Document Type
Report
Date of Original Version
11-1992
Abstract
As part of a multidisciplinary investigation on the sediments of Narragansett Bay, sediment trap materials, surface sediments, hard shell clams (Mercenaria mercenaria) and sediment cores from throughout the bay were analyzed for organic contaminants including: polychlorinated biphenyls (PCBs), petroleum hydrocarbons (PHCs). polycyclic aromatic hydrocarbons (PAHs), substituted benzotriazoles (BZTs), bis (2-ethylhexyl) phthalate (DEHP) and coprostanol (COP). The objectives of this particular study were to provide information on the source, transport and biogeochemical fate of organic contaminants in the bay: while the overall goals of the investigation were to characterize the sediments and sedimentary processes of the Narragansett Bay system based on the results of integrated biological, chemical and geological studies.
Sediment Trap Materials
The sediment trap samples showed a trend of decreasing concentration for most components from Fox Point to Ohio Ledge, suggesting that the source(s) of these contaminants were pollution discharges (e.g., industrial and municipal effluents, CSOs. spills and runoff,) to the upper Providence River and its tributaries. In general, the agreement between the results from the present sediment trap study and older suspended solids data (1985-86) was quite good. However, the PHC concentrations reponed in 1977 were considerably higher than the more recent values. But it is difficult, if not impossible, to determine if changes have really occurred based on the limited number of samples analyzed and procedural variations in the different studies.
In the case of BZT concentrations and their ratios (C10-BZT/Cl-BZT), there was a maximum amount of both BZTs and a minimum ratio in trap material at a Providence River station about 7 km south of Fox Point (i.e., adjacent to the Pawtuxet Cove entrance). This trend with distance is probably due to the source of the BZTs to the Providence River which is the Pawtuxet River via Pawtuxet Cove. The BZT ratio indicates a change in the relative amounts of the two compounds over the distance from the point of entry, i.e., there is a trend of increasing values with distance from Pawtuxet Cove (ratio = 1.5) to Ohio Ledge (ratio = 3.2). The ratio for sediment trap material from the Pawtuxet River was 1.0 - 2.0; since these values are generally lower than any others reported, it suggests that the river is the major source of the material in the Pawtuxet Cove traps.
The relationships between contaminant concentrations in sediment trap material and surface sediments were quite good. For Ar 1254, the sediment levels were about twice the trap values. Conversely, the hydrocarbon concentrations in trap material were about 1.5 times as high for PHCs and about equal in pyrene, relative to the surface sediments. (In all cases, the correlations were significant at the 95% confidence level). In general, these data support the concept that the chemical composition of particulate material and surf ace sediments are closely related, and that the former could be a major source of contaminants to the underlying surface sediments. It is also possible that resuspended surface sediments could contribute to the particulate material in the water column depending on a number of factors such as bottom currents and extent of bioturbation.
There was a significant relationship between the concentration of Cl-BZT in trap material and surface sediments; however, the relationship for the C10-BZT was not as good. and there is presently no explanation for these differences other than that the production of C-BZT started in 1963 and ceased in 1972, while that of the C10-BZT started in 1970 and continued to 1985. (The log Kow for both BZTs are essentially the same, Pruell & Quinn, 1985.) Perhaps, the former BZTs more uniformally distributed in particulate material and sediments than the more recently discharged C10-BZT. Changes in the BZT ratio (C10/Cl) of trap material may be due to a number of reasons including: the preferential retention of the C10-BZT and/or loss of Cl-BZT due to various biogeochemical reactions, and the resuspension of more recent sediments containing higher concentrations of the C10-BZT. This trend would be in agreement with the low ratio in Pawtuxet River surface sediments relative to sediment from the other locations where the ratios generally increase as found in the trap material. For example, the BZT ratio in surface sediments from Ohio Ledge is 7.2, in the mid-bay it is 8.2 and at the west passage the ratio is 9.5 (significant at the 9S% confidence level).
Surface Sediments and Clams
In general, the levels of all contaminants were highest in the Providence River sediments and decreased exponentially with distance downbay, with the lowest values usually found at the mouth of the bay. The major exception was a station in Newport Harbor in the lower east passage, where the concentrations of PHCs, PCBs and COP were relatively high and similar to some of the upper bay stations. The sources of the contaminants at this station may be related to sewage effluent, CSO discharges and runnoff to the harbor area.
One way of evaluating the distribution of various contaminants in surface sediments is to compare the half distances for these components (defined as the distance in which the highest contaminant concentration decreases by 50%). A comparison of values for the east and west transects obtained in this study showed some small differences. For most components (exceptions are BZTs) the half distances were longer in the cast transect, suggesting additional sources of contaminants to that area. This may be due to several reasons including the influence of Newport effluents etc., different water circulation patterns in the two passages, and possible influences from Mount Hope Bay. Organic carbon had the longest half distance due to a large number of sources throughout the bay, and the BZTs had the lowest values since their only source is the Providence River via the Pawtuxct River. Finally, the PH Cs, P AHs. PCBs (sometimes measured as specific chlorobiphenyl (CB) cogeners) and coprostanol had similar half distances, suggesting major sources in the Providence River as well as smaller additional inputs throughout the bay.
Five of the contaminants (CB 101, CB 138, PHCs, C10-BZT) showed significant correlations (95% confidence level or better) between sediment and clam concentrations, and all values decreased with distance from the Providence River. These sediment components are hydrophobic and are strongly partitioned into the clams' lipids. Based on the data from this study, it may be possible to identify areas of the bay from which clams are harvested for commercial use. For example, clams suspected of being taken from areas closed to shellfishing could be analyzed for, these 5 contaminants and the results converted to sediment location using the relationships developed in this investigation.
Trends from older investigations were generally similar to those observed in the present study; namely, highest concentrations in Providence River sediments and clams with decreasing amounts downbay. The range of previous values brackets those obtained in this study. Given then variations in station location, collection procedures, processing techniques and analytical methods, it is impossible to determine at the present time, if the concentrations have significantly changed over the past 15-20 years.
Sediment Cores
Highest levels and deepest penetration of PCBs (measured as CB 138) were in the Fox Point core which had a fairly uniform concentration down to 72 cm, but increased at 110 cm and remained about the same at the bottom of the core (147 cm). The other cores showed less penetration ranging from 42 cm (bottom of core) at Conimicut Point to 13 cm at Fields Point where the CB 138 values reached background (< 0.1 ng/g) concentration. Apparently, the latter core site is a non-depositional area in contrast to Fox Point which has a relatively high sedimentation rate.
The distribution of PHCs and pyrene were similar in some respects. In both cases, Fox Point had the highest level and deepest penetration with relatively constant values down to 147 cm. The PHCs never quite reached background levels in these cores (Ohio Ledge and Fields Point were the closest to background), but the pyrene did reach this level in two cores including the Fields Point core which again had the shallowest penetration (18 cm).
Similar distributions were shown by COP and DEHP in that the Fox Point core had the highest levels and deepest penetration - a slight increase down to 110 cm followed by a prominent decrease to 147 cm. None of the cores reached background values for DEHP, but Ohio Ledge and Conimicut Point came very close. The COP reached background in only the Fields Point core but it rapidly increased again.
It is impossible to adequately explain all of the trends in sediment core distributions with only the organic contaminant results. However, a detailed evaluation of all of the data (i.e., trace metals, organic components, biological species, pollen and geochemical analyses and radiometric dating) is currently being prepared in order to provide information on historical trends as recorded in the bay sediment cores (King et al., 1992).
[Text taken from report summary.]