USGS Studies in Long Island Sound:
Geology, Contaminants, and Environmental Issues

Click here for a larger view.

Figure 2. The distribution of surficial sediments in Long Island Sound and adjacent waters. Triangular and block diagrams explain the map units.

Surficial Sediment Distribution

The regional surficial sediment distribution is extremely patchy, but exhibits textural trends which are related to sea-floor geology, bathymetry, and the effects of currents. In general, gravelly sediments are dominant in easternmost Long Island Sound, where tidal currents are strong, and in areas characterized by glacial tills. Sand occurs across the east-central Sound and along most of the nearshore margins. Silty sand and sand-silt-clay mark transitions within the Sound from higher to lower energy environments, such as on the flanks of bathymetric highs. Clayey silt and silty clay are predominant in low-energy environments, such as on the floors of the central and western basins.

Click here for a larger view.

Figure 5. Shows the regional distribution of the four categories of sea-floor sedimentary environments identified in the Long Island Sound study area.

Sedimentary Environments

The circulation in Long Island Sound, which is controlled by an east-to-west weakening of tidal-current speeds coupled with the westward-directed estuarine bottom drift, has produced a succession of sedimentary environments. The succession begins with erosion or nondeposition at the narrow eastern entrance to the Sound, changes to an extensive area of coarse-grained bedload transport in the east-central Sound, passes into a contiguous band of sediment sorting (where the estuary noticeably widens), and ends with broad areas of fine-grained deposition on the flat basin floor in the central and western Sound. The distribution of these environments provides insight into the long-term fate of contaminants, and is a guide to the future utilization of the sea floor. This information also helps to define the general locations and variability of benthic biological habitats, and gives a long-term perspective of sea-floor conditions in the Sound. Click here to view a USGS fact sheet on this topic.

Click here for a larger view.

Figure 3. Copper (Cu) concentrations in the surface sediments (0-2 cm) of Long Island Sound.

Contaminant Distribution and Accumulation in Sediments

Trace metal contamination of sediments from land-based activities is found throughout Long Island Sound and its watershed. In 1996, the USGS collected samples of surface sediments in the Sound to measure amounts and locations of metal contaminants and to establish a baseline for identifying changing conditions. Average concentrations of silver and copper in the Sound were 4-5 times greater than naturally-occurring background values. Zinc, lead, and manganese concentrations were enriched 1.5-2 times greater than natural background levels. The concentration distributions of these elements correlates with the sedimentary environment, the sediment texture, the organic carbon content, and the abundance of Clostridium perfringens, a bacterium used as a sewage tracer. Strong water currents remove fine-grained sediments, and the contaminants that are associated with them from sandy, reworked environments and concentrate them in the depositional basins. Thus, the greatest enrichment of metals is found in the depositional environments and muddy sediments of the central and western basins, due to both proximity to pollutant sources and the natural movement of sediments and contaminants within the Sound. Click here to view a USGS fact sheet on this topic.

Click here for a larger view.

Figure 4. Concentration profiles of Clostridium perfringens,
mercury, and zinc for a sediment core in western Long Island Sound shown with the regional population increase.

Historical Record of Pollution and Environmental Changes

Sediment cores provide a record of past conditions in Long Island Sound because recently deposited sediments at the top overlie older sediments below. Measurements of mercury and of Clostridium perfringens, a bacterial tracer of sewage, in cores show the onset of anthropogenic contamination two centuries ago and the effects of the increase in a regional human population since then. Concentrations of metal contaminants have decreased in recent decades, but Clostridium perfringens has not. Some areas of the Sound have much faster accumulation of sediment and contaminants than others, but regions of the Sound which now have high contaminant enrichments have also had them in the past. Sediments record changes in pollution and other environmental conditions which may be linked to fluctuations over the same time periods in the populations of small organisms (benthic foraminifera) that live on the sea floor.

Click here for a larger view.

Figure 6. A simulation of bottom bedload transport by currents. Arrows are not scaled; color legend shows magnitude of transport.

Sediment Transport

Three-dimensional, numerical modeling provides a general description of the regional distribution and causes of bottom currents in Long Island Sound in relation to sedimentary environments. Near-bottom current simulations show that the tidal currents, locally enhanced by estuarine circulation and wind events, control transport in the deeper parts of the Sound and promote net westward sediment transport. Our simulations also show that wind- and wave-driven bottom currents control transport along the shallow margins and cause the relatively coarse-grained sediments that occur in these regions, but that the effect of these currents drops off dramatically with increasing water depth.

FeedBack