Modelling potential effects of petroleum exploration drilling on northeastern Georges Bank scallop...

Ecological Modelling 166 (2003) 19–39

Modelling potential effects of petroleum exploration drilling onnortheastern Georges Bank scallop stocks

Peter J. Cranford∗, Donald C. Gordon Jr., Charles G. Hannah, John W. Loder,Timothy G. Milligan, D.K. Muschenheim, Y. Shen

Department of Fisheries and Oceans, Maritimes Region, Bedford Institute of Oceanography, P.O. Box 1006,Dartmouth, Nova Scotia B2Y 4A2, Canada

Received 6 November 2001; received in revised form 5 August 2002; accepted 19 February 2003

Abstract

A set of numerical models was used along with laboratory and field observations to evaluate the potential risk of exploratoryoil and gas drilling on northeastern Georges Bank to sea scallop (Placopecten magellanicus) stocks. The models were used topredict the drilling waste zone of influence and the impact of chronic exposure on scallop growth and reproduction. Growth andreproduction are generally considered to be the most important sublethal effects of chronic contaminant exposure. The highestnear-bottom concentrations of drilling waste (water-based mud) from a hypothetical 92-day exploration well was predictedto occur along the side of the bank (>100 m depth). Laboratory information on drilling mud toxicity threshold concentrationsindicated a potential for 0–48 days of growth inhibition depending upon the site, settling velocity of the mud, and area over whichresults are averaged. Scallop stocks on the side of the bank are relatively sparse, but dense aggregations are found in some areasand it is possible that changes in reproductive output could have detectable effects at the population level. Growth inhibitionin the tidal front region, which has the densest scallop stocks, was predicted to be more localised and confined to a range of0–15 days. Growth loss in the central mixed region (<65 m) was predicted to be negligible (<2 days). These results illustratethe importance of site location and waste settling velocity on potential effects. The magnitude of effects predicted at each sitewas closely related to bottom stress (u∗) as this determines how rapidly drilling mud reaching the seabed are redistributed anddiluted both horizontally and vertically.© 2003 Elsevier Science B.V. All rights reserved.

Keywords:Offshore oil and gas; Drilling wastes; Sea scallop; Growth; Reproduction; Georges Bank

1. Introduction

A long-standing environmental concern related tooffshore oil and gas extraction is the impact of chronic,low-level stresses on marine ecosystems associatedwith the discharge of operational wastes (Boesch et al.,1987). The most important sublethal effects on adult

∗ Corresponding author. Tel.:+1-902-426-3277;fax: +1-902-426-2256.

E-mail address:[email protected] (P.J. Cranford).

organisms exposed to chronic waste discharges, fromboth ecological and fisheries perspectives, are theimpairment of growth and reproduction. Numerousstudies on the potential hazard of operational drillingwastes to marine organisms have been conducted(Neff, 1987; GESAMP, 1993) but relatively few haveaddressed the actual risk of impacts occurring asthis requires knowledge of the expected environmen-tal concentration of wastes. Numerical models arevaluable tools for evaluating the potential environ-mental impact of drilling activities as they provide a

0304-3800/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0304-3800(03)00100-5

20 P.J. Cranford et al. / Ecological Modelling 166 (2003) 19–39

quantitative framework for integrating knowledge onthe intrinsic physico-chemical properties of the dif-ferent contaminants and the extrinsic processes thatcontrol their transport and fate in the environment.Applications of a set of models are presented whichpredict potential impacts of exploration oil and gasdrilling on commercial sea scallop (Placopectenmagellanicus) stocks on northeastern GeorgesBank. Georges Bank, which straddles the UnitedStates–Canadian boundary, is one of the most produc-tive fishing banks in the North Atlantic Ocean and sup-ports the largest offshore scallop fishery in the world.Exploration drilling on Georges Bank is currently pro-hibited under moratoria in both countries. However,as hydrocarbon resources are believed to be extensive,further reviews of this policy are anticipated.

High primary production on Georges Bank supportslarge populations of benthic suspension feeding inver-tebrates (Horne et al., 1989; Thouzeau et al., 1991).In the Canadian sector of Georges Bank (the North-east Peak), bottom-dwelling invertebrates account forup to 70% of the total landed value of all resourcespecies harvested, and the single most valuable fish-ery resource is the sea scallop. Scallops were targetedfor this impact assessment because of their economicimportance, and the availability of information ontheir sensitivity to drilling wastes. Benthic inverte-brates have generally been the focus of studies on thepotential impacts of drilling fluids (muds) and wellcuttings as the bulk of these wastes sediment rapidly.Scallops are sedentary after the juveniles settle on theseabed and could be exposed to contaminants over theentire drilling period. As filter-feeders, scallops obtaintheir food particles (phytoplankton and detritus) fromthe benthic boundary layer (BBL). Resuspension ofbottom sediments is ubiquitous on Georges Bank(Amos and Judge, 1991; Muschenheim et al., 1995),causing frequent reductions in the nutritional value ofthe near-bottom suspended particulate matter (Grantet al., 1997). Sea scallops partly compensate for thisdilution of the food resource by exploiting the resus-pended organic matter (Grant et al., 1997). Resuspen-sion/deposition processes concentrate drilling wasteparticles in suspension near the seabed (Muschenheimand Milligan, 1996), where scallops could be affectedby the chemical toxicity of contaminants, physicaldisturbance to feeding processes, and/or the presenceof non-nutritious materials in their diet.

The high sensitivity of adultP. magellanicustodifferent types of used drilling muds and major con-stituents was shown in chronic exposure studies inwhich cohorts were exposed to low-levels of sus-pended wastes for up to 72 days (Cranford, 1995;Cranford and Gordon, 1992; Cranford et al., 1999).These laboratory studies showed that low-levels ofdrilling wastes can influence food utilisation, growth,reproduction and survival. Threshold concentrationsof drilling mud causing significant impacts on so-matic and reproductive tissue growth varied between0.5 and 10 mg l−1, with the greatest sublethal effectsobserved for a used “low-toxicity” mineral oil-basedmud (Gordon et al., 2000). While this base-oil ap-pears to exhibit chemical toxicity, drilling waste par-ticles (primarily bentonite and barite) also physicallyinterfere with feeding/digestion processes, resultingin growth inhibition (Cranford and Gordon, 1992).Well cuttings particles, which tend to be larger thandrilling mud particles, had a relatively low impacton scallops. Exposures during gametogenesis (gonaddevelopment) tended to show a selective impact ofdrilling wastes on reproductive effort as opposed tosomatic tissue growth.

As part of a multidisciplinary program to improvescientific understanding of the fate and effects of op-erational drilling wastes, numerical circulation, wastedispersion and biological effects models were devel-oped that can be used to predict the spatial and tem-poral extent of environmental impact zones aroundspecific drilling sites on the continental shelf (Gordonet al., 1992). For the present application, numericalmodels for the transport and dispersion of suspendedmaterials in the BBL (Hannah et al., 1995, 1996, 1998)were enhanced to include physical oceanographic in-formation for Georges Bank, the results of field andlaboratory studies on drilling waste particle dynam-ics, and information on operational discharge prac-tices (Gordon et al., 2000; Loder et al., in preparation).This paper attempts to quantify the potential risk tothe production of Georges Bank scallop stocks by in-tegrating numerical model predictions of near-bottomwaste concentrations with laboratory observations ofsublethal effects of drilling wastes, and informationon scallop stock distribution. Model predictions wereused to explore how different oceanographic regimescontribute to the potential spatial and temporal extentof impact zones.

P.J. Cranford et al. / Ecological Modelling 166 (2003) 19–39 21

2. Materials and methods

Our procedure for modelling the potential effectsof drilling wastes on scallops requires a model forpredicting the concentrations of the drilling wastesand a model for converting the concentrations intoeffects on scallops. These models have data require-ments. The required data include a drilling waste dis-charge scenario, an estimate of the fraction of thedrilling waste that reaches the BBL, ocean currents,and exposure–response information for scallops. Themodels and the data are described here.

2.1. Modelling near-bottom drilling wasteconcentrations

A set of models, referred to as benthic boundary-layer transport (bblt) models, have been developed tostudy the dispersion and transport of suspended sed-iment in the BBL of continental shelf environments.Basic model concepts, assumptions and exploratoryapplications are described byHannah et al. (1995,1996, 1998)andLoder et al. (in preparation). A briefoverview is given here. Estimates of the current pro-file and bottom stress are combined with estimatesof the vertical profiles of sediment concentration andvertical mixing to generate estimates of drift and dis-persion. The sediment load is partitioned into discretepseudo-particles or packets each with massm and set-tling velocityw. The packets are advected horizontallyand mixed vertically. Vertical mixing is representedby random exchange (shuffling) of the packets, whichis controlled by a specified mixing time scaletm.

The overall (horizontally averaged) vertical distri-bution of the sediment is assumed to be governed byan equilibrium concentration profile which is used toderive a probability density function for the verticalposition of the packets. The concentration profile,c(z),is taken asc(z) = ca(a/z)p (Rouse, 1937), wherez =0 at the sea floor and is positive upwards,ca is the con-centration at the reference heightz = a, p = w/(κu∗),w is the settling velocity and the von Karman con-stantκ = 0.4. The friction velocityu∗ = (τb/ρ)1/2

whereτb is the magnitude of the bottom stress andρ

is the density of water. The bottom stress was basedon a quadratic drag law,τb = Cd|ub|2, whereub isthe near-bottom current andCd is a drag coefficientwhose value depended on the height above the bottom

of the near-bottom current (Cd = 0.005, 0.0025 forcurrents 1 and 10 m above bottom). The critical shearstress is take to be zero and the material is always insuspension. In addition, the Rouse profile was derivedfor an unstratified fluid and modifications have beenmade to allow the specification of a maximum heightof the profile (hmax) to account for the limiting influ-ence of the water depth and seasonal stratification.

The version of bblt used herein was the ‘local’ ver-sion which neglects spatial variability in the physicalenvironment around the discharge site, but includesthe effects of the horizontal currents, their verticalshear, and vertical mixing that are the primary factorsin short-term dispersion and transport at the releasesite. The horizontal dispersion is generated by the in-teraction of the vertical shear and the vertical mixing.There is no explicit horizontal mixing in the model.This local version can be forced by either a measuredtime-varying current profile or profiles from 3-D cir-culation models.

Preliminary applications of a spatially explicit ver-sion of bblt to Georges Bank showed that the hori-zontal current shears and the spatially variable bottomfriction are important to the drift and dispersion of ma-terial on Georges Bank (Xu et al., 2000). However, thelocal version was shown to provide a good descriptionof the drift and dispersion over the first few days andan accurate characterization of the different regimeson the bank. In addition, the spatially explicit versiondoes not allow for time-varying sediment release andthe current forcing only includes the seasonal meanand M2 tidal currents which does not contain the fullspectrum of time variability of the currents and theirvertical shears.

Local bblt was used to predict the average concen-tration of wastes in the bottom 10 cm of the watercolumn, the approximate layer from which scallopsobtain their food particles, around nine hypotheticaldrilling sites (Fig. 1). Application sites were se-lected to represent the different summertime oceano-graphic regimes on Georges Bank (Loder et al., inpreparation) and include the Hunky Dory and Growlersites identified in a 1987 drilling proposal by TexacoCanada Resources Ltd. One site is in the area on thetop of the Bank (less than 65 m depth) that is verti-cally well-mixed year-round. Three sites are locatedin the area on the side of the Bank that is stratifiedduring summer (greater than 100 m). Five sites are in


Fig. 1. Maps of Georges Bank showing model application sites, bathymetry and oceanographic zones. Site names correspond with mooredcurrent meter stations (�) and calculation nodes from the 3-D circulation model (�).

the transition zone (tidal-mixing front) between themixed and stratified side regions (65–100 m).

Moored current measurements, collected at multi-ple vertical positions from six sites on the NortheastPeak (Fig. 1) during 1988–1989 (Loder and Pettipas,1991; Loder et al., 1993) provided data used toforce summertime bblt applications listed inTable 1.Year-round current records from 1994 to 1995 at theNEP site (Smith, personal communication) providedforcing data during summer and winter (Table 1).

Different start days were chosen reflecting alternativeneap stages (minimal dispersion) of the fortnightlyand monthly tidal modulation cycles. A 3-D seasonaland M2 tidal circulation model (Naimie, 1995, 1996)was used to provide current forcing for additionalbblt model applications (Table 2). For each modelapplication a series of sampling stations were locatedas follows. A preliminary bblt run was performed toassess the primary drift direction inherent in that timeseries. Sampling stations were then located at 0, 2, 5,


Table 1Summary of local bblt applications using observed currents and the hypothetical waste discharge scenario

Region Site Water depth (m) Season Start day f Drift (◦T)

Sections 1–4 (first 62 days)Side GBFS1 155 Summer 189 0.2 60

Summer 217 0.2 50Frontal GBFS2 67 Summer 189 0.4 140

Summer 217 0.4 160NEP 73 Summer 208 0.8 225

Winter 8 1.0 200Mixed GBFS4 63 Summer 189 1.0 180

Summer 217 1.0 180

Section 5 (last 50 days)Side GBFS1 155 Summer 189 0.2 50Frontal GBFS2 67 Summer 189 0.8 140

NEP 73 Summer 208 1.0 225Winter 8 1.0 200

Each was run at two effective settling velocities (1 and 5 mm s−1). Oceanographic regions and site locations are indicated inFig. 1 anddaily releases of mud are summarised inFig. 2. Start day is Julian day. The parameter ‘f ’ represents the fraction of wastes released at10 m below the sea surface in sections 3–5 that is assumed to reach the BBL. Drift indicates the net direction of the sediment plumeduring the simulation.

and 10 km along four orthogonal directions includingthe drift line and up to eight additional locations werechosen at greater distances along the drift line andalong a secondary axis based on drift patterns observedin preliminary runs. The average waste concentrationin the bottom 10 cm was recorded every 30 min.

The vertical shuffling time scale at the different siteswas estimated using observations and models of thevertical mixing rates (Loder et al., in preparation) andthe values oftm ranged from 3 to 8 h, wheretm =3 h maximises the dispersion due to M2 tidal currents

Table 2Summary of local bblt applications using currents predicted by 3-D model and the hypothetical waste discharge scenario

Region Site Water depth (m) Season f Drift (◦T)

Side GBFS1 126 Summer 0.2 65GBFS1 126 Winter 0.4 60Growler 147 Summer 0.2 190Hunky Dory 107 Summer 0.2 5

Frontal GBFS2 74 Summer 0.4 145GBFS2 74 Winter 1.0 90GBFS6 80 Summer 0.4 161NEP 72 Summer 0.8 200ENEP 91 Summer 0.2 148SNEP 91 Summer 0.2 253

Each application was run at two effective settling velocities (1 and 5 mm s−1). These simulations cover wastes released during the first 62days only. Oceanographic regions and site locations are indicated inFig. 1 and daily releases of drilling mud are summarised inFig. 2.The parameters ‘f ’ and ‘drift’ are as inTable 1.

(Hannah et al., 1995). The maximum profile height,hmax, was estimated from observations and models ofthe bottom boundary-layer heights and ranged from17 m at the deep sites to the full water column overthe crest of the bank (Loder et al., in preparation).

The hypothetical drilling waste discharge scenarioused as input to bblt was prepared with the assistanceof Texaco Canada Petroleum Ltd. In this scenario, theexploration wells are drilled using water-based mudsin which the major solid components are bentoniteclay and barite (barium sulfate), and a total of 468 Mt


Fig. 2. Daily drilling mud release used in Georges Bank model applications. This hypothetical waste discharge scenario represents anexploration well drilled using water-based mud.

of mud is released into the marine environment over aperiod of approximately 3 months. The amount of muddischarged in the scenario is in the range reported forthe eight exploratory wells drilled on the US sector ofGeorges Bank in 1981–1982 (Neff, 1987). Drill cut-tings were not included in the model simulations re-ported here because, as discussed before, these largerparticles were observed to have low impact on scallops(Cranford et al., 1999). The drilling scenario is brokendown into five separate sections (Fig. 2). During thefirst two sections (0–850 m depth), drilling muds weredischarged directly at the seafloor. During the deeperthree sections (850–4600 m), material was released ata depth of 10 m below sea surface. The largest dis-charges took place during the first week, but substan-tial bulk dumps occurred at the end of the final twosections. The discharge density generally was held at1.075 kg m−3 for sections 1–4 and at 1.230 kg m−3 forsection 5. A 50/50 mixture of bentonite and barite wasassumed in all bblt simulations. Owing to the limitedduration of some current records and computationalconsiderations, waste drift and dispersion for the first62 days (well sections 1–4 except for bulk discharge atend) and the last 25 days (end of section 4 plus section5) were modelled separately using the same currenttime series. Since at least two-thirds of the discharge

in the BBL occurred during the first 62 days of thescenario, the focus of the impacts evaluation was onthis period.

During the first two sections of the well the mudis discharged at the sea floor and all of it is availablefor transport by the bblt model. However, during thefinal three sections the discharges are at 10 m depth.The fraction,f, of the discharged mud that reaches theBBL during these sections, was estimated as follows.Simulations were carried out using industry standardconvective descent models to determine the depth ofdescent of the waste discharge plume under differ-ent discharge conditions, densities and environmen-tal conditions (Andrade and Loder, 1997). The factorswhich significantly affectedf were the mud density,water depth, BBL depth, initial downward volume fluxof the discharge, current strength, and water columnstratification. Estimates off for the different applica-tion sites are given inTables 1 and 2and range from0.2 in the stratified side region to 1.0 in the verticallymixed central region. Thef value was assumed to behigher during winter based on the assumption that, inthe absence of stratification, the BBL extends higherinto the water column.

Preliminary runs of bblt demonstrated that near-bottom waste concentration was highly dependent on


particle settling velocity (w) (Hannah et al., 1995).This arises from the assumption of a Rouse-typebalance between boundary-layer turbulence, param-eterised byu∗ and w. While local variations inu∗values are generally constrained to within an order ofmagnitude, the range of possible settling velocities forfine particulate matter can vary over three orders ofmagnitude. A major concern was whether barite andbentonite discharges, because of their significantlydifferent physico-chemical properties (e.g. density,particle size spectra, ion exchange capacity) whichcan influencew, should be modelled separately or asa combined discharge. In choosing a representativerange ofw to use in these applications, we relied on(1) laboratory studies of drilling waste settling ratesin both unflocculated and flocculated states (Milliganand Hill, 1998); (2) published literature valuesof in situ observations of settling rates of natu-rally flocculating material (Hill et al., 1998); (3)Stokes’ settling velocities for individual, unfloc-culated barite grains (Gibbs et al., 1971); and (4)field observations of drilling waste vertical distri-butions at the Cohasset and Panouk oil develop-mental sites on Sable Island Bank made betweenJuly 1991 and September 1993 (Muschenheimet al., 1995; Muschenheim and Milligan, 1996). Fromthe field observations it was concluded that the set-tling behaviour in a combined suspension would bedetermined by flocculation processes dominated bythe fine bentonite particle fraction. The settling veloc-ity range for both the barite and bentonite fractions,therefore, becomes similar with an effective settlingvelocity of a 50/50 mixture of bentonite and bariteunder tidally-energetic conditions falling in the rangeof 1–5 mm s−1 (Gordon et al., 2000). As a result,total mud discharge was used as input to the bbltmodel, and each application was run twice using set-tling velocities of 1–5 mm s−1 to bracket the expectedrange.

2.2. Potential effects on scallop growth

The basis for the biological interpretation of totaldrilling waste concentrations predicted by bblt are theresults of laboratory toxicity experiments reported byCranford and Gordon (1992), Cranford (1995)andCranford et al. (1999). These experiments exposed seascallops to different concentrations of various drilling

wastes in raceway tanks and determined chronic lethaland sublethal effects, including effects on reproductiveand somatic tissue growth. Considering that mineraloil-, and synthetic oil-based drilling fluid dischargesare effectively prohibited in the offshore regulatoryregion including Georges Bank, and observationsof the relatively low impact of cuttings particles onscallops, only the bentonite and barite biological ef-fects data are relevant to the model applications inthis study.

Two kinds of effects thresholds were estimated fromthe bentonite and barite exposure data. The first is thezero growth concentration (C0) threshold, at or abovewhich there is no tissue growth. The second is the noobserved effects concentration (C1) threshold, at orbelow which there is no significant effect on scallopgrowth. For pure bentonite, zero growth was observedat 10 mg l−1 and no effect was detected at 2 mg l−1

(Cranford and Gordon, 1992). The effects thresholdshad to be estimated for barite as laboratory experi-ments observed zero growth at 0.5 mg l−1, the lowestconcentration tested. However, this value was used asan estimate ofC1, as observed effects on the physi-ological components of growth indicated that growthwould occur at barite concentrations below 0.5 mg l−1

(Cranford et al., 1999). The no effects concentrationfor barite was estimated at 0.1 mg l−1 by assuming theratio C1/C0 was the same as observed for bentonite.Recent unpublished laboratory observations of drillingwaste impacts on scallops confirm this assumption(Cranford, unpublished data).

The bblt simulations report the concentration of acomposite drilling waste variable. Thus, the first stepwas to convert this into time series of barite and ben-tonitite concentrations. Based on the drilling wastescenario, the material was partitioned 50/50 into bariteand bentonite components. Although the scenario in-dicates some variation in the proportion of each com-ponent with time, the variations were small and the useof a variable proportion of waste constituents wouldrequire additional model refinements.

Sublethal effects on scallops at each site were esti-mated as the total time (days) that growth (somatic andreproductive) is inhibited. The calculation was basedon a linear growth response betweenC0 andC1, as wasobserved in laboratory experiments (Cranford et al.,1999), and additive effects of barite and bentonite.For a particular drill mud componentx, the growth


reduction indexRx due to that component was mod-elled as

Rx = axCx + bx; ax = (Cx1 − Cx

0)−1;

bx =(

1 − Cx1

Cx0

)−1

whereCx is the concentration of componentx, Cx0 is

the no effects concentration for that component, andCx

1 is the no growth concentration.Rx is restricted tohave values in the range [0, 1]; values less than 0 areset to 0 and values greater than 1 are set to 1. The to-tal growth reduction indexR = ∑

Rx andR is againrestricted to the interval [0, 1]. The time-varying con-centrations from bblt give rise to a time-varyingR.The total potential growth days lost during a simula-tion of N time steps withM time steps per day is

PGI =∑N

i=1Ri

M(1)

whereM = 48 for the 30 min time steps used here.Potential growth inhibition (PGI) was computedseparately for high (5 mm s−1) and low (1 mm s−1)effective settling velocities.

Given the equal masses of barite and bentonite inthe discharge scenario and the much lower value ofC0 for barite, the PGI index is dominated by the baritein the simulations reported. However, the formulationpresented here is quite general and can be used formore than two constituents.

3. Results

3.1. Near-bottom drilling waste concentrations

To illustrate the predicted horizontal distributionof drilling mud discharges for the major oceano-graphic regions of Georges Bank, several contour plot‘snapshots’ of near-bed concentration (mg l−1) wereselected from among the model applications listed inTables 1 and 2. More detailed results are availablein Loder et al. (in preparation). Fig. 3 shows wastedistributions on Day 12 at the end of the second wellsection after two large waste dumps at the seabed(Fig. 2), assuming aws of 5 mm s−1. The magnitudeof predicted near-bottom concentrations of drillingwaste was highly dependent upon the geographic lo-cation of the release (Fig. 3). Both the observed and

model current applications indicate that the predictedmean drift of the near-bottom drilling waste plume isgenerally along depth contours except over the Bank’sside where more variability in near-bottom drift di-rection occurs (Fig. 3, Tables 1 and 2). This pattern isconsistent with the residual circulation. Results for theGrowler site indicated that drift from the side of theBank up into the frontal zone is possible under someconditions (Fig. 3, Table 2). The spatial patterns andnear-bottom concentrations of drilling mud predictedby observed and modelled currents were similar inmost cases (Fig. 3). This demonstrates that the 3-Dcirculation model used to force bblt at some sites andseasons provides reasonable current predictions.

To simplify interpretation of the 528 time-seriesplots of near-bed concentrations from the 44 modelapplications listed inTables 1 and 2, an index of thetotal potential “exposure” of benthic organisms todrilling mud was computed for each sampling locationas the time-integrated concentration over the durationof the simulation (

∑C �t). Model results for sam-

pling stations located along the primary drift line andthe first 62 days of discharge are shown based on ob-served current forcing (Fig. 4) and 3-D model currentforcing (Fig. 5). In general, the waste exposure indexdecreased rapidly over distances of 2–10 km from therelease point and was very sensitive to the effectivesettling velocity selected for the discharged drillingmud (Figs. 4 and 5). There is some similarity of thespatial pattern and temporal variability in exposureconcentrations predicted usingws values of 1 and5 mm s−1. However, the higher velocity resulted inconcentrations that are about an order of magnitudegreater than those at the lower velocity (Figs. 4 and 5).At the higher settling velocity, benthic organisms atthe GBFS1, Growler and Hunky Dory drilling sites onthe side of the Bank (>100 m depth) were predictedto be exposed to substantial waste concentrations asfar as 20–40 km from the release point (Fig. 5). Inthe frontal region, exposure to high concentrationswas more localised (within 2 km of the release point)and the sites with the highest potential exposure areNEP, GBFS2, and SNEP. Predicted near-bottom wasteexposure was lowest in the mixed region of the Bank.

Waste exposure predictions for different summer-time starting dates (Julian Days 189 and 217) weregenerally similar at all sites (Fig. 4). Model-forcedapplications for sites in the frontal (GBFS2) and side


Fig. 3. Snapshots of drilling mud concentrations (mg l−1) averaged over the bottom 10-cm of the water column on Day 12 (end of wellsection 2) of the hypothetical drilling waste release scenario (ws = 5 mm s−1). The release is at the origin for each site, and time-series‘sampling’ positions are shown for each site as open circles. Application sites were chosen to be representative of the mixed, frontal andside regions of Georges Bank (Fig. 1) during different times of the year.

(GBFS1) regions of Georges Bank indicate that wasteconcentrations in winter are lower than in summer(Fig. 3). However, waste concentrations at NEP, wherebblt was forced by observed currents, were higher inwinter than summer (Fig. 4), in part due to reduceddrift across the Northeast Peak in winter. The reducedwinter concentrations at the GBFS2 site reflect the in-creased boundary-layer thickness associated with re-duced stratification and increased vertical mixing inwinter and are also expected for other frontal sites nearthe bank edge. The reduced winter concentrations atthe GBFS1 site are associated with the stronger tidalcurrents in winter predicted by the 3-D model, and

are of uncertain reliability (although reduced concen-trations are expected from amplified wave and windinfluences in winter).

3.2. Potential effects on scallop growth

The results of the model applications can be sum-marised as follows, grouped according to the physicaloceanographic zone on the Bank in which the site islocated (Fig. 1). The results were expressed as the to-tal number of days of PGI during drilling of the explo-ration well sections assuming that scallops are presenteverywhere in the model domain and that growth is


Fig. 4. Time-integrated near-bottom concentrations (total potential exposure) at various distances along the primary drift line at bblt modelapplication sites (Fig. 1) forced by observed currents (Table 1) for the first 62 days of the drilling waste release scenario (Fig. 2). Resultsare shown for settling velocities (ws) of 1 mm s−1 (left) and 5 mm s−1 (right). The starting date (Julian) of each bblt run is given inparenthesis by the site name. Note the different scales on the vertical axis.

continuous throughout the year. In actuality, as dis-cussed later, scallops are very patchy in distributionand growth rate varies seasonally so that the predictedimpacts of a given case depends upon the location ofscallop beds relative to the release point and the tim-

ing of drilling. PGI estimates at sampling sites alongthe primary drift direction are shown for cases forcedby observed (Fig. 6) and 3-D model currents (Fig. 7),and aws value of 5 mm s−1. PGI for each site and mudsettling velocity, averaged for all sampling locations


Fig. 5. Time-integrated near-bottom concentrations (total potential exposure) at various distances along the primary drift line at bblt modelapplication sites (Fig. 1) forced by 3-D model currents (Table 2) for the first four sections (62 days) of the drilling waste release scenario(Fig. 2). Results are shown for settling velocities (ws) of 1 mm s−1 (left) and 5 mm s−1 (right). Sites are identified by node numbers fromthe 3-D model and names are given in parenthesis. Note the different scale on the vertical axis.

within different distances from the release point, aregiven inTables 3 and 4.

The model applications for the mixed region on thetop of the Bank (GBFS4) were forced by current meterdata and the net drift of the sediment plume was tothe south (Table 1). There was just one location where

growth inhibition exceeded 1 day and that was at therelease point and this was indicated only at the highersettling rate (Fig. 6, Table 3).

There was no growth inhibition predicted at anyof the sites in the frontal region at the lower settlingrate (Tables 3 and 4). At the higher settling rate, PGI


Fig. 6. Summary plots of the total days of PGI along the primary drift axis during exploration drilling at the model application sites.Potential impacts are shown for the first 62 days (upper) and remainder (lower) of the drilling waste release scenario (ws = 5 mm s−1).Model forcing was by observed currents and the start day (Julian) is shown in parenthesis.

during the first 62 days of the drilling scenario rangedon the order of 2–18 days at the release point and0–5 days within a 10 km radius (Tables 3 and 4).Growth lost during the remainder of the scenario was∼35–45% of the 112 days total PGI (Table 3). The

area in which scallop growth is potentially inhibitedfor more than 10% of the 112-day discharge periodwas approximately 2 km2 for all frontal sites (Fig. 1)except at SNEP during summer and NEP during win-ter, which have potential impact zones (>10% PGI) of


Fig. 7. Summary plots of the total days of PGI along the primary drift axis during exploration drilling at the model application sites.Potential impacts are shown for the first 62 days of the drilling waste release scenario (ws = 5 mm s−1). Predictions are shown for modelsampling locations along the primary drift axis. The bblt model was forced by summer (September and October) and winter (January andFebruary) currents predicted by the 3-D circulation model.

approximately 12 and 78 km2, respectively. Unlike theNEP site, PGI at GBFS2 was lower in winter thansummer (Table 4).

Model applications for sites located in the stratifiedside region indicated that scallop growth inhibitionmay occur even if settling velocity is relatively low.Forws = 1 mm s−1, PGI at the release point during thefirst 62 days of the drilling scenario ranged between

0.2 (Hunky Dory) and 9.5 (GBFS1) days. Forws =5 mm s−1, the predicted impact for this same area anddrilling period was much higher and ranged between14 and 32 days or 22–52% of the discharge period(Tables 3 and 4). Growth effects generally droppedwith distance from the release point, but PGI estimatesbased on the higher settling velocity are on the orderof 10 days as far as 40 km from the drilling site along


Table 3Total days of PGI for sea scallops (P. magellanicus) calculated from output from the local bblt model with observed current forcing

Region Site Start day PGI (days)

0 km 2 km 5 km 10 km Drift line

Sections 1–4 (first 62 days)ws = 1 mm s−1

Side GBFS1 189 ∗9.5 5.8 4.7 3.4 3.2GBFS1 217 ∗7.5 3.7 2.7 2.0 1.7

Frontal GBFS2 189 0.3 0 0 0 0GBFS4 189 0 0 0 0 0GBFS2 217 0.7 0.3 0.2 0.1 0.1GBFS4 217 0 0 0 0 0NEP 208 0 0 0 0 0NEP 8 0 0 0 0 0

ws = 5 mm s−1

Side GBFS1 189 ∗32.5 ∗19.8 ∗15.7 ∗12.7 ∗12.8GBFS1 217 ∗30.0 ∗14.5 ∗10.6 ∗8.4 ∗8.3

Frontal GBFS2 189 ∗10.7 3.7 2.5 1.9 2.7GBFS2 217 ∗10.8 4.0 2.9 2.3 3.5GBFS4 189 1.8 0.5 0.3 0.2 0.2GBFS4 217 4.7 1.9 1.4 1.1 1.0NEP 208 6.0 1.9 1.4 1.1 1.4NEP 8 ∗18.4 ∗9.0 ∗6.8 5.4 ∗6.3

Sections 1–5 (full 112-day discharge scenario)ws = 1 mm s−1

Side GBFS1 189 ∗11.7 7.1 5.7 4.1 3.9Frontal GBFS2 189 0.5 0 0 0 0

NEP 208 0 0 0 0 0NEP 8 0 0 0 0 0

ws = 5 mm s−1

Side GBFS1 189 ∗52.1 ∗29.7 ∗22.6 ∗18.2 ∗17.8Frontal GBFS2 189 ∗18.6 6.4 4.3 3.3 4.4

NEP 208 ∗11.0 3.2 2.3 1.8 2.3NEP 8 ∗32.9 ∗16.4 ∗12.3 9.6 10.6

The different application site locations are indicated inFig. 1. PGI for each application was averaged over a radius of 0, 2, 5 and 10 kmfrom the discharge site, and along the primary drift line out to 30–50 km. An asterisk indicates that growth inhibition was greater than10% over the simulated period.

the primary drift line at GBFS1, Growler and HunkyDory during drilling of the first four sections of thewell (Fig. 7). The spatial extent of sublethal effects atGBFS1 is smaller during winter (Fig. 7) and for thefinal well section (Fig. 6). When PGI estimates forGBFS1 are totalled for the two time periods and allvalues are averaged over sampling locations along the50 km primary drift line, a total of 18 days of growthinhibition is predicted during the 112-day drilling sce-nario. Stations located off the drift line indicated asimilar impact on growth out to a radius of about 2 km.Therefore, assuming the higher settling velocity, thepotential area in which scallop growth is inhibited for

18% of the exposure period may be on the order of150 km2 (π × 2 km× 25 km).

4. Discussion

4.1. Potential impacts on Georges Bank scallopgrowth

The interpretation of the results from these bblt ap-plications on Georges Bank depends upon several fac-tors which include the location of the release site,the distribution of scallop stocks and the time of year


Table 4Total days of PGI for sea scallops (P. magellanicus) calculated from output from the local bblt model with 3-D model current forcing

Season Region Site PGI (days)

0 km 2 km 5 km 10 km Drift line

ws = 1 mm s−1

Summer Side GBFS1 2.2 0.5 0.3 0.2 0.3Growler 4.1 1.8 1.2 0.8 1.2Hunky Dory 0.2 0 0 0 0

Frontal GBFS2 0.1 0 0 0 0NEP 0 0 0 0 0ENEP 0 0 0 0 0SNEP 0.1 0 0 0 0GBFS6 0 0 0 0 0

Winter Side GBFS1 1.9 0.5 0.3 0.2 0.4Frontal GBFS2 0 0 0 0 0

ws = 5 mm s−1

Summer Side GBFS1 ∗23.0 ∗9.9 ∗7.3 5.9 ∗12.9Growler ∗26.2 ∗14.8 ∗12.4 ∗10.6 ∗21.0Hunky Dory ∗14.1 ∗8.8 ∗6.7 5.4 ∗10.8

Frontal GBFS2 ∗12.0 4.2 3.0 2.4 4.7NEP 4.6 1.3 0.9 0.7 1.0ENEP 5.5 2.3 1.6 1.3 2.3SNEP ∗13.8 ∗7.9 6.0 4.7 ∗7.1GBFS6 ∗8.4 3.0 2.1 1.6 2.6

Winter Side GBFS1 ∗13.2 3.7 2.5 2.0 3.7Frontal GBFS2 5.4 1.3 0.8 0.6 0.7

Predictions are for the first 62 days of the hypothetical discharge scenario at different applications sites indicated inFig. 1. PGI for eachapplication was averaged over a radius of 0, 2, 5 and 10 km from the discharge site, and along the primary drift line out to 30–50 km. Anasterisk indicates that growth inhibition was greater than 10% over the simulated period.

at which the wastes are released. Release sites thatwere closest to high scallop densities tended to havea greater chance for impacts, but the impacts also de-pended on waste concentration and net drift direc-tion. The three application sites on the side of theBank, where bottom stress (and hence suspension)and dispersion are lowest, have the highest predictednear-bottom drilling waste concentrations, and there-fore the highest potential for scallop growth impair-ment (Tables 3 and 4). The GBFS1 site is in an areaof low scallop abundance (Fig. 8) and the net drift ofthe near-bottom discharge plume was predicted to benorth east (Tables 1 and 2), generally away from thescallop beds. However, inclusion of spatial variation inthe currents and depth at this site using a spatially vari-able version of bblt (Xu et al., 2000) indicates that driftand dispersion onto the bank may be underestimatedwith local bblt. Growler is also located in an area oflow scallop abundance, but net drift at this site waspredicted to be onto the Bank (Table 2) so some dis-

tant effects are possible. Hunky Dory is located in anarea of moderate scallop density and wastes releasedat this site have a much greater potential of cominginto contact with scallop stocks and affecting growth.Assuming a waste settling velocity of 5 mm s−1, scal-lop growth could be inhibited for up to 17% of thedrilling period at a distance of 40 km along the pri-mary drift line. Given that the waste stream at this sitecan be up to 10 km wide (Fig. 3), this sublethal impactzone may exceed 200 km2.

The greatest scallop densities are located through-out much of the frontal zone with maximum catchesrecorded in the northern area near GBFS2 and GBFS6(Fig. 8). High to moderate densities are also foundnear the ENEP and SNEP sites, but catches have beenrelatively low in the area around NEP. Potential scal-lop growth inhibition by drilling wastes within thefrontal region appeared to be confined primarily tothe release point (Figs. 6 and 7), where a maximumof 19 days of growth inhibition was predicted for the


Fig. 8. Location of the bblt application sites on Georges Bank in relation to scallop stocks. Stock distributions were calculated based onaverage catch statistics from 1993 to 1997. The net direction of the sediment plume at each site during summer is indicated by the arrows.

GBFS2 site for the entire scenario (Fig. 6, Table 3).The results suggested less than 10 days of growthinhibition at distances of 10 km along the drift lineat SNEP and NEP. Despite the presence of moder-ate scallop densities at GBFS4, this site on the topof Georges Bank (Fig. 8), has the lowest potentialfor impacting scallop stocks due to the high bottomstress and strong shear dispersion that rapidly redis-tribute and dilute, both horizontally and vertically, thewastes.

4.2. Confidence in model results

There is a moderate to high degree of confidence inthe reliability of bblt’s representation of the importantphysical processes which control sediment dispersionand transport. Realistic representations of spatial andtemporal variability in bottom shear stress, horizontalcurrents and associated vertical shears, and verti-cal mixing are included (Hannah et al., 1995, 1998;

Loder et al., in preparation). The 3-D circulationmodel has been shown to provide a good represen-tation of the predominant seasonal-mean and tidalflows on Georges Bank (Lynch and Naime, 1993;Naimie, 1995; Horne et al., 1996; Naimie, 1996). Thepresent applications use the local version of bblt inwhich the physical conditions are uniform over theentire model domain, which can extend out from therelease point as far as 50 km. In reality, physical con-ditions on Georges Bank can change markedly overdistances of just a few kilometres, so confidence inmodel output drops with increasing distance from therelease point. Evaluations using the spatially variableversion of bblt indicate that local bblt generally tendsto underestimate dispersion rates, and hence overes-timate waste concentrations (Hannah et al., 1998; Xuet al., 2000). The additional influences of horizontalvariability can result in reduced or increased concen-trations (such as the increased on-bank drift noted forGBFS1) depending on the site.


The effect of spatial variability in physical con-ditions around a particular site on the predictednear-bottom concentrations is considered to be rela-tively small compared to those from settling velocity.Laboratory experiments indicate that drilling wastesflocculate rapidly in seawater, and therefore have higheffective settling velocities (Milligan and Hill, 1998).Observations at the CoPan production site on SableIsland Bank (34 m depth) indicate that drilling wasteflocs are most concentrated near the seabed and canbe seen as far as 8 km from the release point duringdevelopment drilling (Muschenheim and Milligan,1996). The expected range of settling velocities wasestimated using measured drilling waste concentrationprofiles around the CoPan site, but it appears that thesemeasurements did not fully resolve the dense matsof flocculated drilling wastes seen in video images.Thus, higher effective settling velocities (>5 mm s−1)are possible, which would increase near-bottom con-centrations and scallop growth impacts by severalfold above the present model predictions. However,this is unlikely to occur under the tidally-energeticconditions on Georges Bank as the size of drillingmud flocs would be limited by the relatively highturbulence levels (Milligan and Hill, 1998).

The biological effects thresholds are based on theresults of extensive laboratory experiments conductedwith adult scallops. The zero growth threshold forbarite had to be estimated but is reasonable based onmeasured physiological effects (Cranford et al., 1999)and growth measurements from recent chronic expo-sure experiments with barite (Cranford, unpublisheddata). An implication not considered here is how thesuspension of natural sediments, which contains or-ganic matter that may be exploited as food by seascallops (Grant et al., 1997), interacts with the drillingwastes. There is also some uncertainty whetherflocculation, which was limited in the laboratory ex-periments, influences the toxicity of drilling wastes.Considering that the larger water-based mud cuttingshad a much lower impact on scallops than bentoniteand barite, natural aggregation processes may mitigatethe effects of fine particulate wastes on scallop feed-ing behaviour. This is suggested by observations thatsea scallops exposed to aggregated bentonite in thelaboratory did not reduce feeding rate (White, 1997)as was observed for scallops feeding on disaggregatedbentonite (Cranford and Gordon, 1992). However,

field observations of sea scallops feeding on floccu-lated suspensions (Cranford et al., 1998) showed thatthe natural flocs were more fragile than the flocs pre-pared in the laboratory and are easily disrupted by theanimal’s feeding processes. Once the flocs are disag-gregated, the scallop would be exposed to a similarsize spectra of particles as was presented in the labo-ratory experiments, and similar results are anticipated.

As the ‘no observed effects’ threshold concentra-tion for bentonite (2 mg l−1) was greater than the zerogrowth threshold for barite (0.5 mg l−1), all growth im-pacts predicted by the present model applications areattributed to barite concentration. Although barite wasassumed to comprise 50% of the mass of the drillingmud, the proportion of barite can exceed 80% duringdrilling of the final well sections. The predicted im-pact on scallop growth during drilling of the final wellsection is, therefore, underestimated and could exceedthat predicted for the first four sections.

In summary, the greatest uncertainty in predictingimpacts on scallop stocks is related to the vertical dis-tribution of the drilling mud and the parameterisationof its settling velocity. The predicted effects on scal-lops that assume a settling velocity of 5 mm s−1 shouldbe viewed as conservative. Such a conservative ap-proach was recommended byGray and Bewers (1996),who proposed that marine environmental risk manage-ment based on the Precautionary Principal should havea scientific foundation, with risk assessments formu-lated on adequately pessimistic assumptions regardinguncertainties in the prediction of effects.

4.3. Applications of bblt models and results

The bblt models, coupled with biological effectsdata, are valuable quantitative tools that may beapplied in environmental impact assessments, for de-signing environmental effects monitoring programs,to identify major processes controlling the zone ofinfluence of drilling wastes, and for exploring the ef-fectiveness of different impact mitigation options (e.g.changing discharge depth or drilling mud formula-tion). The model applications presented here demon-strate the usefulness of local bblt to address specificquestions related to exploratory drilling at specificsites. The models have also been applied to assessthe environmental impacts of oil and gas produc-tion drilling at offshore sites on the Scotian Shelf


Fig. 9. Number of days of potential scallop growth inhibition(PGI) averaged at the discharge point for all application sites andtimes on Georges Bank as a function of bottom friction velocity(u∗). Friction velocity was averaged over the first 12 days ofthe exposure as the majority of impacts potentially occur duringthis period. The horizontal lines from each data point show themaximum range ofu∗ values at the site over this period. Thecurve was fitted by eye.

(Hannah et al., 1995; MacLaren Plansearch, 1997)and have been implemented by industry as a generictool for assessing the potential effects of explorationdrilling on the Scotian Shelf, Slope and St. PierreBank (LGL, 1999). The models can also be used toexplore other important unanswered questions such asthe potential cumulative impacts of different effluentstreams discharged from single or multiple drillingplatforms. Potential model applications are not lim-ited to petroleum extraction issues but have potentialfor use in assessing impacts from other activities,including the dumping of dredge spoils and seabedaggregate extraction, if information is available onflocculation, effective settling rate, and contaminanttoxicity for the desired target species.

Model results suggest that geographic informationon bottom stress could be useful to environmentalmanagers for screening drilling applications as thesedata would provide an indication of the potential riskof benthic impacts. To illustrate this, mean (over thetidal cycle) friction velocity (u∗) estimates for eachmodel application site are plotted against the predictedimpact on scallops (PGI at discharge site assumingws = 5 mm s−1) in Fig. 9. Sites with highu∗ have alow potential for benthic impacts even when settlingvelocity is high (Fig. 9), as the drilling mud is pre-

dicted to be mixed high into the water column by thehigh bottom stress. The severity of impacts to benthicfauna is predicted to rapidly increase with decreas-ing u∗ values, but would decline onceu∗ falls belowthe critical value for resuspension of drilling wastes(Fig. 9). A value of u∗crit for resuspension ofsandy sediment from GBFS2 was estimated to be2.14 cm s−1 (Grant et al., 1997). Although meanu∗values for this site are less thanu∗crit (Fig. 9), u∗often exceeds this value during each tidal cycle, re-sulting in the frequent resuspension of sand and thesilt-clay fraction (including drilling mud) accumu-lated between sand grains. Geographic variations incurrent regime may help explain why no correlationwas observed between the measured spatial extent ofbenthic fauna effects around oil and gas fields andthe amount and toxicity of drilling wastes discharged(Olsgard and Gray, 1995).

4.4. Implications of growth inhibition to scalloppopulations

Interpretation of the predicted growth impacts atthe population level requires knowledge of annualgrowth trends and the life history of sea scallops onGeorges Bank. Scallops on Georges Bank display asemi-annual reproductive cycle, with spawning occur-ring in May–June and September–October (DiBaccoet al., 1995). The autumn spawn is larger than thespring spawn and, while only mature gametes arereleased during the spring, the scallops are repro-ductively spent after the fall spawn. Gametogenesisis immediately reinitiated after spawning in the fall.Somatic weight tends to decrease during gametoge-nesis as accumulated energy reserves are utilised tosupport gonad growth, but increases outside the re-productive period and when food is abundant. Seascallops appear to invest surplus energy primarilyinto the production of gametes such that reproductiveeffort increases only when conditions are favourable.As a result of this conservative strategy of controlledgrowth and opportunistic reproduction, interannualvariations in environmental conditions greatly al-ter the timing (semi-annual or annual) and nature(synchronised or protracted) of spawning events onGeorges Bank (DiBacco et al., 1995).

Contaminant stress during gametogenesis, result-ing from the presence of non-nutritious and/or toxic


drilling waste particles in the diet, can result in re-duced gonad growth rates (Cranford and Gordon,1992; Cranford et al., 1999) that would result in theproduction of fewer gametes (lower fecundity) and/orsmaller ova having a reduced energy content. Moresevere stress can result in the resorption of gametesand impacts on somatic tissue growth (Cranford et al.,1999). It is likely that drilling wastes would havemore effect on the scallop fishery through changes infecundity (an impact not apparent in the fishery untilreduced recruitment in future years) than in musclesize. However, impacts on somatic tissue growth canalso affect reproductive success as the accumulationof carbohydrate and lipid energy reserves in the mus-cle and digestive gland is believed essential for theinitiation of gametogenesis and the later maturationof gonad (Robinson et al., 1981). Considering thatgametogenesis is nearly continuous on Georges Bank,exposure to drilling wastes would have some impacton fecundity and egg viability regardless of the timeof drilling. The spring and summer are of greatestconcern as the majority of annual gonad productionoccurs between March and August.

The viability of eggs in adults exposed to drillingwastes may also be of concern as the potential con-sequences to larval survival could impact future yearclass strength. It is unlikely, however, that the scal-lops would release non-viable eggs, but would resorband utilize the high nutritive content of some gametesto allow others to reach the critical size for spawning(DiBacco et al., 1995). Scallop populations from re-gions characterised by nutritive stress were observedto produce viable gametes even though reproductiveeffort was low (MacDonald and Thompson, 1986).

Given a primary gonad growth period of 6 months,we estimate that the loss of 10–20 days of gonadgrowth during drilling of a single exploration wellcould reduce scallop fecundity by 5–10%. AlthoughMcGarvey et al. (1993)have demonstrated a correla-tion between egg production and recruitment for scal-lops on the Northeast Peak, it is unlikely that a 10%reduction in fecundity would be detectable in futurestocks unless it occurred over a very large area in aregion of abundant scallop stocks. The only region inthe Canadian sector of Georges Bank that appears tomeet these criteria is located on the eastern side of theBank near the Hunky Dory application site (Fig. 8).Although it is difficult to predict how this level of

growth impairment would alter recruitment to theGeorges Bank scallop fishery because of natural vari-ability, the prediction that the sublethal effects impactzone may exceed 200 km2 at this site suggests thateffects might be detectable at the population level.

Acknowledgements

Funding for this modelling project was provided bythe federal Program on Energy Research and Devel-opment (PERD) and the Department of Fisheries andOceans. We thank the many members of the GeorgesBank Steering Committee who provided input over the6 years of the project. G. Tidmarsh of Texaco CanadaPetroleum Ltd. provided the data upon which thedrilling waste release scenario was based. These bio-logical applications are dependent upon bblt and wethank E. Gonzalez and Z. Xu for their role in its devel-opment and for carrying out the Georges Bank applica-tions. We thank S. Armsworthy for assistance with thebiological effects calculations and G. Robert, and G.Black, for providing scallops stock distribution data.And finally, we thank B. Hargrave and P. Boudreaufor constructive comments on the manuscript.

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