Environ. Sci. Technol. 1997, 31, 1093-1101
Reliability of Salt Marshes as “Geochemical Recorders” of Pollution Input: A Case Study from Contrasting Estuaries in Southern England A N D R E W B . C U N D Y , * ,† IAN W. CROUDACE,‡ JOHN THOMSON,‡ AND JAMES T. LEWIS‡ Department of Geography and Earth Sciences, Brunel University, Borough Road, Isleworth, London TW7 5DU, U.K., and Southampton Oceanography Centre, European Way, Southampton, SO14 3ZH, U.K.
Temperate, mesotidal salt marshes are usually good “geochemical recorders” of pollutant input. Dated salt marsh cores from the Hamble, Itchen (Southampton Water), and Beaulieu estuaries (southern U.K.) are assessed. Sediments show clear labeling from effluents, which varies depending on their proximity to major urban or industrial areas. For elements where input is dominantly from a single source and periods of peak discharge are known (i.e., Cu, 137Cs, and 60Co), historical records of pollutant input can be reconstructed, provided redistribution through sediment mixing or early diagenetic processes is minimal. Where the pollutant has a range of sources (i.e., Pb) or where physical mixing in-estuary produces a time-integrated signal, it can prove extremely difficult to relate concentration depth profiles to discharge histories. Using concentration and stable Pb isotope data, the observed temporal input of Pb to these marshes is shown to reflect a complex, mixed marine/ atmospheric input from regional (automobile emission) and local (urban/industrial) sources. While general trends in pollutant loading may still be observed, it is extremely difficult to reconstruct accurately temporal trends in Pb input and sources of Pb in these estuaries using salt marsh records due to the importance of local, poorly-defined Pb sources and in-estuary mixing processes.
Introduction Following the development of radiometric dating using isotopes with half-lives suitable for the study of recent anthropogenic processes (210Pb, 137Cs, 241Am), vertical cores of estuarine and coastal sediments have been increasingly used to reconstruct the history of coastal pollution (e.g.. 1-7). Salt marsh sediments in particular have been used in numerous studies as due to their clay-rich nature and consequently large adsorption capacity they represent a major repository for contaminants (8). Their stabilized, vegetated nature and dense root system also make them less susceptible to post-depositional disturbance than adjacent mudflat areas. For sediments to provide a useful history of anthropogenic input to the coastal environment, cored material must be * Corresponding author telephone: +44 181 891 8235: fax: +44 181 891 8237; e-mail:
[email protected]. † Brunel University. ‡ Southampton Oceanography Centre.
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1997 American Chemical Society
dominantly fine-grained and clay-rich (adsorptive), dateable, have reasonably high rates of sediment accumulation (>0.3 cm yr-1), be relatively undisturbed, and there should not have been significant redistribution of elements through mixing/ early diagenetic processes. Where these conditions have been met, salt marsh sediments have been successfully used to reconstruct contaminant chronologies at various coastal sites (see review by Valette-Silver; 8). In the U.K., contaminant chronologies have been derived from salt marsh deposits in a number of areas, e.g., the Severn estuary (e.g., refs 6 and 9), Cornwall (10), and Cumbria (11). In many cases, pollutant profiles retained by sediments have shown good correlation with local industrial activity/urbanisation, allowing their use as a dating tool (e.g., refs 9 and 11-13). Despite the success of these studies, in the dynamic estuarine environment, sediment mixing and redistribution prior to deposition presents a major problem to sediment-based historical reconstructions, where varying time lags between input of the pollutant and its deposition on the marsh surface may produce a time-integrated discharge record. MacKenzie et al. (14) examined the vertical distribution of discharge products from the Sellafield nuclear reprocessing plant in salt marsh sediments from SW Scotland and found that the dominant transport mechanism of Sellafield waste to the marsh sites involved redistribution of contaminated silt. The marsh sediments thus retained a record of the time-integrated Sellafield discharges rather than of temporal variations in individual releases. Early diagenetic remobilization of radionuclides and heavy metals may also perturb contaminant chronologies, with a number of studies finding evidence of metal mobility following deposition (see review by Farmer; 15). The present study examines the suitability of salt marsh sediments from the Solent estuarine system, southern U.K. as historical recorders of anthropogenic input. Sites have been chosen to represent contrasting estuaries, a comparatively undeveloped estuary (Beaulieu) and a heavily industrialized and urbanized estuary (Hythe and Hamble, Southampton Water). Elemental analysis focuses on Cu and Pb due to the widespread release of both elements into the local marine environment. Cu has a relatively well-defined historical input from industrial processes on the west side of Southampton Water, and although sources of Pb are less welldefined, source discrimination is often possible through measurement of its stable isotopic composition (discussed later). Study Area. The sites studied are located in the Solent region of southern England (Figure 1). The Beaulieu estuary is situated within the New Forest, a tourist and conservation area. Large sections of the land surrounding the estuary are privately owned, and the area has comparatively little industrial/residential development. In contrast, the Southampton Water area (Itchen estuary) has experienced significant industrial, residential, and leisure development during this century. The Hamble Estuary (situated on the eastern side of Southampton Water) has undergone significant development for recreational purposes (construction of four marinas), while the Hythe area is intensively developed, with the large Esso oil refinery at Fawley and related industries that use the feedstock from the refinery (Figure 1).
Methods Cores (0.4 m × 10 cm diameter) were taken using a handdriven PVC tube from salt marsh sites at Beaulieu, Hythe, and Hamble in spring 1992. The marshes in this area have been shown to exhibit slight variation in geochemical conditions, pollutant fluxes, and sediment accumulation rate
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TABLE 1. Net Sediment Accumulation Rates at Each Site, Derived Using 137Cs, 60Co, and 210Pb Data sediment accumulation rate (mm y-1) derived from site Beaulieu Hythe Hamble
210Pb
dating 3 5 5
137Cs
dating
2-5 6 2-3
60Co
dating
