Composition of estuarine colloidal material: major and trace elements

Composition of estuarine colloidal material: major and trace elements ANNE C. SIGLI:O* and GEOKGE R. HELZ Chemistry

Department,

University of Maryland. MD 20742. U.S.A.

College

Park.

Colloidal-sized material from the Patuxent River estuary. Maryland. was analyzed for more Abstract than 30 elements by instrumental neutron activation analysis. Sample data from stations ranging in salinity from 0.04 g/kg to 12 g/kg indicate that the winter colloidal material is dominated upstream by poorly crystallized clay minerals and iron oxide. but above a salinity of 10 g/kg. it consists primarily of carbonaceous material. Winter colloidal material decreases non-conservatively seaward from 29 to 0.6 mg/l. The summer colloidal material. however, is carbon-rich throughout the estuary and the amount of colloidal material in the upper water column does not change appreciably with increasing salinity. Many trace elements (Cs. Hf, Rb, SC. Th. V and the REE) covary with Al and probably are associated with the clay component. Other elements (As. Ba, Sb, Se. U and Zn) are considerably enriched relative to Al and may covary with the organic component. The results suggest that the freshwater winter colloidal system is a product of continental weathering processes. whereas the summer colloidal material is derived primarily from estuarine biological processes.

INTRODUCTION

THE GEOCHEMICALbehavior of trace elements during estuarine mixing of rivers with the ocean is strongly influenced by the chemical form in which the elements occur. While a conventional distinction is made between dissolved and particulate forms, many authors have observed that much of the ‘dissolved’ (i.e., 0.45 pm filter-passing) material actually may consist of sub-micron particles and organic macromolecules (BENESand STEINNES,1974; BOYLE et al., 1977; KKOM and SHOLKOVITZ, 1978; MURRAY and GILL, 1979; MOORE er al.. 1979). As yet, little is known about the origin, composition and nature of this colloidal-sized material. although the flocculation of ‘dissolved’ material in contact with seawater has received some attention (ANDREN and HAKRISS, 1975; SHOLKOVITZ. 1976, 1978; ECKERT and SHOLKOVITZ. 1976: BOYLE et a/., 1977; WOLLAST and PETERS, 1978). In order to describe the general chemical properties of estuarine colloidal matter, we have studied samples collected by ultrafiltration (after removal of particulates) from the Patuxent River estuary in Maryland. This paper deals with major and trace elements components, while companion papers discuss the isolation and identification of organic compounds (SIGLEO et al., 1980b; SIGLEO rt al., in preparation). SAMPLING

AND

of the area). farms (35’!,,) and suburbs (l5”,,). There IS no industrial development in the Patuxent watershed. except for a large power plant located near Station 3. Water samples (10 I.) were collected at 0.5 m depth with Teflon pumps from five stations (Fig. I) on February 6 and June 26. 1979. in order to obtain data representative of seasonal extremes. In particular. biological productivity is expected to be much lower in winter and to exert a greater influence in summer. A single sample (Station 6) was collected near the mouth of the Patuxent on July 2X. 1979. Water depth ranged from 6 to 60m. Durmg February Stations l-3 were under as much as lOcm of ice. The stations were selected to obtain samples with a range of salinities from zero (Station I) to 12 g.kg (Stations 5 and 6). Samples were collected on single high slack tldc runs by following the tidal crest. This procedure was used to mini-

ANALYSIS

The Patuxent, with a discharge of I.4 to 370 x lO* I/day. is a major tributary to Chesapeake Bay. Land use in the Patuxent watershed is primarily undeveloped forests (50”,,

* Present address: U.S. Geological Center, Reston. VA 22092. U.S.A.

Survey,

431 National Fig. 1. Sample 2501

location

map.

2502

A\UNI C‘ SIGLEO and GkoH(it

mize tidal variations In suspended load over a tidal cycle (ARTHUR and BALL, 1979). Stream gage data indicate that the discharge was similar for both days the (5.26 x 10” I/day on February 6 and 6.10 x IO’l/day for June 26; U.S. Geological Survey, 1980). The magnitude of river suspended material is affected also by wind. which was not considered. The samples were stored at 4 C in 201. Nalgene contamers which had been leached with 6N HNO,. rinsed with distilled water. and finally rinsed with the sample itself. The samples were prefiltered through 0.40~tm Nucleopore filters to remove suspended sediment and microorganisms. The filtered samples were concentrated and desalted using an Amicon UM2 membrane (1.2 nm nominal pore size) with 3.5 atm NZ pressure. rinsed with I I. distilled water and the concentrate freeze-dried. Particulate-size fractions (>0.40 pm) for summer samples, only. were collected from the Nucleopore filters and freeze-dried. linfortunately, there are no particulate comparative data for the winter. Freeze-dried samples (approximately IO mg) were packaged in acid-leached (6N HNOj) polyethylene in a class 100 clean room and irradiated in the National Bureau of Standards reactor in a thermal neutron tlux of 6 x IO’” n cmm2 set-’ for periods of 5 min and 4 hr. The resultant activity was counted on Ge(Li) gamma detectors connected to a Tennecomp data collection and analysis system (TP5000) with on-line data reduction capability. NBS coal Standard Reference material 1632a was used to check the analytical procedure. Concentrations were determined by comparison of activities with multielement standards as described by GEKMANI et al.(1980). Relative errors based on counting statistics were less than 5”,, for most elements, with the exceptions of Cs. Hf. and Th (lo”,,). Lu. Ta. V and Yb (15”:,) and Mg (20”“). Salinities were calculated from chloride concentrations determined with a Corning 920M chloride meter. Carbon measurements on the freeze-dried products were made with a Perkin Elmer Model 240B elemental analyzer. Dissolved organic carbon (DOC) in unfiltered samples was measured on an Oceanography International instrument using the persulfate oxidation method of MENXL and VACCAKO (1964). Table

1.

R. Htt./ RESULTS

for more than 30 elements arc given 111Tables I 3 for the winter and summer sample groupb. lnspection of these tables indicates a large range in the character of the materials collected. As illustrated III Fig. 2, the winter colloidal material in the Patuxcnt River varied from an aluminum-rich product ,+1 the head of the estuary to a carbonaceous produc; nca~ the mouth. This shift in composition is accompdnicd bq a great reduction in the amount of colloidal material recovered (Fig. 2). The high concentration of colloidal materrul from Station 1 was also observed in replicate samples (SIGLIX) rt u/., l980a) and thus does not appear to be an artifact produced by leakage during the first filtration. X-ray diffraction patterns of winter samples from Stations I and 2 indicate that the inorganic nta:crial is poorly crystallized kaolinite with some illitc. Thz\e minerals are also reported to bc the principal cla!, minerals in Patuxent River sediments (ROREKTS.197 I j. If all the K and an appropriate amount of Ai 111the winter sample from Station 1 is assumed to rn lllite LK,,5A15,sSih,5020(OH)4] and if the remaining ,A1 i\ in kaolinitc [Al,Si,0S(OH)4], then these mmet-aI\ plus Fe(OH), would comprise #I”,, of the sample. This is in good agreement with the observed ash content of 77”,, (Table I). In Fig. 3. the concentrations of winter colloidal aluminum. iron and carbon are plotted as a function of salinity. The concentrations of Al and Fe dccreasc by a factor of 500 between Stations 1 and 5. whereas the concentration of C declines only by a facto1 of 10 If dilution were the sole process involved, then the concentrations would fall on straight lines connecting the concentrations in the mixing end-members For Data

Chemical characteristics and dry concentrates.

of the water

Freeze-dried

Salinity Station* 4.

Winter Colloiddi 1 2 3 4 5

B.

3 4 5 6 C.

2.9 3.8 2.a 2.2 1.9 Fraction

0.05 0.11 2.66 5.53

:

Ash('.)

(mg/L)

CC”)

28.6 9.4 2.86 1.76 0.60

6.4 7.6 10.1 23.8 32.8

76.7 71.6 68.4 36.6 16.9

26.4 34.6 34.3 35.7 36.R 38.5

19.5 10.9 a.0 9.5 13.2 5.0

11.0 13.2 10.7 9.9 10.4

67.1 69.8 63.4 56.0 36.0 11.1

(1.2 nm to 0.40 urn)

4.0 ::f 2.9 3.4

a.98 10.9

Sumner Particulate

3 4

I'aterial Recovered

Material

o.oa 0.73 3.23 1.36 12.30

Sumner Colloidal 1

TOCI (mg/Li

(g/kg)

Concentrate

1.9 2.2 1.7 1.6 n.7 1.4

Fraction

(a 0.40 i,m)

3.2 4.0 2.1 2.9 3.4 3.0

31.2 32.7 33.5 24.4 8.7 3.4

0.11 0.05 2.66 5.53 a.98 10.9

* The station locations are (1) Lower Marlboro, (2) Holland Cliffs, (3) Trueman Point, (4) Long Point, (5) Soloman's Island, and (6) PR-Bouy. i TOG-total

organic carbon

in unfiltered

water samples.

Composition

of estuarine

1

8.

Major Elements

(values in weight

2.81 4.50 1.73 _-

2:;9 0.17 __

0.18

0.17

0.65

0.04

0:;2

0:os

0:04

Mg I( Na Ca Cl

Trace Elements

::0 15 140 21.6 230

::1 71 137 16.4 158 5.5 2.2 2.7 76 0.49 965 131 0.98

514 1;117 138 16.5 180 5.1 1.9 3.7 65 0.66 576 143 0.67

561 1;‘ 243 109 14.3 160 4.3 1.56 3.4 51 0.43 363 108 0.96

::5 12.5 0.98 ::4

15 G!B 0.77 ;:9

::fr 7.7 0.71 10

3.41 493

3.07 348

Z"9 372

::: kl" 0.60 1390 133 0.87 22 <1 12

ii‘

296 3.55 543

Yb Zn

1-P' Mdjar Al 'e Mg

K

Na Cl Trace

l-C2

Elements 6.19 4.92 0.81 1.28 0.37 0.39 Elements

Composition

(

by weight

in freeze-dried 3.90 4‘71 0.55

0.72 0.60 __

1.28

0.32 0.94 2.15

1.68

0.65 3.17 5.06

1.39 1.85

222 12.4 116 42 0.58 0.24 0.57 9 _-

168 __ co.3 1.7 2.1 1.12

__

1.2 18 e0.5

208

1980).

and particulate

3-P

<50

fractions

from Patuxent River Summer Samples.

3-c

4-P

4-c

5-F'

5-c

6-P

6-C

6.37 4.80 P.CO I.63 0.91 1.79

0.83 0.57 1.05 0.20 0.66 0.28

6.30 4.96 1.14 1.58 0.54 0.68

0.99

3.07 2.94 1.43 1.35 0.68 0.68

0.66 0.85 0.50 -1.85 2.70

0.70 0.77 2.45 0.22 0.98 1.14

0.36 0.34 0.60 __

::0

;5 682 11

12 819 175 63 15

6 260 240 16 36 287 0.5 0.3 0.3 7 __

10 1290 242 16 12 68 n.21 0.13 1.3

3

1209 __

125 6 3.0 1.4

material)

0.69 0.36 0.33 2.87 4.50

2.09 3.38

(us/g dry weight)

17 925 33 108 18

10 570 338 31 94

Cr CS Eu tif La LU fln Rb :,"

88 3.9 1.7 2.3 57.7 0.37 3378 110 0.72 12

:PO 0.53 0.8 20 -745 4.1 1:9

Se Sm Ta ::

2.5 8.7 0.53 10 0.77

5.8 2.18 0.22 3.1 .-

i:

:13

Yb In

2.55 279

P designates

2-c

2.15 1.51

As B.3 Br Ce CO

1

of the colloidal

2-P

1.07 0.51 0.61 0.53 3.75 0.62 6.55

<0.4pm) material with increasing salinity has afso been noted for other estuaries (ASRX and CHESTER, 1973; HOLLIDAY and LISS, 1976; SHOLKOVITZ. 1976; BOYLE PI al.. 1977; WOLLAST and PETERS. 1978; ARTHUR and BALL. 1979; SHOLKOVIX and PRICE.

these elements tlx intermediate points all fall below such lines, suggesting that rapid removal by Rocculation and settling is occurring. At salinities above 7g/kg. the more linear relationship suggests simple dilution of the remaining colloidal material. The flocculation and rapid removal of colloidal (i.e. Table 3.

material)

___

1.2

V

5

(values in pg/g dry weight)

5.3

Th

-

percent freeze-dried 7.89 5.98 1:90

13.9 6.98

2503

4

3

2

14.0 7.15 2:;4

Al Fe

Ag As Ba Br Ce co CF Cs ELI Hf La LU Mn Rb Sb SC Se Sm Ta

material

Caposition of winter colloidal (1.2 nm - 0.45 urn) material.

Table 2.

A.

colloidal

6

40 7.7

::0 65 94 19 92 4.6 1.75 2.7 52 0.29 4110 79 0.32 12.6 4 6.2 0.70 0.91 10 0.3

1.3 4.9

1.3 600

?9 334

z9 305

particulate

fraction.

494 15 ?? 0.33 0.30 0.3 :::o 861 21 5.3 1.6 5.4 1.1 0.13

2

9"; 21 80 4.0 1.6 2.0 55 0.28 10190 94 __ ::4 6.5 0.61 0.74 ::: 132 1.9 350

C designates

:0 :,:1 0:3 7.0 0.06 321 __ 6.0 1.5 5 0.87

__

18 1088 173 104 18 104 5.3 1.75 3.4 56 0.23 1248 98 1.6 14 5.5 6.9 0.83

1:; 1.9

9.9 0.69

ii46 247

952.6 526

colloidal

fraction.

4 93 301 12 6.1 279 0.5 0.17 0.5 9.5 __ 230 __ 3.2 1.7 6 0.83 __

86

4 0.97 3.1 z13 1420 95 1.3 10 3.5 4.1

0.33 1.8 4 0.70

a __

L

1.6 __

1:;

7:;

1:;

1:;

IS0.68

B;--

2.8 _" __

1:

875

793

196

61t8

__

280 5.8 3.4 101 0:;3 0.3 5 __ 466 __ 0.9 0.79 2 0.37 __ 0:; 0.9 4.3 0.38 143

Material

Al %

Recovered mg/L

Carbon %

30-

15-

30-Winter ~Slirrmff

20-

IO-

O-

1 Ilr b

c,

I

2

Fig. 2. Comparison

3

20-

5-

IO-

o-

mm 4

of winter

IO-

1234

5

2

4 Salinity

Fig. 3. Concentrations

6

5

O- d I

I 2

il

and summer collotdal material (“,, on a dry weight basis). Ntlmhcrx ordinate refer to stations in Fig. 1.

Iron appears to be the major element controlling color in these samples. The freeze-dried products range in color from brown at the freshwater Stations 1 and 2, to white at Stations 5 and 6. The brown samples contain tap to 7.IY;, dry weight Fe. whereas the light colored samples contain only 0.51 Fe. The correlation between sample color, iron content, and aluminum content is to be expected if iron is present primarily as oxyhydroxide coatings around clay minerals (FOLLETT,1965). In contrast to the winter samples. the summer colloidal material is comparatively carbon-rich and aluminum-poor throughout this part of the estuary (Fig. 2). Further, there is no dramatic decrease in the

0

b,

8

IO

12

(g/kg)

of colloidal Al. Fc and C in winter.

ozi

seaward direction in the amount of colloidal matertat recovered. Inspection of the summer data from the Patuxenr River indicates that there is a distinct contrast between the composition of the coltoidal and particulate fractions. Considerably more particulate relative to colloidal material was recovered. and the particulate fraction was comparatively low in carbon and high in Al and Fe. The contributions of the colloidal and particulate fractions to the total amount of certain elements in Patuxent waters is presented in Fig. 4. For each element. the amount in the particulate fraction is greater than the amount in the colloidal fraction in the upper part of the estuary. Howcvcr. bq. a sahnity of 10 g/kg, the two phases contribute almost an equal proportion to the concentration of the clements shown. In comparison to winter particulate material, which was brown in color. the summer particulate fraction was bright green indicating the presence of photosynthetic organisms. These diRerences in suspended load composition also correlate well with seasonal patterns in nutrient and biomass concentrations (BOYNTONrt ul.. 1980). Scanning electron photom~crographs oi these samples indicate that the summer particulate fraction ( > 0.4 /lrn) contains numerous microorganisms (primarily centric diatoms) and clay mineral particles. The freshwater to brackish colloidal maternal ( ~0.4 pm) is also composed in large part of suhmicron clay particles, along with diatom fragments and some polymeric organic material. In contrast, the colloidal material from brackish water (=_IO g/kg) consists entirely of filamentous organic poiymers. probably derived from extracellular products and planktonic debris {SIGLEOetrri.. 1980b). DISCUSSiOI\; The data suggest that the Patuxent River colloidal material is a mixture of two primary components: an

Composition

of estuarine

colloidal

material

2505

0 0

3

6

9

0

12

3

6

9

I2

‘P

0.001

\ \

0

3

6

9

12

o-L7==lJ-r 0

3

6

SALINITY

9

12

0~ 0

3

6

\

9

12

(g/kg)

Fig. 4. Comparison of particulate and colloidal concentrations of scleclcd elcmenrs per Iltw of satcla function of salinity (summer data). Marked devlatlons from otherwise smooth WI-vcs at a sal~nll! 5 glkg result from increased turbity caused by a power plant cooling cxmtl.

inorganic clay-rich component and a carbonaceous one (Fig. 2). Since iron and many trace elements decrease along with aluminum during mixing in the estuary. it is likely that they are bound to the atuminurn-rich material by either surface sorption. incorporation into grain coatings. or incorporation into poorly crystallized clay mineral structures. Ftocculation induced by dissolved salts may account for the disapp~~~n~e of the atlimi~urn-rich com~onellt in estuarine water (ECKEKT and SHOKOLVITZ. 1076): although ZABAWA (1977) has noted that biological processes. such as fecal pellet formation. may also contribute to removal of fine-grained matter in estuaries. The lower abundance of At. Fe and tithophile trace elements such as the rare earths in the summer vs. winter samples could be regarded as evidence for the importance of biological removal proccsses: however. it could be argued equally well that this simply reflects stower erosion when the watershed is under an active vegetation cover. The most reasonable hypothesis for the origin of the clay component is that it is an ultrafine-grained product of continental weathering. The rare earths (REE) are among those elements which are likely to be associated with the aluminous component. The winter colloidal fraction and the summer particulate fraction. both of which are high in Al at Station 3, have a normalized REE pattern typical of evolved crustal rocks (HASKIN t’f rd.. 196X). The summer colloidal fraction is also similar to the sbate with regard to relative abundances of the rare earths. but the absolute abundances are all lower.

a< of

emphasizing the point that the clay component comprises a smatter proportion of the sample relative to carbon. The normati7cd data in Fig. 5 indicate that ZREE are lower in the summer colloidal fraction relative to the winter colloidal material by nearly a factor of IO. which is reasonably consistent with the rcspective ash contents of X and 6X”,, on ;t dry weight basis (Table 1). In contrast to the inorganic corn~~~ll~nt. preliminary work (SIGLKOc’r LI/.. 19XOb) suggest5 that the organic fraction of these samptos is derived primarily from iiTsiflr aquatic microorganism. and nol from ierrestrial runoK The arguments for this conctus~on are that: (1) the colloidal material consists of carbohydrate and protcinaccous polymers. known cxlracettutar products from phytopt~~nkton (13tr.1. and SAKSHA~G. 1980); (2) detailed amino acid analyses failed to find any of those amino acids indicative of soil bacteria (SKUO tat cti.. in preparation); and (3) there were no traces of tipnin. or other, evidence of woody plants. These findings are consistent with fhosc of McDower.r. and FISHHK11976) who reported that the majority of terrestrial carbon was processed rapidly locally. and released as atmospheric Cc), or dissolved bicarbonate. The hchavior of the trace elcmcnts may he compared and classified if their concentrations are normalized in the form of enrichment factors (EF), defined as foltows for a given element x’ :

ANNE C. SIGLEOand GEORGE R. HELZ

2506

La Fig. 5. Rare earth

ce

Sm

abundances

at Station

Tb

Eu

Yb

3 normahred to abundances composite (NASCI.

Lu

in the North

Amc~-scan 4x&:

Indicate that the seasalt contribution to colloidal and particulate material is significant in samples from brackish waters. probably due to occlusion. Tantd~m was depleted relative to crustal abundances 111both sets of samples. Tantalum displays an affinity for LI~ZInium minerals and may be retained in heav? m~t,~~rals during the weathering process. Group B consists of elements that 111man;~ caxeh are more than tenfold enriched relative to alummum. Evidence that some Group B elements ma) hc WOCIated with the non-clay fraction of the colloidal matter can be obtained by examining the variation m onrichment factors for some representative elements ITable 4). In winter as the organic component increases relative to the clays toward the lower part of the estuary. the enrichment factors for the Group B elcmet~ts 11s.

This index is used conventionally to compare on a single scale elements that encompass a large range in geochemical abundance (DUCE et ul.. 1975; Gladney et al.. 1976; LEPEL et al., 1976). In this paper WEDEPOHL’S (1968) values for averge crust have been used to calculate the reference ratios, although averages from other crustal or argillaceous standard rocks could have been used with similar results (LEPEL rt d., 1976). As indicated in Fig 6, most of the trace elements can be subdivided into two groups. Group A consists of elements with enrichment factors near unity (such as SC, V and the REE), which covary with Al and are most likely associated with the clay mineral or terrigenous fraction of the sample. Large crustal enrichment factors for the elements Na, Cl, Ca and Mg

I o3

I

I I

f’ ; IO2

tl

I 0’

GROUP A h

)

\

IO0 c

/

Y

GROUP B

10-l

1

I,

CeCsEuR

Fig. 6. Crustal

11,

,,I

HfLaLu

,

,

RbScSmWThV

1

1

I,

Yb

1

1

AsBoCoCrMn

I

I

h

1

SbSeU

I

L

Zn

enrichment factors for colloidal material. Bars represent total range of data: left bar in each pair is for winter and right bar for summer data.

Composition of estuarine colloidal material Table 4.

2507

Crustat enrichment factors for selected elements in estuarine colloidal material.

SUMMER

WINTER

1

2

3

4

5

1

2

3

4

5

1.1 1.0 0.9

1.8

1.1 0.9 0.9 1.4

1.7 1.3 1.1 1.6

3.5 2.8 2.4 1.4

1.1 1.1 0.9 1.4

1.6 1.4 1.1 1.6

1.8 2.3 1.2 2.7

1.6 1.5 1.0 2.2

1.5 1.0 1.0 1.5

2.9 2.5 1.5 ---

4.6

4.9

6.9

la

---

21

::,

40

18

41

Se Sb ZIl

:."5 5:1

2.7 6.7 4.6

7.9 3.3 5.7

13 ::

i:, 25

34 170 36

470 55

390 288 40

3ao 124 115

:;0 37

Salinity (g/kg)

0.1

0.7

3.2

7.4

12

0.05

0.1

2.7

5.5

9

Group A Fe EU SC V Group B As

Sb, Se and Zn also increase slightly. whereas, those for Group A elements remain essentially unchanged. In summer when the colloidal material was uniformly carbon-rich, the enrichment factors for these Group B elements are uniformly high. A possible source for As, Sb and Se enrichments may be very fine particles or gases discharged to the atmosphere from the power plant near Station 3 (GLADNEY et al., 1976). Since there is evidence that the organic component is a product of biological productivity within the estuary, its trace element composition may partially reflect the plankton composition. Planktonic marine organisms can accumulate certain trace metals greatly in excess of the concentrations in seawater, particularly during summer bloom conditions. For example, MORRIS (1971) found that the concentrations of both particulate and dissolved Zn increased during algal blooms. The Zn increase in the dissolved phase was thought to be caused by membrane rupture during the seawater filtration step. Enrichment factors computed from published plankton compositions (MARTIN and KNAUER, 1973; MOORE and BOSTROM,1978). give values between 200 and 400 for Zn. These calculations suggest that most of the Zn in the colloidal fraction could be associated with the organic component. In addition, Fig. 4 shows that Zn in the particulate fraction follows the pattern for particulate carbon. There is a suggestion. then, that some Zn in the particulates is also organically bound, although it is also known to coprecipitate with Fe in oxide coatings (CARPENTERand HAYES, 1980). Similarly, the enricliment factors for Ba from published plankton analyses vary from 32 to 58. which is within the range observed for Patuxent colloidal and particulate data. For some elements, biological processes may not account adequately for the enrichments observed. For example, in lower marine organisms, chromium has an average enrichment factor of 16 (from MOORE and BOSTROM,19781, which is somewhat lower than the 49 observed in the highest colloidal summer sample. The graph of Cr vs salinity (Fig. 4) shows that the particu-

late Cr distribution varies with that of Fe. suggesting that particulate Cr occurs primarily in Fe-oxide mineral coatings. Similar arguments can be made for Co. which is also reported to coprecipitate with Fe (CAKPENT'ER and HAYES, 1980). The enrichment factors for Mn in plankton vary from 2.5 to 5 for both sets of published analyses (MARTIN and KNAUER, 1973; MCQRE and BOSTROM. 1978). suggesting that the plankton contribution toward enrichment of this element is small. Since Mn enrichment occurred only in summer. when seasonal anoxia in the surface sediments is most widespread, the enrichment of Mn in the colloidal fraction may be due to reprecipitation of Mn that had become mobilized from reduced sediments (EATON. 1979; SUNDBY et al., 1981). The distribution patterns in Fig. 4 suggest that Fe and Mn are in separate phases. which is consistent with selective dissolution experiments of Fe-Mn-oxide coatings (CARPENTER and HAYI:S, 1980) and data from other estuaries (HOLLIDAY and Lrss, 1976).

CONCLUSIONS Although the distinction between particulate and colloidal-dissolved constituents in river load is operationally defined by filter size. we have observed that for our samples. there are in fact some fundamental differences in the nature of the material recovered. Specifically, for summer samples from the Patuxent River, the particulate fraction is composed of mineral particles and planktonic organisms. The colloidal fraction. however. consists primarily of biogenic products with a minor mineral component. In contrast. the winter freshwater colloidal material consists primarily of poorly crystallized clay minerals. The relatively large amount of colloidal kaolinite-illite in the winter suggests that during periods of low biological productivity, products from terrestrial runoff provide the majority of material for the colloidal system. During periods of high biological productivity, however, the products from biological processes predomi-

nate. Thus, within the colloidal fraction one can find a range of composition depending on location and season. Enrichment factor calculations suggest that the majority of trace elements vary with, and thus likely associate with, the mineral component. The elements As. Ba, Sb, Se. II, and Zn are enriched relative to their predicted values and ma! be selectively enhanced in the organic fraction. Cr and possibly Co and Zn. may be associated with iron oxide surfaces on clay minerals, wherea Mn shows evidence of some reniobili~~ti~~tl ft-om sediments. Ta is the only element which is dcplctcd. These results indicate that an t~iiderstandil~~ of the sources and cycles of the major and tract elements in estuarine waters requires studies of seasonal and compositional variations of the colloidal. as well as particulate phases. il

c,li,lo~~~lct/g~r,l~,~7~.~

We arc gratcfui to Professor 6. for CHN analyses. to M. S. G~:KMAYI and J. PHAt.bN for analytical assistance. to Professors G. E. C;OKL)UN and W. H. Z~LLER for making the actication analysis facilities available. and to A. Y. CAXTI~.IO for collecting the sampie from Station 6. The crew of the R,%’ Olktn provided expert assistance in sample collection. We also thank J. A. ‘~IH~l~S~~ and W, R. Bou&ros at the Chesapeake Biological Laboratoryfor many informative discussIons. This work was supported hq a grant from Maryland Power Plant Siting Program. KASL~K

REFERENCFX ANoat:N A. W. and HARRISS R. C. (1975) Observations on the association between mercury and organic matter dissolved in natural waters. Gcoc~hiltr. C’o.st~oc,/tirt~.it~c~ 39, 1253 l’57. ARTHUR J. F. and BALI. M. II. (1979) Factors intluencing the entrapment of suspended material in the San Francisco Bay-delta estuary. In Satr Fictrrci,scc~ BL~!.. 7‘f7~, ~~~bf~fzf~~,f~ ~s~~ff/~~ (ed, T. J. Conom~~s). pp, I43 174. Pacific Division, Am. Assoc. Adv. Sci. (San Francisco), ASTON S. R. and CHESTIXRR. (1973) The influence of suspended particles on the precipitation of iron in naturai waters. t‘\tiitrri~~ Constcli nlfir. .%I‘. 1. 725 231. BI:LI W. H. and SAKSHAIX; E. (1980) BacterIaI utilization of algal extracellular products2. i\ kinetic stud) of natural populations. Linniol. Orcc~zoqr. 25, 1071 1033. BENIS P. and STEINN~S E. (1974) I/r S&r dialysis fol- the determination of the state of trace elements !n natural waters. Wrrter Rcs. 8, 947 951. BOYLE E. A., EDMONI) J. M. and SnoLsovt’rz E. R. (1977) The mechanism or iron removal in estuaries. Gc~&in~. Co.\rtio&irn. Acru 41, 1313 1324. BOYNTON W. R., KEMP W. M, and OS~ORX c‘. G. (1980) Nutrient fluxes across the sedimel~t-water interface in the turbid zone of a coastal plain estuary. In ~.s~f/~?~i~i~~ Pc,r.spcwi~-cs ied. V. S. Kennedy). pp. 93 I IO. Academic Press. CARPENTER R. H. and HAYES W. B. (IYXO) Annual accretion of FeeMn-oxides and certain associated metals in a stream environment. Chetn. G&. 29, 249-259. DUCE R. A., HOFFMAN C. I_. and ZOI.LER W. H. (1975) Atmospheric trace metals in remote northern and southern hemisphere sites: Pollution or natural. .Sckww 187, 59-61. EATON A. (1979) The impact of anoxia on Mn fuxes in the Chesapeake Bay. Gro&n. (‘o.sntoc~/ti~~r. .4crcc 43. 429 432.

f;c~~txr

lation

J. M. and SHOLKOVUL E. K. (1970) l’be IIMuof iron, aluminum and humatea from rixer writes

by electrolytes. FOI.LFTl’

J!.

A.

Grwhiru.

Cowtod~ir~r.

Acrr~ 40, X47 ~48.

C.

(1965) The retention of amorphou\ cohydroxide b\ knolinite. ,I Sfjil $1 ‘. 16.

loidal ferric 3.14 341. _ GEKMANI M. S., GOICM~N I.. SICU o A. C’.. K~~w.~.c./\‘fi G. S.. OLIMI:I.1.. SMALL A.. ANDERSON 11, L.. I. .\II t ? M P.. c;t:to\‘z~l M. C.. CHOQUETTt C. E.. Lt.!+I F’. :\., (;()I
233, 421 -428.

MVRRAI’ J. W. and GILL. G. (1978) The geochernlbtrq 01 l(.lll JZ, iron in Puget Sound. Gtwchim. (‘osrwwhi~e 9 19. KOBE.RTSW. P. (1971) Sediments and suspended parttculatc matter of the Patuxent River drainage basin, Maryland. Unpublished Thesis. George Washington Univ.. 761) pp. Snot.novrrz E. R. (1976) Flocculation of dissolved organic and inorganic matter during the mixing of river watci

Mn. Al. Cu. Ni, Co and Cd during estuarinc mixmg Earth Phe. Sci. Lrtt. 41, 77 86. SHCILKOVITZ.E. R. and PRICE N. B. (1980) The mqnr-elcment chemistry of suspended matter in the Amaron estuary. &o&n. Cosmorhim. Acta 44, 163 17 I SIC~LEOA. C.. HELZ G. R. and ZOLLI~R W. H. (IYXO~J Organit-rich colloidal material in estuaries and its alteration by chlorination. En&on. Sci. Techno/. 14, 673 67U. S&o A. C.. HOERING T. C. and HAKI, P. F.. (IYXOh) The colloidal organic matter in waters of the Chrzapeakc Bay and Patuxent River. C‘urnryit, In>f. M/rtrl~inqtort Ytwh. 19. 394 399.

Composition

of estuarine

SK;LI:O A. C.. HAKt: P. E. and HELZ G. R. (in preparation) Amino acid composition of estuarine colloidal material. SK;L~ A. C.. HOERIX; T. C. and HELZ G. R. (in preparation) Composition of estuarine colloidal materiai: organic components. S~ILIIHY B.. SILVIRREIIG N. and CHESSELET R. (1981)Pathways of manganese in an open estuarine system. Geeclnrtt. Cosmoc/~int. A<,tir 45, 293307. I!.S. Geological Survey (1980) Water resources data for Maryland and Delaware. 12%ifrvDfrru Rep. MD-DE-79-l. 39x pp.

colloidal

material

2509

WEUEP~HL R. H. (1968) Chemical fractionation in the sedimentary envtronment. In Origin und D~.~trjb~riof~ CT/the ~~f~~~~~~zts fed. L.. Ii. Ahrens), pp. 999..-1016. Pergamon Press. WOLLAST R. and PETERS J. J. (1978) Biogeochemical properties of an estuarine system: The river Scheldt. In Biogrochemi.str~~ qf Estuarinr Sediments (ed. E. I>. Goldberg), pp. 279. 293. UNESCO. ZARAWA C’. F. (1978) Microstructure of agglomerated suspended sediments in northern Chesapeake Bay Estuary, Scirrtw 202, 49.-5 1,