The biogeochemistry of 210Pb and 210Po in fresh waters and sediments

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January 1988The focus of this work was geochemical cycling of 210Pb in lakes, including the water column, sediments, and their interactions with each other. A special goal was to elucidate processes that might influence the distribution and fluxes of the radionuclide in ways that could effect 210Pb sediment dating. A mass balance for the epilimnion showed that 210Pb inputs by precipitation were matched by outputs on settling particles, indicating that direct uptake by bottom sediments was inconsequential. Below the epilimnion, vertical eddy diffusion was calculated by the heat flux gradient method including corrections for both radiant heating and heat loss to sediments. Vertical mixing was very low because of stability imparted by a steep temperature/density gradient extending right to the sediment water interface. Anoxic conditions caused remobilization of reduced iron, which reprecipitated at the oxycline and returned to the bottom via settling. 210Pb followed the same pattern except that, at the interface, it was scavenged rather than precipitated. Below the zone of precipitation, both 210Pb and iron distributions could be described by a model consisting of constant release from anoxic sediments, horizontal transport, and simple dilution in the water column. Cycling of 210Po was complicated by unidentified additional factors. A finite difference model (SEDIMIX) was used to find the combination of sedimentation and Fickian redistribution that provided the best fit to the 210Pb sediment data. Sedimentation rates were found to increase linearly with overlying water depth. The magnitude of the Fickian component of 210Pb transport was equal to calculated rates of pore water diffusive flux, which is probably more important than sediment mixing in this lake. 210Pb, 210Po, and ancillary geochemical parameters were measured on the solid fraction and pore waters of two cores taken from the deepest basin in August and September. The radionuclides were two orders of magnitude higher than in overlying water and had steep concentration gradients that could support substantial diffusive fluxes. Fe, Mn, S(II), and alkalinity did not have similar gradients. 210Pb partition coefficients ranged from 1500 to 15000, decreasing with depth, and seemed to be controlled by sorption on iron oxides. Remobilization to the water column apparently comes from a thin layer of iron-rich floc near the sediment water interface. Deeper in the cores, diffusive transport can cause redistribution of 210Pb to an extent that can affect 210Pb dating.Financial support for parts of this work has come from a National Science Foundation graduate research fellowship, a U.S. Geological Survey water resources research grant, three National Wildlife Federation environmental conservation fellowships, the Massachusetts D.W.P.C., and a Geological Society of America graduate award

Biogeochemistry and Contaminant Geochemistry of Marine and Estuarine Sediments, New Haven, Connecticut (USA)

The urbanized shore areas of Long Island Sound in the vicinity of New Haven, Connecticut (USA) have a long history of exposure to point and non-point sources of pollution, New Haven having been one of the birthplaces of the industrial revolution. As an unintended consequence of such activities, the region\u27s sedimentary systems have incorporated a complex mixture of organic and inorganic contaminants. With its long and varied pollution history and the multiplicity of sedimentary environments (fluvial, estuarine, intertidal, marsh, etc.) present in a compact geographical area, the region is ideal natural laboratory for field testing new contamination assessment techniques. The residents of this densely-populated region continue to exploit the local waterways for recreation and economic benefit, including the harvest of seafood. A comprehensive, systematic evaluation of organic and inorganic contamination thus also addresses public health concerns. New Haven harbor, an embayment approximately 3 by 5 km, is an active terminus for international marine cargo. The tidally-influenced Quinnipiac, Mill and West Rivers empty into the harbor, the shoreline of which is the site of docking facilities, a petroleum tank farm, a power generation station, sewage treatment facilities, a busy interstate highway, housing and park land. The shipping channel leading into the inner harbor is maintained by periodic dredging. A series of shallow core samples were taken in open water, in the shipping channel, in the inner harbor and at the river mouths. For comparative purposes, several additional sediment samples were taken in waters near Guilford, a residential suburb of New Haven with low density housing and a pleasure boat marina. For molecular organic analysis, we employed pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), using milligram quantities of dry, whole sediment. The compounds thus detected form a complex mixture of thermally extractable components, plus the products of the thermal decomposition of (bio)polymers present in the sample. The Py-GC/MS technique as used here offers a practical alternative to high cost, time-consuming standard analytical procedures, particularly in a regional survey employing a large number of specimens. The New Haven harbor samples were also analyzed for trace metals. The Py-GC/MS results indicate a predominance of aquatic organic matter (OM) in the open water sediment samples at both sites, as evidenced by the relatively high abundance of nitrogen compounds in the pyrolyzate, including pyrroles, pyridines and indoles. These compounds are characteristic pyrolysis products of proteins and degraded proteinaceous matter, in this case largely from marine algae and bacteria. In contrast, estuarine samples show a larger terrestrial OM component, including the phenolic pyrolysis products of lignin and thermally-extracted long chain, odd carbon-numbered normal alkanes. A subset of the samples (from the river mouths and, to a lesser extent, from the inner harbor in New Haven) shows significantly higher relative concentrations of thermally-extractable polycyclic aromatic hydrocarbons (PAHs) and petroleum-derived hopanes. The Guilford and open water New Haven harbor samples appear relatively uncontaminated by PAHs. Our analytical approach permits the recognition of biogeochemical differences indicative of the parent OM and the depositional environment, as well as potentially hazardous anthropogenic contributions to the sediment

Beryllium-7 Elucidate Sediment Dynamics of the Branford River Estuary, Connecticut, USA

Berrylium-7 elucidates sediment dynamics (i.e., sources, sinks, deposition, and resuspension) in a Connecticut estuary. Average 7 −2−1 annual atmospheric deposition of Be is 290 mBq cm year. Sediment samples from 43 locations within the estuary show that 7Be deposition is spatially complex, but were statistically indistinguishable a year apart. Weekly time series of sediments indicate 7 that levels are nearly constant on this shorter time scale on ceradio active decay is taken into account. Be levels in sediments area balance between steady losses through radioactive decay and periodic pulse inputs following rainstorms. The water column was measured intensively during three rain events, showing that 7Be is removed rapidly from the water column, with a rate constant −1 7 averaging 1.00 ± 0.12 day. A mass balance shows that Be is supplied about equally by direct precipitation onto the estuary’s surface and inflow from the watershed. Losses from the water column are split between net sedimentation (43%) and tidal flushing (57%). Variations in sedimentary 7Be levels at very short (meters) and longer (km) distances, and changes at time scales from hours to years, indicate that a large number of samples are required to capture all the variability in these highly dynamic systems. The current study differs from previous research in that a large number of measurements were conducted on a smaller system, and a full mass balance was developed

Beryllium-7 Elucidate Sediment Dynamics of the Branford River Estuary, Connecticut, USA

Berrylium-7 elucidates sediment dynamics (i.e., sources, sinks, deposition, and resuspension) in a Connecticut estuary. Average 7 −2−1 annual atmospheric deposition of Be is 290 mBq cm year. Sediment samples from 43 locations within the estuary show that 7Be deposition is spatially complex, but were statistically indistinguishable a year apart. Weekly time series of sediments indicate 7 that levels are nearly constant on this shorter time scale on ceradio active decay is taken into account. Be levels in sediments area balance between steady losses through radioactive decay and periodic pulse inputs following rainstorms. The water column was measured intensively during three rain events, showing that 7Be is removed rapidly from the water column, with a rate constant −1 7 averaging 1.00 ± 0.12 day. A mass balance shows that Be is supplied about equally by direct precipitation onto the estuary’s surface and inflow from the watershed. Losses from the water column are split between net sedimentation (43%) and tidal flushing (57%). Variations in sedimentary 7Be levels at very short (meters) and longer (km) distances, and changes at time scales from hours to years, indicate that a large number of samples are required to capture all the variability in these highly dynamic systems. The current study differs from previous research in that a large number of measurements were conducted on a smaller system, and a full mass balance was developed