Environ. Sci. Technol. 2002, 36, 854-861
Records of Change in Salt Marshes: A Radiochronological Study of Three Westerschelde (SW Netherlands) Marshes F . M . D Y E R , † J . T H O M S O N , * ,† I. W. CROUDACE,† R. COX,‡ AND R. A. WADSWORTH‡ Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, U.K., and Centre for Ecology and Hydrology, Monks Wood, Abbots Ripton, Huntingdon PE28 2LS, U.K.
Three salt marshes on a 50-km transect along the north bank of the Westerschelde Estuary were investigated to determine whether salt marshes in the estuary had responded to shipping channel modifications in recent decades. Marsh accretion rates were estimated mainly from 137Cs profiles with further evidence from 241Am because changes in both rate of deposition and nature of the accreting material precluded use of standard 210Pbexcess dating models. The 137Cs profiles usually show peaks corresponding to atmospheric deposition from the 1963 fallout maximum and sometimes from the Chernobyl accident, although intervening enhanced 137Cs activities derived from the nuclear reprocessing marine discharges of Sellafield and La Hague are clearly discernible. In all three marshes (Ritthem at the mouth of the estuary and Zuidgors and Waarde at 20 and 45 km upstream), a marked, near-coincident change in the rate of accumulation and in the grain size of material deposited occurred around 1980. This may be related to a combination of channel deepening and straightening operations undertaken in the mid-1970s and/ or natural changes in winter wave climate.
Introduction The navigation channel of the Westerschelde Estuary (SW Netherlands; Figure 1) has undergone major deepening modifications in recent decades. One dredging operation in the mid-1970s reduced the overall channel length by 2 km and increased the channel depth from 10 to 13 m, while a second dredging operation in 1997-1999 increased it from 13 to 16 m. Modification of the cross-sectional aspect of an estuary in this manner reduces the friction experienced by incoming water on the flood tide, which allows more water to enter the estuary and causes an accelerated increase in the mean high tide level in the estuary. Salt marshes tend to accrete so that the high marsh remains close to the level of mean high water at spring tides (MHWS; 1). Because of regional factors such as eustacy, the general sea level trend around the North Sea, including the Westerschelde, is upward (2; Figure 2a). If the increase in height of MHWS is accelerated, * Corresponding author e-mail:
[email protected]; phone: +44 23 80596548; fax: +44 23 80596554. † Southampton Oceanography Centre. ‡ Centre for Ecology and Hydrology. 854
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the local salt marsh accretion rate should therefore also increase. Besides the changes in MHWS recorded at all Westerschelde tidal gauges between 1975 and 1980 (Figure 2a), the one at Bath has shown a sustained increase in average annual tidal range relative to the other Westerschelde tidal stations since that time (Figure 2b). As a result of the 19971999 dredging, mean high water levels were predicted by model to increase by ∼2 cm at the mouth of the estuary (Vlissingen) and by as much as 5-10 cm at Bath, 70 km upstream near the Dutch/Belgian border (Figure 1). The primary objective of the ISLED (Influence of Sea Level rise on Ecosystem Dynamics of salt marshes) project was to determine the effects the predicted rise in mean sea level from the 1997-1999 dredging would have on the salt marshes in the Westerschelde Estuary. ISLED thus aimed to utilize the dredging-induced sea level rise in the Westerschelde as an analogue for the effects of global sea level rise under climate change on coastal salt marshes. A radiochemical investigation was also carried out by ISLED to determine whether evidence of increased accretion rates from the 1970s dredging was recorded in the marsh sediments. This was undertaken by applying the 210Pbexcess and 137Cs methods that are often employed for marsh dating (3-6). 210Pb is a natural radionuclide in the 238U decay chain. Disequilibrium between 210Pb and its grandparent nuclide 226Ra arises through diffusion of the gaseous intermediate radionuclide 222Rn from the land into the atmosphere. When this 222Rn subsequently decays, the 210Pb atoms produced are removed from the atmosphere by wet and dry deposition processes and become incorporated into accreting sediments. Within the sediment column, this 210Pb in excess of radioactive secular equilibrium with 226Ra (210Pbexcess) decays with depth, achieving the equilibrium level with 226Ra after ∼5 half-lives (t1/2 210Pb ) 22 yr) or ∼100 yr. 210Pbexcess data are used to estimate rates of accumulation with assumptions of constant sediment accumulation rate or constant input flux of 210Pb excess (7). For reasons discussed below, neither of these methods is appropriate for the Westerschelde marshes, although it does appear that the constant flux/constant sedimentation model might be applied over parts of records. This model is
(210Pbexcess)z ) (210Pbexcess)0e-λt where (210Pbexcess)z and (210Pbexcess)0 are the specific activities at depth z and at the surface, λ is the disintegration constant of 210Pb, and s is the constant accumulation rate. Time t is evaluated as z/s from the slope of the exponential best fit of the data versus depth. The artificial fission radionuclide 137Cs (t1/2 ) 30 yr) was first released into the global atmosphere from the testing of high-yield nuclear weapons in the early 1950s, with significant levels of fallout occurring for the first time in 1954 (8). Maxima in global fallout occurred in 1958-1959 and particularly in 1963-1964, after which there was a discontinuation of atmospheric testing of large-yield devices resulting from the International Test Ban Treaty (9). In 1986, the Chernobyl reactor in the Ukraine failed, releasing a cloud of radioactivity that included 137Cs into the troposphere. Wet deposition of this 137Cs occurred patchily over Europe during the subsequent few days. Chernobyl fallout was identified unequivocally in Dutch marshes through the accompanying presence of the shorter-lived 134Cs (t1/2 ) 2.06 yr) although this has since decayed (6, 10, 11). In favorable cases, the timings of initiation and of maximum 137Cs deposition over time can 10.1021/es0110527 CCC: $22.00
2002 American Chemical Society Published on Web 01/26/2002
FIGURE 1. Westerschelde Estuary, SW Netherlands, showing the marshes under study (italics) and the locations of the tide gauge stations referred to in the text.
FIGURE 2. Average annual high tide and average yearly tidal range data for four tidal stations in the Westerschelde Estuary (from bottom to top in panels a and b: data from Vlissingen, Terneuzen, Hansweert, and Bath stations). Note that the tidal range underwent a large increase at Bath in the 1970s, after which the range in this part of the estuary remained elevated. be used to calculate accumulation rates as s ) z/t (where s ) accumulation rate, z ) depth, and t ) time), assuming that no post-depositional migration of the radionuclide has occurred. The reliability of 137Cs dating has been questioned due to demonstrated mobility of Cs (12, 13), especially in anoxic conditions where sorbed Cs+ may be replaced competitively by NH4+ (14). Fallout from nuclear weapons testing, however, contained a variety of Pu isotopes, including 241Pu (t 241Am (t 1/2 ) 14.4 yr) that decays by β emission to 1/2 ) 432 yr). Both Pu and Am are considerably less mobile than 137Cs, so that the daughter 241Am in-growth peak can be used to verify the position of the 1963 fallout maximum horizon (13). A significant proportion of Pu exists in seawater as soluble Pu (15); therefore, nuclear fuel reprocessing releases may affect the inventory of 241Am in marshes due to decay of marine-delivered 241Pu.
Study Sites and Methods The salt marshes at Ritthem, Zuidgors, and Waarde are all located on the northern shore of the Westerschelde Estuary, the mouth of which is usually referenced at Vlissingen (Figure 1). Ritthem is an embayment marsh close to the mouth, while Zuidgors and Waarde are estuarine fringing marshes 20 and 45 km upstream. An aerial photograph time series of all three marshes taken on nine occasions between 1944 and 1998 was assembled from various sources to gain an historical perspective. Comparison of the areas of the marshes when this series of photographs was rectified and georegistered to a common coordinate system revealed that the extent of the Zuidgors and Waarde marshes had fluctuated between 1944 and 1998 because of advances and retreats on their seaward faces (16). These advances and retreats appeared to be VOL. 36, NO. 5, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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approximately synchronized in time at these marshes, although the largest areal changes occurred at Zuidgors where the rate of retreat of the marsh front was up to 10 m per year. The period 1944-1980 was a phase of continuous advance of the Zuidgors and Waarde Marshes. After 1980, marsh retreat commenced, which was more rapid on the west than the east. In the field, this erosion is evidenced by the development of a cliff up to 1m in height on the western ends of both marshes. Cores were collected on relatively flat, undisturbed areas of marsh, well away from creek channels, and near the areas where the ecological and physiological ISLED field experiments were performed. Transects of three cores were taken from the marsh edge to ∼60 m landwards from the western end of Zuidgors Marsh (Z1, Z3, and Z4) and the eastern end of Waarde Marsh (W2, W3, and W5). Other cores taken were W4 (marsh front, replicating W2), Z2 (sand flat immediately in front of Z1), and ZE from the eastern end of Zuidgors and WW2 from the western end of Waarde. Two cores were taken from the smaller Ritthem Marsh, one in a low/middle marsh area (R2) and the other 30 m landwards in a high marsh area (R3). Sediment cores were taken using 1-m lengths of 10 cm i.d. polyacrylate tubing. The length of the core retrieved depended on the depth at which the core tubing struck sandy material, which effectively stalled penetration. Compression of the cores was minimal (∼3%). Following retrieval from the marsh, the cores were capped, sealed, and stored upright at 4 °C. Cores were sectioned at 1-cm intervals using a screw extrusion device. Samples were freeze-dried and then ground in a TEMA gyratory swing mill using agate grinding barrels. Major and trace element data were obtained on fusion beads and powder pellets using an automatic sequential wavelength-dispersive XRF instrument (Philips PW 1400). Calibrations were based on a range of international geochemical reference materials and depended on the use of R coefficients for major elements (beads; 17) and on the scattered radiation method for trace elements (pellets; 18). Data for 210Pbexcess, 137Cs, and 241Am were obtained from low background, high-resolution planar γ detectors (2000 mm2 × 20 mm) with relatively low counting efficiencies. Twenty-gram samples of ground sediment were compressed into 47 mm diameter pellets, sealed into 50 mm diameter polystyrene Petri dishes with epoxy resin, and stored for >3 weeks before counting to ensure that radioactive secular equilibrium developed between 226Ra and its shorter-lived daughters 222Rn, 214Pb, and 214Bi. Count times of 3 days per sample were used to collect γ spectra from 30 to 670 keV, which provided data for 226Ra (via 214Pb and 214 Bi photopeaks) and for 137Cs, 241Am, U (via 234Th), and 210Pbexcess (calculated as total 210Pb measured - 226Ra measured). Spectra were processed using the Fitzpeaks γ software package (J. F. Computing Services, Stanford in the Vale, Oxfordshire). Comparisons of the 241Am data with others collected by chemical separation and R spectrometry confirm that the γ counting data are accurate (19).
Results and Discussion Atmospheric deposition can usually be taken to have provided the major radionuclide input for steady-state dating methods using 210Pbexcess (3, 7, 20) including salt marsh applications (21) and for impulse signal methods based on 137Cs from nuclear device testing and the Chernobyl accident (4, 11, 13). The principles behind these methods are well-established, but none of the profiles measured in this work was amenable to straightforward interpretation. A mean atmospheric deposition of 210Pb of 73 Bq m-2 a-1 has been measured in The Netherlands (22), which corresponds to a steady-state inventory of 2300 Bq m-2. The 210Pb excess inventories measured in the cores from this study 856
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TABLE 1. 137Cs and Cores
210Pb
excess
Inventories in the Salt Marsh 210Pb
core
137Cs inventory (Bq m-2)
W2 W4 W5 WW2 ZE Z1 Z3 Z4 R2 R3 N. Hemisphere deposition U.K. soils
4620 3566 4388 3249 3805 4858 6048 5097 5252 4333 1275 (8) 2450 (23)
5187
0.89
4321 7971
1.02 0.4
13193 9135 7549 7199 5322 2300 (22) (2300)
0.37 0.66 0.67 0.73 0.81 0.55 1.06
excess
inventory (Bq m-2)
137Cs/ 210Pb
excess
range from 5200 to 13200 Bq m-2 (Table 1). The 137Cs inventories in all cores (3300-6000 Bq m-2; Table 1) also exceed both the mean Northern Hemisphere fallout of 1275 Bq m-2 in 1998 (from ref 8) or the higher value of 2450 Bq m-2 in 1998 determined at a similar latitude in U.K. soils (23). An additional source term besides atmospheric delivery is therefore necessary for delivery of both 210Pbexcess and 137Cs to the Westerschelde Marshes. Given that the Zuidgors and Waarde Marshes are retreating, the additional radionuclide inputs might be from recycling of eroded marsh back on to the residual marshes or from an enhanced input from the Westerschelde, possibly related to the occasional channel deepening or the regular maintenance dredging. It has been shown (24) that the sediments of the North Sea are now contaminated with radionuclides (including 137Cs) discharged from the BNFL Sellafield and COGEMA La Hague nuclear reprocessing plants. Radionuclides delivered by the river Scheldt were mainly deposited in the vicinity of Antwerp, so that the particulate material in the Westerschelde water column is contaminated with 137Cs and Pu introduced from the North Sea (25). It is therefore possible that the deposition of particulates from the North Sea may have contributed to the Westerschelde Marsh 137Cs and 241Am (via decay of 241Pu) inventories. A hydrodynamical model of the North Sea (26) reveals that sea-borne activities of 137Cs introduced by fallout and reprocessing sources were at maximum levels in the late 1970s and early 1980s in the southeast North Sea (Box 59 in Neilsen’s model, ref 26; Figure 3). Note that these high 137Cs activity levels in solution between 1975 and 1985 from reprocessing releases occur between the 1963 fallout maximum and 1986 Chernobyl impulse inputs. Several cores in this study have 137Cs profile shapes with elevated levels between two peaks; this profile shape is interpreted as a combination of atmospheric and marine 137Cs records of Figure 3. Plutonium isotope (238Pu/239,240Pu) activity ratios of ∼0.21 measured in a few selected samples from Zuidgors indicate that reprocessing radionuclides are present in the marshes, (fallout has a ratio of only 0.024; 27). This measured ratio is similar to values seen in the Westerschelde particulate material (25). The presence of these R-emitting Pu isotopes also indicates that a marine input of 241Pu must have occurred, as the 241Am activity in the marsh sediment decreases with distance up the estuary, with Waarde displaying lower activities than Ritthem and Zuidgors. In the case of 210Pb, there is an additional possibility of a contribution from phosphogypsum processing plants at Zandvliet near Antwerp and at Vlissingen, which between them discharge 800-1800 GBq a-1 210Pb to the Westerschelde (22). If discharged 210Pb were to become associated with fine sediments, then it might be deposited on the marshes, as
FIGURE 3. Data used in reconstructing salt marsh accretion rates: (a) 137Cs fallout in the Northern Hemisphere (39) and (b) 137Cs in the SE North Sea (Box 59 in ref 26), modeled using data from atmospheric fallout and Sellafield and La Hague known discharges. Data courtesy of S. P. Nielsen. they are the principal sediment sinks in the estuary (28). The finest ( 40 Bq kg-1) 226Ra content. The negative 210Pb excess-specific activities calculated in these cores were also coincident with the maximum 226Ra levels in each case. It is not clear whether some contribution from the phosphate processing works is present or whether some diagenetic rearrangement of 226Ra is occurring.
Historical Change in Sedimentation Rates All the radionuclide data gathered from these cores is complex, and caution is required in the interpretation of the data. The combination of the 137Cs and 210Pb dating techniques, however, has shown that there was a widespread change in salt marsh sedimentation in the Westerschelde around 1980. The compositional and radionuclide data demonstrate that an increase in accumulation rate was VOL. 36, NO. 5, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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simultaneous with a coarsening of the material deposited on the Zuidgors and Waarde Marshes, while the increased accumulation of finer sediment on Ritthem Marsh was preceded by an accumulation rate change when it was embayed. It is probable that the recent coarse sediment on the Zuidgors and Waarde Marshes is deposited as a result of storm events, as high wave energies are required to move sand-sized material (35, 36). A possible source of this coarse material is the eroding salt marsh itself, with the finer material winnowed away by water movement and the coarser material preferentially redeposited. The enhanced sandiness exhibited at the eastern end of Waarde Marsh relative to the western end is possibly from the adjacent dredge-spoil dump in the estuary. The high 210Pbexcess and 137Cs inventories for all cores also suggest that material may be being reworked, with eroded salt marsh sediment being placed back on the marsh, as observed on a Massachusetts (United States) marsh (37). A short-term increase in mean tidal range was observed in all Westerschelde tide gauge records from 1976 to 1980, which may be related to the mid-1970s phase of channel deepening and realignment. It is therefore possible that the channel deepening caused the change in sedimentation rate due to increased inundation periods and frequencies. Enhanced accretion rates were observed in the Oosterschelde following an increase in inundation frequency as a result of engineering works that enclosed part of the embayment (10). It was also found that a large diurnal tidal range contributes to high accretion rates. The retreat of the western ends of Zuidgors and Waarde Marshes that occurred some time between 1970 and 1980 has been ascribed alternatively to an increased frequency of WSW and W winds at force 8 and greater rather than to channel deepening (16). Due to the orientation of the Westerschelde, winds from these directions increase the fetch of waves in the estuary. Dredging may have augmented the impact of this change in wind direction by allowing high wave activities to penetrate further up the estuary in the newly straightened channel. The marsh could therefore respond to this increased storminess by retreating by cliff erosion, as seen elsewhere (38). The predominantly fine nature of the recent sediment accumulated at Ritthem is consistent with the location of this marsh in a harbor. Low wave energies will predominate in this sheltered environment, allowing more fine sediment to settle out than would be expected on an estuarine fringing marsh with its inherently higher energy environment. Increased inundation frequencies are therefore important in enhancing the accretion of embayment marshes, while estuarine fringing marshes will tend to accumulate relatively greater amounts of sediment during storm events. If the tidal range is increased following the 1990s dredging, therefore, it is possible that marsh accumulation rates will also increase. Because of the sediment-rich nature of the estuary, requiring regular dredging to maintain the shipping channel depth, it is likely that the sediment supply is great enough to support an increase in marsh accumulation rates. This is countered by the landwards retreat of the marshes that has occurred in the past two decades.
Acknowledgments This work was partly supported by ISLED (Influence of Sea Level rise on Ecosystem Dynamics of salt marshes), Project ENV4-CT97-0582 of the EU Environment and Climate programme. We thank Dr. Tom Cappenberg (Programme Coordinator, CEMO Yerseke), ISLED partners, and Dr. Dick de Jong (RIKZ, Middelburg) for numerous discussions; Dr. S. P. Nielsen (Risø National Laboratory, Denmark) for data from his 137Cs model; and the journal referees for constructive critiques of the earlier version. 860
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Literature Cited (1) Orson, R.; Panageotou, W.; Leatherman, S. P. J. Coastal Res. 1985, 1, 29-37. (2) Tooley, M. J. Recent Sea Level Changes. In Salt Marshes: Morphodynamics, Conservation and Engineering Significance; Allen, J. R. L., Pye K., Eds.; Cambridge University Press: Cambridge, 1992; pp 19-40. (3) Armentano, T. V.; Woodwell, G. M. Limnol. Oceanogr. 1975, 20, 452-456. (4) DeLaune, R. D.; Patrick, W. H.; Buresh, R. J. Nature 1978, 275, 532-533. (5) McCaffrey, R. J.; Thomson, J. Adv. Geophys. 1980, 22, 162-236. (6) Zwolsman, J. J. G.; Berger, G. W.; van Eck, G. T. M. Mar. Chem. 1993, 44, 73-94. (7) Appleby, P. G.; Oldfield, F. Application of 210Pb to sedimentation studies. In Uranium Series Disequilibrium: Applications to Earth, Marine and Environmental Sciences, 2nd ed.; Ivanovich, M., Harmon, R. S., Eds.; Oxford Science Publications: Oxford, 1992; pp 731-778. (8) Cambray, R. S.; Playford, K. Lewis, G. N. J.; Carpenter, R. C. Radioactive Fallout in Air and Rain: Results to the End of 1987; AERE-R 13226; HMSO: London, 1989. (9) Ritchie, J. C.; McHenry, J. R. J. Environ. Qual. 1990, 19, 215233. (10) Oenema, O.; DeLaune, R. D. Estuarine, Coastal Shelf Sci. 1988, 26, 379-394. (11) Callaway, J. C.; DeLaune, R. D.; Patrick, W. H. Limnol. Oceanogr. 1996, 41, 444-450. (12) Anderson, R. F.; Schiff, S. L.; Hesslein, R. H. Can. J. Fish. Aquat. Sci. 1987, 44, 231-250. (13) Appleby, P. G.; Richardson, N.; Nolan, P. J. Hydrobiologia 1991, 214, 35-42. (14) Comans, R. N. J.; Middelburg, J. J.; Zonderhuis, J.; Woittiez, J. R. W.; de Lange, G. J.; Das, H. A.; van der Weijden, C. H. Nature 1989, 339, 367-369. (15) Nelson, D. M.; Lovett, M. B. Nature 1978, 276, 599-601. (16) Cox, R.; Wadsworth, R. Manuscript in preparation. (17) Croudace, I. W.; Williams-Thorpe, O. Archaeometry 1988, 30, 227-236. (18) Croudace, I. W.; Gilligan, J. X-ray Spectrom. 1990, 19, 117-123. (19) Warwick, P. E.; Croudace, I. W.; Oh, J.-S. Anal. Chem. 2001, 73, 3410-3416. (20) Koide, M.; Soutar, A.; Goldberg, E. D. Earth Planet. Sci. Lett. 1972, 14, 442-446. (21) Cochran, J. K.; Hirschberg, J.; Wang, J.; Dere, C. Estuarine, Coastal Shelf Sci. 1998, 46, 503-522. (22) Beks, J. P. The 210Pb Budget of the North Sea: Atmosperic Input vs. sediment flux. In Radionuclides in the Oceans Part 1: Inventories, Behaviour and Processes; Germain, P., Guary, J. C., Guegeniat, P., Metevier, H., Eds.; Radioprotection-Colloques Vol. 32, C2; 1997; pp 219-224. (23) Cawse, P, A.; Horill, A. D. A Survey of 137Cs and Pu in British soils in 1977; AERE-R10155; Harwell, HMSO: London, 1986. (24) Beks, J. P. Cont. Shelf Res. 2000, 20, 1941-1964. (25) Martin, J. M.; Wollast, R.; Loijens, M.; Thomas, A.; Mouchel, J. M.; Nieuwenhuize, J. Mar. Chem. 1994, 46, 189-202. (26) Nielsen, S. P. J. Mar. Syst. 1995, 6, 545-560. (27) Harley, J. H. J. Radiol. Res. 1980, 21, 83-104. (28) Verlaan, P. A. J. Estuarine, Coastal Shelf Sci. 2000, 50, 627-638. (29) Rutherford, P. M.; Dudas, M. J.; Arocena, J. M. Sci. Total Environ. 1996, 180, 201-209. (30) van der Pluijm, A. M.; de Jong, D. J. Historich Overzicht Schorareaal in Zuid-West Nedeland: Oppervalkte Schorren in de Jaren 1856, 1910, 1938, 1960, 1978, 1988 en 1996. Werkdocument RIKZ/OS-98.860x (in Dutch). (31) Pethick, J. S. J. Sediment. Petrol. 1981, 51, 571-577. (32) Dijkema, K. S.; Bossinade, J. H.; Bouwsema, P.; DeGlopper, R. J. Salt Marshes in The Netherlands Wadden Sea: Rising High Tide Levels and Accretion Enhancement. In Expected Effects of Climatic Change on Marine Coastal Ecosystems; Beukema, J. J., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1990; pp 173-188. (33) Stapel, J.; de Jong, D. Sedimentatiemetingen op het Schor bij Waarde en het Verdronken Land van Saeftinge, Westerschelde (ZW Nederland). Rapport RIKZ-98.022 (in Dutch). (34) Baeyens, W.; Van Eck, G. T. M.; Lambert, C.; Wollast, R.; Goeyens, L. General Description of the Scheldt Estuary. In Trace Metals in the Westerschelde Estuary: A Case Study of a Polluted, Partially Anoxic Estuary; Baeyens, W., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1998; pp 1-14. (35) Ehlers, J.; Nagorny, K.; Schmidt, P.; Stieve, B.; Zietlow, K. J. Coastal Res. 1993, 9, 698-701.
(36) Chmura, G. L.; Kosters, E. C. Estuarine, Coastal Shelf Sci. 1994, 39, 33-44. (37) Roman, C. T.; Peck, J. A.; Allen, J. R.; King, J. W.; Appleby, P. G. Estuarine, Coastal Shelf Sci. 1997, 45, 717-727. (38) Pethick, J. S. Salt Marsh Geomorphology. In Salt Marshes: Morphodynamics, Conservation and Engineering Significance; Allen, J. R. L., Pye, K., Eds.; Cambridge University Press: Cambridge, 1992; pp 41-62.
(39) Playford, K.; Lewis, G. N. J.; Carpenter, R. C. Radioactive Fallout in Air and Rain: Results to the End of 1990; AEA-EE-0362; HMSO: London, 1992.
Received for review June 12, 2001. Revised manuscript received November 9, 2001. Accepted November 21, 2001. ES0110527
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