EXAMPLE: The Distribution of Lead in Gulf of Maine Sediments- Query results from the Contaminated-Sediment Database

RESULTS

A series of questions are presented with plots and maps that are created from data contained in the CONTAMINATED SEDIMENTS DATABASE FOR THE GULF OF MAINE.

Where are the highest concentrations of metal contamination in Gulf of Maine sediments?
Both Lead (Fig 6) and Copper (Fig 7) in modern sediments have highest concentrations near urban centers. For most common industrial contaminants in New England, the magnitude of pollution in marine sediments is closely related to the local population density (Fig 8). This occurs because most industrial activities are located near cities, sewage disposal requirements are a direct function of population, and transportation-related pollution is greatest where there are a lot of people. This differs from some situations in the western parts of the U.S., where the highest metal concentrations are associated with sites of historical mining and smelting activities.

What concentrations of lead occur naturally in sediments and how can one tell if sediments are contaminated with lead?
Most coastal surface sediments in the Gulf have lead concentrations no greater than 10 times that of background (Fig 9). Naturally occurring, i.e. "background", values of an element differ for differing types of minerals and sediment types. Samples that are very far from contaminant sources or samples that were deposited prior to the Industrial Revolution (those from deep in cores, Fig 10) are used to determine regional background values. Concentrations that are greater than background are "enriched" and, if enrichment is sufficiently great, may be considered contaminated or polluted. In muddy sediment, background values are around 18 µg/g for Pb and 15 µg/g for Cu. Backgrounds are lower in sands.

Can the concentrations of lead found in these sediments cause toxic effects?
One way of estimating the potential for biota to experience detrimental effects when exposed to particular sediments is to compare concentrations of individual contaminants with guidelines for sediment quality criteria that are determined in laboratory tests. The color coding of Pb (Fig 6, Fig. 11 for area insets) and Cu (Fig 7) concentrations show locations of sediment that have: little or no enrichment in Pb or Cu (green = background); those which are contaminated but unlikely to cause any toxic effects (blue < ER-L); those where some detrimental effects may occur (orange between ER-L and ER-M); and those with contaminant concentrations so enriched that toxic effects are likely to occur (red between ER-L and ER-M). The greatest enrichment observed for Pb and Cu is 400 times the naturally occurring background values. All urban areas have some samples with sediment concentrations that may cause some toxic effects in the ecosystem.

Can the patterns of lead enrichment be used to approximate the distribution of other contaminants?
Many of contaminants are often introduced to the environment from common sources. Certain classes of contaminants also have similar chemical behavior in the ecosystem and will have a common mobility and fate. When contaminants are shown to correlate (Fig 12), one contaminant can indeed be used as a proxy for another if no other data are available.

What do differences between patterns for various elements indicate?
Differing patterns indicate a variation in sources or partitioning during contaminant transport. Changing elemental ratios, e.g. Pb/Cu, in samples from different times or places can be used to identify processes. In Fig 13, the greater input of atmospheric sources for Pb than for Cu is reflected in a weak trend of increasing Pb/Cu ratio with distance offshore from population centers. The use of histograms (Fig 14) allows statistical comparison of patterns for differing areas, which might have very different sample coverage.

What are the dominant factors controlling the distribution of lead in Gulf sediments?
The present concentration results from a complex interaction of the input type and location, the variability and sorption capacity of the sediment substrate, and the mobility of deposited Pb in situ. The presence of fine-grained sediment is indicative of areas of low energy (hence net deposition) and high particle surface area (hence large sorption capacity). Sediment texture (Fig 15) is consequently often used to estimate regions of contaminant accumulation and for normalization to remove effects of natural variability in the sediments. Normalization of the Pb concentration to sediment grain size (Fig 16) does not markedly reduce the spatial heterogeneity of the data in the Boston Harbor sediments, suggesting that source proximity and other factors are more important than texture for controlling or predicting Pb concentrations in this region near shore. The effects of substrate, source, and mobility cannot be separated in the offshore Gulf of Maine because there are not enough samples that have both Pb and texture data.

Have lead concentrations in sediments of the Gulf of Maine decreased with time? The range in lead concentrations within the Gulf is so large, ranging from background to highly polluted, that temporal changes are masked by spatial variability unless the query is carefully restricted to avoid or normalize for variations in substrate composition. Inspection of either historical data from a depositional region (Fig 17), or profile data from cores in the database (Fig. 18, Bothner, et al, 1998) show that concentrations in muddy, surface sediments near urban centers have decreased slightly over the last 20 years. The decrease in surface sediment Pb concentration is less than reductions in pollutant sources might suggest because sediments deposited in the past can be moved up to the sediment surface by biological mixing and sediment transport processes. There is insufficient data to say what the situation is in the deeper basins of the Gulf where the primary source of lead contamination is atmospheric deposition.