Reports
Pollution history of the Chesapeake Bay: trace metal and nutrient study
1996, Hill, J.M., and Park, J.
File Reports, Coastal and Estuarine Geology, File Report 1996-04
Executive Summary
Cores were taken at six sites throughout the main stem of the Chesapeake Bay as part of the NOAA Status and Trends program. The sites were selected to encompass the range of geochemical environments found in the Bay, where good temporal resolution was expected. This report is part of this multi-disciplinary project to examine the historical record, focusing primarily on recently deposited sediments, to assess natural processes operating in the Bay, and the extent of man's influence on these processes. The scope of this project was to measure a range of physical properties and inorganic chemical species as indicators of natural and anthropogenic processes with in the sediment column.
Some of the results of the study are:
The two northern most cores are strongly influenced by input from the Susquehanna, both in regard to physical transport, based on terrigenous carbon loading, and geochemical processes, indicated by the variations sulfur and iron speciation. In addition, the cores' geochemistry varies from normal marine sediments in that gas is present. The presence of gas tends to preserve AVS and organic carbon, and alter the suite of diagenetic mineral that may form.
The two mid-Bay cores also have gas present and have high sedimentation rates, as a result of being located in paleochannels. Fluvial influence appears to be negligible, and the primary source of carbon is from plankton. The diagenetic processes with the last 30 years appears to be well-behaved. Showing little variation in the carbon and sulfur chemistry.
The southern cores have lower sedimentation rates. Gas is not present in the southern most core, while gas occurs over a meter into the sediment for the other southern core. As a result, organic carbon, from plankton, is not well-preserved and the fraction of AVS sulfur is low. Diagenetic processes with the last 30 years appears to be well-behaved.
In regard to trace metals, most of the metals analyzed for all of the cores there is little change in the enrichment factors over the past thirty years. Generally, when variations in metal behavior occur, they follow environmental conditions, in regard to fluvial influence or variations in geochemical conditions.
In the northernmost site there is a trend of increasing metal concentrations, starting in the mid 60's and peaking ~ 1981 then decreasing rapidly in the mid 80's: the metals which show this trend are Pb, Zn, and Cu.
Zn and Cu are significantly elevated throughout the Bay with respect to "pristine" sediments collected at the same site. Pb is elevated to some extent in all of the cores but two cores.
Comparison of Baywide variations in loading of recent sediments, as referenced to pristine sediments shows the influence of the Susquehanna River. Mn, Zn, Ni, Cr, and Cu have elevated levels in the northernmost samples decreasing to background levels mid-Bay. South of this point, enrichment factors of Mn, Ni, and Cr near one, indicating that recent sediments are not enriched compared to pristine sediments. On the other hand both Cu and Zn show an enrichment factor of 2 - 2.5 throughout the entire Bay indicating anthropogenic loading of these metals. The ubiquitous nature of the loading indicates atmospheric loading as a possible source of these metals; equivalent in magnitude to background loading.
