Sediment samples are routinely collected at randomly selected locations within the deep basins of each reservoir (Fig. 23, Fig. 24 and Fig. 25). Deep basins were chosen for sampling to minimize the disturbance and particle mixing effects of current and wave action that occur at shallower depths. Also, many of the analytes of interest tend to concentrate in the fine sediments that settle in the deep reservoir basins; thus, measurements from these areas would typically represent the highest levels that might be expected for a given reservoir.

Sediments were collected to depths of 5-10 cm using a grab sampler or Eckman dredge, to obtain > 6 L of sediment at each sampling site. In the field, > 5 L of each sample was sealed in a pre-cleaned plastic bucket and transported to CEMRC for preparation prior to analyses.

In the laboratory, the sediment samples were homogenized and split into aliquots for various analyses. Samples destined for radiochemical analyses were dried at 105°C to a constant weight, pulverized and homogenized prior to analysis. Samples analyzed for elemental and inorganic constituents were dried at 60 ^oC, and pulverized prior to analysis. A 250-g aliquot of each sample collected in 1998 was sent to Duke Engineering & Services (Bolton, Massachusetts) where it was analyzed for the alpha-emitting radionuclides ^239,240Pu, ²²⁸Th, ²³⁰Th, ²³²Th, ²³⁴U, ²³⁵U and ²³⁸U. Inorganic analyses were produced by CEMRC using IC, ICP-ES, ICP-MS and AAS, with methods described elsewhere in this report.

In addition to the above analyses, 1-L aliquots of the 12 sediment samples collected in 1999 were sent to the NMSU SWAT laboratory for particle size and texture analysis (Soil Conservation Service, 1972. Soil Survey Laboratory Methods and Procedures for Collecting Soil Samples. SCS, USDA; Gee, G. W. and J. W. Bauder, 1986. Particle-size Analysis. In Kline, A. (ed.) Methods of Soil Analysis. Part I. Physical and Mineralogical Methods - Agronomy Monograph no. 9. American Society of Agronomy, Madison, WI).

For 1999 studies, surface water was collected at one location within each reservoir. The surface water samples were taken in the same general area as the sediment samples. At each sampling location, one sample was collected from the surface (~ 0.5 - 1 m depth) and a second sample was collected from approximately 0.5 - 1 m above the sediment bed. The sample volume collected for radiological analyses was approximately 5 L. In addition, two 1-L samples were collected for inorganic analyses.

In the laboratory, surface water samples collected for radiological analyses were vacuum-filtered to 0.2 µm, acidified with HNO₃ to a pH < 2, and a 3-L aliquot removed for analysis of actinide and gamma-emitting radionuclides. Samples collected for elemental analyses were prepared according to the applicable EPA standard methods for the instrumentation used.

Baseline actinide analyses (^239,240Pu, ²³⁴U, ²³⁵U, ²³⁸U, ²²⁸Th, ²³⁰Th and ²³²Th) have been completed on 29 deep basin sediment samples collected through the end of 1998 including 13 sediment samples from Brantley Lake, eight samples from Lake Carlsbad and eight samples from Red Bluff Reservoir. Across all three reservoirs, baseline ^239,240Pu activity concentrations in individual sediment samples ranged from 0.07 to 0.52 mBq g^-1 with a regional mean (± SE) concentration of 0.26 (± 0.02) mBq g^-1. ANOVA showed there were significant differences (p < 0.05) in the mean ^239,240Pu activity concentrations between the individual reservoirs (Fig. 26 and Table 16) with the lowest mean concentration in Lake Carlsbad and highest mean concentration in Red Bluff reservoir. Within reservoirs, the mean ^239,240Pu activity concentration has remained relatively stable over the baseline sampling interval with none of the reservoirs exhibiting an obvious increasing or decreasing trend over time.

Sediment activity concentrations of ²³⁴U, ²³⁵U and ²³⁸U ranged from 9.9 to 123.0 mBq g^-1, 1.2 to 3.7 mBq g^-1 and 7.5 to 71.7 mBq g^-1, respectively. Mean sediment activity concentrations for all three U isotopes were similar between Lake Carlsbad and Brantley Lake but were almost a factor of 2 higher in Red Bluff Reservoir (Fig. 27). (U concentrations in sediment samples measured by ICP-MS were also approximately twice as high at Red Bluff Reservoir as at the other two lakes.) U activity concentrations in sediment across all reservoirs have exhibited a slight increase over time in all reservoirs, but the number of data points is too small to document a significant trend.

Across all reservoirs, Th sediment activity concentrations in individual samples ranged from 11.1 to 82.1 mBq g^-1, 14.4 to 96.9 mBq g^-1 and 11.9 to 96.0 mBq g^-1 for ²²⁸Th, ²³⁰Th and ²³²Th, respectively. However, the mean activity concentration for all of the Th isotopes was relatively similar across the baseline sampling period in all three reservoirs (Fig. 28).

Although the sediment concentrations of the U and Th isotopes were variable within and between reservoirs, the isotopic ratios were very similar across all three reservoirs. All three reservoirs appeared to be slightly enriched in ²³⁴U compared to ²³⁸U, with mean ²³⁴U/²³⁸U activity ratios ranging from 1.4 to 1.6. Mean ²²⁸Th/²³²Th ratios were close to unity, ranging from 0.75 to 1.1, indicating that these isotopes are in secular equilibrium within the sediments. A discussion of this phenomenon and the behavior of U and Th in freshwater systems is included in the CEMRC 1998 Report.

Filtered surface water samples collected during 1998 were analyzed by gamma spectroscopy for 11 naturally occurring and 12 anthropogenic gamma-emitting radionuclides. With the exception of two Red Bluff Reservoir samples that had detectable levels of ⁴⁰K, all results were below detection limits. However, radioactivity may be present in the water column in association with the seston (suspended particulate or colloids larger than 0.2 µm) that was filtered out and not included in the analyses.

A 200-L sample of unfiltered surface water was collected in Brantley Lake and Red Bluff Reservoir in 1998 and sent to Los Alamos National Laboratory for ²³⁹Pu analysis using thermal ionization mass spectroscopy (TIMS). ²³⁹Pu was not detected at MDC’s of 1.3 µBq L^-1 for the Brantley Lake sample and 2.2 µBq L^-1 for the Red Bluff Reservoir sample.

In 1999, two surface water samples (surface and deep positions) were collected from a single location in each reservoir. The samples were filtered in the laboratory and analyzed by gamma spectroscopy for gamma-emitting radionuclides. All results were below detection limits for these analytes. Actinide analyses of the samples are still pending and results will be reported, as they become available.

Baseline Non-Radiological Analyses of Surface Water and Sediment