Marine Chemistry in the Coastal Environment

7 Methane and Radon-222 as Tracers for Mechanisms of Exchange Across the Sediment-Water Interface in the Hudson River Estuary D. E. HAMMOND, H. J. SIMPSON, and G. MATHIEU

Downloaded by AUBURN UNIV on April 19, 2017 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0018.ch007

Lamont-Doherty Geological Observatory of Columbia University, Palisades, N.Y. 10964

One problem in formulating material balances for aquatic systems is predicting the rates of mass transport across the. sediment-water and water-atmosphere interfaces. An empirical relationship based on wind speed (1) has been developed to predict the rate of exchange across the upper interface, but exchange across the lower interface is usually calculated by use of reaction-molecular diffusion models. Almost no information exists regarding the accuracy of these models in calculating fluxes. This paper seeks to examine such processes in the Hudson Estuary by examining the distribution of CU4 and Rn222 , two naturally-occurring tracers which are generated primarily in sediments and migrate into the overlying water column. Due to limitations of space, this paper is a brief preliminary summary of research reported by Hammond (2). Future papers will present a more detailed discussion of the results summarized here. Description of the Hudson Estuary The Hudson Estuary is shown in Figure 1 with several regions of interest labelled. The standard mile point (mp) reference system used in the text is indicated. This index represents statute miles north of the southern tip of Manhattan along the river axis. A distinct difference in bedrock geology has resulted in the development of a broad, shallow channel profile through the quaternary deposits of the Tappan Zee region (mean depth « 5.3m) and a narrow, deep profile through the crystalline rocks of the Hudson Highlands (mean depth 12.8m). A summary of the regional geology can be found in Sanders (3) and references he cites. This distinct morphology change is important to the discussion of methane distribution. The Hudson has been classified as a partially-mixed estuary (4). Bottom salinities are typically 20-40% greater than surface salinities. The estuary is tidal from the Narrows (mr> -8) to the Green Island Dam (mp +154), with an amplitude of about one meter throughout. The 0.1°/ o o isohaline penetrates to about mp 25 119 Church; Marine Chemistry in the Coastal Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1975.


MARINE CHEMISTRY

I 74030*

U 74°00*

i

1 73°30'

Figure 1. Map of lower Hudson Estuary. The mile point reference index is abbreviated as MP. The Tappan Zee region has a broad, shallow channel, and the Hudson Highlands has a narrow, deep channel. Large amounts of sewage from New York and New Jersey are discharged to the Upper Bay.

Church; Marine Chemistry in the Coastal Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

HAMMOND

7.

Methane and Radon-222 as Tracers

ET AL.

121

during periods of high fresh water discharge (spring) and to about mp 60 during periods of low discharge (late summer). Circulation within the estuary has been discussed in terms of a one-layer advection-diffusion model by a number of authors (see _5 and references cited there) and a two-layer advective model (6). On the basis of s a l i n i t y distribution, the horizontal eddy d i f f u s i v i t y calculated from the f i r s t type of model i s 3-6 x 10 cm /sec (25-50 kra2/day). An estimation of the rate of exchange of dissolved gases across the atmosphere-water interface can be made on the basis of the Lewis and Whitman stagnant film model (7) recently reviewed by Broecker and Peng (8). This model envisions gas flux to be limited by molecular diffusion through a stagnant film of water at the air-water interface. The surface of this film i s in equilibrium with the atmosphere (C^) and the base of the film has the same composition as the bulk solution (Cft). Film thickness (Z) is a function of wind speed, and the evasive flux of gas C per unit area i s then: 6


2

*c - |

+

X

(

°s

q

-

[2]

where X i s the decay constant of radon. The solution to [2] i s : q

r

(C| - C ) = Me"*!* + ?!e l

x

[3]

s

where r i «y /^s* A

C

s

C

q

C

" s =

s

88

0

^PPly^S

t n e

boundary conditions

at x » «> at x - 0 where C = overlying x*ater concentration w

and differentiating, the flux of radon to the overlying water is: Jsed

(Cf/l - C^)

Measurements of Cg^ were made by making a slurry from a known volume of wet sediment and d i s t i l l e d water. This slurry was sealed in a glass kettle, purged of radon, stored, and the re-growth of radon was measured. For Tappan Zee sediments C§
X

2

^X9 /D . * Applying boundary conditions: in

at x • 0

w

+ ?T

e

83 2

r i x e

r

r

= P ~ 2y + Q 2* e

r

r

at x « d

e

x

r

y

r i ( N l * - Me" l ) - r (O r2y - P "" 2 ) e

2

CgT

" )

=

2 0 0

The flux from

2

atoms/M sec

The model calculated d i f f u s i v i t y w i l l depend on the value of d chosen for the thickness of the mixed zone, but d = 2 cm i s reasonable on the basis of the soupiness c r i t e r i a and indicates D = 7.2 x 10""^ cm /sec. This i s about seven times the molecular d i f f u s i v i t y in the layer below and indicates that substances dissolved in i n t e r s t i t i a l waters may migrate through the upper few centimeters of sediment at rates above their molecular 2

g


MARINE CHEMISTRY

130

38

d i f f u s i v i t i e s . Selection of d ° 5 cm would Indicate Dg 3*0 x 10~5 cm /sec. The source of turbulence for this s t i r r i n g could be small polycheate worms which populate these sediments or i t could be s t i r r i n g by t i d a l currents. Data collected i n winter months, when bioturbation i s minimal, may provide an answer to this question. It should be pointed out that alternative models could explain the "enhanced" flux of radon. One p o s s i b i l i t y i s a sediment-reworking model i n which localized erosion to several centimeters occurs periodically at random locations. The distribution of shortlived radionuclides i n the sediment may be useful i n distinguishing between among possible alternatives. Downloaded by AUBURN UNIV on April 19, 2017 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0018.ch007

2

Conclusions 1. Bubbles of methane which escape from sediments and p a r t i a l l y dissolve as they r i s e through the overlying water column appear to dominate the transport of methane across the sedimentwater interface i n the Hudson Istuary. Thus, methane i s apparently not useful for tracing the process of diffusion through surface sediments i n estuaries. 2. A radon excess of 1.33 + 0.28 dpm/fc exists during summer months i n the waters of the Hudson Estuary between mp 16 and mp 60. Most of this excess i s supported by input from the sediments at a rate which exceeds evasion to the atmosphere. Radioactive decay of radon i n the water column accounts for over half the loss of this sediment input. 3. Summertime radon flux from the sediments i s about twice as great as a molecular diffusion model would predict. The mechanisms controlling the flux cannot be uniquely constrained. A model which can account for the flux envisions the sediments to be composed of two layers. In the upper layer, uniform s t i r r i n g by currents and organisms creates an eddy d i f f u s i v i t y . In the lower layer, molecular diffusion controls transport. Choosing the thickness of the upper layer as 2 cm indicates an eddy d i f f u s i v i t y during summer months approximately seven times greater than molecular d i f f u s i v i t y . Acknowledgements The authors thank J. Goddard, J . Rouen and T. Torgersen for assistance i n sample collection and analysis, P. Biscaye, J. Sarmiento, T. Torgersen and S. Williams for reviewing the manuscript, K. Antlitz for typing i t , and D. Warner for drafting the figures. The research reported here was supported In part by the Environmental Protection Agency, contract No. R803113-01. t



7. HAMMOND ET AL.

Methane and Radon-222 as Tracers

131

Abstract Methane concentrations in Hudson Estuary waters range from ~0.2 μmol/l to ~1 μmol/l. North of mp 20, concentrations are dominated by the balance between input from sediments and evasion to the atmosphere. Seasonal and regional distribution characteristics indicate that transport across the sediment-water interface is dominated by bubbles of methane which are produced in sediment and partially dissolve as they escape and rise through the water column. Radon-222 is a noble gas with a four day half-life. It is produced primarily in sediments and may migrate into overlying waters where it decays or escapes to the atmosphere. Estimates of the rate of the latter process, combined with measurement of the former, indicates that in the Hudson Estuary migration across the sediment-water interface occurs at twice the rate predicted by a molecular diffusion model developed by Broecker (20). To account for the observed radon flux, a two-layer model developed by Peng et al. (22) can be applied to sediments. The model employs an upper layer in which uniform stirring by currents and organisms creates an eddy diffusivity about seven times the rate, of molecular diffusivity in the layer below. Literature Cited 1. Emerson, S.R., The Gas Exchange Rate in Small Canadian Shield Lakes, Limnology and Oceanography, (in press), 1975. 2. Hammond, D.E., Dissolved Gases and Kinetic Processes in the Hudson River Estuary, Ph.D. Thesis, Columbia University, 161 pp., 1975. 3. Sanders, J.E., Geomorphology of the Hudson Estuary, in Roels, O.A., cd., Hudson River Colloquium, Annals of theNewYork Academy of Sciences, vol. 250, pp. 5-38, 1974. 4. Pritchard, D.W., Estuarine Circulation Patterns, Proc. Am. Soc. Civil Engrs., 81, pp. 717/1-717/11, 1955. 5. Simpson, H.J., R. Bopp and D. Thurber, Salt Movement Patterns in the Lower Hudson, Third Symposium on HudsonRiverEcology, 1973, Hudson River Environmental Society, New York, 1974. 6. Abood, K.A., Circulation intheHudson Estuary, in Roels, O.A., ed., Hudson River Colloquium, Annals of the New York Academy of Sciences, vol. 250, pp. 39-111, 1974. 7. Lewis, W.K. and W.G. Whitman, Principles of Gas Absorption, Industrial and Engineering Chemistry, 16, pp. 1215-1220, 1924. 8. Broecker, W.S. and T.H. Peng, Gas Exchange Rates Between Air and Sea, Tellus, 26, pp. 21-35, 1974. 9. McCrone, A.W., The Hudson River Estuary: Sedimentary and Geochemical Properties Between Kingston and Haverstraw, New York, J. Sed. Petrol., 37, pp. 475-436, 1967.



132

MARINE CHEMISTRY

10. Claypool, C.E. and I.R. Kaplan, The Origin and Distribution of Methane in Marine Sediments, in Kaplan, I.R., ed., Natural Gases in Marine Sediments, Plenum Press,NewYork, pp. 99-139, 1974. 11. Schubel, J.R., Gas Bubbles and the Acoustically Impenetrable, or Turbid, Character of Some Estuarine Sediments, in, Kaplan, I.R., ed., Natural Gases in Marine Sediments, Plenum Press, New York, pp. 275-298, 1974. 12. Worzel, J.L. and C.L. Drake, Structure Section Across the Hudson River at Nyack, New York, from Seismic Observations, Annals of the New York Academy of Sciences, 80, pp. 1092-1105, 1959. 13. Reeburgh, W.S., Observations of Gases in Chesapeake Bay Sediments, Limnology and Oceanography, 14, pp. 363-375, 1969. 14. Martens, C.S. and R.A. Berner, Methane Production In the Interstitial Waters of Sulfate-depleted Marine Sediments, Science, 185, pp. 1167-1169, 1974. 15. Sackett, W.M. and J.M. Brooks, Origin and Distribution of Low-molecularWeight Hydrocarbons in Gulf of Mexico Coastal Waters, (this volume). 16. Nissenbaum, A., B.J. Presley and I.R. Kaplar, Early diagenesis in a Reducing Fjiord, Saanich Inlet, British Columbia. I. Chemical and Isotopic Changes in Major Components of Interstitial Water, Geochimica and Cosmochimica Acta, 36, pp. 10071027, 1972. 17. Cappenberg, T.E., Ecological Observations on Heterotrophic, Methane Oxidizing and Sulfate Reducing Bacteria in a Pond, Hydrobiologia, 40, pp. 471-435, 1972. 18. Rudd, J.W.M., R.D. Hamilton and N.E.R. Campbell, Measurement of Microbial Oxidation of Methane in Lake Water, Limnology and Oceanography, 19, pp. 519-524, 1974. 19. Swinnerton, J.W. and V.J. Linnenbom, Gaseous Hydrocarbons in Sea Water: Determination, Science, 156, pp. 1119-1120, 1967. 20. Broecker, W.S., The Application of Natural Radon to Problems in Ocean Circulation, in Ichiye, T., ed., Symposium on Diffusion in Oceans and Fresh Waters, Lamont -Doherty Geological Observatory, Palisades, N.Y., pp. 116-145, 1965. 21. Rona, E., Diffusionsgrösse und Atomdurch-messer der Radiumemanation, Z. Physik Chemie, 92, pp. 213-218, 1917. 22. Peng, T.H., T. Takahashi and W.S. Broecker, Surface radon measurements in the North Pacific Station Papa, Journal of Geophysical Research, 79, pp. 1772-1730, 1974. 23. Li, Y.H. and S. Gregory, Diffusion of ions in sea water and deep-sea sediments, Geochimica and Cosmochimica Acta, 38, pp. 703-714, 1974.


Marine Chemistry in the Coastal Environment

Recommend Documents