Estuarine and Coastal 3Iarine Sdence (x98o) xO, 75-83

Chemical Tracers for Particle Transport in the Chesapeake Bay

Andrew Eaton, Virginia Grant and M. Grant Gross Chesapeake Bay Institute, Johns Hopkins University, Baltimore, ~faryland 2x2x8, U.S.A.

Received x2 September x978 and in revised form x5 February x979

Keywords: estuaries; particle transport; trace elements; tracers; Chesapeake Bay

Particulate material from the Susquehanna River is enriched in Fe (5-7%), allowing it to be distinguished from other sources of particles in surface waters of the Chesapeake Bay. Although the turbidity maximum removes most of the flyer-borne detritus, chemical composition of particles (SPM) indicates that Susquehanna River materials dominate the inorganic sus- pended matter in surface waters of the Bay at least 2o-4o km further. Fe concentrations, Fe/Ti, and Fe/Zn ratios in surfacewater SPM still farther down the Bay indicate that those particles are a mixture of river derived and biological material and that shore erosion is not normally a source of surface water suspended sediments as far as the mouth of the Potomac.

Seasonal variationstof elemental ratios such as Fe/Zn can also be used to demonstrate scavenging of soluble trace elements by particles during summer conditions.

Introduction

Numerous contaminants, including metals, pesticides, and nutrients may be associated with suspended particulate matter in an estuary (Munson, x976; Taft & Taylor, i976; Turekian x977). In most estuaries most river-derived suspended sediment is trapped in the turbidity maximum (Schubel, x968; Nichols, x974). However, measurable amounts of suspended matter escape this trap to travel farther down the estuary.

In the Chesapeake Bay estuary suspended particulate matter (SP~I) below the turbidity maximum consists of river-derived material, material produced from shore erosion and biologically produced material (Biggs, x97o ). Establishing the magnitude of each source would aid in establishing mass balances for constituents associated with suspended matter such as metals (Helz, x976) and radionuclides (Goldberg et al., x978 ). Biggs (x97o) suggested that only 8% of the suspended particulate matter farther than 5 ~ km from the river mouth was derived from Susquehanna River discharge, with the rest derived equally from shore erosion and in situ biological production. This conclusion was based entirely on measure- meat of total suspended sediment loads and particulate carbon measurements throughout the Bay minus a load calculated from long term rates of shore erosion, although shore erosion in a highly episodic process.

Carpenter et al. (x975) demonstrated that the Susquehanna SPM was especially enriched in Fe, Mn, Zn, Cu, and Ni during winter. This material is always enriched in metals com-

75

76 .'t. Eaton, V. Grant & i~l. G. Gross

pared to other particle sources such as shore erosion (Helz, i976; Ferri, I977) or biota (Martin & Knauer, x973). Here we note the constancy of certain elemental ratios in Susque- hanna SPM and the use of the chemical composition of Chesapeake Bay surface water SPIM to trace the transport and alteration of river-derived material.

Figure x. Station locations in the Chesapeake Bay.

Particle transport, Chesapeake Bay 77

Materials and methods

To determine the composition of river-borne material, we obtained 20 liter samples of water just downstream of the Conowingo Dam on the Susquehanna River at approximately bi- weekly intervals from December 1976 through January 1978 . These samples were obtained by submerging an acid-cleaned polyethylene bottle near the shore where the suspended particu- late matter was derived from Susquehanna River flow over and through the hydroelectric dam. We obtained 2o to 5 ~ liter tri-monthly samples along the axis of the Bay at approxi- mately 20 km intervals (Figure I) using an all-plastic pumping apparatus.

Particles were separated allowing all particles larger than 0- 5 lam equivalent spherical diameter (ESD) to settle out (Carpenter el al., I975). The supernatant was decanted and the settled particles collected on several 3 l~m Nuclepore filters. Less than 5~/o of the settled solids pass this filter.

The particles and filter were digested in hot 6 N-HCI for three days, to recover more than 9o% of the metals (Eaton et aL, I977). The digest was freeze dried and taken up in o.o 3 M- HCI. Fe and Zn were determined by flame atomic absorption spectroscopy on the solution. Precision of analyses of replicate samples is better than -t-5% for Fe and + 1 o % for Zn. To confirm the fact that coagulation of 'soluble' Fe would make a minimal contribution to our measured particle-assoclated Fe, we made measurements of filter passing (Whatman G F / F - - nominal pore size 0. 7 lain) Fe on fresh samples using the method of Stookey (I97o). The maximum concentration measured downstream of the turbidity maximum was less than

5 lag I -x. Samples from the December x976 cruise were analyzed for Ti using a thin film X-ray

fluorescence technique (Cann & Winter, 197 r ; Price & Skei, 1975). Precision of Ti deter- minations is -r xo%.

Results

The concentrations and seasonal variability of Fe in the SPM in the Susquehanna River (Figure 2) confirms the results of Carpenter et al. (I975). During most of the year, Fe concentration in suspended particles exceeds 50/0 and is as high as 8~o during winter.

In order to use the chemical composition of SPM as a transport tracer one must demon- strate that the composition does not change as a result of chemical reactions or that such changes are predictable in direction and magnitude. For Fe, chemical alterations are clearly

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Figure 2. Fe concentrations in Susquehanna River SPM 1976-77.

78 A. Eaton, V. Grant ~ M. G. Gross

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Figure 3. Fc concentrations in Chesapeake Bay surface water SPM z976-77 .

undirectional, from soluble material to particles (Boyle et al., i977) and most of the alteration occurs at very low salinities. We noted earlier that the soluble Fe in the Chesapeake Bay estuary is only a small fraction of the total load and will thus not alter the particle Fe concen- trations through coagulation.

Variations in Fe concentration in the SPM downstream from the mouth of the Susque- hanna River show two seasonal patterns (Figure 3). In most seasons the.Fe concentration in particles is high and constant (5-7%) for a distance of 6o-8o km from the river mouth (at least 2o km farther down the Bay than the turbidity maximum) and then shows a rapid decrease by a factor of z - 3 within the space of zo--4o km. The other pattern (mid-summer) is of continuously decreasing Fe concentrations in particles as a function of increasing distance doxwtstream. The non-summer data demonstrate that river derived material is the source of surface water particulate matter much farther along the Bay than had been previously suggested.

Lower Fe concentrations can be derived from mixing of river derived material with either shore erosion material or biological material. To distinguish among the 3 sources one must use other data on the composition of the particles in addition to Fe concentrations. We have used Ti and Zn as additional indicator elements since concentrations of these elements and Fc differ markedly among various particle sources.

Ti is clearly useful as a tracer element since it is normally associated only with particles (Kennedy et aL, x974) and is therefore not likely to be altered during transport.

Zn, in contrast, is a comparatively mobile element. However, its behavior is generally predictable in terms of direction.In fresh waters it is adsorbed on particles and in saline waters, as Carpenter et al. (z975) have noted for the Chesapeake Bay, it is desorbed as

Particle transport, Chesapeake Bay 79

salinities increase. This is similar to observations in other estuaries (Evans & Cutshall, I973; Fukai et al., 1975; Thomas & Grill, x977). Observations of Zn concentrations in Susquehanna River SP~i indicate seasonal variations which may be due to sorption of Zn. Fe/Zn ratios vary from 78+x 5 (n=8) in December x976-February I977 to a significantly different 15o-t-46 (n=I7) for the rest of the year. The relatively higher Zn concentrations in the winter are probably due to sorption on freshly precipitated Fe oxide surfaces, since Lewis (t976) has shown that soluble iron travels farther downstream at low winter temperatures than in other warmer seasons.

Shore erosion contributions of SP~I are evaluated by analysis of the Coastal Plain deposits, the source of eroded material. Coastal Plain deposits average x-8% Fe, o.4% Ti and 26 parts/xo ~ Zn (Ferri, i977) , but are mainly quartzose coarse sand, which acts to decrease the trace element concentrations relative to the easily suspendible materials (Eaton, I979a). For proper calculations one must know the Fe, Ti and Zn concentration in the fine fraction of shore erosion material which would potentially contribute to the middle Bay suspended material. Since quartz sands are generally depleted in all the possibly useful metals, ratios such as Fe/Ti or Fe/Zn will reflect the fine fraction no matter how much the fine grained sediment is diluted by the sand.

It is possible to use information on the clay mineralogy of the Coastal Plain deposits coupled with the reported Fe concentrations in various clay minerals to estimate the Fe concentration in the suspended fraction.

TABLE I. Composition of various particle end members which could contribute to suspended sediment in Chesapeake Bay surface waters (~4- xtz)

Component % Fe Fe /Ti Fe/Zn

Susquehanna River 5"94-z'3 (n=25) z2 (n=z) (this work)

Coastal Plain Deposits z ' 8+o '9 ( n = 3 0 4"34-2"2 ( n = 3 0 (Ferri, x977)

Biological material o 'o24-o.lo (n=z3) x64-7 (n=8) (Martin & Knauer, x972)

z25 4-5 ~ (n=25)* Geometric mean = I z$ log Fe/Zn = 2-o64-o'25 Geometric mean = 660 log Fe]Zn = 2"824-0"35

(n=3I ) Geometric mehn = z4 log Fe/Zn = z'z3 4-o'28

(.=xx)

*The overall average is used since winter material would be mixed with material from other seasons in the turbidity maximum of the upper Chesapeake Bay.

TABLE 2. Elemental ratios in surface water SPM from the Chesapeake Bay

Fe/Zn Fe/Ti

/~lean of samples >5 ~ km from iMean

Date Mean4- ztr Range river mouth Mean+ ztr Range >5okra

December, z976 (n----8) zzo+z7 9o--z47 Io74-9 zz 4-z. 4 8"5-z2 zo'94-z'7 March, z977 (n=9) x394-37 79-z85 I27-t-37 No data July, z977 (n=8) 8 z + 3 t 34-x4x 64+23 No data October, x977 (n=8) zzo4-zz 7o--z43 zo3 4-2o No data December, x977 (n=5) 89+2z 68-zzz 69 No data l~Iarch, z978 (n=5) zo54-3o 7z-x45 804-8 No data

80 A. Eaton, V. Grant ~.~ 2~X. G. Gross

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Figure 4. Fe versus Zn for Chesapeake Bay surface water SPM z976-77. July, z977 data shown in open circles (O)- Approximate composition of end-members shown. For all data except July regression of Zn on Fe is statisticaUysignificant at P<o'o 5.

This method of estimation is limited by the range of Fe concentrations recorded for illite, one of the dominant clay minerals of the Coastal Plain deposits (Steffanson & Owens, I97o; Hathaway, I97.; Ferri, I977). The median concentration of Fe in 24 illite analyses reported by Deer et al. (x962) and Grim (I968) is 3"5%. This includes glauconite illites, which are rich in Fe. The other clay sized constituents which are dominant in the Coastal Plain deposits are quartz and kaolinite, both of which act as dilutants of the Fe content of clay and silt sized material (Deer et al., x962; Grim, 1968 ). Thus the Fe content of the material eroded from Coastal Plain deposits is probably less than 3~o. This could be increased as a result of surficial F%Oa, but we have no immediate way of measuring this, nor can we partition it between the coarse and fine fraction. Analyses of the fine fraction alone would provide much useful data.

Concentrations of these elements in the biological end member are based on analyses of plankton by Martin & Knauer (x973). It is apparent that the 3 mixing end members differ markedly in both Fe concentrations and elemental ratios (Table I). Thus these elements can be used to trace the transport and alteration of Chesapeake Bay surface water sPAr.

As we have noted, the constant high Fe concentration in Chesapeake Bay surface water SPA{ for 6o-8o km indicates that shore erosion is not a significant contributor at least that far down the Bay. Further confirmation is provided by elemental ratios (Table *.) which are constant and never close to those of shore erosion material, even when one only considers samples farther than 5o km from the river mouth. Only in July, x977 is there not a statistically significant relationship between Fe and Zn (Figure 4) in SPAI. In non summer months the data reflects a decrease of the river derived load during mLxing with metal poor biological material.

The different pattern observed in July indicates that other factors are affecting Zn concen- trations on the particles. During the summer there is a large flux of soluble Mn from bottom sediments of the mid-Bay (Eaton, I979b) with a subsequent reprecipitation in surface waters to produce Mn rich particles. This fres.h!y precipitated � 9 acts as a sorption surface for other soluble metals such as Zn.

Particle transport, Chesapeake Bay 8z

Art alternative explanation for the constancy of the Fc/Zn ratio in the spring, fall, and winter would be the sorption of zinc onto particles which were a mixture of the two detrital sources. We consider this unlikely since (x) Carpenter et al. (I975) demonstrated that Zn is generally desorbed at high salinities in the Chesapeake; (2) Fe/Ti ratios suggest that shore erosion is not significant in at least one case and (3) unless new surfaces are provided (e.g. fresh Mn) one cannot readily adsorb large quantities of Zn since sorption sites will already be filled.

Discussion

Variations in the chemical composition of suspended sediment in surface water in the Chesa- peake Bay provide a valuable tracer for mixing processes and alterations in the character of suspended sediment as a result of biogeochemical processes. Three questions can be addres- sed through these data; (x) how far down the Bay is Susquehanna River material the major contributor to surface water suspended particulates ? (2) how far down the Bay is Susque- hanna River material transported under typical conditions ? (3) in the open Bay (e.g. farther than 6o-8o km down the Bay) is the decreasing significance of Susquehanna River material in the SPM due to a dilution with other material or due to active removal of river derived material via coagulation or other mechanisms ?

The constancy of Fe concentrations in surface water particles for a distance of 6o-8o km and similarity of those concentrations to Susquehanna River SPM suggests that the river is the dominant source 2o-4o km farther downstream than had been previously assumed.

g lore significant, shore erosion is not a contributor to surface water SP~i even as far as the mouth of the Potomac. If it can be shown that the SPg~[ is a product of mixing only two components, one with high Fe (river material) and one with essentially no Fe (biological material), the relative contribution of the Susquehanna at any given location can be assessed from Fe concentrations in particles and particle loads in surface waters. One can then estimate the transport of Susquehanna River material along the Bay. The covariance of Fe and Zn reflects the fact that surface water SPM is normally only a product of two end members, riverine material and biological debris. Thus, we conclude that Susquehanna River material is transported at least as far as the mouth of the Potomac River.

Biological contributions to surface layer SPM, measured by particulate carbon and nitro- gen concentrations, show little variation from 5 ~ to x4o km from the river mouth for a given season (Taft et aL, x976; I977). Therefore, the decrease in Fe concentrations in particles below 60-80 "Inn from the river mouth does not reflect increasing quantities of biological material, but must be due to active removal of river material, presumably due to salinity induced coagulation or some similar phenomenon. We should note that surface water salinities reached IO~oo at about 7 ~ km from the river mouth during the periods of study and increased very gradually going farther down the Bay. Sholkovitz (I976), in mixing experiments, observed a maximum in the'coagulation of many constituents at xo--i2~o o. Tiffs suggests, but does not prove, that salinity induced coagulation is responsible for the change in composition of surface water SPM observed at a distance of 60-80 km from the river mouth.

Transport of Susquehanna River-derived suspended sediment almost the entire length of the upper and middle Chesapeake Bay under typical conditions has important implications for the fate of pollutants attached to suspended matter. Susquehanna River suspended sediment is enriched in metals (Fe, gin, Zn, Cu, Ni) relative to other local sources (Turekian & Scott, I967; Carpenter et aL, x975; Goldberg et aL, x978 ). Increased coal mining in the Susquehanna River basin might result in a further increase in this enrichment due to the

82 .4. Eaton, V. Grant & 3I . G. Gross

weathering of sulfides associated with coal deposits and the subsequent removal of soluble metals onto Susquehanna River particulate material (Lewis, I976 ). The impact of this enrichment could be felt quite far down the Bay. Other particle associated toxic constituents such as pesticides ~ iunson , i976 ) could also be transported quite far downstream if they were sorbed by Susquehanna River suspended material. Thus although the majority of the River derived material is removed within 3 ~ km of the river mouth, the remainder could have persistent impact on the entire Bay as a source of toxic material during typical conditions.

Acknowledgements

Partially supported by grant no. EY76-SO2-3292 from the Department of Energy to The Johns Hopkins University. We appreciate comments from Bob Biggs, Owen Bricker, and Karl Turekian. Contribution no. 273 from the Chesapeake Bay Institute.

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