Estuarine and Coastal Marine Science (1979) 9, 643-651

Tracer Study of Sediment-Water Interactions in Estuaries

Nile A. Luedtke and Michael L. Bender University of Rhode Island, Graduate School of Oceanography, Nawagansett Bay Campus, Kingston, RI 02881, U.S.A.

Received p October 1978 and in revisedform zzJanuary 1979

Keywords: estuaries; isotopes; tracers; sediment-water interface; water exchange; Narragansett Eay

Rates of sediment bioturbation and advective exchange of water across the sediment-water interface in Narragansett Bay, Rhode Island were studied by radioactive tracer experiments. The ‘biological pumping rate’ of water across the interface (0.7 ho.3 cm8 cm- 2 day - ‘) was calculated from a two box model in which the decrease in 22Na concentration in water overlying a box core was ascribed to animal pumping of water across the sediment-water interface. The ‘ diffusion coefficient’ of solid sediment was determined to be 3 f I x IO-' cm8 s-’ from the depth distribution at the end of the experiment of lrlCe tagged microspheres and 58Fe (which was quantitatively scavenged onto particles at the start of the experiment). The pore water profile deter- mined for 54Mn in the cores was similar to stable Mn profiles previously determined for the site. Of the radioisotopes added to the water overlving the box core, nearly 100% of the Na and 500/b of Sr remained in solution, while only 10% of the Ba, Mn, and Hg and <I% of the Cd, Zn, and Fe remained in solution when the experiments were terminated.

Introduction

Physical mixing of solid sediment by organisms is a well documented phenomenon. Dapples (1942) concluded that in regions of high population sediment could be ingested many times before it was eventually buried. Rhoads (1963) made laboratory determinations of the amounts of sediment processed by individual Yoldia Zimatulu. The use of temperature, climatic, and population data allowed him to estimate that 0.57 cm3 cm -2 year -1 of sediment was moved by Yoldia in Buzzards Bay, Mass., with 2.3 and 5.2 cm3 cm -2 year-l being determined for two Long Island Sound areas. Studies of the polychaete Pectinaria gouZdi by Gordon (1966) indicated that these worms would work the surface sediments of Barnstable Harbor several times before burial. Other workers have employed naturally occurring or pollutant radin- isotopes to study sediment movement. Aller & Cochran (1976) measured 234Th distribution in Long Island Sound sediments. Their results indicated that reworking of the upper 4 cm was rapid relative to the 234Th half-life, and showed that there was some mixing down to depths of at least I 2 cm. Robbins & Edgington (1975) used 210Pb and 13’Cs to estimate recent sedimentation rates and depict bioturbation in Lake Michigan.

Recently, several groups have concluded that organisms transport water across the sediment-water interface at rates great enough to influence sediment-water exchange fluxes.

643 0302-3524/79/l 10643 $09 $02.00/O @ 1979 Academic Press Inc. (London) Ltd.

644 N. A. Luedtke & M. L. Bender

Aller (Ig77), Goldhaber et aE. (Ig76), Korsec & Hammond (Ig77), and McCaffrcy et al. (1978) ascribed constant pore water metabolite concentrations with depth, to advective water exchange. Korsec & Hammond (1977) and McCaffrey et al. (1978) found that 22Na injected into bottom waters invaded sediments faster than predicted from ionic diffusion, and again ascribed this observation to advcctive exchange driven by organisms. In this study, we have used radioisotopes to trace the dispersion of dissolved and particulate metals across the sediment-water interface and into the solid sediment and its associated pore waters. The results are used to calculate the rate of advective water exchange and the ‘diffusion cocffi- cient’ of sediments at one site in Narragansett Bay. We also consider the implications for trace metal geochemistry in anoxic sediments.

Our study site is located under about 5 m of water, just north of Jamestown Island in Narragansett Bay. Pore water chemistry at this site has been extensively studied by McCaffrcJ- et al. (1978) and benthic fluxes have been measured and modeled by Nixon et al. (1974),

and McCaffrey et al. (1978). A vigorous bottom community occupies the site, and there are no indications that it is seriously polluted.

Methods

Plexiglass box cores (30 cm high ~9 cm x 30 cm) of the same type used for pore water analysis by McCaffrey et al. (1978), were collected from the Jamestown North sampling site in November of 1975 and again in September of 1976. At each time the cores were placed in a holding tank and kept at room temperature (2oT). In-lab flux determinations compared to in situ flux determinations have given similar results using this mode of experimentation (Elderfield et al., Ig7ga). Air was bubbled through each supernatant during the entire experiment. Individual cores were kept from 6 to 14 days after radioisotopes had been introduced. A IO pCi aliquot of each isotope was added to each supernatant. The isotopes added are indicated in Table I, along with the ionic form as introduced, the half-life, and the y-rays monitored. In this work radioisotopes and elements will be discussed interchangeably.

TABLE I. Isotopes introduced to the experimental cores with their ionic forms and monitored radiation

Ionic form Isotope Half-life y-rays

monitored (Kev) Experiment

1975 1976

Na ‘~I Sri2 Ba+2 Mn +2 Zn +? Hg+” Cd +2 Fe+3 Ce”

22Na 8 %r

2.62 years 1275 64.7 days 514 10.7 years 81 & 356 312 days 835 243 days 1116 46.6 days 279 453 days 88

59 days 1 099 8~ 1292 32.5 days ‘45

a As a microspheric particle.

Samples were periodically collected from the supernatant solutions of the cores, filtered, and analyzed. At the termination of each experiment the box cores were taken and separated by half-centimeter or centimeter layers. Sub-samples of the bulk sediment were counted. The remaining sediment was placed into a centrifuge bottle and centrifuged to separate the pore water and solid. A IO ml aliquot of the pore water was counted following filtration through a 0.45 pm Nucleopore filter.

Sediment-water interactions 645

After collection and separation, each sample was counted on a Ge(Li) detector feeding into a Nuclear Data zzoo multi-channel analyzer and a magnetic tape unit. Quantification of peak areas was accomplished by using the program GAMNL with the IBM-370 computer system of URI. Peak areas for each isotope were calculated in units of counts per minute per gram of sample. Solution count rates were calculated relative to initial supernatant activity= 1.00. Total fraction counts were determined relative to the count rate of the 0-112 cm sample taking the activity of that sample as 1.00.

100

50

f I-

T3-76

Sr.

Hg l

Boo Mnn

Znr Cd0

0 40 60 120 160 200 0 40 60 120 I60 200

TIME,h

Figure I. Percent of initial tracer activity remaining in the supernatant solution veraua the time of incubation.

Results

Figure I shows plots of the relative per cent activity in the supernatant solutions versus the time of incubation for the various experiments. Rates of removal increase in the order Na<Sr<Ba-Hg<Mn<CdzZn<Fe. Cd and ‘Zn are undetectable in the supernate after 40 to 60 h, reflecting total removal well before the end of the experiment. The Fe introduced is undetectable in the initial sampling which may be due to immediate precipitation as iron hydroxy-oxides.

The pore water profiles obtained from these experiments are shown in Figure 2, along with the overlying supernatant values at the time of separation. Zn was present above the detection limit only in core TI-76. Otherwise Zn, Cd, and Fe were undetectable in all samples. The Cd and Zn are removed from the supernatant more slowly than Fe, but in the sediment they,

646 N. A. Luedtke &f M. L. Bender

TI-75

F ‘- Na o ‘-‘g . Bo 0 Mn d Zn A

T3-76 T2-76 0,5F: t I I b 1 I I F: I I !?A? I I I ,

0 4 8 12 16 0 4 8 12 16

DEPTH, cm

Figure 2. Percent of initial supernatant tracer activity in the pore waters of the cores at the time of separation versus sample depth.

like Fe, arc associated only with the solid phase. As in the supernatant samples Ba and Hg behave similarly in the pore water. Pore water concentrations of both decrease with depth, although the o-1/2 cm sample values are higher than the supernatant value. Mn is of parti- cular interest because 54Mn shows a definite concentration maximum between o cm and 2 cm depth, as does stable pore water Mn in samples from the same site (McCaffrey et al., 1978). The great increase above bottom water values reflects MnO, reduction (soluble Mn * -t production) by organic matter, while the downcore decrease in dissolved manganese reflects incorporation into an unidentified solid phase (Elderfield et aE., 19@). Na and Sr are, as expected, the most soluble elements added to the system. For reasons to be considered later, Na concentrations are uniformly higher than Sr values.

Total sample profiles are plotted versus depth in Figure 3. It is helpful to examine these data with the knowledge that all the Ce and Fe can be associated with the solid phase and all the Na with the aqueous phase. Thus, a relative affinity for the solid phase can be determined. Transport of Zn, Cd, Hg, and Mn follow Fe closely, because they are principally tied to the solid phase. The Ba, Sr, and Na profiles deviate from Fe profiles, reflecting partial transport in solution.

A mass balance reveals the fractional distribution of activities between supernatant, pore water, and solid sediment (Table 2). Activity in the initial and final supernatant was deter- mined by multiplying the activity in a sample aliquot by the volume of supernatant. Pore water activities were calculated by multiplying the sample aliquot activity by the total sample fraction volume times the porosity. Porosities were taken as 0.8, 0.75 and 0.70 for

Sediment-water interactions 647

O-I/Z cm, I/~-I cm, and for the remainder of the core, respectively (R. McCaffrey, personal communication). The solid phase activity is calculated by difference.

0.5 ’ ’ ’ ’ ’ ’ ’ \L

’ ’ ’ ’ ’ ’ ’ ’ 1 0 4 6 12 16

DEPT 0 4 8 I2 16

H, cm

Figure 3. Variation of total sample isotope activity with depth, relative to the activity observed in the first half-centimeter of sediment. A represents the Zn activity in core TI-75 and T3-75 but is an average value for the Zn, Cd, and Fe activities observed in cores Tz-76 and T3-76,

TABLE 2. Average per cent distribution of isotope activities. Supernatant and pore water averages have been determined from the experiments, xhile the solid phase activity is calculated by subtracting the supernatant and pore water activities from the total initial supernatant activity

Per cent initial activity Final Final Final

supernatant pore water solid phase -__

Na 66+rz 33i8 I Sr 62~~13 18+6 20

Ba 23fro 4tr 73 Mn 4*3 7*7 89 Hg zoi6 4*x 76

Zn-Cd-Fe <I ir 99

Discussion

The 22Na is totally associated with the solution, and can be used to determine the rates at which water is transported across the sediment-water interface by the physical activity of organisms. 141Ce and 5sFe distributions in the sediment allow us to calculate mixing coeffi- cients of the solid phase due to bioturbation.

648 N. A. Luedtke & M. L. Bender

An upper limit to 2zNa transport into the pore waters by Fickian diffusion may be calculated from the equation of Crank (‘975)-d escribing diffusion from a constant concentration (taken as C,) plane source into a diffusive medium with zero initial concentration,

Co-Gw x -____ c

==erf -----= ‘I) 2Z/Dt

1.0

0.6 F

\ 4 0 11-75

P A T3-75 l T4-75

0 71-76 0 TZ-76

0.6 0 73-76

,”

; 0.4

DEPTH , cm

Figure 4. Penetration through simple diffusion processes compared to our experi- mental 22Na pore water profiles. C=concentration at depth, C,+nitial concentra- tion in the supernatant. Theoretical diffusion lines are calculated for a ; da\- incubation period.

We take D as one half of the Na diffusion coefficient in dilute solutions at 25°C (5.8 x 10-6 cm2 s-l, Ii & Gregory, 1974) (see McCaffrey et al., 1978). The distance below the sediment water interface is X, C, is the initial supernatant concentration (taken as I.OO), and C (x,tj is the concentration in the pore water normalized to the initial supernatant value. The 22Na pore water profile thus predicted for Fickian diffusion after a 7 day incubation is shown in Figure 4 along with experimentally determined values of 22Na US. depth. The comparison shows that 22Na has been transported into the pore waters more rapidly than can be accounted for by Fickian diffusion. Entrainment or subduction of the 22Na during sampling would demand up to 50’3’~ of the surface radioactivity be mixed with sediments at depths of 8-12 cm, which can not be the case here. The additional transport must be due to benthic biological activity. We make the extreme simplifying assumption that all 22h’a transport is due to benthic organisms, and use the data to calculate parameters describing transport rates.

We have developed a two-box model for calculating the transport of solution across the sediment-water interface (McCaffrey et al., 1978). In our model, organisms are assumed to pump water between the two well mixed reservoirs (sediment and overlying water). The 22Na data is used to estimate the ‘ biopumping rate’ (the volume of water exchanged each day, per unit surface area, between the pore water and the overlying solution).

The decrease of 22Na in the supernatant solution is described by the following differential equation:

dG(t) - =--KC,(t)+K 2 C,(o)--K ; C,(t) dt P P

Sediment-water interactions 649

Where C,(o)=the normalized 22Na concentration in the overlying solution at t=o (taken as I.OO), C,(t)=the normalized 22Na concentration in the overlying solution at time=t (i.e., ratio of the concentration at time t to the concentration at time zero), K=exchange constant, V,=supernatant volume, and Vp=pore water volume. The solution to this differential equation is :

C,(t)= (vs,;p+I) rPS/~P) Cs(o)+e (- XI) (V IV +l)

s P I.

This is rearranged as:

WC,(t) [1+~~/~~1-~~~/~,) Cs(o)l=--K[~+~s/~pl t

TABLE 3. Summary of data used to calculate the water exchange rate from z*Na content of the supernate

Core t

(days)

Tr-75 6.0 T3-75 7’0 T4-75 13’0 TI-76 6.2 Tz-76 8.1 T3-76 7’2

a’ lb (4 (4

12.1 17-o 6.4 18.0

8.4 14’0 19.5 10’5 12’1 17’4 11’0 18.5

( V,/ V,Y (da%

1‘02 0.08

0.51 0.04 0.86 0’11

2.65 0’02

0.99 0.06 0.85 0.09

Exchange rate (cm” cm --2 day -I)

1’0

0.3 0.9 0.4 0.7 1’0

’ Height of solution. * Height of sediment column. ’ Calculated assuming porosity=o.7o.

Parameters for calculating C,(t) as a function of t are given in Table 3, and plots of the natural log function versus time are shown in Figure 5. K is calculated from the straigh: lines drawn through the experimental results.

TIME , days TIME , days

0 0 2 2 4 4 6 6 8 EXCHANGE

RATE

cm’ cm-‘d-’

IO

Figure 5. Change in the term In C, [ (I+:) - Cs@) ($)I

as a function of time after 22Na addition to overlying water. Values of k and hio- pumping rates (in cm3 cm -* day -‘) are derived from the slope of the lines.

650 N. A. Luedtke & M. L. Bender

K is the fractional rate at which the supernate must be pumped across the sediment- water interface to account for the observed decrease of C, with time. The ‘ biopumping rate’ is then given by the product of K and the volume of supernate per square centimeter of sediment. Values of the biopumping rate calculated from this relation are included in Table 3 and Figure 5. From the average of the values for the six cores, we conclude that organisms pump 0.7 cm3 of water back and forth across each cm2 of sediment-water interface each day.

Utilizing the differential diffusion equation described by Crank (1975) :

where x=depth in the sediment column, C=concentration at depth x, t=time of mixing, D=diffusion coefficient [our mixing coefficient (II,,,)], and A=a constant, we have estimated mixing coefficients for the solid phase of the box cores. The model employed for these determinations assumes an instantaneous plane source diffusing in one dimension with a constant mixing coefficient (DJ. The solid radioactive sources of 141Cd and 59Fe arc also bounded on one side of the plane source by the water column, and the total amount of these sources has to remain constant over its area of distribution within the sediment. This total amount of source, M, mixing in the core must be given by:

M= m s

C dx II

and if the distribution follows equation (I) then

(2)

or upon integration :

M=A 2/nD,,

Substitution of (3) back into equation (I) gives:

(3)

M

‘= d\/RD,t e - XZ/4D,’

By selecting M=I.oo, taking the time of separation of the core as t, and selecting a D,, it is possible to calculate a theoretical concentration at each depth. Theoretical profiles are inte- grated over sampling depths and expressed as fractional parts of the integrated total. Com- parison of these fractions with the experimental data allow best-fit mixing coefficients to be selected.

TABLE 4. Comparison of our mixing coefficients with those of other investigators

Reference D,,, cd s -I Species

investigated Location

Rhoads (1963) I’S x10-7 Yolrlia limatula Long Island Sound 3’2 x 10-T

Gordon (I 966) 076 x IO -7 Pectinariu god&i Barnstable Harbor Aller & Cochran (1976) 1.2 x IO-~ 231Th Long Island Sound

2’5 x10-6 This work 2’3 x10-7 lilCe Narragansett Bay

2.5 x IO-'

4.0 x IO-' 5gFe

Sediment-water interactions 65’

Estimated mixing coefficients from this work are given in Table 4 along with those of other investigators. In evaluating the data we must first be aware of the animal population of the area McCaffrey et al. (1978) report that the majority of the benthic population including several bivalve species, a gastropod species and some smaller polychaetes, are found in the upper 4-6 cm. The larger polychaete population, namely Spiochaetopterus and Maldanopsis which mix sediment to greater depths, are less abundant. Due to the 14-17 cm length of our cores, we have probably cut polychaete burrows and greatly reduced their excavation activities by our procedures. Thus most of the sediment reworking we observed can be attributed to the animal community living in the upper 4-6 cm of our box cores. The mixing coefficients determined show close agreement with those determined by Rhoads (1963) for Yoldia limatula and Gordon (1966) for Pectinaria gouldia. Values determined by Aller & Cochran (1976) in Long Island Sound using natural 2 54Th are an order of magnitude higher. Our low values relative to Aller and Cochran may reflect the general disturbances inherent in any laboratory experiments.

Conclusions

We have used radioactive tracers to determine several meaningful factors concerning the area of study. 22Na remains completely in solution; its rate of removal from the supernate implies a ‘biological pumping’ rate of 0.7 cm3 cm -2 day - l. 14iCe and 6sFe are associated with the solid phase and have been used to estimate sediment mixing coefficients of 3% I x IO-~

cm2 s -l.

References

Aller, R. C. 1977 The influence of macrobenthos on chemical diagenesis of marine sediments, Ph.D. Thesis, Yale University, 600 pp.

Aller, R. C. & Cochran, J. K. 1976 2S4Th/e38U Disequilibrium in near-shore sediment: particle rework- ing and diagenetic time scales. Earth and Planetary Science Letters 29,37-50.

Crank, J. 1975 The Mathematics of Diffusion, second ed. Clarendon Press, Oxford. Elderfield, H., Luedtke, N. A., McCaffrey, R. J. & Bender, M. L. r979a Benthic flux studies in Nar-

ragansett Bay. Submitted to AmericanJournal of Science. Elderfield, H., McCaffrey, R. J., Luedtke, N. A., Bender, M. L. & Truesdale, V. W. r979b Chemical

diagenesis in Narragansett Bay sediments. Submitted to AmericanJournal of Science. Goldhaber, M. B., Aller, R. C., Cochran, J. K., Rosenfeld, J. K., Martens, C. S. & Berner, R. A. 1977

Sulfate reduction, diffusion, and bioturbation in Long Island Sound sediments: Report of the FOAM group. AmericanJournal of Science 277,193-237.

Gordon, D. C. 1966 The effects of the deposit feeding polychaete Pectinaria gouldii on the intertidal sediment of Barnstable Harbor. Limnology and Oceanography 11,32j'-332.

Hale, S. 1974 The role of benthic communities in the nutrient cycles of Narragansett Bay. M.S. Thesis, University of Rhode Island, 129 pp.

Korsec, M. A. & Hammond, D. E. 1977 Nutrient trace across the sediment water interface in San Francisco Bay. EOS, Trans. Am. Geophys. Union 58, (I z), I 165.

Li, Y. H. & Gregory, S. 1974 Diffusion of ions in sea water and in deep-sea sediments. Geochimica et Cosmochimica Acta 38,703-714.

McCaffrey, R. J., Myers, A. C., Davey, E., Morrison, G., Bender, M. L., Luedtke, N. A., Cullen, J. D,. Froelich, P. & Klinkhammer, G. 1978 Benthic fluxes of nutrients and manganese in Narragansett Bay, Rhode Island. Submitted to Limnology and Oceanography.

Nixon, S. W., Oviatt, C. A. & Hale, S. S. 1976 Nitrogen regeneration and the metabolism of coastal marine bottom communities, pp. 269-283 in The Role of Terrestrial and Aquatic Organisms in Decomposition Processes (Anderson, J. M. & MacFayden, A., eds). Blackwell Scientific Publications, Oxford.

Rhoads, D. C. 1963 Rates of sediment reworking by yoldia limatula in Buzzards Bay, Mass., and Long Island Sound. Journal of Sedimentary Petrology 33 (3), 723-727.

Robbins, J. A. & Edgington, D. H. I975 Determination of recent sedimentation rates in Lake Michigan using Pb-are and (h-137. Geochimica et Cosmochimica Acta 39,28j-304.