Heterogeneity of Depositional Environments

The short-distance-scale heterogeneity of net particle deposition rates in the Hudson has been recognized for decades. Examples based on the distribution of 137Cs activity with depth in sediment cores were reported by Olsen et al. (1978). Bopp and Simpson (1989) found 137Cs penetration depths ranging from 0 (no detectable 137Cs in the surficial sediment) to 200 cm in sixteen cores from a six-mile reach of the river near Kingston, New York (Fig. 26.1). A range of 0 to >64 cm of sediment with 137Cs activity had previously been found at three coring sites separated by a total distance of less than half a mile (Bopp, 1979). In the Arthur Kill (site 10; Fig. 26.1) cores Kill 20 and Kill 21, collected less than 20 meters apart, had detectable levels of 137Cs to depths of 4 cm and 160 cm, respectively. Analyses of samples from Kill 21 are being used to develop detailed contaminant chronologies for this area of the New York/New Jersey Harbor complex.

Spatial heterogeneity of depositional environments greatly complicates any attempt to interpret spatial distributions of particle-associated contaminants in the Hudson (Gibbs, 1994; Feng et al., 1998). The approach that we have employed consistently over the past twenty-five years is to focus our contaminant analyses on cores from specific sites. The ideal site is one of continuous, fairly rapid deposition of fine-grained sediments. Such sites are often found in coves, marshes, and other sheltered areas of the river and can be thought of as "sampling" the fine-grained suspended matter transported with the main flow. Our best cores are composed of homogeneous mud that is dominated by silt and clay-sized particles with little down-core variation in organic matter content. The 137Cs approach significantly constrains the estimate of timing of particle deposition over the past fifty years (Fig. 26.2). Sites like these are neither easy to locate nor so rare as to exclude detailed interpretation of spatial distributions of contaminant levels. Data from eighteen such sites in the Hudson River, its major tributaries, and the New York/New Jersey Harbor complex have been used to summarize the spatial and temporal distributions of PCBs, dioxins, and chlorinated hydrocarbon pesticides (Bopp et al., 1998). Similar data for several trace metals are presented below.

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