The influence of the pycnocline on the oceanic settling of manganese nodule mining waste

Marine Environmental Research 12 (1984) 127-142

The Influence of the Pycnocline on the Oceanic Settling of Manganese Nodule Mining Waste E. Ozturgut Ozturgut Oceanographics, 3006 N.E. 194th St., Seattle, WA 98155, USA

& J. W. LaveUe NOAA/Pacific Marine Environmental Laboratory, 7600 Sand Point Way N.E., Seattle, WA 98115, USA (Received: 14 November, 1983)

ABSTRACT The likelihood that manganese nodule mining discharge (essentially deepseabed clays and some nodule fragments) would reside on the pycnocline for a long period of time was investigated by introducing mining waste particles into a two-layer laboratory settling column illuminated by laser. The experiments were repeated with polystyrene particles of uniform shape and size to further study the effect of a density interface on settling. The results indicate that mining particulates do not have sufficiently low density to accumulate on the pycnocline although a density interface can temporarily concentrate settling particles. A review of related studies of accumulation of inorganic particles on density interfaces suggests that the available evidence for pycnocline accumulation of inorganic particles is slight. A measurement of the wet density spectra of any oceanic discharge is necessary to accurately assess the possibility of particles rafting on a density surface.

INTRODUCTION The residence time of inorganic particles in the upper oceanic water column determines the availability of particles for ingestion by organisms 127 Marine Environ. Res. 0141-1136/84/$03-00 © Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Great Britain

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E, Octurgut, J. 14. Lavelle

or exchange with ambient water of the toxic and non-toxic substances often associated with particles. Previous studies of particles from both natural sources and ocean waste disposal activities have suggested that the pycnocline, the region of large vertical density gradients near the ocean surface, strongly influences the residence time. Manganese nodule mining waste (Ozturgut et al., 1981) has been one ocean discharge not studied within this context. Field observations had suggested, however, that flocculation was a factor in determining settling velocities of the discharged material (Lavelle et al., 1982) and therefore particles of sufficiently low density to remain in the pycnocline could be present in the discharge plume. Here we present results of laboratory experiments designed to study the possibility of the long-term residence of manganese nodule mining discharge at the pycnocline and we discuss the results in relation to earlier studies. ~Fhe impetus for this work comes from previous observations ot" the influence of the pycnocline on inorganic particle distributions. These studies have been made in relation to sand and gravel mining (Hess & Nelsen, 1975); discharges of chemical waste (Orr & Hess, 1978; Orr & Baxter, 1983; Brown & Kester, 1983); sewage sludge (Proni & Hansen, 1981); dredge spoil (Proni et al., 1976; Proni & Hansen, 1981); drilling mud (Brandsma et al., 1980) and pharmaceutical wastes (Proni & Hansen, 1981). In some of these cases, claims have been made that particles ~accumulate' in, or at the top of, the pycnocline. The implication is one of long-term or permanent residence. To be specific, 'accumulation' here means permanent residence, possible in actuality only if biological processes (e.g. pelletization, coagulation; Zabawa, 1978), which aggregate particles in the upper ocean and which lead to settling, are disallowed. Residence time in the euphotic zone (or in the pycnocline), rather than accumulation, would be a more useful, quantitative approach to the biological exposure problem, however. In this discussion, we implicitly focus on the vertical movement of particles, although horizontal processes in the ocean are acting simultaneously. These horizontal processes (e.g. horizontal diffusion, current shear, gravity flow) change particle concentrations and sometimes provide the appearance of accumulation, although, in fact, particles continue to settle downward. F'ive factors determine the vertical location and movement of"inorganic particles: the manner of introduction or discharge; the particle characteristics, including size and density; the salinity and temperature structure of the receiving environment; the vertical turbulence distri-

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bution and vertical advection. The first of these is of short-term importance only. It determines the distribution of particles at the time single particle settling and diffusion begins to determine the vertical distributions. For example, material might be discharged at a sufficiently high rate that convective plummeting through the pycnocline results (e.g. Proni & Hansen, 1981) or particles might rise with the positively buoyant discharge from an outfall to reach the pycnocline from below (e.g. Fischer et al., 1979), or particles might be carried along with a water-mass which is intruding laterally, regardless of the particle load (e.g. Drake, 1971 ; Pak et al., 1980). The second and third factors are linked, for spherical particles of submillimeter size, through Stokes' settling law (see Lerman (1979) for effects of shape on settling velocity). Accordingly, the settling velocity of particles depends on particle size, the density difference between the particle and the surrounding fluid and the fluid viscosity. For an equatorial pycnocline temperature change with depth from 25 ° to 10 o, there is an increase of the molecular viscosity of water by about 45 ~ , and a resulting decrease of about 30 ~ in the Stokes settling velocity. Thus a thermocline alone could retard (but not prevent) settling. Reduced density is important only when the density of the particles is close to that of the surrounding fluid (generally 1-02-1.03 g/cm3). Density differences across the pycnocline can be as much as 5 mg/cm 3. The particle's positive, neutral or negative buoyancy at any density level will depend on its density relative to the fluid below. Densities for oceanic particles have rarely been measured, although organic particle densities very near those of seawater are likely. For inorganic particles, one might expect densities higher than those of organics. However, clay particles, which may have individual densities of 2.6-2.7 g/cm 3, are known to flocculate in seawater, and the resulting aggregates are thought to have densities in the range 1.05-1.10g/cm 3 (Krone, 1963, 1976). Flocculated particles, which include those inferred to be part of the manganese nodule mining discharge (Lavelle et al., 1982) might therefore be candidate materials, if densities near 1"03 g/cm 3 were achievable, for slowed settling or pycnocline accumulation. Another candidate is acid-iron waste colloid which has a laboratory measured density of 1.03g/cm a for large flocs (Lagvankar & Gemmell, 1968). Unfortunately, in the previously cited field measurements in which the pycnocline has been observed to play a r61e in the vertical distribution of particles, there have been no measurements of particle density. The fourth factor determining vertical particle movement is vertical

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diffusion that acts to reduce vertical concentration gradients. In the turbulent mixed layer above the pycnocline, gradients are sustained with more difficulty than in the pycnocline where reduced turbulence is coincident with increased fluid stability. In neither region can turbulence prevent settling. For slowly settling particles, however, the differences of diffusivities in the mixed layer and pycnocline may temporarily result in the appearance of a barrier to downward transport (for dissolved material, a diffusion floor: Csanady, 1973). However, very small particles ( < 4#m) do not form a lens at the top of the pycnocline but are nearly uniformly distributed over the whole mixed layer - a result discussed by Lavelle & Ozturgut (1981 ). Vertical advection, the filth factor, may be important if the velocity is sufficient to offset the downward motion of a particle due to gravitational settling (e.g. Ichiye & Carnes, 1981). In oceanic divergence regions with upward velocities in the range of 10- 4 cm/s, particles with small settling velocity might be affected. For manganese nodule mining wastes, density and size have been the principal unknowns: their composite effect on settling through a density and viscosity gradient region has now been examined in laboratory experiments. The results do not show any support for pycnocline accumulation of this material. Furthermore, after closely examining claims made for other ocean waste discharges, we think that the previous field and laboratory studies have generally also not provided evidence ~ hich allows the conclusion that particles accumulate on the pycnocline, although, of course, temporary lenses of higher particulate concentration have been observed. The discussion here is limited to discontinuous discharge, LABORATORYSTUDY The potential tbr mining discharge to accumulate on a density interlace was examined in the laboratory settling experiments (details in Ozturgut, 1982) using a stratified settling column and deep ocean mining particles. The column consisted of a glass container, 50cm high, with sides of 2(I x 20 cm. A two-layer structure in the tank was created by first adding O the top layer (salt-water mixture with a salinity of 35.b,,, density of 1.022g/cm 3) and then gradually introducing a lower layer (10"/o by volume glycerol-water mixture) with a siphon placed at the bottom of the column. The 10Yo concentration of glycerol solution at a room

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temperature of 22°C approximated the density (l.0265g/cm 3) and the viscosity (0.0125 poise) of the waters in the deep ocean mining region at 150-200 m depths. The column was illuminated on a vertical plane with an argon ion laser light source which provided good optical contrast of particles and fluid. Time-lapse photographs were taken as the material settled. The mining particles were released from a specially designed holder so that no downward initial velocity was caused by the water level differences between the column and the release chamber (Ozturgnt, 1982). The mining discharge sample used for this study came from the pilot mining tests conducted by Ocean Management Inc. in the spring of 1978 in the equatorial Pacific (Ozturgut et al., 1981). The discharge samples (bottom water, fine sediment and some small nodule fragments) were obtained and frozen during the monitoring studies of that mining test. The salinity of the water in which the particles were stored was 34.7~. These laboratory experiments were performed in December, 1981. Prior to the settling experiment reported on here, size and standard settling velocity analyses (e.g. McCave, 1979) were conducted on five aliquots of the mining discharge sample. The results showed that the fraction of the sample coarser than 64 #m constituted, on average, 8.5 of the sample. A comparison of size distributions determined by Coulter Counter for a chemically dispersed and for an untreated sample from a second~ mining test showed very little difference (Lavelle et al., 1982), indicating few or no floes or a disaggregation of loosely bound floes in the counting process (Gibbs, 1982). On the other hand, earlier standard settling experiments showed concentration-dependent settling which is indicative of flocculation (Lavelle et aL, 1982). The experiment reported in this paper was undertaken at a concentration where floceulation might still occur and in a manner where floes that had densities close to that of the oceanic pycnocline (1.026 gm/cm a) might be detected. For the settling experiment in the stratified column, 20 ml of the mining discharge sample were diluted to 100ml, giving an estimated particle concentration of 200 mg/liter, which was then released using the sampleintroduction device. Figure 1 shows photographs selected from the timelapse photography conducted at intervals during this experiment. The largest mining discharge particles reached the interface (29 cm from the surface) in about 108s, a settling velocity of 0.28cm/s. According to the size analyses, less than 9 ~ of the sample had this, or faster, velocities. Some of the particles could be tracked, especially in the lower layer, from

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b

c

II d

e

t

Fig. I. Settling of manganese nodule mining discharge in a two-layer column, with the density-viscosity interface marked with an arrow. The time sequence is (a) 2 s; (b) 100 s; (c) 2.5 min; (d) 5 rain; (e) 10 rain and (f) 30 min. Graduations are every 2 cm. Densities in the upper and lower layers were 1.022 and 1-026, respectively, identical to those in the mixed layer and pycnocline in the deep-ocean mining area.

Influence of the pycnocline on settling

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one frame to another, allowing estimates of settling velocities to be made. Four such trackings resulted in an average velocity of 0.116 cm/s in the lower layer, and 0-16 cm/s in the upper layer. This reduction of settling velocity in the lower layer by about 27 ~ is almost entirely due to the increased viscosity. No accumulation at the interface was discernible. Even in observations made 20 h later, when more than 97 ~o of the mining discharge would have settled to the interface level, accumulation on the interface or in the lower layer was not evident. This experiment was repeated with uniform-size and low-density polystyrene particles. These particles, 42.5/an and 90/an in diameter, are those used for Coulter Counter calibration; their specific gravity, as determined after these experiments, was 1.065. In these experiments the upper layer was distilled water and the lower layer, glycerol solution with a density of 1.0505g/cm 3 and a viscosity of 0.018 poise. In the experiment, the 42.5 #m polystyrene particles settled as a band (Fig. 2) because of their uniform spherical shape and very narrow size distribution. Initially, the polystyrene particles appeared to settle at a faster velocity than that expected from Stokes' law, although the Reynolds number was very much smaller than 1. Since single particles should settle according to Stokes' law at low Reynolds number, the particles appear to have been settling convectively immediately after their introduction into the settling tube. This more rapid settling proceeded down to an initial mixing depth of 14cm. Below this depth, however, down to the interface level, the average settling velocity of the particles derived from the time-lapse photographs was 6.7 x 10 -3 cm/s, which is very close to the Stokes settling velocity of 6.4 x 10-3cm/s. Similarly, below the interface, the settling velocity inferred from the photographs was in good agreement with Stokes' law. For example, in Fig. 2, the particles appeared to have settled about 3.4cm between hours 1 and 2, giving a settling velocity of 9.4 x 10-4cm/s, near the Stokes velocity of 7-5 x 10-4 cm/s. Samples were taken for Coulter Counter analyses in the vicinity of the concentrated band of the second layer to provide a quantitative estimate of concentration: the numbers of particles per milliliter at 1 cm above and below the concentrated band were 540 and 80, respectively, whereas, at the nominal center of the band, the particle number was 3040 particles per milliliter. The maximum corresponds to a concentration of approximately 130mg/liter and an inter-particle separation of the order of seventeen particle diameters. No convective settling of particles at these concentrations was observed.

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a

d

b

c

e

f

Fig. 2. Settling of polystyrene spheres (diameter 42.5/~m) in a two-layer column at times: (a) 2 s; (b) 3.5 min: (c) 13 rain; (d) 20 min; (e) 1 h and (f) 2 h. Densities in the two layers were 1-0 and 1.0505 g/cm 3, respectively, while particle density was 1.065 g/cm 3. The particles are supplied by the manufacturer in a dyed solution (visible as an interface at approximately 14cm in (e) and (f)) which remained in the upper layer as the particle fraction settled.

Influence

of the pycnocline

on settling

135

A theoretical model can summarize the essential features observed. Let the indices 1 and 2 represent the upper and lower layers of a two-fluid settling column. Since the laboratory experiment represents discontinuous discharge, a time-dependent model is appropriate. If material is injected at a point, z', above the interface (z = 0) at t = 0, the settling of the material is described by: 0C1 ~t

0C1 wl ~ = Co 6(0 6(z' - z)

c~C2

0C2 w2 ~ = 0

Ot

(1)

where C i and wl (i = 1,2) are the concentrations and the Stokes settling velocities in each of the two layers, t is time, z the vertical co-ordinate (positive upward) and 6 is the Dirac delta function. It is presumed that the fluid in the tank is still; diffusion is molecular and unimportant relative to settling 6f particles of typical oceanic size. At the boundary (z = 0), the fluxes are matched: w l c I = w2 C 2

z=0

(2)

The solutions to eqn. (1) with the conditions of eqn. (2) are: C~ = Co 6(z' - w i t - z)

C 2 = Co(wffw2) 6(z' - w i t - ( w f f w 2 ) z )

(3)

Suppose the material is uniformly distributed from the surface (which is at height, h, above the interface) to a depth 1. That initial distribution can be written: Q(z') = Co

h - l < z' < h

=0

z'
(4)

If, in addition, the settling velocity of the material in the first layer were log-normally distributed with a mean velocity of wo and variance ~r2: P(Wl) = 1/(2~0"2) 1/2 e-(l°gt°Wl --10810W0)2/20"2

(5)

the concentrations in each layer at points z and time, t, would be: C 1 = C o loglo e

~

h - zl/t

P ( w 1) d w f f w l

d (h - 1 - z)/t

C2 = Co(Wl/W 2) logxoe " r -p')/t J ( h - t - pz)/t

P ( w l )dW ffW 1

(6)

E. O:turgut, J. W. Lavelte

136

k

t~

I CD W

I

~

f FJZ~

i

t:O

t = 24

- -4-2

:t44

'

30L

1~2

0 !0 IO%o C

Fig. 3. M o d e l results in s i m u l a t i o n o f the settling o f polystyrene spheres (w o = 6.7 x 10 -3 cm/s, a = 0.02, w J w 2 = 10-0, h = 21 cm, l = 14cm). C o n c e n t r a t i o n s are scaled l o g a r i t h m i c a l l y a n d time is i n d i c a t e d in minutes.

where fl WI/W2, a fixed ratio. These formulae (eqn. (6)) have been evaluated for the case of polystyrene particles in a configuration similar to that of the experiment (w 0 = 6 . 7 x 10-3cm/s, a =0.02, wl/w 2 = 10.0, h = 21 cm, l = 14 cm). The ratio of velocities is the result of the density of the lower layer (1-0505 g/cm 3) being close to that of the settling particles (1.065 g/cm3). A time sequence of results (Fig. 3) shows a collapse of the cloud of particles on the interface which then slowly settles through the second layer. Thus, a pycnocline can concentrate settling particles, but unless w2 = 0 (eqn. (6)), no permanent enhanced concentration will occur. w2 can be zero only if the density of the particle is less than, or equal to, that of the lower layer, or if the lower layers were advecting upward at the same speed with which the particle settles. Vertical diffusion, which might be another significant vertical transport process in some environments, will only broaden the concentration profiles and slow the clearing of the upper layer. =

DISCUSSION Observations of increased concentrations of inorganic materials at a density interface (not restricted to the main pycnocline) have generally been of two types: lateral intrusions from distant sources and from point discharges. Drake (1971) demonstrated that the off-shore vertical distribution of particulates, which emanated as flood discharge of the

Influence of the pycnocline on settling

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Santa Barbara River, exhibited lenses of higher concentrations, and these were associated with depths of measurable thermal stratification. These lenses were presumably created intrusively as Pak et al. (1980) have argued for mid-water column particle maxima off Oregon. Intermediate density water bearing particulates (which may themselves have no significant effect on density) intruding between the upper and lower layers of stratified water is also common at depth in reservoirs (e.g. Fischer et al., 1979). Similarly, Proni et al. (1976) observed dredge material advecting seaward on isopycnal surfaces just offshore from the Miami ship-channel. Although the end result of an intrusion is a more concentrated lens of particulate at some depth, the existence of the lens does not require its longevity or permanence. Although particles may be impeded by the stratification from vertically diffusing, settling will deplete the excess concentration in the lens, unless the supply of new material is continuous. The time-scale for the disappearance of the layer will depend on the magnitude of the settling and might be quite long. For 1 #m diameter spherical quartz particles, the Stokes settling is approximately I0 m in a year, a time-scale in which biological processes may dominate. Nonetheless, intrusions generally cannot lead to accumulations unless the settling velocity at some intermediate depth is nil. Intrusions can be associated with point discharges as well. Acoustic studies of the ocean dumping of sewage sludge (primarily organic) in New York Bight by Proni & Hansen (1981) revealed a lens of particles emanating from the column-core of the dump along the thermocline. The column is the result of a high bulk density discharge convectively plummeting to the bottom. The lens may result from clearer water being entrained along the circumference of the column, causing part of the fluid-sediment mixture to have a density near that of the pycnocline. The mixture could then intrude along the isopycnal interface, retarded only by the small frictional resistance offered by the reduced vertical turbulence in the region of stratification. Unless the particles in the plume are neutrally buoyant, however, this intrusive lens will not be sustained as a particlerich layer. Density spectra for the particles in a sewage discharge are, to our knowledge, yet unmeasured, so that a conclusion of accumulation is unjustified. Pharmaceutical waste disposal off Puerto Rico has also been studied acoustically (Proni & Hansen, 1981). These authors claimed that the vertical movement of the dumped material (2 % solids) was limited by temperature gradients, although their published measurements are

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E. Ozturgut, J. W. Lavelle

ambiguous in showing a density gradient effect. While they conclude that 'gradient surfaces act as zones of accumulation', no measure of the density of the particles was given. Since tracking a plume at sea generally admits to only short-term observations, a conclusion about pycnocline accumulation will generally not be possible without particle density measurements. Hess & Nelsen (1975) used sphalerite and glass particles in the form of a slurry to simulate the overflow of a hydraulic dredge during sand and gravel mining in Massachusetts Bay. They also reported effects on the vertical distribution as a result of a region of stratification. Since sphalerite and glass beads have a specific gravity of 4-0 and 2.65, respectively, however, the absence of tracer particles below the pycnocline was more probably the consequence of vertical current shear moving particles off to unsampled regions or of insufficient vertical resolution in the sampling. High-frequency acoustic backscattering returns were used to study the dispersion of a precipitate formed during the disposal of chemical wastes (Orr & Hess, 1978). After 10-20 h past discharge, the lighter components are reported to have gradually concentrated on the isopycnal structure located at the mixed layer boundary and in between this boundary and the seasonal thermocline. On occasion, particles were also observed to penetrate the seasonal thermocline and form a diffuse cloud in the main thermocline. Whether negligible density contrast between the particles and the thermocline led to accumulation, or whether the material continued to settle and disperse, cannot be judged from the measurements. Orr & Baxter (1983) investigated the distribution of particles subsequent to the disposal of industrial waste from the two Dupont chemical plants. Particles found after discharge were hydrous iron flocs in one case, and magnesmm hydroxide flocs in the second. Orr & Baxter reported that the seasonal difference in the vertical distribution of the particles was caused by the density gradient between a shallow summer mixed layer and a deeper winter mixed layer. They noted that 'the shallow summer pycnocline traps the particles and appears to limit the vertical distribution to the mixed layer'. The density and size distributions of the particles involved are not known. In the case of hydrous ferrous hydroxide, however, laboratory studies show floc density greater than that of the fluid in which the measurements were conducted by only 3-7 x 10 - 3 g/cm 3. Although flocculation in the laboratory was induced in

Influence of the pycnocline on settling

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high alkalinity, simulated ocean water, discharge of the material into the near ocean might lead to nearly the same density result, which makes conceivable the particles being neutrally buoyant at the density interface. On the other hand, a recent laboratory experiment in a temperature stratified salt-water tank with hydrous ferrous oxide (Brown & Kester, 1983) does not support that conjecture. Particles which formed after the introduction of the acid-iron waste into the upper layer of a two temperature-layer column passed (though slowly) into the lower layer through settling. No accumulation of the material at the interface occurred. The experimental situation was somewhat like that modelled here, although each layer was uniformly mixed and turbulent and a wider range of settling velocities was involved. In summary, the evidence presented to date for pycnocline accumulation of inorganic particles is slight. We think a number of the reported conclusions have been misinterpretations, or based on observations over insufficient periods of time. It also has become apparent that residence time in the vicinity of the pycnocline, rather than accumulation, is a more appropriate approach to the problem. Residence time, however, for particles subjected to horizontal and biological processes, as well as vertical transport, is not easily defined or measured.

CONCLUSIONS Inorganic particles from discontinuous discharge can reside at a pycnocline on a long-term basis only when either the density of the discharged particle is such that it is neutrally buoyant at that level or if the downward settling is opposed by upward convection, as might occur in oceanic divergence regions. In the latter case, horizontal divergences would result in advective dispersion, however. For other conditions, where there is a net downward settling, however small, lenses of higher particulate concentrations may temporarily develop on pycnoclines, as the vertical distributions adjust to reduced settling and turbulent diffusion in the lower layer. This situation is, however, impermanent and not adequately described as an accumulation. Consequently, laboratory and field investigations directed toward pycnocline accumulation are fundamentally studies of particle densities, inasmuch as upward fluid convection is a factor only in limited geographical areas. In the laboratory study reported here, it has been

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E. Ozturgut, J. W, Lavelle

shown that no component of the manganese nodule mining waste discharge has a density which would cause it to raft on the pycnocline. Since the experiments were conducted in salt water and the particulates were primarily clay, at concentrations of approximately 200 mg/liter, few, or no, flocs existed or were created with a density less than 1.030 g/cm 3. Based on these laboratory experiments, manganese nodule mining discharge will not accumulate on the ocean pycnocline. For the other discharge measurements reviewed here, the evidence is weak for the discharges having a fraction with density which would be neutrally buoyant at an intermediate level in the water column, Nor, in general, can settling velocity spectra be inferred from the available data which would permit an estimate of the residence time of the particle on the pycnocline. Apparent accumulations, as observed in some of the earlier studies, can occur for several reasons, Along-isopycnal flow of particle-laden water may present the appearance of a vertically static lens of particles, which can, in fact, be settling. Additionally, shear flow between the mixed layer and the pycnocline may cause material in the pycnoline to be advected in an unexpected and unsampled direction, giving the appearance that the pycnocline is a barrier to vertical particle transport. Sampling of plumes in the open ocean is also difficult because plumes generally have small physical dimensions, and concentrations are quickly diluted by horizontal processes below the level of instrument detectability. In most cases, the plume is undetectable or lost to observation before sufficient time has elapsed to judge whether true accumulation is possible. Hence, the measurement of the wet density spectra of any discharged material is essential before claims can be made that discharged particles can raft on a density surface. ACKNOWLEDGEMENTS We thank Mr L. Kimrey of the National Oceanic and Atmospheric Administration, Office of Marine Pollution Assessment, for his assistance in the experiments and for the Coulter Counter measurements. We appreciate the encouragement of Mr John Padan of the National Oceanic and Atmospheric Administration. This work was supported by the Office of Minerals and Energy and the Office of Marine Pollution Assessment of the National Oceanic and Atmospheric Administration.

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REFERENCES Brandsma, M. G., Davis, L. R., Ayers, R. C. & Sauer, T. C. (1980). A computer model to predict short term fate of drilling discharges in the marine environment. Proceedings of the Symposium 'Research on Environmental Fate and Effects of Drilling Fluids and Cuttings', Lake Buena Vista, FL., January 21-24, 1980, 588-610. Brown, N. F. & Kester, D. R. (1983). Fate of ocean dumped acid-iron waste in a MERL stratified microcosm. In: Wastes in the ocean. Vol. 5. Deep-sea waste disposal. (Kester, D.R., Burr, W.V., Park, P.K., Duedall, I.W. & Capuzzo, J. W. (Eds)), Wiley-Interscience, New York. Csanady, G. T. (1973). Turbulent diffusion in the environment, D. Riedel, Boston, MA. Drake, D. E. (1971). Suspended sediment and thermal stratification in Santa Barbara Channel, CA. Deep Sea Res., 18, 763-9. Fischer, H. B., List, E. J., Koh, R. C. Y., Imberger, J. & Brooks, N. H. (1979). Mixing in inland and coastal waters, Academic Press, New York. Gibbs, J. R. (1982). Floc stability during Coulter counter size analysis. J. Sed. Petrology, 52(2), 657-60. Hess, W. N. & Nelsen, T. A. (1975). A test particle dispersion study in Massachusetts Bay. Proceedings of the Offshore Technology ConJerence, Houston, Tx, 1975, OTC 2160. Ichiye, T. & Carnes, M. (1981). Sediment dispersion and other environmental inputs of drag ocean mining in the eastern tropical Pacific Ocean. In: Marine environmental pollution. II. (Geyer, R.A. (Ed)), Elsevier, Amsterdam, 475-517. Krone, R. B. (1963). ,4 study of rheologic properties of estuarine sediments. Hydraulic Engin. Lab. & Sanitary E~gin. Res. Lab., Univ. of Cal., Berkeley, 91 pp. Krone, R. B. (1976). Engineering interest in the benthic boundary layer. In: The benthic boundary layer. (McCave, I. N. (Ed)), Plenum, New York, 143-56. Lagvankar, A. L. & Gemmell, R. S. (1968). A size-density relationship of flocs. J. ,4mer. Water Works Assoc., 60, 1040-6. Lavelle, J. W. & Ozturgut, E. (1981). Dispersion of deep-sea mining particulates and their effect on light in ocean surface layer. Marine Mining, 3, 185-212. Lavelle, J. W., Ozturgut, E., Baker, E. T. & Swift, S. A. (1982). Discharge and surface plume measurements during manganese nodule mining tests in the North Equatorial Pacific. Mar. Environ. Res., 7, 51-70. Lerman, A. (1979). Geochemical process: water and sediment environments, Wiley & Sons, New York. McCave, I. N. (1979). Suspended sediment. In: Estuarine hydrography and sedimentation handbook. (Dyer, K.R. (Ed)), Cambridge Univ. Press, London, 131-85. Orr, M. H. & Hess, F. R. (1978). Acoustic monitoring of industrial chemical waste released at deep water dump site 106. J. Geophys. Res., 83, 6145-54.

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Orr, M. H. & Baxter II1, L. (1983). Dispersion of particles alter disposal of industrial and sewage water. In: Wastes in the oceans. Vol. 1. Industrial and sewage wastes in the ocean. (Duedall, I. W., Ketchum, B. H , Park, P, K: & Kester, D. R. (Eds)), Wiley-Interscience, New York, 73-86. Ozturgut, E. (1982). Results of experiments on potential pycnocline accumulation by particulates discharged during mining of manganese nodules and by other particles. (Unpublished manuscript.) Ozturgut, E., Lavelle, J. W. & Burns, R. E. (1981). Impacts of manganese nodule mining on the environment: Results from pilot-scale mining tests in the North Equatorial Pacific. In: Marine environmental pollution, 2. Dumping and mining. (Geyer, R. A. (Ed)), Elsevier, Amsterdam, 437-74. Pak, H., Zaneveld, J. R. V. & Kitchen, J. (1980). Intermediate nepheloid layers observed off Oregon and Washington. J. Geophys. Res:, 85, 6697-708. Proni, J. R. & Hansen, D. V. (1981). Dispersion of particulates in the ocean studied acoustically: The importance of gradient surfaces in the ocean. In: Ocean dumping of industrial wastes. (Ketchum, B. H., Kester, D. R. & Park, P. K. (Eds)), Plenum, New York, 161-75. Proni, J. R., Newman, F. C., Rona, D. C., Drake, D. E., Berberian, G. A., Lauter, C.A., Jr. & Sellers, R. L. (1976). On the use of acoustics for studying suspended oceanic sediment and for determining the onset of the shallow thermocline, Deep Sea Res., 23, 831-7. Zabawa, C. F. (1978). Microstructure of agglomerated suspended sediments in Northern Chesapeake Bay estuary, Science, 202, 49-51.