Sampling And Analysis

The Ledbetter embayment of Kentucky Lake (Figure 9.1) was selected for this study. The sampling site (lat. 36° 45' 02" N; long. 88° 08' 19" W) in the Ledbetter embayment is situated apart from the mainstream of water current in Kentucky Lake so that the natural sedimentation process is not disturbed. The sediment core collected from this location should thus represent a continuous sequence of sedimentation and consequent accumulation of contaminants over the time frame of interest. A stainless steel inner liner (6 ft long and 2 in. in diameter) was used to collect the sediment core. The inner liner was inserted into a custom-made cast-iron sediment core sampler. It was pushed manually from a boat through the sediment under water to collect the sediment core. The sediment core collected was about 1 m long.

After collecting the sediment core, the inner liner was taken out of the core sampler and immediately transported to the laboratory. A stainless steel iron rod was used to push the sediment core out of the inner liner. The cores were cut into 5-cm sections and were collected in preweighed wide-mouth glass bottles. For chemical analysis, an aliquot of each core section was freeze-dried using a Freezone Freeze Dry System (model: 77535) for 60 h. Aliquots of each section were used for total organic carbon and radiochronological study.

9.2.1 Chemical Analysis

Sediment samples were digested with nitric, perchloric, and hydrofluoric acids. After digestion, the samples were analyzed for Cr, Ni, Cu, Zn, As, Se, Ag, Cd, Sn, Sb, and Pb. The samples were analyzed using Fisons/VG Model PQII+ inductively coupled plasma-mass spectrometry (ICP-MS). Indium was used as an internal standard because it has a major isotope at mass 115, which is approximately in the middle of the mass range (0 to 240 amu). Also, its natural occurrence in environmental samples is negligible. The concentration of the internal standard was in the range of 10 to 100 times greater than the other analytes in the sample.

All standards and samples were spiked with the internal standard of known concentrations. The samples were concentrated or diluted so that all analytes were bracketed by the calibration standards. Each sample batch of 20 samples included one or more certified reference standards, matrix spike and matrix spike duplicate, and continuing calibration standards. Total mercury in sediment was determined by isotope dilution-cold vapor (ICPMS) as described by Smith [14]. The samples are spiked with an enriched 201Hg isotope prior to microwave digestion with HNO3. An aliquot of the digest was reduced with sodium borohydride and the resulting mercury vapor was swept into the torch of the ICPMS.

9.2.2 Organic Carbon and Nitrogen

The sample preparation for the organic carbon and nitrogen analysis in sediments was performed following the procedure described by Wong et al. [15]. The analysis was carried out using a CHN analyzer (Perkin Elmer Series II CHNS/O Analyzer-2400 Series II with Perkin Elmer Auto Balance-AD4). Blanks (tin cups), conditioning reagents (sulfamic acid; Perkin Elmer-N241-0501), and standards (CHN standard; acetanilide; organic analytical standard; 0240-121; C = 71.09%, H = 6.71%, O = 11.86%, and N = 10.38%) were run to meet the analytical quality control and quality assurance criteria. After every ten samples, a duplicate, a blank, and a standard were run to verify the instrumental quality control criteria.

9.2.3 Radiochronology

An aliquot of each of the 5-cm sections of the sediment core from the Ledbetter embayment site were collected in preweighed glass beakers and weighed and dried in an oven at 60°C for 72 h. The activity of 210Pb and 137Cs within each sample was determined concurrently using two low-

background, planar intrinsic germanium detectors, a computer-based multichannel analyzer, and Maestro-II analysis software [5]. The sediment accumulation rate was calculated by producing a depth profile of total 210Pb activity within the sediment core. The calculated 210Pb accumulation rate was verified using 137Cs as a complementary radiotracer.

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