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| Sediment Chronology
The techniques used in sediment chronology are derived from universities and from scientific studies. Although many techniques have been developed, only measurements of man-made Cs-137 and natural Pb-210 and Be-7 have emerged as practical approaches to the chronology of sediments accumulating within the last 100 years for site characterisation and remediation studies.
Laboratory analyses of sediment cores can determine sedimentation rates and the age (calendar dates) associated with various depths within sediments. These chronology results can be used for characterising the deposition environment of a water system, which is pertinent to the planning of dredging operations. The methods are particularly useful for water systems which contain buried toxic substances. Measurements of different radioactive species give chronology information for time frames from a half year to 100 years before the present.
The accumulation of sediment in a water system is one of the most common situations which requires dredging. Consequently, a knowledge of sedimentation rates is often useful in planning dredging operations. The pattern of sedimentation rates can be used to calculate when various areas will require work. This information can also be used to plan the time intervals between successive operations. In shipping channels and other locations which are frequently dredged, sedimentation rates can be calculated from the history of periodic depth surveys. There are instances, however, for which no history of depth surveys exist.
In recent years there has been a growing emphasis on characterisation and remediation of contaminated waterways. Often a long stretch of river and the associated terminal estuary has been contaminated by industries over a period of decades. The toxic substances released by these industries become progressively buried in the sediments, such that profiles of these substances in the sediment become a record of the contamination process. These water systems may have only a fragmentary history of depth surveys, or none at all. Such water systems require another method to measure sedimentation rates and to determine the age (calendar dates) associated with buried toxic substances. Chronology information of this kind is useful in determining which industries caused the contamination, for industrial production or release records can be compared to the dates of buried materials. Chronology information is also useful for determining whether buried substances are migrating or degenerating.
Trace quantities of radioactivity, primarily of natural origin, are found in most substances. The earth, the oceans, the atmosphere and living things have always contained a number of radioactive species. Scientific methods for measuring recent sedimentation rates by analysing trace quantities of radioactivity were developed by universities beginning more than thirty years ago. These methods were subsequently expanded and applied to lakes, ocean environments, and rivers. With increasing interest in the environment, the techniques were applied to contaminated river and estuary systems. The procedures involve taking sediment cores and analysing samples at various depths. The distribution of natural or artificial radioactive species in the cores can often be interpreted to produce a chronological history of the sediments and their associated contaminants.
Present studies of contaminated river systems often include the taking of a pattern of sediment cores to characterise the site. The number of cores may be fewer than 10 or greater than 100. Cores are often 4 to 10 cm in diameter and 1 to 7 m in length. The cores are generally cut lengthwise to enable a sedimentary geologist to make a visual study of the sediments, sometimes including grain size measurements. Transverse sections of the cores are then sampled and sent to a laboratory to measure the concentrations of contaminants, resulting in profiles of the contaminants as a function of depth. Other sections of the core, often 2 cm in thickness, are sent to a radiochemistry laboratory to measure trace radioactivity at various depths. The radioactivity data are interpreted to determine sedimentation rates and the age (dates) associated with different depths in the sediment. The chronology data are then linked to the contaminant profiles in order to characterise the site.
The Pb-210 Method: Chronology Studies on the 100 Year Time Frame:
The Pb-210 (lead-210) method performs best in relatively quiet deposition areas such as marsh lands , bays, lakes and the backwaters of river systems. For example, in the Passaic River (New Jersey), more than 100 sediment cores were analysed for Pb-210. Of those, the three cores taken from quiescent tributaries exhibited more regular profiles. Fast flowing rivers may produce intermittent deposition which is better measured by the Cs-137 (cesium-137) method described in the next section. Nevertheless, the Pb-210 method is often used in conjunction with the Cs-137 method in active rivers in order to obtain maximum chronology information. Lead-210 is a natural radioactive form of lead which is found in small quantities in most soils as part of the uranium (U-238) decay series. It is also produced as natural fallout from the atmosphere by radioactive decay of Rn-222 (radon-222) gas. Minute quantities of Pb-210 fall constantly onto the earth's surface. This material accompanies and mixes with sediments which accumulate at the bottoms of water systems. For a given locality, the supply of Pb-210 is often at a steady rate, being derived from direct deposition, from upstream transport, and from decay of Rn-222 in the water. The result is a relatively high concentration of Pb-210 in the shallow sediments. Tolerances of the measurements are based on detection uncertainties at the 2 sigma (95% confidence) level.
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