An important conceptual advance in our understanding of the controls on sediment and contaminant deposition in estuaries arose with the articulation of the 'equilibrium surface' concept by Curtis Olsen and his colleagues (Olsen et al., 1993). Based on a large amount of data from the Hudson River estuary and elsewhere, they argued that sediment accumulation in estuaries is governed by a dynamic equilibrium among the processes responsible for transporting sediment, including river discharge, tidal mixing, and waves. Sediments accumulate until a balance among the various physical and biological transport processes is attained, after which little net accumulation takes place. Thus estuaries may have areas that are naturally in or out of equilibrium as a consequence of morphologic changes or seasonal variations in physical processes (tides, storms, river flow) that are unique to each estuary. Areas that are out of equilibrium can be characterized by rapid accumulation of fine-grained sediments and associated contaminants.
The Hudson River estuary presents a classic example of the application of the dynamic equilibrium concept of sediment and contaminant accumulation. The flow of the Hudson River changes dramatically over the course of the year, with strong flows occurring during the spring freshet. Variations in tidal currents as well as the occurrence of storms also can change the equilibrium of the system. Dredging is also a major factor affecting the equilibrium of the Hudson system with respect to sediment accumulation. Areas that have been dredged show large net rates of accumulation until a new equilibrium is established. Piers and other marginal structures have a similar perturbing effect on the dynamic equilibrium existing among the processes that transport sediment, and high rates of sediment accumulation are often found in such areas.
The large numbers of 137Cs measurements made in Hudson River sediment cores since 1975 by C. Olsen, J. Simpson, R. Bopp, S. Chillrud and colleagues at the Lamont-Doherty Earth Observatory permit broad patterns of sediment accumulation to be ascertained. In general, the 137Cs depth distributions and inventories (total amount of 137Cs in the core) show that there is little accumulation in the main navigation channel and in areas such as Haverstraw Bay (Olsen et al., 1984). In contrast, broad shallow areas in coves and other marginal areas show distributions of137 Cs to greater depths, suggesting enhanced sediment accumulation (Fig. 6.2). The lower estuary, including the western margin off Manhattan and especially the inner harbor, shows 137Cs distributed to depths of 1-2 m in the sediment, indicating rapid rates of sediment accumulation. However, localized rates may be quite variable in such areas, ranging from 1 to 5 cm y-1 (with an average of about 3 cm y-1) in non-dredged areas and 4 to 70 cm y-1 (with an average of about 9 cm y-1) in dredged areas (Fig. 6.3; Olsen etal., 1981b, 1984).
Our research characterizing distributions of the natural radionuclides 234Th, 7Be and 210Pb supports the high degree of spatial as well as temporal variability in the dynamic equilibrium governing sediment accumulation in the Hudson system (Hirschberg et al., 1996; Feng, et al., 1998). For example, collection of a localized set of cores in a non-dredged portion of the estuary off Manhattan (km point ~10) shows low activities of all three radionuclides in surface sediments in a transition area between the western margin of the estuary and the main channel (Fig. 6.4). The absence of the short-lived radionuclides 234Th and 7Be is particularly striking in that these radionuclides tag sediment that has recently been in contact with the overlying water column. In contrast activities of all three radionuclides are greater in cores taken in the western margin area, although there is significant spatial variability even in the relatively small area sampled. Sediment accumulation rates calculated from the 7Be profiles range from ~6 to 26 cm y-1 (Fig. 6.5). Longer term patterns of accumulation are indicated by excess 210 Pb profiles to depths of ~50 cm and show that the rapid rates of accumula-tionindicatedby7Be are not sustained over time intervals much greater than a year. Indeed, the equilibrium governing sediment accumulation in this area likely changes seasonally in response to river flow. The collected cores were taken severalmonths after the spring freshet, which typically takes place in April/May of each year. Large variations in river flow and suspended sediment transport occur with these events, perturbing the equilibrium governing sediment accumulation. Thus, in this portion of the estuary, short-term, rapid sediment accumulation occurs along the western margin but longer term accumulation is prevented by annual changes in river flow as well as seasonal changes in tidal flow.
These results are reinforced by the recent work of Rocky Geyer, Jonathan Woodruff and colleagues from the Woods Hole Oceanographic Institution. Woodruff et al. (2001) used changes in sediment properties, X-radiography and 7Be distributions to show that sediment was deposited in the lower estuary during the spring freshet, butwas transported and redeposited upriver, in the vicinity of the es-tuarine turbidity maximum, several months later. Woodruff etal. (2001) also argued that the high rates of sediment accumulation in this area were not sustained on the long term (decadally).
Sediment can provide information about accumulation history through the use of other techniques in conjunction with radioactive dating. Cores of fine-grained sediment, composed of especially small grains of clay and silt, often contain subtle variations in the arrangement of the grains that can be interpreted by geologists to reveal the conditions under which the particles settled from the overlying river water. This is often referred to as the fabric of the sediment: small structures, often
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