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Retention of riverine iron in estuaries
GwcMmka *I czI!wmxhindca Acla vol. 46, pp. 1003-1009 0 Pergamon F’reu Ltd. 1982. F’rintcdin U.S.A.
Retention of riverine iron in estuaries LAWRENCE M. MAYER Oceanography Program 6t Department of Geological Sciences, Ira C. Darling Center, University of Maine at Orono, Walpole, Maine 04573 (Received September 28, 1981; accepted in revised form January 27, 1982) Abstract-Retention of Fe floes, resulting from the mixing of river water and seawater, was examined in three Maine estuaries. Riverine Fe was found to remain fairly conservative with salinity, implying that the process of flocculation does not necessarily remove Fe from water parcels. Laboratory experiments corroborated the field data by demonstrating that neither gravity nor suspended sediment were very effective in removing flocculated Fe from suspension. However, input of a tannery effluent did appear to result in scavenging of Fe from estuarine waters. Flocculated riverine Fe was found to increase considerably the Fe concentrations of estuarine bottom sediments, with the amount of iron per sediment specific surface area dependent on mean river flow entering an estuary. While no long term retention efficiencies could be calculated for these estuaries, it seems likely that a significant portion of flocculated riverine Fe escapes to shelf waters. INTRODUCIION A NUMBER of studies during the past decade have documented the nonconservative behavior of filtrable iron during the estuarine mixing of seawater and river waters. These studies have demonstrated the geographical ubiquity of the phenomenon and the nature of the colloidal flocculation process responsible. Little attention has been given, however, to aspects of the retention of iron in the estuarine zone. The effect of the flocculation of iron-rich colloids on the actual removal of these colloids from water parcels has rarely been addressed. The fact that riverine iron-rich colloids smaller than, say, 0.45 pm aggregate to form floes larger than 0.45 Frn during mixing with seawater does not necessarily imply that these newly-formed large floes will settle quickly from the water columns. The filtrability of estuarine colloids does not equate to their tendency to sediment. In this study, both the dissolved and particulate iron concentrations were examined in a series of estuarine water parcels, under low turbidity conditions, in order to determine if the iron removed from a filtrable state was actually lost from the water parcel. The potential roles of gravity and suspended sediment in removing iron from water parcels were examined by experimentation. The influence of iron colloid flocculation on the iron contents of estuarine sediments has also received little attention. If iron is removed from water parcels it may be detectable in the estuarine sediments. Coonley et al. (1971) searched for iron enrichments in the sediments of the Mullica estuary but failed to find incremental iron of the magnitude to be expected from the observed flocculation process. Eisma et al. ( 1966) found evidence for iron accumulation in the coastal sands offshore of the Rhine estuary. Can iron colloid flocculation produce a measurable increase in the concentrations of iron in estuarine sediments? Such an increase was sought in this study by comparing sedimentary iron concentrations in a series of
estuaries receiving varying amounts of river input, using a textural normalization technique to remove grain size effects. The study area employed was the Gulf of Maine and three of its associated estuaries (Fig. 1). The estuaries are all drowned river valleys, receiving freshwater runoff from watersheds similar in climate and vegetation. Very few carbonate rocks are found in the drainage basins; the rivers are thus soft water in nature. The estuaries differ primarily in extent of freshwater inflow, with the Damariscotta receiving only about 2 m3 - see-’ and the Sheepscot and Saco increasing in roughly order-of-magnitude increments, respectively. Sediment delivery by the rivers of New England estuaries are low relative to other U.S. estuarine systems (Folger, 1972), optimizing the use of these systems in detecting riverine contributions of iron in colloidal as opposed to suspended sediment form. METHODS AND MATERIALS Removal of f?ocs from water parcels The experimental approach consisted of determining the amount of iron removed from a water parcel by both centrifugation and filtration, both in the presence of varying amounts of suspended sediment. The sediment was collected from the top I cm of a mudflat in the Damariscotta estuary and wet-sieved to remove particles larger than 44 pm. Sheepscot River water and Damariscotta estuary seawater (salinity = 29-31%) were prefiltered through- Millipore 0.45 pm Type HA filters. These orefiltered solutions were mixed in a i:l ratio in the presence of 0, 10, 20, 30, 50, and 69 mg ~1~’ sediment. The resultant suspensions were placed on a slow reciprocating shaker for 5 hr at room temperature. Each suspension was then centrifuged for 20 min at 1120 g, which sufficed to remove the sediment from suspension. An aliquot was then removed from each suspension and filtered through a Millipore 0.45 pm Type HA filter. Roth the centrifuged and the centrifuged plus filtered aliquots were then analyzed for iron. The field study involved examining both the dissolved and particulate iron in quasi-synoptic estuarine profiles. The Saco estuary was sampled within +45 min of slack high
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removed of oxide and organic coatings and carhonutc m;~ teriai, which formed an insignificant fraction of the sediments in this study area. The analytical coefficient cri‘v:lri. ation for surface area was better than f15”r.
RESULTS Removal
FIG. 1. Map of Damariscotta, Sheepscot, and Saco estuaries and their location in the Gulf of Maine. Bottom sediment sample sites shown as dots. tide. Although samples were taken from most depths, complete sets of analyses were performed primarily on surface water samples, as this layer dominates seaward transport of iron in the highly stratified Saco. All samples were filtered through acid-washed and pre-weighed Gelman A/E glass fiber filters underlain by a Millipore 8.0 pm Type SC filters. Calibrated against a series of varying size Nuclepore membrane filters, this filter combination had an effective pore size of 0.5 pm for filtrable iron. Distilled water was rinsed through each filter at the end of filtration to remove seasalt. The filtrates were analyzed for iron and salinity. The Gelman filters were weighed to obtain the suspended particulate concentration and then leached with a 4MHN03/0.7M-HCI solution for 2 hr at 70°-90’. The leachates were analyzed for iron and chromium by atomic absorption spectrophotometry. Filtrable iron was measured in duplicate using the Ferrozine technique of Stookey (1970). The analytical coefficient of variation ranged from +2.5% at freshwater iron concentrations to f7.596 at seawater concentrations. Salinities were measured on a Beckman RS-7B salinometer.
qf iron from
nuter parcels
The relative efficacies of suspended sediment, gravitational settling, and filtration in removing iron from a suspension of - 15% salinity are indicated in Fig. 2. Gravitational settling was simulated by centrifugation for a time period long enough to remove all particles >0.35 pm, assuming a density of 2.5 g. cm-‘, from suspension. Filtration was at a nominal pore size of 0.45 pm. Beginning with 104 kg* 1 ’ of iron in suspension, centrifugation was able to remove 24.1 pg - 1-l in the absence of suspended sediment while filtration was far more effective, removing an additional 59.3 pg.l-‘. Coonley et al. (1971) found similar results for riverine and very low salinity samples. Clearly, much of the flocculated material formed at intermediate salinities during the mixing of river water and seawater must have a density lower than 2.5 g - cm-3. Under the influence of a 1 g gravitational field, the material which was removed by filtration but not by centrifugation would be expected to settle less than 1 meter in a 15 day period, which is greater than the flushing time of many estuaries. The addition of suspended sediment in concentrations similar to those typically found in estuaries resulted in a measurable but only a small enhancement of removal of iron from suspension. The incremental removal by suspended sediments was approximately the same for solutions clarified by either filtration or centrifugation plus filtration. The lack of incremental iron removal at suspended sediment concentrations above 30 mg - I-’ suggests that colloid scavenging by sediment was limited by the availability of colloids susceptible to adsorption rather than the availability of adsorption sites on sediment particles. These results are quite different than those of Aston and
Iron accumulation in estuarine sediments Sediment samples were collected by Shipek grab in subtidal areas and by hand with a plastic scoop in intertidal areas from the Gulf of Maine, and the Damariscotta, Sheepscot, and Saco estuaries (Fig. 1). Only the top centimeter of sediment was sampled. For iron analysis, approximately 1 g of sediment was heated to dryness in concentrated HNO, twice, follo&d by dissolving the residue in 5% HNO, for analysis. The resultant digestates were analyzed for iron by atomic absorption spectrophotometry. Specific surface areas were measured by a modification of the cetyl pyridinium bromide (CPB) technique (Greenland and Quirk, 1964; Mayer and Rossi, 1981). This technique measures specific surface areas of aluminosilicate grains
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FIG. 2. Effects of centrifugation, centrifugation plus filtration, and suspended sediment concentration on the removal of iron from suspension.
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Chester (1973), who found natural suspended sediment to remove 75% of a~ificially prepared iron coiloids from seawater, and lend support to the contention of Sholkovitz (1976) that Aston and Chester’s conclusions are not applicable to estuarine situations. The possibility that flocculated iron colloios are mixed conservatively with seawater, rather than removed from the water column as a result of floeculation, was also examined using field data collected from the Sac0 estuary. Figure 3 shows plots of the salinity dependence of 0.5 pm filtrable and total (filtrable plus particulate) iron for two sampling dates. Also plotted are hypothetical total iron concentrations calculated as the sum of ( 1) conservative mixing of riverine total iron into seawater, and (2) the iron estimated to be contained in the suspended particulate matter (SPM) resuspended from estuarine sediments or from outside the mouth of the estuary. The riverine total iron was dominated by filtrable iron; suspended sediment levels from the Saco River are generally very low (co.5 mg +1-l) because the river is impounded immediately above its juncture with the estuary. The particulate iron found in the river on the two sampling dates probably consisted largely of colloidal iron aggregated during river flow, as found by Eaton (1978); the concentrations of iron in the SPM collected in the im~unded part of the river ranged from 15% to 25%-Fe by weight. The second term was calculated by multiplying the SPM concentration by 20 mg-Fe-g-sediment-‘, a value estimated from the following three considerations. First, SPM in the seawater outside the estuary had iron concentrations of 5-23 mg-Fe * g-sediment-‘, and this seaward source appeared to contribute a major fraction of the SPM in the surface waters of the estuary. Second, the SPM in the turbidity maximum presumably represents material derived primarily from resuspension within the estuary; SPM from this zone showed concentrations of lo-20 mg-Fe - g-sediment-‘. Third, iron analyses of a series of grain size separations (Mayer and Fink, 1980) yielded values of 6-50 mg-Fe- g-sediment-’ for the silt and clay fractions (unpublished data), which are the sizes most likely to be resuspended. The contribution to the SPM pool by resuspension was minimized by sampling on a calm day at slack high tide, at which time SPM levels should have been relatively low (Anderson, 1970). Estuarine suspended sediment levels in the surface waters varied from ~1 mg-1-l in the low-salinity zone to -4 mg - 1-l at the seaward end of the estuary, with values in the deeper turbidity maximum reaching 13 mg - 1-r. Different responses for riverine total iron to the mixing process with seawater were observed on the two sampling dates. Roth data sets show the expected, negative nonconservative mixing for the Jiltrable iron, resulting in W-95% flocculation by 15% salinity. The g/22/79 data set shows, however, congruence between the hy~thetica1 and the actual total iron, implying essentially conservative mixing of
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SALINITY (%e)
FIG. 3. Filtrable iron (O), hypothetical total iron (A), and measured total iron (A,) plotted versus salinity for two sampling dates in the Saco estuary.
riverine total iron with the seawater endmember. The discrepancies are most likely due to the average iron concentration of 20 mg-Fe - g-sediment-’ assigned to the SPM; this value is high for SPM derived from offshore and probably low for sediment resuspended from upper estuarine mudflats. The total iron data from the 7/23/79 collection, on the other hand, show a significant removal of iron from the water column, as indicated by the consistent negative deviation of observed total iron from the predicted trend. The uncertainty in the hypothetical total iron trend, due to the unknown true concentration of iron in the SPM, precludes an accurate calculation of the removal percentage, but it appears that most of the riverine iron was withdrawn from the water parcels. The probable cause for these two different mixing patterns is indicated by an examination of the particulate chromium data (Fig. 4). A tannery is present at the upper limit of tidal incursion, which releases a waste into the river consisting largely of chromium and organic matter. The data from the 7/23/79 sampling show an order of magnitude higher concentration of this effluent at the head of the estuary than those of the S/22/79 collection. The rapid decrease in chromium concentration with salinity in the 7f23f 79 data indicates a rapid precipitation of the effluent
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mium ratio of the tannery sludge found in the sediments of this estuary, about 40: 1 (Mayer and Fink. 1980) a considerable amount of organic matter can be expected to have been associated with this chromium. The nature of this organic matter was not determined in this study, but likely is a combination of polyphenolic tanning agents, proteins, and fats. A scavenging of the iron colloids by the organic-rich tannery effluent appears, then, to be the mechanism for removal of the iron from the water column. Iron in estuarine sediments Figure 5 shows the concentrations of iron in sediments from the Saco, Sheepscot, and Damariscotta
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FIG. 4. Suspended particulate chromium concentration versus salinity for two sampling dates in the Saco estuary.
estuaries as well as offshore sediments from the Gulf of Maine, plotted against the specific surface area of the sediments. Plotting the iron concentrations against surface area allows removal of grain size as a variable in comparing iron concentrations from different areas. It is evident that iron concentrations, normalized against surface area, increase in the order Gulf of Maine < Damariscotta < Sheepscot < Saco. This estuarine trend is related to the amounts of freshwater inflow received by the estuaries. Figure 6 is a plot of the approximate mean river flow entering each estuary versus the average incremental thickness of the iron coating in the sediments over the Gulf of Maine ‘baseline’ iron thickness. These incremental coating thicknesses were calculated by subtracting the average Gulf of Maine iron to surface area ratio from the ratio for each of the estuaries. Such a calculation assumes that the increased iron concentrations found in the estuaries are not due to
, material in the estuary, while roughly conservative mixing of particulate effluent chromium is evident in the g/22/19 data. Given the high carbon to chro-
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FIG. 5. HNO,-leachable iron concentrations versus sediment specific surface area for sediment samples from the Gulf of Maine and three estuaries. Sampling sites shown in Fig. 1.
FIG. 6. Mean river flow versus incremental iron:surface area ratios (explained in text) for the (A) Damariscotta, (B) Sheepscot, and (C) Saco estuaries.
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IRON IN ESTUARIES
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FIG. 7.Iron:surface area ratios for intertidal (upper numbers) and subtidal (lower numbers) sediments in the Saco estuaries. Numbers in parentheses are the specific surface. areas of the sediments. Intertidal data are means + standard deviations for several samples from each of four mudflats. Also shown is the high tide ha&line (10-20460) from g/22/79 sampling trip.
mineralogical effects; this assumption is justified on the bases of similar mineralogical composition among the estuaries and the use of a textural normalization technique. Further corroboration that the incremental iron is derived from riverine material comes from consideration of organic carbon levels in the Damariscotta and Sheepscot estuaries. Mayer et al. (198 1) have shown that organic carbon levels, if plotted versus specific surface area, in the Sheepscot are higher than those of the Damariscotta in a manner very similar to the case of iron. The 8°C values of the organic matter in the two estuaries indicate that the additional organic matter in the Sheepscot estuary is terrigenous. If the incremental organic carbon concentrations, per unit surface area, are calculated for the Sheepscot over the Damariscotta, a value of 830 pg-organic carbon - meter-* results. The incremental organic carbon to incremental iron ratio, then, is 13, on a molar basis. Analysis of iron-humic floes collected in river water-seawater mixing experiments have yielded carbon:iron ratios ranging from 1.3 to 59 (Mayer, in prep.); similar ratios have been obtained by Sholkovitz (1976), Sholkovitz et al. (1978), and Boyle et al. (1977). These calculations, then, are consistent with an iron-humic colloid origin for both the incremental iron and carbon in the Sheepscot sediments. A similar calculation cannot be performed for the Saco estuary because of the organic pollution in that system (Mayer and Fink, 1980). The distribution of iron in the Saco sediments is shown in Fig. 7, expressed as the concentration of iron per unit surface area. The most intense accumulations, on a per unit surface area basis, were found in the central part of the estuary. This pattern is evident in both the intertidal and subtidal sediments, in spite of their quite different grain sizes. The mid-estuarine iron maximum in the intertidal sediments contrasts with the highest pollutant chromium loadings in the upper estuarine intertidal mudflats (Mayer and Fink, 1980), and presumably reflects the higher salinity required for iron flocculation than for pollutant chromium settling (Figs. 3 and 4). This mid-estuarine maximum may instead be due to the coincidental location of the subtidal turbidity maximum, the typical upper limit of which is denoted
by the typical high tide hahxline shown in Fig. 7. Accumulation of river-derived Kepone has also been found at an estuarine nodal point (Nichols and Cutshall, 198 1); such accumulations in this zone may result from the high suspended sediment levels and consequent scavenging of materials- from the water column. DISCUSSION The “removal” of filtrable iron during estuarine mixing should not be interpreted to mean removal from estuarine water parcels. The data presented in this work suggest, instead, that although the initially colloidal iron does increase its particle size during flocculation, it does not necessarily settle rapidly from suspension in the absence of anthropogenic perturbations. Rather, much of the larger flocculent material may remain essentially conservative with the water parcel. This conclusion is corroborated by the lack of success by Coonley et al. ( 197 1) in finding the flocculated iron from the Mullica River in the sediments of its estuary, and obviates the need to invoke tidal scour for erosion of iron floes from the sediments, as suggested by these authors. These lowdensity floes may well comprise the iron-rich filtrable but not settlable suspended particulate matter observed by Duinker et al. (1974) in the coastal waters near the estuary of the Rhine River. Of course, estuaries with very long flushing times will likely have increased retention efficiencies. Windom (1975) concluded that riverine iron delivered to Southeastern U.S. estuaries is completely retained in the estuarine sediments, based upon a sedimentation rate calculation for iron compared with the annual total iron delivery from the rivers. However, his analysis did not take into consideration that much of the sediment accumulating in these estuaries derives from offshore rather than from rivers (Pevear, 1972); such a budgetary approach is therefore unjustified. It would be of interest to calculate this type of budgetary analysis using incremental iron concentrations in the Sheepscot estuary; however, sedimentation rate data averaged over the entire estuary are not available. Nevertheless, the analysis of estuarine sediments indicates that estuaries do act as at least a partial
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sink for flocculated iron. The removal of iron from estuarine water parcels is likely effected by both gravitational settling of larger or denser iron floes and scavenging by SPM. This latter mechanism may play a more important role than that indicated by the small effect shown in Fig. 2. Anderson (1970) demonstrated that SPM levels in a shallow New England estuary exhibited a periodicity with high levels at low tide and low levels at high tide, which he suggested was due to wind wave resuspension of tidal flat sediment under shallow water conditions. Such a periodicity would imply that once each tidal cycle bottom sediment will be introduced to and removed from a parcel of fresh water flushing through an estuary. This water parcel may then lose a small amount of iron colloids to SPM scavenging many times, depending on the flushing time of the estuary. Such a removal mechanism will be maximized in either shallow estuaries or those with extensive tidal flats. Benthic biodeposition by filter feeders such as bivalves may also be important in removing iron colloids (Lowman et al., 1971). For example, Anderson et al. (1981) have shown filter feeders to constitute a major sedimentation pathway for a mudflat in the Damariscotta estuary. However, most bivalves show poor filtration efficiencies for particles smaller than 4 pm (Newell, 1979), which includes most of the iron floes formed (Mayer, in prep.; Bale and Morris, 1981). Iron-rich floes flushed from the estuaries out into shelf waters may be removed from the water column by gravitational settling, given the longer residence times of water on the shelf relative to the estuaries. Alternatively, the fine-grained iron-rich floes may be removed by a planktic biodepositional mechanism. McCave (1975) has pointed out the importance of large particle formation in facilitating downward material flux in the oceans. Large particle formation can occur either biogenically or nonbiogenically. Non-biogenic aggregation, i.e., further flocculation, is an unlikely process, as flocculation ceases after a few hours (Sholkovitz 1976; Boyle et al., 1977; Mayer, in prep.). In addition, the dilution of the iron colloids during mixing with seawater to higher salinities will further inhibit the rate of interparticle contact and therefore the flocculation rate. Biogenic particle formation, and biodeposition, may occur via phytoplankton uptake of colloidal iron (Harvey, 1937) followed by zooplankton grazing and fecal pellet formation (Elder and Fowler, 1977; Prahl and Carpenter, 1979), or occlusion of iron-rich floes into planktonic carbonate shells (Turekian et al., 1973). As evidence for the first pathway, coastal iron concentrations have been shown to decrease during periods of high phytoplankton productivity (Thompson and Bremner, 1935). Acknowledgements--I thank L. Schick and P. Rossi for their able and patient laboratory assistance, and E. R. Shol-
kovitL for reviewing an early draft of this manuscript. Ihc work upon which this publication is based was supported in part by funds provided by the Office of Water Research and Technology (Grant No. B-016ME), U.S. Department of the Interior, Washington, D.C., as authorized by the Water Research and Development Act of 1978
REFERENCES Anderson F. E. (1970) The periodic cycle of partrcuiate matter in a shallow temperate estuary. J. Sedimenf. Petrol. 40, 1 I28- 1135. Anderson F. E., Black L.. Watling L. E., Mook W. and Mayer L. M. (1981) A temporal and spatial study of mudflat erosion and deposition. J. Sedimenr. Petrol. 51, 729-736. Aston S. R. and Chester R. (1973) The influence of suspended particles on the precipitation of iron in natural waters, Est. Coastal Mar. Sci. 1, 225-231. Bale A. J. and Morris A. W. (1981) Laboratory simulation of chemical processes induced by estuarine mixing: The behaviour of iron and phosphate in estuaries. Est. Coastal Shelf Sci. 13, l-10. Boyle E. A., Edmond J. M. and Sholkovitz E. R. (1977) The mechanism of iron removal in estuaries. Geochim. Cosmochim. Acta 41, 13 13-l 324. Coonley L. S., Jr., Baker E. B. and Holland H. D. (1971) Iron in the Mullica River and Great Bay, New Jersey. Chem. Geol. 7, 51-63. Duinker J. C., Van Eck G. T. M. and Nolting R. F. (1974) On the behavior of copper, zinc, iron and manganese, and evidence for mobilization processes in the Dutch Wadden Sea. Neth. J. Sea Res. 8, 214-239. Eaton A. (1978) Removal of ‘soluble’ iron in the Potomac River estuary. Est. Coastal Mar. Sci. 9, 41-49. Eisma D., Das H. A., Hoede D., Van Raaphorst J. G. and Zonderhuis J. (1966) Iron and trace elements in Dutch coastal sands. Neth. J. Sea Res. 3, 68-94. Elder D. L. and Fowler S. W. (1977) Polychlorinated biphenyls: penetration into the deep ocean by zooplankton fecal pellet transport. Science 197, 459-460. Folger D. W. (1972) Characteristics of estuarine sediments of the United States, U.S. Geol. Surv. Prof: Paper 742. 94 PP. Greenland D. J. and Quirk J. P. (1964) Determination of the total specific surface area of soils by adsorption of cetyl pyridinium bromide. J. Soil Sci. 15, 178- 19 1. Harvey H. W. (1937) The supply of iron to diatoms. J. Mar. Biol. Ass. U.K. 22, 205-219. Lowman F. G., Rice T. R. and Richards F. A. (1971) Accumulation and redistribution of radionuclides by marine organisms. In Radioactivity in the Marine Environmenr. National Academy of Sciences. Mayer L. M. (1982) Kinetics and mechanism of iron flocculation in Maine estuaries, in prep. Maver L. M. and Fink L. K.. Jr. (1980) Granulometric dependence of chromium accumulation in estuarine sediments in Maine. Est. Coastal Mar. Sci. 11, 491-503. Mayer L. M., Macko S. A., Mook W. H. and Murray S. (198 1) The distribution of bromine in coastal sediments and its use as a source indicator for organic matter. Org. Geochem. 3, 37-42. Mayer L. M. and Rossi P. M. (1981) Application of the cetyl pyridinium bromide (CPB) technique to the determination of specific surface areas of Maine coastal sediments: Relationship of surface area with other sediment textural factors. Tech. Rept. B-016-ME-I, Univ. Me. McCave I. N. (1975) Vertical flux of particles in the ocean. Deep-Sea Res. 22, 49 l-502. Newell R. C. (1979) The Biology of Intertidal Animals. Marine Ecological Surveys, Inc. Nichols M. M. and Cutshall N. H. (1981) Tracing Kepone
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contamination in James Estuary sediments. Rapp. P.-v. R&n. Cons. Int. Explor. Mer 174, 102-I 10. Pevear D. R. (1972) Source of recent nearshore marine clays, southeastern United States. In Environmental Framework of Coastal Plain Estuaries (ed. B. W. Nelson), pp. 317-335, GSA Mem. 133. Prahl F. G. and Carpenter R. (1979) The role of zooplankton fecal pellets in the sedimentation of polycyclic aromatic hydrocarbons in Dabob Bay, Washington. Geochim. Cosmochim.
Acta 43, 1959- 1972.
Sholkovitz E. R. (1976) Flocculation of dissolved organic and inorganic matter during the mixing of river water and seawater. Geochim. Cosmochim. Acta 40, 831-845.
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Sholkovitz E. R., Boyle E. A. and Price N. B. (1978) The removal of dissolved humic acids and iron during estuarine mixing. Earth Planet. Sci. Lett. 40, 130-l 36. Stookey L. L. (1970) Ferrozine: a new spectrophotometric reagent for iron. Anal. Chem. 42,779-781. Thompson T. G. and Bremner R. W. (1935) The occurrence of iron in the waters of the north-east Pacific Ocean. J. Cons. Int. Explor. Mer. 10, 39-47. Turekian K. K., Katz A. and Chan L. ( 1973) Trace element trapping in pteropod tests. Limnol. Oceanogr. 18, 240249. Windom H. L. (1975) Heavy metal fluxes through saltmarsh estuaries. In Estuorine Research (ed. E. L. Cronin), Vol. 1, pp. 137-152. Academic Press.
Retention of riverine iron in estuaries LAWRENCE M. MAYER Oceanography Program 6t Department of Geological Sciences, Ira C. Darling Center, University of Maine at Orono, Walpole, Maine 04573 (Received September 28, 1981; accepted in revised form January 27, 1982) Abstract-Retention of Fe floes, resulting from the mixing of river water and seawater, was examined in three Maine estuaries. Riverine Fe was found to remain fairly conservative with salinity, implying that the process of flocculation does not necessarily remove Fe from water parcels. Laboratory experiments corroborated the field data by demonstrating that neither gravity nor suspended sediment were very effective in removing flocculated Fe from suspension. However, input of a tannery effluent did appear to result in scavenging of Fe from estuarine waters. Flocculated riverine Fe was found to increase considerably the Fe concentrations of estuarine bottom sediments, with the amount of iron per sediment specific surface area dependent on mean river flow entering an estuary. While no long term retention efficiencies could be calculated for these estuaries, it seems likely that a significant portion of flocculated riverine Fe escapes to shelf waters. INTRODUCIION A NUMBER of studies during the past decade have documented the nonconservative behavior of filtrable iron during the estuarine mixing of seawater and river waters. These studies have demonstrated the geographical ubiquity of the phenomenon and the nature of the colloidal flocculation process responsible. Little attention has been given, however, to aspects of the retention of iron in the estuarine zone. The effect of the flocculation of iron-rich colloids on the actual removal of these colloids from water parcels has rarely been addressed. The fact that riverine iron-rich colloids smaller than, say, 0.45 pm aggregate to form floes larger than 0.45 Frn during mixing with seawater does not necessarily imply that these newly-formed large floes will settle quickly from the water columns. The filtrability of estuarine colloids does not equate to their tendency to sediment. In this study, both the dissolved and particulate iron concentrations were examined in a series of estuarine water parcels, under low turbidity conditions, in order to determine if the iron removed from a filtrable state was actually lost from the water parcel. The potential roles of gravity and suspended sediment in removing iron from water parcels were examined by experimentation. The influence of iron colloid flocculation on the iron contents of estuarine sediments has also received little attention. If iron is removed from water parcels it may be detectable in the estuarine sediments. Coonley et al. (1971) searched for iron enrichments in the sediments of the Mullica estuary but failed to find incremental iron of the magnitude to be expected from the observed flocculation process. Eisma et al. ( 1966) found evidence for iron accumulation in the coastal sands offshore of the Rhine estuary. Can iron colloid flocculation produce a measurable increase in the concentrations of iron in estuarine sediments? Such an increase was sought in this study by comparing sedimentary iron concentrations in a series of
estuaries receiving varying amounts of river input, using a textural normalization technique to remove grain size effects. The study area employed was the Gulf of Maine and three of its associated estuaries (Fig. 1). The estuaries are all drowned river valleys, receiving freshwater runoff from watersheds similar in climate and vegetation. Very few carbonate rocks are found in the drainage basins; the rivers are thus soft water in nature. The estuaries differ primarily in extent of freshwater inflow, with the Damariscotta receiving only about 2 m3 - see-’ and the Sheepscot and Saco increasing in roughly order-of-magnitude increments, respectively. Sediment delivery by the rivers of New England estuaries are low relative to other U.S. estuarine systems (Folger, 1972), optimizing the use of these systems in detecting riverine contributions of iron in colloidal as opposed to suspended sediment form. METHODS AND MATERIALS Removal of f?ocs from water parcels The experimental approach consisted of determining the amount of iron removed from a water parcel by both centrifugation and filtration, both in the presence of varying amounts of suspended sediment. The sediment was collected from the top I cm of a mudflat in the Damariscotta estuary and wet-sieved to remove particles larger than 44 pm. Sheepscot River water and Damariscotta estuary seawater (salinity = 29-31%) were prefiltered through- Millipore 0.45 pm Type HA filters. These orefiltered solutions were mixed in a i:l ratio in the presence of 0, 10, 20, 30, 50, and 69 mg ~1~’ sediment. The resultant suspensions were placed on a slow reciprocating shaker for 5 hr at room temperature. Each suspension was then centrifuged for 20 min at 1120 g, which sufficed to remove the sediment from suspension. An aliquot was then removed from each suspension and filtered through a Millipore 0.45 pm Type HA filter. Roth the centrifuged and the centrifuged plus filtered aliquots were then analyzed for iron. The field study involved examining both the dissolved and particulate iron in quasi-synoptic estuarine profiles. The Saco estuary was sampled within +45 min of slack high
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1004
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removed of oxide and organic coatings and carhonutc m;~ teriai, which formed an insignificant fraction of the sediments in this study area. The analytical coefficient cri‘v:lri. ation for surface area was better than f15”r.
RESULTS Removal
FIG. 1. Map of Damariscotta, Sheepscot, and Saco estuaries and their location in the Gulf of Maine. Bottom sediment sample sites shown as dots. tide. Although samples were taken from most depths, complete sets of analyses were performed primarily on surface water samples, as this layer dominates seaward transport of iron in the highly stratified Saco. All samples were filtered through acid-washed and pre-weighed Gelman A/E glass fiber filters underlain by a Millipore 8.0 pm Type SC filters. Calibrated against a series of varying size Nuclepore membrane filters, this filter combination had an effective pore size of 0.5 pm for filtrable iron. Distilled water was rinsed through each filter at the end of filtration to remove seasalt. The filtrates were analyzed for iron and salinity. The Gelman filters were weighed to obtain the suspended particulate concentration and then leached with a 4MHN03/0.7M-HCI solution for 2 hr at 70°-90’. The leachates were analyzed for iron and chromium by atomic absorption spectrophotometry. Filtrable iron was measured in duplicate using the Ferrozine technique of Stookey (1970). The analytical coefficient of variation ranged from +2.5% at freshwater iron concentrations to f7.596 at seawater concentrations. Salinities were measured on a Beckman RS-7B salinometer.
qf iron from
nuter parcels
The relative efficacies of suspended sediment, gravitational settling, and filtration in removing iron from a suspension of - 15% salinity are indicated in Fig. 2. Gravitational settling was simulated by centrifugation for a time period long enough to remove all particles >0.35 pm, assuming a density of 2.5 g. cm-‘, from suspension. Filtration was at a nominal pore size of 0.45 pm. Beginning with 104 kg* 1 ’ of iron in suspension, centrifugation was able to remove 24.1 pg - 1-l in the absence of suspended sediment while filtration was far more effective, removing an additional 59.3 pg.l-‘. Coonley et al. (1971) found similar results for riverine and very low salinity samples. Clearly, much of the flocculated material formed at intermediate salinities during the mixing of river water and seawater must have a density lower than 2.5 g - cm-3. Under the influence of a 1 g gravitational field, the material which was removed by filtration but not by centrifugation would be expected to settle less than 1 meter in a 15 day period, which is greater than the flushing time of many estuaries. The addition of suspended sediment in concentrations similar to those typically found in estuaries resulted in a measurable but only a small enhancement of removal of iron from suspension. The incremental removal by suspended sediments was approximately the same for solutions clarified by either filtration or centrifugation plus filtration. The lack of incremental iron removal at suspended sediment concentrations above 30 mg - I-’ suggests that colloid scavenging by sediment was limited by the availability of colloids susceptible to adsorption rather than the availability of adsorption sites on sediment particles. These results are quite different than those of Aston and
Iron accumulation in estuarine sediments Sediment samples were collected by Shipek grab in subtidal areas and by hand with a plastic scoop in intertidal areas from the Gulf of Maine, and the Damariscotta, Sheepscot, and Saco estuaries (Fig. 1). Only the top centimeter of sediment was sampled. For iron analysis, approximately 1 g of sediment was heated to dryness in concentrated HNO, twice, follo&d by dissolving the residue in 5% HNO, for analysis. The resultant digestates were analyzed for iron by atomic absorption spectrophotometry. Specific surface areas were measured by a modification of the cetyl pyridinium bromide (CPB) technique (Greenland and Quirk, 1964; Mayer and Rossi, 1981). This technique measures specific surface areas of aluminosilicate grains
0
IO
zc
30
SUSPENDED
40 SEDIMENT
50
60
70
(mq I-‘)
FIG. 2. Effects of centrifugation, centrifugation plus filtration, and suspended sediment concentration on the removal of iron from suspension.
RETENTION
OF RIVERINE
Chester (1973), who found natural suspended sediment to remove 75% of a~ificially prepared iron coiloids from seawater, and lend support to the contention of Sholkovitz (1976) that Aston and Chester’s conclusions are not applicable to estuarine situations. The possibility that flocculated iron colloios are mixed conservatively with seawater, rather than removed from the water column as a result of floeculation, was also examined using field data collected from the Sac0 estuary. Figure 3 shows plots of the salinity dependence of 0.5 pm filtrable and total (filtrable plus particulate) iron for two sampling dates. Also plotted are hypothetical total iron concentrations calculated as the sum of ( 1) conservative mixing of riverine total iron into seawater, and (2) the iron estimated to be contained in the suspended particulate matter (SPM) resuspended from estuarine sediments or from outside the mouth of the estuary. The riverine total iron was dominated by filtrable iron; suspended sediment levels from the Saco River are generally very low (co.5 mg +1-l) because the river is impounded immediately above its juncture with the estuary. The particulate iron found in the river on the two sampling dates probably consisted largely of colloidal iron aggregated during river flow, as found by Eaton (1978); the concentrations of iron in the SPM collected in the im~unded part of the river ranged from 15% to 25%-Fe by weight. The second term was calculated by multiplying the SPM concentration by 20 mg-Fe-g-sediment-‘, a value estimated from the following three considerations. First, SPM in the seawater outside the estuary had iron concentrations of 5-23 mg-Fe * g-sediment-‘, and this seaward source appeared to contribute a major fraction of the SPM in the surface waters of the estuary. Second, the SPM in the turbidity maximum presumably represents material derived primarily from resuspension within the estuary; SPM from this zone showed concentrations of lo-20 mg-Fe - g-sediment-‘. Third, iron analyses of a series of grain size separations (Mayer and Fink, 1980) yielded values of 6-50 mg-Fe- g-sediment-’ for the silt and clay fractions (unpublished data), which are the sizes most likely to be resuspended. The contribution to the SPM pool by resuspension was minimized by sampling on a calm day at slack high tide, at which time SPM levels should have been relatively low (Anderson, 1970). Estuarine suspended sediment levels in the surface waters varied from ~1 mg-1-l in the low-salinity zone to -4 mg - 1-l at the seaward end of the estuary, with values in the deeper turbidity maximum reaching 13 mg - 1-r. Different responses for riverine total iron to the mixing process with seawater were observed on the two sampling dates. Roth data sets show the expected, negative nonconservative mixing for the Jiltrable iron, resulting in W-95% flocculation by 15% salinity. The g/22/79 data set shows, however, congruence between the hy~thetica1 and the actual total iron, implying essentially conservative mixing of
IRON IN ESTUARIES
1005
SALINITY (%e)
FIG. 3. Filtrable iron (O), hypothetical total iron (A), and measured total iron (A,) plotted versus salinity for two sampling dates in the Saco estuary.
riverine total iron with the seawater endmember. The discrepancies are most likely due to the average iron concentration of 20 mg-Fe - g-sediment-’ assigned to the SPM; this value is high for SPM derived from offshore and probably low for sediment resuspended from upper estuarine mudflats. The total iron data from the 7/23/79 collection, on the other hand, show a significant removal of iron from the water column, as indicated by the consistent negative deviation of observed total iron from the predicted trend. The uncertainty in the hypothetical total iron trend, due to the unknown true concentration of iron in the SPM, precludes an accurate calculation of the removal percentage, but it appears that most of the riverine iron was withdrawn from the water parcels. The probable cause for these two different mixing patterns is indicated by an examination of the particulate chromium data (Fig. 4). A tannery is present at the upper limit of tidal incursion, which releases a waste into the river consisting largely of chromium and organic matter. The data from the 7/23/79 sampling show an order of magnitude higher concentration of this effluent at the head of the estuary than those of the S/22/79 collection. The rapid decrease in chromium concentration with salinity in the 7f23f 79 data indicates a rapid precipitation of the effluent
L. M. MAYER
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I
1
.
7/23
+
!
30
mium ratio of the tannery sludge found in the sediments of this estuary, about 40: 1 (Mayer and Fink. 1980) a considerable amount of organic matter can be expected to have been associated with this chromium. The nature of this organic matter was not determined in this study, but likely is a combination of polyphenolic tanning agents, proteins, and fats. A scavenging of the iron colloids by the organic-rich tannery effluent appears, then, to be the mechanism for removal of the iron from the water column. Iron in estuarine sediments Figure 5 shows the concentrations of iron in sediments from the Saco, Sheepscot, and Damariscotta
1
6-
5
I
8/22
l
-
54,
l
3 ‘*
l
IO
20
30
SALINITY
(%o)
FIG. 4. Suspended particulate chromium concentration versus salinity for two sampling dates in the Saco estuary.
estuaries as well as offshore sediments from the Gulf of Maine, plotted against the specific surface area of the sediments. Plotting the iron concentrations against surface area allows removal of grain size as a variable in comparing iron concentrations from different areas. It is evident that iron concentrations, normalized against surface area, increase in the order Gulf of Maine < Damariscotta < Sheepscot < Saco. This estuarine trend is related to the amounts of freshwater inflow received by the estuaries. Figure 6 is a plot of the approximate mean river flow entering each estuary versus the average incremental thickness of the iron coating in the sediments over the Gulf of Maine ‘baseline’ iron thickness. These incremental coating thicknesses were calculated by subtracting the average Gulf of Maine iron to surface area ratio from the ratio for each of the estuaries. Such a calculation assumes that the increased iron concentrations found in the estuaries are not due to
, material in the estuary, while roughly conservative mixing of particulate effluent chromium is evident in the g/22/19 data. Given the high carbon to chro-
n v
1007
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i 0
1 “E
4 u.
.
a Y E SAC0
.
SHEEPSCOT
0
OFPSWORE
0 OAYARISCOT TA 0
IO
20
SURFACE
0
IO:
0 I+ 0.1
, 1.0
?4
30 AREA
(rn*.g-‘)
FIG. 5. HNO,-leachable iron concentrations versus sediment specific surface area for sediment samples from the Gulf of Maine and three estuaries. Sampling sites shown in Fig. 1.
FIG. 6. Mean river flow versus incremental iron:surface area ratios (explained in text) for the (A) Damariscotta, (B) Sheepscot, and (C) Saco estuaries.
RETENTION
OF RIVERINE
IRON IN ESTUARIES
1007
1 Km
FIG. 7.Iron:surface area ratios for intertidal (upper numbers) and subtidal (lower numbers) sediments in the Saco estuaries. Numbers in parentheses are the specific surface. areas of the sediments. Intertidal data are means + standard deviations for several samples from each of four mudflats. Also shown is the high tide ha&line (10-20460) from g/22/79 sampling trip.
mineralogical effects; this assumption is justified on the bases of similar mineralogical composition among the estuaries and the use of a textural normalization technique. Further corroboration that the incremental iron is derived from riverine material comes from consideration of organic carbon levels in the Damariscotta and Sheepscot estuaries. Mayer et al. (198 1) have shown that organic carbon levels, if plotted versus specific surface area, in the Sheepscot are higher than those of the Damariscotta in a manner very similar to the case of iron. The 8°C values of the organic matter in the two estuaries indicate that the additional organic matter in the Sheepscot estuary is terrigenous. If the incremental organic carbon concentrations, per unit surface area, are calculated for the Sheepscot over the Damariscotta, a value of 830 pg-organic carbon - meter-* results. The incremental organic carbon to incremental iron ratio, then, is 13, on a molar basis. Analysis of iron-humic floes collected in river water-seawater mixing experiments have yielded carbon:iron ratios ranging from 1.3 to 59 (Mayer, in prep.); similar ratios have been obtained by Sholkovitz (1976), Sholkovitz et al. (1978), and Boyle et al. (1977). These calculations, then, are consistent with an iron-humic colloid origin for both the incremental iron and carbon in the Sheepscot sediments. A similar calculation cannot be performed for the Saco estuary because of the organic pollution in that system (Mayer and Fink, 1980). The distribution of iron in the Saco sediments is shown in Fig. 7, expressed as the concentration of iron per unit surface area. The most intense accumulations, on a per unit surface area basis, were found in the central part of the estuary. This pattern is evident in both the intertidal and subtidal sediments, in spite of their quite different grain sizes. The mid-estuarine iron maximum in the intertidal sediments contrasts with the highest pollutant chromium loadings in the upper estuarine intertidal mudflats (Mayer and Fink, 1980), and presumably reflects the higher salinity required for iron flocculation than for pollutant chromium settling (Figs. 3 and 4). This mid-estuarine maximum may instead be due to the coincidental location of the subtidal turbidity maximum, the typical upper limit of which is denoted
by the typical high tide hahxline shown in Fig. 7. Accumulation of river-derived Kepone has also been found at an estuarine nodal point (Nichols and Cutshall, 198 1); such accumulations in this zone may result from the high suspended sediment levels and consequent scavenging of materials- from the water column. DISCUSSION The “removal” of filtrable iron during estuarine mixing should not be interpreted to mean removal from estuarine water parcels. The data presented in this work suggest, instead, that although the initially colloidal iron does increase its particle size during flocculation, it does not necessarily settle rapidly from suspension in the absence of anthropogenic perturbations. Rather, much of the larger flocculent material may remain essentially conservative with the water parcel. This conclusion is corroborated by the lack of success by Coonley et al. ( 197 1) in finding the flocculated iron from the Mullica River in the sediments of its estuary, and obviates the need to invoke tidal scour for erosion of iron floes from the sediments, as suggested by these authors. These lowdensity floes may well comprise the iron-rich filtrable but not settlable suspended particulate matter observed by Duinker et al. (1974) in the coastal waters near the estuary of the Rhine River. Of course, estuaries with very long flushing times will likely have increased retention efficiencies. Windom (1975) concluded that riverine iron delivered to Southeastern U.S. estuaries is completely retained in the estuarine sediments, based upon a sedimentation rate calculation for iron compared with the annual total iron delivery from the rivers. However, his analysis did not take into consideration that much of the sediment accumulating in these estuaries derives from offshore rather than from rivers (Pevear, 1972); such a budgetary approach is therefore unjustified. It would be of interest to calculate this type of budgetary analysis using incremental iron concentrations in the Sheepscot estuary; however, sedimentation rate data averaged over the entire estuary are not available. Nevertheless, the analysis of estuarine sediments indicates that estuaries do act as at least a partial
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1. M MAYER
sink for flocculated iron. The removal of iron from estuarine water parcels is likely effected by both gravitational settling of larger or denser iron floes and scavenging by SPM. This latter mechanism may play a more important role than that indicated by the small effect shown in Fig. 2. Anderson (1970) demonstrated that SPM levels in a shallow New England estuary exhibited a periodicity with high levels at low tide and low levels at high tide, which he suggested was due to wind wave resuspension of tidal flat sediment under shallow water conditions. Such a periodicity would imply that once each tidal cycle bottom sediment will be introduced to and removed from a parcel of fresh water flushing through an estuary. This water parcel may then lose a small amount of iron colloids to SPM scavenging many times, depending on the flushing time of the estuary. Such a removal mechanism will be maximized in either shallow estuaries or those with extensive tidal flats. Benthic biodeposition by filter feeders such as bivalves may also be important in removing iron colloids (Lowman et al., 1971). For example, Anderson et al. (1981) have shown filter feeders to constitute a major sedimentation pathway for a mudflat in the Damariscotta estuary. However, most bivalves show poor filtration efficiencies for particles smaller than 4 pm (Newell, 1979), which includes most of the iron floes formed (Mayer, in prep.; Bale and Morris, 1981). Iron-rich floes flushed from the estuaries out into shelf waters may be removed from the water column by gravitational settling, given the longer residence times of water on the shelf relative to the estuaries. Alternatively, the fine-grained iron-rich floes may be removed by a planktic biodepositional mechanism. McCave (1975) has pointed out the importance of large particle formation in facilitating downward material flux in the oceans. Large particle formation can occur either biogenically or nonbiogenically. Non-biogenic aggregation, i.e., further flocculation, is an unlikely process, as flocculation ceases after a few hours (Sholkovitz 1976; Boyle et al., 1977; Mayer, in prep.). In addition, the dilution of the iron colloids during mixing with seawater to higher salinities will further inhibit the rate of interparticle contact and therefore the flocculation rate. Biogenic particle formation, and biodeposition, may occur via phytoplankton uptake of colloidal iron (Harvey, 1937) followed by zooplankton grazing and fecal pellet formation (Elder and Fowler, 1977; Prahl and Carpenter, 1979), or occlusion of iron-rich floes into planktonic carbonate shells (Turekian et al., 1973). As evidence for the first pathway, coastal iron concentrations have been shown to decrease during periods of high phytoplankton productivity (Thompson and Bremner, 1935). Acknowledgements--I thank L. Schick and P. Rossi for their able and patient laboratory assistance, and E. R. Shol-
kovitL for reviewing an early draft of this manuscript. Ihc work upon which this publication is based was supported in part by funds provided by the Office of Water Research and Technology (Grant No. B-016ME), U.S. Department of the Interior, Washington, D.C., as authorized by the Water Research and Development Act of 1978
REFERENCES Anderson F. E. (1970) The periodic cycle of partrcuiate matter in a shallow temperate estuary. J. Sedimenf. Petrol. 40, 1 I28- 1135. Anderson F. E., Black L.. Watling L. E., Mook W. and Mayer L. M. (1981) A temporal and spatial study of mudflat erosion and deposition. J. Sedimenr. Petrol. 51, 729-736. Aston S. R. and Chester R. (1973) The influence of suspended particles on the precipitation of iron in natural waters, Est. Coastal Mar. Sci. 1, 225-231. Bale A. J. and Morris A. W. (1981) Laboratory simulation of chemical processes induced by estuarine mixing: The behaviour of iron and phosphate in estuaries. Est. Coastal Shelf Sci. 13, l-10. Boyle E. A., Edmond J. M. and Sholkovitz E. R. (1977) The mechanism of iron removal in estuaries. Geochim. Cosmochim. Acta 41, 13 13-l 324. Coonley L. S., Jr., Baker E. B. and Holland H. D. (1971) Iron in the Mullica River and Great Bay, New Jersey. Chem. Geol. 7, 51-63. Duinker J. C., Van Eck G. T. M. and Nolting R. F. (1974) On the behavior of copper, zinc, iron and manganese, and evidence for mobilization processes in the Dutch Wadden Sea. Neth. J. Sea Res. 8, 214-239. Eaton A. (1978) Removal of ‘soluble’ iron in the Potomac River estuary. Est. Coastal Mar. Sci. 9, 41-49. Eisma D., Das H. A., Hoede D., Van Raaphorst J. G. and Zonderhuis J. (1966) Iron and trace elements in Dutch coastal sands. Neth. J. Sea Res. 3, 68-94. Elder D. L. and Fowler S. W. (1977) Polychlorinated biphenyls: penetration into the deep ocean by zooplankton fecal pellet transport. Science 197, 459-460. Folger D. W. (1972) Characteristics of estuarine sediments of the United States, U.S. Geol. Surv. Prof: Paper 742. 94 PP. Greenland D. J. and Quirk J. P. (1964) Determination of the total specific surface area of soils by adsorption of cetyl pyridinium bromide. J. Soil Sci. 15, 178- 19 1. Harvey H. W. (1937) The supply of iron to diatoms. J. Mar. Biol. Ass. U.K. 22, 205-219. Lowman F. G., Rice T. R. and Richards F. A. (1971) Accumulation and redistribution of radionuclides by marine organisms. In Radioactivity in the Marine Environmenr. National Academy of Sciences. Mayer L. M. (1982) Kinetics and mechanism of iron flocculation in Maine estuaries, in prep. Maver L. M. and Fink L. K.. Jr. (1980) Granulometric dependence of chromium accumulation in estuarine sediments in Maine. Est. Coastal Mar. Sci. 11, 491-503. Mayer L. M., Macko S. A., Mook W. H. and Murray S. (198 1) The distribution of bromine in coastal sediments and its use as a source indicator for organic matter. Org. Geochem. 3, 37-42. Mayer L. M. and Rossi P. M. (1981) Application of the cetyl pyridinium bromide (CPB) technique to the determination of specific surface areas of Maine coastal sediments: Relationship of surface area with other sediment textural factors. Tech. Rept. B-016-ME-I, Univ. Me. McCave I. N. (1975) Vertical flux of particles in the ocean. Deep-Sea Res. 22, 49 l-502. Newell R. C. (1979) The Biology of Intertidal Animals. Marine Ecological Surveys, Inc. Nichols M. M. and Cutshall N. H. (1981) Tracing Kepone
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contamination in James Estuary sediments. Rapp. P.-v. R&n. Cons. Int. Explor. Mer 174, 102-I 10. Pevear D. R. (1972) Source of recent nearshore marine clays, southeastern United States. In Environmental Framework of Coastal Plain Estuaries (ed. B. W. Nelson), pp. 317-335, GSA Mem. 133. Prahl F. G. and Carpenter R. (1979) The role of zooplankton fecal pellets in the sedimentation of polycyclic aromatic hydrocarbons in Dabob Bay, Washington. Geochim. Cosmochim.
Acta 43, 1959- 1972.
Sholkovitz E. R. (1976) Flocculation of dissolved organic and inorganic matter during the mixing of river water and seawater. Geochim. Cosmochim. Acta 40, 831-845.
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Sholkovitz E. R., Boyle E. A. and Price N. B. (1978) The removal of dissolved humic acids and iron during estuarine mixing. Earth Planet. Sci. Lett. 40, 130-l 36. Stookey L. L. (1970) Ferrozine: a new spectrophotometric reagent for iron. Anal. Chem. 42,779-781. Thompson T. G. and Bremner R. W. (1935) The occurrence of iron in the waters of the north-east Pacific Ocean. J. Cons. Int. Explor. Mer. 10, 39-47. Turekian K. K., Katz A. and Chan L. ( 1973) Trace element trapping in pteropod tests. Limnol. Oceanogr. 18, 240249. Windom H. L. (1975) Heavy metal fluxes through saltmarsh estuaries. In Estuorine Research (ed. E. L. Cronin), Vol. 1, pp. 137-152. Academic Press.