RELATIONS BETWEEN SUSPENDED MATTER AND SALINITY IN

ESTUARIES OF THE ATLANTIC SEABOARD, U.S.A. C1)

Robert H . MEADE U.S. Geological Survey

Woods Hole, Massachusetts, U.S.A.

ABSTRACT

Despite the suggestion by laboratory experiments that suspended matter in rivers is flocculated and deposited where it reaches salt water, the concentrations of suspended matter in estuaries associated with some moderately large rivers of the Atlantic seaboard show no accelerated decrease that can be attributed to salt flocculation. Velocities of the estuarine waters are apparently sufficient to obscure the expected effects of salinity and keep the material in suspension. The general decrease in suspended matter associated with increasing salinity in the seaward direction can be attributed to the simple dilution of river suspensions by sea water.

Superimposed on the general seaward decrease in suspended concentration is a maximum concentration near the upstream limit of sea salt. Suspended concentrations decrease both upstream and downstream of the maximum. This maximum has been found at one time or another in all the estuaries that have been sufficiently studied. It seems to reflect the accumulation of suspended matter near the salt limit by net land­ward flow along the estuary bottom.

INTRODUCTION

The material suspended in estuaries is a complex mixture of organic and inorganic constituents, and its behavior is influenced in complex ways by the dissolved constituents and dynamic characteristics of the estuarine waters. The purposes of this paper are (1) to summarize the available information on the composition of suspended matter and its relation to salinity, and (2) to isolate unsolved problems associated with the com­position, transportation, and deposition of suspended matter in the major estuaries of the Atlantic coast of the United States between Cape Cod and Cape Canaveral (fig. 1). The conclusions reached in this paper are mostly the same as those stated earlier by Nelson (1959, 1960) on the basis of his studies of the estuaries of the York and Rappa­hannock Rivers, and by Postma (1967) in his excellent summary of sediment transport and sedimentation in estuaries. Most of the field evidence presented here, however, was not available to Nelson or Postma.

In this paper, I use the term "salinity" in a descriptive and qualitative sense rather than in its precise chemical sense as a measure of the total inorganic solids dissolved in water. Specific proportions of dissolved salt are expressed in terms of chloride concentrations.

COMPOSITION OF SUSPENDED MATTER

Because the composition of suspended particles affects their relation to salinity, we should first summarize the available information on the composition of suspended matter in estuaries and rivers of the Atlantic seaboard. Table I summarizes data on the proportions of organic matter in estuaries, as indicated by the weight lost when dry suspended matter is ignited at 500° to 700 °C. The three estuaries in the table are listed from north to south and also coincidentally in order of increasing influence of river

(1) Publication authorized by the Director, U.S. Geological Survey. Contribution No. 2003 from the Woods Hole Océanographie Institution.

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Fig. 1 — Map of Atlantic seaboard of United States, showing locations of estuaries and rivers mentioned in text.

inflow. The Long Island Sound sampling station was purposely located to minimize the effects of rivers, whereas Charleston Harbor receives most of the flow of the Santee River which (according to Dole and Stabler, 1909) carries the largest suspended load of any river on the Atlantic seaboard. The important thing to notice is that large proportions of combustible organic matter are always present in suspension in estuaries, even where inorganic concentrations are large and even in waters near the bottom where much less organic matter is produced than at the surface.

The identity of the noncombustible material suspended in the Altantic coast estuaries—the ash that remains after ignition at 500° to 700°—is poorly known. According to a recently published report (Bond and Meade, 1966), the ash component

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TABLE I

Average concentrations of total suspended matter and average proportions of organic suspended matter in estuaries of the Atlantic seaboard

Estuary Depth

Total Suspended

Matter (mg per liter)

Organic Suspended

Matter (percent dry weight)

lost on ignition)

Long Island Sound (Riley, 1959)

Chesapeake Bay (Bond and Meade, 1966) (Patten, Mulford, and Warinner, 1963; Patten and Warinner, 1961). (Patten, Warinner, and Eayrs, 1961 ; Patten Young, and Roberts, 1963, 1966) Charleston Harbor (Federal Water Pollution Control Administration, 1966)

Surface 4.5

Surface Surface

Surface Bottom

Surface Bottom

2.6 9.7

10.0 34.9

32.3 51.7

40

50 34

35 21

28 27

of the suspended matter in surface waters of Chesapeake Bay consists mostly of diatom frustules and amorphous aggregates of oxides of iron and perhaps other elements. Only a small proportion of the suspended matter, 2 to 11 percent of the total, consists of recognizeable mineral grains. Similar data that we have obtained in other estuaries along the Atlantic seaboard (Manheim and others, 1966) tend to confirm the obser­vation that mineral grains are not the major noncombustible component of the suspen­ded matter.

Perhaps more to the point is the composition of suspended matter in rivers—the material that is subject to flocculation as it flows into estuaries—but published infor­mation on its composition in the Atlantic drainage is scarce. A series of measurements of suspended matter made frequently throughout the year in the Middle Oconee River, a tributary of the Altamaha (Nelson, 1957, pp. 98-100), gave an average of about 13 percent combustible organic matter. A similar series of measurements made by Weber and Moore (1967) in the Little Miami River of Ohio (not in the Atlantic Drain­age, but close enough that results should be applicable) gave an average of about 16 percent. Samples collected near the peak of the spring diatom flowering in the Roanoke River and the fresh waters of Albemarle Sound (Manheim and others, 1966) yielded 24-33 percent combustible organic matter. These few data suggest that the proportion of organic matter suspended in rivers during most of the year is about half the proportion suspended in estuaries. Although the mineral assemblages have been determined in some of the rivers (Griffin and Ingram, 1955; Heron and others, 1964; Neiheisel, 1966; Powers, 1957), little is known of the proportions of mineral grains in suspension. Measurements made in fresh waters that we collected during the springs of 1965 and 1966 from the Roanoke River (Manheim and others) and the Altamaha and Potomac Rivers (unpublished) suggest that mineral grains make up only a small fraction, on the order of 10 percent, of the total suspended matter.

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FLOCCULATION OF SUSPENDED MATTER

When a salt solution is added to a suspension of fine-grained particles in the laboratory, the particles usually flocculate and settle out of suspension at a rate greater than their rate of settling in salt-free water. This effect must be evaluated in any study of the relations between salinity and suspended matter.

A closer look at the process of flocculation shows that it depends on two effects, collision and cohesion. That is, particles must be brought together and, once together^ must stay together. The important factors that influence these effects in nature are the salinity, the composition and concentration of the suspended matter, and the agitation of the water. Increasing salinity reduces the repulsion and thereby enhances the cohesion between particles. Increasing suspended concentration enhances the proba­bility of collision. Increasing agitation of the water promotes flocculation by increasing the rate of collision, and it inhibits by disrupting the larger floes. More thorough discussions of these effects are given by Krone (1962, pp. 11-15,46-50), Overbeek (1952), and Verwey and Overbeek (1948).

Experimental evidence

The relations between salinity and suspended matter in some experiments conducted in still water are shown in fig. 2. Suspended particles are flocculated mostly at chloride concentrations between 0 and 3 parts per thousand and further increases in salinity beyond about 3 parts per thousand usually do not cause significant further increases in the degree of flocculation as reflected in the rate of settling. This conclusion is supported

2 0 ~2

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1000 MG

. 530 MG PER L

l , - ^ 220 MG PER L

fc- ' . . • 1 1 I 1 1 1 1 1 1 1 1

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A.

520 MG PER L

: 1 1 1 : • 1

K A O L I N I T E

M O N T M O R I L L O N I T E

CHLORIDE /PARTS PER THOUSAND)

Fig. 2 — Results of laboratory experiments on relations between settling velocities of fine-grained materials and chloride content of water. A, Different initial concentra­tions of mud from San Francisco Bay, consisting of illite, montmorillonite, kaolinite, quartz, chlorite, organic matter, and amorphous iron oxides (Krone, 1962, p. 21). B, Different clay minerals in suspensions of 2000 mg clay per liter of water at 26° C (Whitehouse and others, 1960, p. 34).

99

by similar results in other experiments, one on the flocculation of Australian river muds (Rochford, 1951, pp. 35-36) and another on estuarine sediments from British Guiana (Postma, 1967, p. 161). The effects of the concentration and composition of the suspen­ded matter show in figure 2 as greater settling rates in suspensions of greater initial concentrations (A) and differences in the settling velocities of different clay minerals (B).

The interactions of the salinity of the water and the concentration and composition of the suspended matter are complex. Note, in the most concentrated suspension repre­sented in figure 2A, that the settling velocity continues to increase with increasing chloride concentrations well above the 2 or 3 parts per thousand at which the less concentrated suspensions reach their maximum settling velocities. The observation that the more concentrated suspensions continue to increase in settling velocity with increasing salinity is supported by Gripenberg's experiments (1934, pp. 75-76) on the coagulation of concentrated suspensions (9,400 to 10,600 mg per liter) of fine mud from the Baltic Sea. Another complication is brought out by the report by Whitehouse and his coworkers (1960, pp. 32-33) that, in their experiments on pure clay minerals (fig. 2B), the settling rates were not affected by changes in the concentration of the suspension in the range between 0 and 3600 mg per liter. Why their results should differ so markedly from Krone's results shown in figure 2A is uncertain: perhaps suspensions of pure clay minerals behave differently from suspensions (such as those studied by Krone) that include other mineral and organic constituents.

Flocculation and settling of suspended particles are also subject to the effects of strong flow. In flume studies made by Krone (1962, pp. 41-50) on flowing suspensions of mud from Sap Francisco Bay, the internal shear in the water limited the maximum size and, therefore, the maximum settling velocity that the floes could reach. In these experiments Krone used suspensions less concentrated than 300 mg per liter and chloride concentrations of about 9 parts per thousand. A result of these experi­ments was a lack of progressive accumulation of sediment on the bottom of the flume when the shear on the bottom sediment was greater than about 0.6 dyne per square cm—a condition that was reached at flow velocities of about 20 cm per second. At these velocities, the rate of deposition of suspended matter was equaled by the rate of scour and resuspension of the bottom sediment.

To sum up all these experiments: The effects of dissolved salts on suspended matter are complicated by the influences of the internal shear of the water and the concentration and composition of the suspended matter itself. Whether or not river-borne suspended matter is precipitated by flocculation where it meets salt water at the head of an estuary is determined by the interplay of all these factors. Figure 3 compares (1) the relation to chloride that one might expect if suspended matter were precipitated by salt floccu­lation to (2) the relation to be expected if the suspension were merely diluted by sea water that contained little suspended matter of its own. Neither of these simple relations, as we shall see, completely describes the observed distributions of suspended matter in estuaries.

Fig. 3 — Digrammatic comparison of expected effects of precipitation of river-borne suspended matter at low salinity to expected effects of simple dilution of suspensions by sea water. A, Suspended matter versus chloride. B, Changes in suspended matter and chloride along the length of an estuary.

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Field example: Savannah River estuary

Some of the impressions gained from the experiments on flocculation can be tested with data collected by the U.S. Army Corps of Engineers in the Savannah River estuary. Most of the data are similar to those shown in figure 4: hourly measurements of velocity and salinity and less frequent measurements of suspended matter at different depths through a tidal cycle at stations in the estuary. The suspended matter was determined by filtration, salinity (which 1 converted to chlorinity) by conductivity measurements, and velocity by current-meter measurements. These results show that suspended matter is stirred off the bottom during peak velocities and that it settles down again near slack water.

ki 5?

k, :\

St

ki

10-

-t-^S900'~rh -,—i i i 5000^°^

SUSPENDED MATTER

(MG PER L I T E R )

V E L O C I T Y (CM PER SEC)

CHLORIDE (PARTS PER THOUSAND)

16 20

TIME (HOURS AFTER MIDNIGHT)

Fig. 4 — Relations of suspended matter to depth, velocity, and chloride through a tidal cycle at station 16 km above mouth of Savannah River estuary, 20 July 1950 (modified after U.S. Army Engineer District, Savannah, 1961, app. B, pt. 2, sheet 58C). Positive velocities are seaward; negative velocities are landward.

The concentration of suspended matter near the surface (fig. 4) seems to be inversely related to chloride. Following this relation a step further, the two variables can be graphed against each other, using two types of data. Each graph in figure 5 contains data that were collected in a given cross section of the estuary at different times through a tidal cycle. Data in figure 6 were collected at different stations up and down the estuary at the same stage of the tide. Data in both figures were taken from near-surface samples collected in sections of the estuary, such as the one represented in figure 4, where the

101

water was deep enough that only a minimal effect of stirring of the bottom was felt at the surface.

M

A.

-k

CHLORIDE (PARTS PER THOUSAND)

Fig. 5 — Relations between suspended matter and chloride in waters collected within a meter or two of the surface through tidal cycles at stations in the Savannah River estuary, 1950. A, 16 km above mouth, 20 July. B, same location as A, 14 September. C, 19 km above mouth, 28-29 September. Data from U.S. Army Engineer District, Savannah (1961, app. B, pt. 2, sheets 58B, 58C, 38, 56B, 56C). Compare with fig. 8.

B. 200-

100-

O-

\ Low \ slack

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A High

s lack

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0 2 4 6 tO 12 14

200-

100-

0-

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. — - * Low slack

a \

High slack

8 |

CHLORIDE (PARTS PER THOUSAND)

Fig. 6 — Relations between suspended matter and chloride during slack tide in waters collected within a meter or two of the surface of the Savannah River estuary. A, at stations 0 to 16 km above mouth, 28 July 1950. B, at stations 6 to 19 km above mouth, 13 December 1951. C, same stations as A, 12 October 1950. Data from U.S. Army Engineer District, Savannah (1961, app. B., pt. 2, sheets 37, 41, 43, 45, 47, 49; 9, 13, 17, 19, 21 ; 39, 42, 44, 46, 48, 50). Compare with fig. 7B.

The graphs in figures 5 and 6 give a misleading impression of the true relations be­tween suspended matter and salinity. Taking them at face value, one might say that the salt does indeed cause the suspended matter to flocculate at chloride concentrations between 0 and 2 parts per thousand, or just as expected from the experimental results shown in figure 2. This conclusion, however, is doubtful because it involves the assump­tion that the maximum concentration of suspended matter shown in each graph represents the concentration in the river water flowing into the estuary—and the assump­tion is not true. Measurements made daily above the head of tide in the river, at a station 90 to 95 km upstream of the head of salt, during the week previous to the dates of the surveys showed that the concentrations of suspended matter in the inflowing river were substantially less than the maximum concentrations measured in the estuary (compare the data in table 2 with those in figs. 5 and 6). That is, the suspended matter may be more concentrated near the upstream limit of sea salt than in the inflowing fresh water.

102

TABLE 2

Concentrations of suspended matter in Savannah river water at Clyo, Georgia, 105 km above mouth

(U.S. Army Engineer District, Savannah, unpublished data)

Daily suspended concentration p . during week preceding date

Date of Survey • . . . of survey (mg per liter) in this paper

Maximum Mean

20 July 1950 5A 112 86 14 September 1950 5B 134 87 28-29 September 1950 5C 62 52 28 July 1950 6A 148 89 13 December 1951 6B 52 46 12 October 1950 6C 37 34

MAXIMUM SUSPENDED CONCENTRATIONS NEAR THE UPSTREAM LIMITS OF SEA SALT

Nelson (1959, 1960) observed that the concentration of suspended matter in two estuaries of the Atlantic seaboard, the Rappahannock and York Rivers, reached its maximum near the transition between fresh and salt water and diminished upstream and downstream of the transition. Postma (1967, pp. 172-173) has summarized the observations of similar turbidity maxima in estuaries of northern Europe and British Guiana.

Maxima of suspended concentration in three river-mouth estuaries of the Atlantic seaboard are shown in figure 7, a series of one-dimensional plots of near-surface concentrations of chloride and filtered suspended matter. In the observations made in the estuaries of the York and St. Johns Rivers (figs. 7A and 7C), the concentrations of suspended matter (dark circles) are greatest where the chloride concentrations (open circles) begin to increase. In the Savannah River estuary, although no data were avail­able for a synoptic longitudinal transect of the fresh water-salt water transition zone, a transect could be approximated by combining data taken at low and high slack water at four stations (fig. 7B); and again a maximum concentration of suspended matter is apparent near the transition from fresh to salty water.

The maximum concentration of suspended matter follows the fresh water-salt water transition as it moves up and down the estuary with the tides. Figure 8 shows the variations in chloride and suspended concentration with time through a tidal cycle at a station in the Savannah River estuary. The peak concentrations of suspended matter passed the station at the same time that the water began to be salty on the incoming tide and to be fresh on the outgoing tide.

Zones of maximum suspended concentration extend from the surface of an estuary to the bottom as shown in a series of sectional plots (fig. 9) of turbidity and chloride in Chesapeake Bay between the head of the bay and a point about 30 km up the bay from the mouth of the Potomac River. This part of the bay is essentially the estuary of the Susquehanna, which discharges more fresh water than any other river on the Atlan­tic seaboard of the United States. The data are not as certain as those in figures 7 and

103

3 lu

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<0

100-

50-

0-

A.

•-"" / „^*

A / \ / \ /

Low slack

High\ slack y

25 20 (5 25 20 (5

ivX

15 Uj

5 •to s

DISTANCE ABOVE RIVER MOUTH (KILOMETERS)

Fig. 7 — Maximum concentration of suspended matter near upstream limits of sea salt in three estuaries. Dark circles represent suspended matter; open circles represent chloride. A ,York River estuary, surface (?) concentrations, 24 March 1960; modi­fied after Nelson (1960). B, Savannah River estuary, near-surface concentrations at low and high slack water, 14 September 1950; data from U.S. Army Engineer District, Savannah (1961, app. B, pt. 2, sheets 32, 34, 35, 38). C, St. Johns River estuary, near-surface concentrations at high slack water, 16 November 1955; data from Pyatt (1959, app. E). Note different distance scale (by factor of 2) in B and different suspended-matter scale (by factor of 5) in C.

It1 i * ' - iso—I

g S; loo-j

x8

r /

/ X \

/

1 1 1 1

1 5.

4 8 12 16 TIME (HOURS AFTER MIDNIGHT)

Fig. 8 — Changes in suspended-matter concentration, chloride concentration, and velocity of near-surface waters through a tidal cycle at midchannel station, 16 km above mouth of Savannah River estuary, 14 September 1950. Data from U.S. Army Engineer District, Savannah (1961, app. B, pt. 2, sheet 38). Compare with plot of same suspended-matter and chloride data in fig. 5B.

104

8 because (1) the suspended matter was measured by light extinction rather than by filtration and (2) the surveys were made over periods of a few days and at random stages of the tide. However, the sections do show how the turbidity maxima are distributed in two dimensions with respect to chloride, and how the turbidity in the maximum zones decreases generally from the bottom to the surface.

t! /I \.05-:08- *\ : <.05

l 1 1 ' 1

300 250 200 -150 DISTANCE ABOVE BAY MOUTH (KILOMETERS)

Fig. 9 — Relations between turbidity and chloride along axial sections of northern half of Chesapeake Bay between mouths of Susquehanna and Potomac Rivers. A, 10-13 October 1949. B, 24 October-2 November 1950. C, 18-23 January 1951. Turbidity expressed as extinction of wavelengths 700 mn (A) and 600 mn (B and C) ; chloride in parts per thousand. Modified after Stroup and Wood (1966, p. 60, 142, 159) and Whaley and Hopkins (1952); data from Chesapeake Bay Institute (1950, 1951a, 1951b). Bay-bottom profile greatly simplified. Vertical exaggeration 600 times.

105

Maximum concentrations of suspended matter have been found at times near the limit of sea salt in the most extensively studied estuaries of the Atlantic seaboard. They were present in upper Chesapeake Bay, either at or within 15 km downstream of the salt limit, in all the thoroughly-sampled surveys summarized ;by Stroup and Wood (1966). Nelson (1960) stated that he observed them consistently throughout the year near the surface of the Rappahannock and York Rivers, regardless of whether the fresh-water inflow was great or small. Morris Brehmer of the Virginia Institute of Marine Science (M.M. Nichols, written commun., 1967), in a series of 12 surveys of the James River, observed suspended maxima near the bottom in 7 of the surveys and near the surface in 3 or 4 of the surveys; the surface maxima were observed mainly in winter and spring. Judging from the data in figures 5-8 the suspended maxima seem to be common in the Savannah River estuary. Just how prevalent they are in other estuaries of the Atlantic coast—especially in estuaries having different physical and dynamic characteristics— remains to be. found out in further surveys.

The composition of the suspended matter within the zones of maximum concen­tration is poorly known. Surveys of inorganic phosphate and chlorophyll that accom­panied some of the surveys of turbidity in Chesapeake Bay (Stroup and Wood, 1966) show no particularly consistent evidence that the material suspended in the maximum

i l : 30 25 20 15 10 5 0

DISTANCE ABOVE RIVER MOUTH (KILOMETERS)

Fig. 10 — Relation of flow pattern to sediment accumulation in Savannah River estuary (Simmons, 1965). A, Flow predominance, as indicated by contours of percent of net flow that moves downstream through a tidal cycle under average conditions. B, Average accumulation of sediment in navigation channel, as indicated by dredging records. Note that most of the sediment accumulates near convergence of upstream- and downstream-moving bottom flows (where 50-percent flow contour intersects bottom).

106

is predominantly either organic or inorganic. Considering the generally large propor­tions of organic matter suspended in the Atlantic seaboard estuaries (table 1), more details of the identity of the material suspended in the maximum zones might shed some light on the causes and effects of the maxima.

At the present state of our knowledge and understanding, the cause of the maximum concentrations of suspended matter seems to be the unique pattern of nontidal circu­lation that is characteristic of estuaries. In the saline reaches of moderately stratified coastal-plain estuaries—which includes most of the estuaries discussed in this paper— the salty water moves landward along the bottom, mixes with fresher water from farther upstream, and flows back down the estuary near the water surface (Pritchard, 1967, pp. 38-39). The point at which the upstream-moving bottom water ot the estuary converges with the downstream-moving bottom water of the inflowing river is marked by the proximity of the upstream limit of sea salt. The point where the two flows converge also serves as a partial trap for the sediment that moves on or near the bottom. Some of the sediment is deposited at the convergence, as shown by comparing the flow predominance and the sediment dredging in the Savannah River estuary (fig. 10); and some of the sediment is carried vertically upward with the converging waters to form a zone of maximum suspended concentration. (Tidal mixing and other diffusion processes may dislocate the maximum suspended concentration, the limit of sea salt, and the upstream limit of net upstream bottom flow from each other, but they are usually fairly close together).

One might expect the intensity of the maximum suspended concentration to be directly related to the intensity of the two-layer estuarine circulation. The three pairs of sections in figure 9 have been selected and arranged to suggest that the greater the development of the two-layer flow (as indicated by the degree of stratification of the chloride distribution), the more intense the turbidity maximum. Postma (1967, p. 172), however, suggests that the amount of suspended matter in the river or the sea is the primary factor influencing the intensity of the turbidity maximum whereas the strength of the estuarine circulation is only secondary. The influences of these two factors are difficult to separate in field studies because both are directly related to the quantity of inflowing fresh water.

CONCLUSIONS

The available evidence supports B.W. Nelson's earlier conclusions that suspended matter is not necessarily precipitated from suspension in estuaries when fresh water meets salt water and that the seaward decrease in suspended concentration can be attributed to progressive downstream dilution by sea water. Although the suspended matter may flocculate at low salinities, as predicted from laboratory experiments, it may not be deposited in estuaries where velocities and internal shears are strong enough to inhibit settling or to disrupt the flocculated aggregates. Velocities of the required strength seem to be present in most or all of the estuaries of the Atlantic seaboard associated with rivers of moderate to large size. When one subtracts the superimposed maximum concentrations near the limit of sea salt, the downstream decrease in suspen­ded concentration from the river water through the estuary can be accounted for by simple dilution.

The maximum suspended concentration and the upstream limit of sea salt in an estuary often coincide. Their coincidence, however, does not mean that one is the cause of the other. Rather they both seem to be the results of a third factor—the pattern of dynamic nontidal circulation of estuarine waters.

107

ACKNOWLEDGMENTS

The Savannah District of the U.S. Army Corps of Engineers, particularly Mr. J.W. Harris and Colonels W.L. Barnes and P.W. Ramee, have patiently satisfied my several requests for copies of information that was either unpublished or distributed in a limited number of copies. I have profitted from discussions with J.D. Milliman, C.G.H. Rooth, and D.A. Ross of the Woods Hole Océanographie Institution, and from reviews of the manuscript by K.O. Emery of WHOI, F.T. Manheim of the Geological Survey, and M.M. Nichols of the Virginia Institute of Marine Science.

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