Collection and preparation of lake sediments for dating and trace element analysis by nuclear techniques

Aquatic mediums are the recipients of the liquid effluents from variety of industries. Both the suspended and bottom sediments of the mediums, owing to their great absorption capacity for many of the toxic pollutants, could become effective indicators of the pollution status of the medium when sampled, stored and analyzed with appropriate care. The determination of metal levels in sediments can play a key role in detecting the sources of pollution. The analysis of pollutants coupled with measurement of sedimentation rates in the medium using radiometric methods can reconstruct the history of discharge into the medium. Sampling modes of suspended solids bottom surface sediments and bottom core samples are presented in this presentation. The significance of sample storage procedures for the maintenance of sample integrity is also discussed. The choice of container materials for the storage of samples should receive utmost attention, especially for wet samples. Many type of the containers made from relatively inert materials like Teflon, quartz, polyethylene, hard glass and so forth have both attractive and undesirable attributes. The duration of storage and the requirements of short term and long term storage are also discussed. The pre-treatments of the samples, i.e. preparation of samples ready for analysis is a crucial step in generating representative data. The problems associated with pretreatment procedures are presented in order to avoid contamination of the sample during sampling operation, through operating personnel and sample preparation. The importance of control analysis with standard reference materials and participation in inter laboratory comparison exercises are emphasized. The dating techniques using natural Pb-210 and anthropogenic Cs-137 applied to determine recent sedimentation rates in Lake Naini, in Uttar Pradesh, India are discussed in detail. Recent sedimentation rates, estimated by Pb-210 has found to be fairly constant at one location (mean dry mass sedimentation rate being 0.112 ± 0.010 g cm-2 a-1) but varying in other locations in the lake (the dry mass sedimentation rates varying from 0.026 ± 0.010 to 0.421 ± 0.050 g cm-2 a-1). At all locations the short term rates (for the last three decades) derived from the fall out radio nuclide Cs-137 have been observed to be marginally higher compared to long term (last 120-150 yr.) deduced from Pb-210. The spatial and depth wise distribution of both the radio nuclides obtained from sediment cores of the lake along with their textural properties like porosity and water content provide preliminary information on the existence of differential depositional zones throughout the lake and on the physico-chemical nature of the sedimentation process in the lake. (author)