areas are reworked, relict, or perhaps locally-derived glacial or glaciofluvial

12 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles. sediment. Around Fair Isle, strong tidal and wave action also aids in the winnowing of fines from the sediment.

Finer, muddy sediments are concentrated in seabed depressions such as the Southern Trench, Witch Ground Basin and the Fladen Ground. Areas of muddy sands also occur in: (i) on the southern side of Moray Firth, (ii) across an irregular zone 220 km east of Aberdeen, (iii) two areas 120 km east of Shetland and (iv) the Firth of Forth, where deposition is controlled by the outflow of the river, and sheltered from strong waves (Pantin, 1991) (Figure 6).

The processes of sediment transport in the North Sea are summarised below: (1) Advective movement of sediment due to tidal currents. (2) Advective transport of sediments due to wave action. (3) Advective transport of sediment from river outflow, wind drift and storm-surges. (4) Eddy-induced diffusive movement of suspended sediments via tides and waves. (5) The diffusive movement of bed-load transport due to near bottom currents. Advection describes the lateral movement of sediment through the movement of water whilst diffusion is sediment transport along a concentration gradient, therefore can be governed by temperature, salinity or turbidity (Pantin, 1991).

In the North Sea tides and currents have the greatest influence on sediment movement. In some areas the frontal region between coastal and offshore waters is reduced during neap tides compared to springs. Therefore the greatest exchange of Suspended Particulate Matter (SPM) occurs during neap tides. During spring tides the SPM stays within coastal waters (Stevenson et al., 1995). During storms, most North Sea seabed surface sediments, with the exception of the very deepest regions, become resuspended and dispersed due to the action of waves and currents. Threshold fluid stress necessary to generate transport of cuttings is around 0.1-0.2 Nm-2 (ALTRA, 1996). North Sea tidal bed shear stress is an order of magnitude lower and therefore transport of cuttings in-situ is unlikely. Decadal- scale, powerful storms may produce wave stress, even in 130 m of water, strong enough to induce temporary erosion of cuttings piles, however the mounds are, on average, depositional.

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14 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

2. History of Cuttings in the central and northern North Sea

Previous reviews on cuttings piles in the North and Central Sea (e.g., CORDAH, 1998; ALTRA, 1996) have compiled pre-existing data on cuttings piles. This review will only highlight those areas concerning contaminant leaching. More detailed information on cuttings history can be found in earlier reviews.

2.1 Origin of Cuttings

“Drill cuttings” is a general term used to describe the resulting mixture of drilling mud fluids and solids, rock fragments, sediment, and speciality chemicals from the drilling of exploration, appraisal and production wells. Cuttings are brought up to the platform, separated from the drilling mud, and then discharged to the seabed where "…up to 75 % of the material discharged from an installation may accumulate in the pile; the remaining material is more widely dispersed" (Altra, 1996). Discharge practices (depth of discharge and rate), the type of mud used when drilling, current regime and platform construction, are all-important in determining the development of a cuttings pile (ERT, 1994; ALTRA, 1996). In areas of shallow water (<50 m) and strong currents (e.g., southern North Sea) the cuttings will rapidly disperse, allowing biodegradation of the remaining material. In depositional areas with relatively weak currents (e.g., in the basins of the central and northern North Sea) the cuttings may flocculate and accumulate under and around platforms. The shape and volume of cuttings piles around central and northern North Sea platforms (2-15 m in height with a predominant orientation aligned with the prevailing tidal flow) along with the results from previous work (Mobil, 1998) indicate a high dependence on cuttings deposition with current regime. Once deposited the cuttings become consolidated and more resistant to erosional processes. (ALTRA, 1996).

As noted by several reports (ALTRA, 1996; AURIS, 1993; CORDAH, 1998 and Nyguyen, 1998) the determination of the amount and volume of drill cuttings deposited on the North Sea is variable. These reports tend to concentrate on development wells and estimate 1,470,000 tons of cuttings deposited onto the seabed. However, if the number of exploration, appraisal and production wells for the UK sector from 1963-1993 (4783

15 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles. wells, de Groot, 1996) is used then the estimate increases to 4,783,000 tons based on the assumption of 1000 tons of cuttings per well as used by ALTRA (1996) (Figure 7).

2.2 Physio-chemical description of cuttings piles

Once the cuttings pile begins to accumulate on the seafloor, microbial reactions sustained by the organics in the drilling mud, decaying plankton detritus from the overlying water column, and to some extent, residual material in the rocks themselves, begin to consume available oxidants in the microbes quest for energy. A predictable sequence of oxidant utilisation, commencing with oxygen consumption, through nitrate to sulphate reduction can be observed. The depth of these reactions in the cuttings pile is related to the benthic infaunal activity (irrigation), accumulation rate of the cuttings, organic substrate content, and physical sedimentary factors, but will usually occur in the shallow subsurface within a few centimetres of the sediment-water interface with sharp concentration gradients. A particular consequence of the reactions is to fix heavy metals as relatively insoluble sulphides when free sulphide becomes available through the reduction of sulphate in the porewater water. The physio-chemical characteristics of cuttings piles vary from site to site. Generally the piles have 20-60 % water content, a bulk density of 1.6-2.3 g/cm-3, particle size ranging from 10 mm – 2 cm, and show a distinct stratification and maintain high stability (ALTRA, 1996; CORDAH, 1998). The morphology of cuttings is influenced by the drilling mud used which can affect the size of the particle and its ability or tendency to aggregate. Some of the first coring of drill cuttings piles in the North Sea was conducted by the British Geological Survey in 1992, for Amoco on the NW Hutton platform. Five cores from 0.88 m to 2.35 m in length were taken between 7 m and 30 m from the platform. Also obtained were 12 surface sediment samples from an aligned transect north of the platform, at nominal distances of 100-7500 m distance (Skinner, 1992). The cores recovered sequences of drill slurry of variable thickness from 0.06 m to over 0.60 m.

These drill slurries were found to be a coarse admixture of drill cuttings in a gelatinous matrix with an ‘oily’ feel and sheen. Units were either weakly (<0.01 m) banded or

16 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

17 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles. homogeneous in character with a faint ultra-violet luminescence. Shear strengths varied downcore from <5 kPa to over 10 kPa, where the underlying seabed was cored. The drill slurries varied in colour from grey (Munsell Chart colour 5Y5/1), brown and black to olive and olive yellow. All but the most proximal core end with a shelly sand horizon overlying diamicts, assumed to be the seabed. Particle size analysis was used to distinguish the drill cuttings (slurries) from the basal layer (the seabed sediments).

The total oil concentrations measured via Gas Liquid Chromatograph (GLC), from the cores varied from 78 to 74000 mg g-1. In all the cores the basal sediment was contaminated by weathered diesel, amounts of which progressively decreased towards the surface of the cuttings piles. The layer of diesel contamination is overlain by low- toxicity base oils, which become increasingly weathered towards the surface of the pile. This rough lithostratigraphy can be related to the early use of diesel-based mud followed by the intensive use of low toxicity base oils (AURIS, 1993).

Metal concentrations, measured using a twin acid extraction method, displayed a variable trend across the five cores. Generally concentrations of copper, iron, manganese, nickel, lead, and zinc were found to decrease with depth through the piles, whilst only nickel and iron were found to be correlated to depth. Cadmium was recorded from only one core with concentrations in the range of 5 to 6 µg g-1, while mercury was found in all cores in the range of <0.02 µg g-1 to 0.45 µg g-1. Recorded barium concentrations were 123µg g-1 to 277µg g-1 although the method used to determine the metal is thought to under- represent this value (AURIS, 1993).

The cores described above represent the pile conditions from one pile at the Northwest Hutton platform. It must be stressed that each cuttings pile has individual physio- chemical characteristics that are unique to each platform.

2.3 Composition of Cuttings

A typical cuttings pile will contain the insoluble portion of the drilling mud that coats the cuttings along with the rock fragments drilled from the strata overlying and combined with the target reservoir. Thus the cuttings pile will contain barite and bentonite from the 18 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles. drilling mud along with the fraction of drilling fluid not partitioned into the water column.

2.3.1 Drilling mud types

When a well is drilled, the waste cuttings are carried up the drill string, to the drilling rig/platform, by drilling mud. The function of the drilling mud is to lift formation cuttings to the surface, control subsurface pressures, lubricate drill strings, bottom hole cleaning and cooling, and maintenance and stability of uncased sections of the bore hole (Okpokwasili and Nnubia, 1995). Drilling muds are a suspension of solids (e.g., clays, barite, small cuttings, etc.) in liquids (i.e., water or oil) or in liquid emulsions, with chemical additives as required to modify its properties. Additives include viscosifiers, emulsifiers, lubricants, wetting agents, corrosion inhibitors, surfactants, detergents, caustic soda, salts and organic polymers (Hudgins, 1994). Synthetic drilling muds (SM); oil-based muds (OBM) and water-based muds (WBM) are used throughout the North Sea. Drilling mud content is strictly regulated. The use of lubrication is only allowed to help stop drilling difficulties, such as drill stem sticking. The lubricants used have ranged from diesel to synthetic oil.

· Water Based Mud These can be seawater muds, KCl/polymer muds, saturated saltwater muds, and lime muds. At the present time the discharge of WBM over the side is permitted (Hudgins, 1994).

· Synthetic Mud The lubricants in synthetic muds are 'synthesised' from products such as ethylene. They basically contain carbon, hydrogen and oxygen atoms in different configurations, selected for their low toxicity and ability to biodegrade. As of 31st December 2000 discharge of synthetic mud contaminated cuttings will be restricted in the UK with broader regional restrictions under consideration.

19 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

· Oil Based Muds These muds use base oil plus water or brine as an emulsion. (Hudgins, 1994). PARCOM Decision 92/2 called for a reduction in the discharge of contaminated cuttings from wells drilled with mineral OBM to below 1% oil-on-cuttings. Overboard disposal of OBM is greatly restricted in all sectors of the North Sea (Hudgins, 1994).

2.3.2 Metals

Metals entering the North Sea through both anthropogenic and natural processes including in situ, fluvial and aeolian pathways disperse with current speed and direction and may accumulate in depositional areas such as the deeper regions of the northern North Sea (see Figure 6). This can lead to differing spatial and temporal patterns on the background metal concentrations observed. A majority of the surface marine sediment within UK waters consists of sandy sediments with low organic matter content over which frequent flushing of bottom water occurs. These conditions create well-oxygenated sediments and positive redox potentials in the upper few centimetres of the sediments. Exceptions are estuaries, some near shore deposits or around oil platforms (Stevenson et al., 1995). Metal concentrations in the cuttings piles are from a combination of accumulation and migration from the natural sediment, from discharged barite and speciality chemicals in the drilling muds, the platform itself (e.g., paint chips, corrosion etc…) and from aeolian input. Elevated concentrations of metals (Cr, Cu, Ni, Pb and Zn), and especially Ba (from drilling mud barite, BaSO4) relative to the natural sediment have been recorded or can be expected for piles in the central and northern North Sea (Table 1). Positive correlation’s observed between metals such as barium and total oil in the sediment surrounding platforms in the North Sea support the suggestion that cuttings may be a primary source for the elevated levels found (Gardline 1996; Hartley, 1996). However, the differing sample collection and analytical methods used in obtaining metal concentrations from cuttings in the North Sea leads to a decreased confidence in present values in the literature and the ability to compare data (Hartley, 1996; Nguyen, 1998).

20 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

Table 1. Maximum total metal concentrations for the northern and central North Sea and from various cuttings piles in the North Sea.

Central and North Sea *Beryl "A" *1523a *1523/b **Beatrice **NWH ***Fulmar Sediments µg g-1 µg g-1 µg g-1 µg g-1 µg g-1 µg g-1 µg g-1 Ba 860 2,080 3,220 24,436 1,055 2,778 2,833 Cr 70 125.3 43 82.9 72 - 101 Cu 14 75.5 13 35.9 192 110 160 Ni 23 26.3 110 35.6 0.25 43 62 Pb 24 17.2 52 160 0.25 313 146 V 34 180.5 61 63.7 0.25 - 144 Zn 155 251.8 - 1,308 715 916 1,110 Data from: *Gardline, ** Auris, ***BGS.

2.3.3 Hydrocarbons

Davies et al., (1984), ALTRA (1996) and CORDAH (1998) have provided extensive reviews on hydrocarbon data for the North Sea. ALTRA (1996) estimate that 68,000 tons of oil has been discharged during the major developments in the central and northern North Sea. Elevated hydrocarbon concentrations, up to 10,000 times background, have been found in the sediment and cuttings surrounding oil production platforms in the North Sea. The primary source of these hydrocarbons is through the use of OBM and SM muds during drilling operations and the subsequent discharge, after washing, of these cuttings onto the seabed. Hydrocarbons found in relation to drilling operations are C15- nC22 alkanes (diesel), nC10-C27 alkanes (low tox) and UCM (unresolved complex mixture) with a concentration range of 4 µg g-1 to 30000 µg g-1 (ALTRA, 1996). As with the metal concentrations, the direct comparison of data sets for hydrocarbon concentrations are hindered by the many differing analytical methods used resulting in a range of background concentrations (Davies et al., 1984).

2.3.4 Speciality chemicals

Hudgins (1994) describes how the use and discharge of speciality chemicals in the North Sea is extensive and provides a detailed breakdown of the chemicals used. These can include viscosifiers, emulsifiers, lubricants, wetting agents, corrosion inhibitors, surfactants, detergents, caustic soda, salts and organic polymers. These chemicals are 21 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles. added to OBM and WBM to maintain the required chemical and physical properties (Hudgins, 1994). Table 2 gives a breakdown of the type and quantity of speciality chemicals used and discharged.

Table 2. Drilling chemical use and discharge data from 1989 (from Hudgins 1994). NR signifies no reported use or discharge. Chemicals used (tonnes y-1) Chemicals Discharged (tonnes yr-1) Chemical OBM WBM TOTAL OBM WBM TOTAL Weighting agents 1,941 12,497 14,438 1,130 12,070 13,200 Bentonitic agents 621 7,467 8,088 71 6,592 6,663 Alkaline chemicals 1,554 1,135 2,689 436 894 1,330 Salinity 2,169 9,193 11,362 1,161 8,206 9,367 Lost circulation 45 317 362 10 180 190 Lignosulfonates 220 267 487 154 218 372 Lignites 65 33 98 21 22 43 Polymers/viscosifier 490 1,152 1,642 202 1,062 1,264 Filtrate reducers 128 952 1,080 23 770 793 Gilsonite 212 4 216 22 3 25 Defoamers NR 143 143 104 104 Biocides NR 13 13 6 6 Corrosion inhibitors NR 30 30 12 12 Scale inhibitors NR 2 2 1 1 Drilling lubricants NR 63 63 62 62 Pipe-release agents 5 20 25 5 5 Dispersants NR NR NR Oxygen scavengers NR 7 7 5 5 Emulsifiers/detergent 1,766 178 1,944 429 126 555 Cuttings wash 1,028 0 1,028 110 0 110 Shale inhibitors NR 26 26 20 20 Wetting agents 34 NR 34 15 0 15 Thinning agents 7 90 97 3 67 70 Base oils 23,693 NR 23,693 5,690 5,690 TOTAL 33,978 33,589 67,567 9,477 30,425 39,902

22 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

2.3.5 Radioisotopes

Waste streams from oil and gas production and processing operations also contain naturally occurring radioactive materials (NORM) (Veil and Smith, 1999; Chambers et al., 1994). The radioactive material is naturally present in shales from which oil and gas are extracted. The long-lived radionuclides 238U and 232Th with time decay into other radionuclides, or daughter products (Figure 8). With respect to drill cuttings, important radionuclides in the 238U and 232Th decay series are 226Ra, 228Ra. Other radionuclides of concern include those that form from the decay of 226Ra and 228Ra, e.g. 210Pb (see Figure 8).

Radium is slightly soluble and is mobilised and transported in the brines associated with subsurface formations. During extraction, a number of chemical transformations can occur (Houghton et al., 1981) which result in dissolved radium remaining in solution in the drilling fluids or precipitating and becoming incorporated into the solid components of drill cuttings. Drill cuttings contain radium that coprecipitates with other alkaline earth elements, such as barium, strontium, and calcium forming metal sulphates (Veil and Smith, 1999). Conditions that affect radium solubility and precipitation during transport through the borehole include water chemistry (primarily salinity), temperature, and pressure.

The presence of radionuclides in both liquid and solid wastes generated during oil and gas production is well known (Veil and Smith, 1999; Chambers et al., 1994). Radionuclide concentrations and their impacts on the environment have been documented with respect to produced water discharges (Meinhold, 1996; Hart et al., 1995). Radionuclides contained in deposits on processing equipment, known as Low Specific Activity scale has also been documented (OLF, 1998). However, during the literature search for this project, no studies were located that specifically address the behaviour of radionuclides associated with deposits of drill cuttings on the seafloor.

Previous investigations of drill cuttings piles have focused mainly on oil hydrocarbons and speciality chemicals associated with cuttings discharges (for example, Bakke et al.,

23 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

24 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

1989a; 1989b; Grahlnielsen et al., 1989; Daan & Mulder, 1993; 1994; 1995). Although there is a lack of data on radionuclide levels in drill cuttings piles, the long-term fate of these materials will be considered based on available information (presented in previous sections of this report). This includes information on both physical and chemical changes through time, which can be expected to take place in drill cuttings piles. Both barium and radium behave similarly in the environment (Legeleux and Reyss, 1996, Carroll et al., 1995). Thus information on the long-term fate of barium in drill cuttings deposits provides another indication of the expected long-term chemical fate of radioisotopes of radium.

This conclusion is in agreement with studies of U-series radionuclides in natural deep-sea sediments (Colley and Thomson, 1990). Colley and Thomson (1990) observed that advective transport of radionuclides was absent and that radionuclides of uranium and thorium are essentially immobile. Comparing the diffusion coefficients of uranium and thorium (D < 5 x 10-13 cm2 s-1) to radium (D = 10-9 – 10-10 cm2 s-1), radionuclides of radium are only slightly mobile. Low diffusion coefficients for these radionuclides are maintained by the reducing nature of deep-sea sediments (Laul et al., 1981).

In addition to losses due to chemical transformations, radionuclides undergo radioactive decay. The concentrations of radionuclides present in drill cuttings piles will diminish through time as a result of the physical laws which govern radioactive decay (IAEA, 1996). These laws dictate that only about 3 % of the original radioactivity remains after five half-lives have passed since the time that the radionuclide was first separated from the parent radionuclide (in this case removal from the borehole). For radionuclides with long half-lives, e.g. 226Ra (half-life = 1600 yr.), radioactive decay will not result in significant losses over timescales of hundreds of years. For 228Ra (half-life = 5.75 yr.),

25 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles. five half-lives represents 30 years and for 210Pb (half-life = 22.3 yr.)) five half-lives represents approximately 100 years.

We can expect through time however, that the cuttings piles will be re-colonised by benthic organisms (Hartley and Watson, 1993; Bakke et al., 1989). Benthic organisms burrow into the seabed resulting in increasing downward fluxes of oxygen into sediments and increasing exchanges of porewater and sediments back to the sediment surface (Aller and Aller, 1998). On continental shelves where benthic biomass and bioturbation rates are high we can expect that as recovery of seabed infauna occurs with increasing time since deposition, the presence of organisms will result in a greater transfer of contaminated porewater and drill cuttings materials back to the sediment surface. However, benthic organisms operate primarily within the upper sediment layers (world- wide mean = 9.8±4.5 cm (Boudreau, 1998)) so that the tendency toward increases in radionuclide exchange rates with time will be moderated by the rate of sediment deposition at individual sites. The dynamic interplay of these three processes, benthic colonisation, oxygen penetration into sediments, and sedimentation rate thus governs the overall impact of NORM associated with individual drill cuttings piles. For example, consider two identical drill cuttings piles on the seabed. Location 1 is in a low sedimentation environment with high rates of benthic colonisation and high oxygen penetration and location 2 is in a high sedimentation environment with low rates of benthic colonisation and low oxygen penetration. These two scenarios represent the two ends of the spectrum of possible conditions associated with the drill cuttings piles. The interaction of organisms and the environment with NORM will be highest at location 1 and lowest at location 2.

Exposure of marine biota to radioactivity may occur through surface contact or ingestion and may result in either acute or chronic effects to organisms (Simmonds et al., 1995). In the case of drill cuttings piles, acute exposures of organisms to radioactivity are highly improbable. Additional information is needed to determine whether chronic exposure of organisms to NORM may potentially occur. Chronic exposure to radiation sources may result in mortality, sterility or decreased fertility for organisms - all of which are important to the reproductive success of a species (Simmonds et al., 1995). To obtain quantitative estimates of the impacts to biota from NORM associated with individual drill 26 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles. cuttings piles, information is needed on the following: a) radionuclide concentrations, b) benthic colonisation rates and composition, c) oxygen penetration and sediment turnover rates, d) sedimentation rate, and e) food web relationships. In the case of drill cuttings piles which contain a number of potentially hazardous chemicals, synergistic effects from multiple contaminants should also be considered.

Based on knowledge gained through previous investigations on seabed sources of marine radioactivity, direct exposure to radionuclides either contained in or released from drill cuttings piles is not likely to result in significant doses to humans (AMAP, 1998; ANWAP, 1997). Ingestion of significant quantities of seafood exposed to radioactive contaminants in drill cuttings is also not likely to occur. However, a quantitative estimate of the risk to humans from seafood ingestion can be derived from using the suite of information collected for the determination of impacts to biota (see previous paragraph).

2.3.6 Contaminants from similar environments

Metal chemistry in the porewaters of drill cuttings has received very little attention. To gain an understanding of the processes controlling early metal diagenesis in this unique environment and the ultimate fate of the metals within the piles (source or sink) it is necessary to study a range of constituents in the solution and solid phase. The magnitude of porewater concentration gradients can be used to calculate the flux of ions within the porewater and across the sediment-water inter-face (Sundby et al., 1986). Unfortunately, there is no published data available1 with respect to these parameters in drill cuttings piles, however extensive work has been done on mine tailings in Canada (Pedersen, 1984; Pedersen, 1985; Pedersen and Losher, 1988; Pedersen et al., 1993). These tailings have somewhat similar characteristics to cuttings in that they are: (1) deposited in aquatic environments, (2) accumulate into piles, (3) organic enrichment occurs, (4) benthic faunal disruptions are observed, (5) addition of speciality chemicals takes place during the drilling operation and, (6) high sulphide concentrations occur within the tailings piles. Conclusions from these studies are that no serious perturbations

1 An NERC-industry research programme, Managing Impacts in the Marine Environment (MIME), has produced some data on metal leaching rates which is in preparation for peer-review and publication. The Appendix contains abstracts of papers presented that have not been critically reviewed. 27 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles. occur to the host waters and there was little or no release of a number of metals from the tailings (see Table 3). However, upon disruption of the tailings piles, large concentrations of metals may be released into the water as a result of oxidation of metal sulphide complexes. Conceptual considerations, along with the limited published data seem to suggest that the metal concentrations associated with drill cuttings will be perpetually bound within the cuttings. However, until detailed fieldwork is performed on the necessary parameters this hypothesis remains to be validated.

Table 3. Estimated benthic copper fluxes in seawater from mine tailings (HOL 14, RUP 3 & RUP 1) and in natural sediment (Resurrection Bay) to overlying seawater in Holberg and Rupert inlets, British Columbia (from Pedersen, 1985)

d [Cu] dZ D in situ (10°C) Flux (J) -4 2 -1 -2 -1 Core or Location ?(0-2cms) (nmol cm ) (cm year ) (nmol cm year ) ______HOL 14 0.8 3.9x10-3 113 0.4 RUP 3 0.7 8.5x10-3 100 0.6 RUP 1 0.7 3.5x10-3 100 0.3 Resurrection Bay, Alaska (Heggie, 1983)a 0.8b 45x10-3 85c 3 ______

a Fluxes are calculated using Fick’s First Law; d[Cu]/dx is the gradient between the sediment-water -1 interface and the subsurface maximum in each core assuming [Cu] bottom water = 11 nmol L (as in HOL 14 supernatant water). The benthic flux for Resurrection Bay is estimated by the same method, using [Cu] -1 -1 bottom water = 10 nmol L and [Cu] 2.5cm depth = 122 nmol L . This calculated flux differs from Heggie’s estimate based on the copper balance in fjord water because it does not include rapid Cu remobilization from decomposing organic matter in the surficial ‘fluff’ layer. b Estimated c At 5°C

28 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

3. Status of Existing Cuttings

3.1 Biological Effects Research conducted on the Dutch sector of the continental shelf characterises different degrees of impact on the physical environment and biological resources for three types of depositional settings (Daan and Mulder, 1993,1994,1995). These settings are an erosional site, a transitional site, and a depositional site. Biological effects associated with drill cuttings deposited around platforms at these locations were still detectable 8 years after termination of the discharges of oil-based drilling muds. Specific long-term impacts from drill cuttings observed at the sites are:

· Decrease in time of the spatial extent of pollution around platforms and of associated biological effects · Clear signs of recovery observed at > 500 meters from platforms · High oil and barium contents in subsurface sediment layers (10-30 cm) <500 meters from the platform · Biological effects detected up to 250 m

According to Daan and Mulder (1995), differences in the concentration of barium over time can largely be explained by the patchy distribution of material in the close-in vicinity of the platform. In other words, losses through time of barium from the drill cuttings piles has thus far been negligible. We would expect therefore that radium initially present in drill cuttings deposits is also still present. The biological impacts of drill cuttings on the benthic community is well documented (Daan et al., 1990, 1991, 1992, 1994, 1996; Daan and Mulder, 1993, 1994, 1995; Davies, et al., 1984; Ferm, 1996; Kroncke et al., 1992; Olsgard et al, 1997; Plante-Cuny, et al., 1993). There is a consistent pattern to the faunal changes observed following a gradient of increased volume of cuttings. Logically, an increased volume of cuttings will produce a detrimental decrease in the diversity and volume of benthic communities. This pollution gradient is a result of the combined effects of physical smothering, organic enrichment, chemical contamination (by hydrocarbons, heavy metals and sulphide). Field studies in the North Sea have demonstrated that effects are principally confined to benthic

29 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles. communities within a 1-2 km radius of platform sites (Figure 9)(Addy et al., 1987; Dicks et al., 1987; Davies et al., 1984; Reiersen et al., 1989; Kingston 1987, 1992).

The sulphide - oxidising bacteria Beggiatoa spp., commonly grows on anoxic marine sediments (Gundersen et al., 1992). These bacteria obtain their energy for chemoautorophic growth by oxidation of sulphide through elemental sulphur to sulphate. Beggiatoa spp., are known to be associated with severely perturbed environments due to organic enrichment and sulphide release (Figure 10).

30 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

31 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

3.2 Contaminant behaviour

The term "contaminant" in this report is used to refer to the constituents measured in cuttings piles whose concentrations are above natural background concentration as a result of anthropogenic activities. The ability to measure an amount of contaminant in a drilling discharge, relative to the background concentration, is a reflection of variations in a number of certain parameters. These are: (a) the expected dilution of the drilling discharges in the environment, (b) the statistical power to detect changes based on the observed variability of each contaminant and (c) the chemical or physical transformation (e.g., solubility) of each contaminant during transport processes (Steinhauer et al., 1994).

3.2.1 Hydrocarbons

A large proportion of the oil and other hydrocarbons first deposited onto the seabed in North Sea are still unchanged with time (AURIS, 1993). The loss to the water column during settlement of cuttings, and further loss due to microbial and chemical degradation of hydrocarbons, have not resulted in a substantial decrease in quantities following discharge (AURIS, 1993). The quantity and integrity of hydrocarbons still remaining in the piles is a result of the piles’ depleted dissolved oxygen content, the type of drilling fluids used, the low temperatures encountered, and the resulting decimation of benthic communities present due to cuttings deposition (smothering) and composition. The balance between oxygen transport processes and the chemical reactions occurring within the cuttings piles determines the rate of degradation and ultimate fate of hydrocarbons in the piles. In marine sediments the dominant mass transport mechanisms are often large benthic animals that move particles and fluids during feeding, burrowing, tube construction, and irrigation (Aller, 1998). This important mechanism is missing from the piles; therefore oxygen transport is limited to diffusion or tidal advection and pumping. As a result oxygen is quickly consumed by the breakdown of organic matter. The rapid consumption of oxygen (see Table 4) in the piles means bio-degradation is much slower for the hydrocarbons present, so the cuttings still contain historic mud residues that have not been broken down thoroughly over time, as was initially thought. Laboratory experiments by Dow et al., (1990) have shown that sediments contaminated with a high percent of oil based cuttings become anaerobic and fail to support hydrocarbon 32 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles. degradation. In addition to the anoxic conditions slowing down degradation, Okpokwasili, and Nnubia, (1995) have shown that oil spill dispersants and drilling fluids affect the ability of marine bacteria to metabolise these substrates.

3.2.2 Metal concentrations

Differentiating between anthropogenic versus natural background concentrations for metals in seabed sediments can be difficult and lead to problems in an interpretation of the sources of the contamination. An assessment of the published data, followed by analysing subsurface (below bioturbation and anthropogenic influence) sediments and the measurement of similar type sediments known to be uncontaminated, are standard procedures to obtain natural background levels. Table 1 presents data on the recorded background concentration levels for metals in the North Sea. It is likely that the natural sediment will ultimately provide a sink for metals bound up in the cuttings pile. However there are numerous biogeochemical pathways that exist: (1) adsorption and desorption from the surface of particles, particularly oxyhydroxides of iron and manganese, (2) incorporation in diagenetic mineral phases (such as sulphides) that may be metastable under different redox conditions, (3) adsorption into organic matter, (4) assimilation into the gut of benthic infauna and further concentrations down the food chain. These pathways may allow recycling of metals from the cuttings piles either naturally, or through physical/chemical disturbance. The reliability of any water quality model depends on its inputs. In order to model the contaminant dispersion for cuttings the concentrations and fluxes must be determined. Determination of metal fluxes and metal desorption rates from particles is an important parameter for modelling cuttings dispersion and resulting impact on water quality.

Particle size distribution, organic matter content, the type of benthic fauna and the local sedimentation rate are all important factors when considering the fate of both dissolved (<0.2 mm) and total metal concentrations within the cuttings piles.

Sediment texture, deposition and resuspension all have a large influence on the distribution of metals in seabed sediments. Particle size will effect the volume to surface ratio available for binding of metals, with the fine-grained particles usually having the

33 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles. highest metal concentration (anthropogenic or natural). To compensate for element variability with respect to grain size and dilution by secondary phases, the application of normalisation factors should be applied. Such factors can be made with respect to different size fractions, or by comparison to a conservative element such as Al or Li. (Stevenson et al., 1995; Szefer, et al., 1996). Sediment textures range from medium silt (10 mm) to fine pebbles (2 cm) in cuttings piles in the northern and central North Sea (ALTRA, 1996).

The production, transport and destruction of organic matter plays a major role in the geochemical cycle of marine trace metals (Westerlund et al., 1986). The bacterially generated degradation of organic matter decreases the redox potential with depth below the sediment surface. As the degradation continues, a certain point will be reached, depending on composition and accumulation rate of sediment, at which the redox potential falls below values at which oxidised forms of manganese and iron oxides are stable and reductive dissolution takes place. The dissolution increases the porewater concentrations of dissolved manganese, iron, and associated trace metal ions causing them to migrate through the pore water towards the sediment surface (Sundby et al., 1986). The typical sequence of organic matter oxidation is shown in table 4.

Table 4. Diagenetic Scheme for the remineralization of organic matter (from Shimmield and Pedersen, 1990)

Aerobic Respiration

138O2 + (CH2O)106(NH3)16(H3PO4) ® 106CO2 + 16HNO3 + H3PO4 + 122H2O

Denitification

+ 94.4 HNO3 + (CH2O)106(NH3)16(H3PO4) ® 236 N2 + 106CO2 + 16 NH3 + H3PO4 + 366H2O

Manganese Reduction

+ 2+ 236MnO2 + (CH2O)106(NH3)16(H3PO4) + 472H ® 106CO2 + 55.2Mn + H3PO4 + 177.2H2O

Iron Reduction

34 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

+ 2+ 212Fe2O3 + (CH2O)106(NH3)16(H3PO4) + 848H ® 424Fe + 106CO2 + 16NH3 + H3PO4 + 530H2O

Sulphate Reduction

2- 2- 53SO4 + (CH2O)106(NH3)16(H3PO4) ® 106CO2 + 16NH3 + 53S + H3PO4 + 106H2O

Disproportionation

(CH2O)106(NH3)16(H3PO4) ® 53CO2 + 53CH4 + 16NH3 + H3PO4

This degradation sequence proceeds via bacterially mediated electron-transfer reactions where the oxidants are sequentially consumed with depth as oxidant demand exceeds supply. In areas of high organic matter deposition, this sequence will rapidly reach sulphate reduction, creating sulphide within the porewaters. This reaction will normally bind up metals into sulphide phases (Pedersen et al., 1993; Shimmield and Pedersen, 1990; Froelich et al., 1979). These diagenetic processes seen in the natural environment may be perturbed within cuttings piles. The high content of organic matter, presence of oil, speciality chemicals and the lack of bioturbation will combine to have an effect on the porewater chemistry. The high concentrations of organic matter in the cuttings piles enables the diagenetic sequence to reach sulphate reduction more rapidly within the top few centimetres (Figure 11). Oil in the sediment will displace water as it migrates through the pile, leaving a void that will be filled either through vertical or horizontal migration with either more oil or porewater water. The added presence of speciality chemicals in the cuttings have the possibility of creating organic complexation with dissolved metal species hindering the processes of diagenetic precipitation of minerals. The limited benthic communities near the cuttings pile result in reduced bioturbation (Daan et al., 1994; Daan et al., 1996). Therefore little mixing of the sediments leads to little introduction of oxygen, which inhibits the degradation of hydrocarbons and therefore the cuttings pile itself.

3.2.3 Leaching

Seafloor processes play a major role in the regulation of the chemical composition of water masses in the oceans. These processes are important contributors to the biogeochemical cycles of elements in aquatic systems. (Tenberg et al., 1995). These 35 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

36 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles. processes include the migration of ions through the porewaters, the movement of sediment particles, the cycling of metal oxides such as Fe and Mn across the sediment oxic / anoxic boundary, and organic matter degradation and cycling. The control these processes have on the hydrocarbon and metal concentrations in the cuttings piles is not known at present. The data available on the leaching and loss of contaminants from central and northern North Sea drill cuttings piles is limited and mainly addresses hydrocarbons, the work being confined mainly to laboratory experiments (Nguyen, 1998) or artificial environments (Dow et al., 1990; Davies and Tibbets, 1987). Nguyen (1998) states that leaching occurs as a reflection of the combination currents, surface tension and buoyancy effects on the oil. Laboratory tests have shown a release of oil from cuttings at a rate of 75 mg m-2 cuttings/day and decreased to 1 mg m-2 cuttings/day after 24 hours. Dow et al., (1990) also showed a small initial loss of oil to the overlying water (approximately 3 % by weight of added oil within the first 7 months). However Davies and Tibbets (1987) showed the opposite; over 80 % of the oil was lost from the sediment within the first 37 days (Figure 12). This decrease in oil was due to physical means (release of oil droplets and dissolution) and not due to biodegradation. The conclusion from this experiment was that after the first initial loss the remaining oil left on the particles will be tightly bound to the sediment and will therefore be very slow to degrade (Davies and Tibbetts, 1987). From the limited available evidence this would indicate that the majority of cuttings related oil remaining after initial deposition will remain within the pile. It does not leach out in any substantial quantities over time, but instead will stay bound to the sediment particles, trapped within the porewater water and degrade very slowly.

37 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

38 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

4. Conclusions

S No drill cuttings pile is the same, each represents a unique combination of sediment signature, contaminants and benthic community and each is affected by the local hydrodynamic regime.

S Although cuttings piles display a rapid decrease in contaminant concentrations with distance from the source, a considerable proportion of contaminants are likely to remain within the cuttings pile.

S Disturbances, in the form of physical reworking and biological irrigation of cuttings piles will increase exchanges of porewater and solids back to the seabed surface resulting in pathways of exposure for organisms.

S Although there is uncertainty as to the extent to which oil concentrations reduce over time it appears that most oil is dispersed during initial deposition. Thereafter, further degradation of oil in the pile will be very slow.

S Undisturbed drill cuttings piles will not result in increased exchanges of radium and other related radionuclides into the surrounding environment.

S The most likely pathway leading to exposure to radioactivity from drill cuttings is through the exposure of organisms that over time colonise drill cuttings piles.

S The cuttings piles will continue to act as a repository for contaminants for the duration of their existence before ultimate burial by natural sedimentation processes.

39 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

ACKNOWLEDGEMENTS

The authors acknowledge the invaluable help of Alan Stevenson and Nigel Fannin, at the British Geological Survey, Marine and Petroleum Geology Division, Edinburgh, in the production of this report. Thanks also to Kate Hoare at BMT and Niall Bell at Cordah. AURIS Environmental are thanked for allowing access to report material. Jason Smith and Bhavani Narayanswamy at the Dunstaffnage Marine Laboratory, Oban provided advice on the geochemistry and biology respectively. The constructive comments by the referee's.

40 Breuer et al., (1999) A review of contaminant leaching from drill cuttings piles.

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