PLUTONIUM IN ROCKY FLATS FRESHWATER SYSTEMS 647 



to the high variability of plutonium concentrations in the environment, numerous 

 samples were collected and analyzed by a modified solvent-extraction liquid-scintillation 

 counting procedure (Keough and Powers, 1970). Counting yield was 96%. Overall 

 chemical recovery was 90%. The minimum detectable activity (MDA) was 0.30 d/min per 

 sample for a 100-min count. Modifications and additional analysis information are 

 presented in a report by Johnson, Svalberg, and Paine (1974). Unless otherwise stated, all 

 references to plutonium in this chapter include both ^^^Pu and ^'^"Pu plutonium 

 isotopes because the analytical procedure did not discriminate between plutonium 

 isotopes. 



From 20 to 30 surface-sediment cores (approximately the top 5 cm) were obtained 

 during each sampling period (~1 per month) from each pond. The procedure followed 

 that outlined by Hakonson (1972). Additionally, core samples were extracted from the 

 sediment beds of the pond, and the procedure defined by Johnson, Svalberg, and Paine 

 (1974) was used to determine vertical distribution of plutonium within the pond 

 sediments. Incremental samples were composited for analysis. The coefficient of variation 

 determined from composited sediment samples was approximately 30%. 



Surface-water samples were initially taken at each sediment sampUng location from 

 each pond. Later samples were taken not only at the surface but also at 0.5-m increments 

 to the sediment- water interface. A mechanical water sampler was used to collect the 

 subsurface water samples. 



Water samples were filtered in a MiUipore filtering apparatus that was modified by 

 adding a brass screen with a pore size of 250/Lim to the top of the water intake funnel. The 

 screen removed most of the zooplankton and large organic material from the water 

 sample. The water was pulled through a glass-fiber filter (Whatman GF/A, 4,7-cm 

 diameter) with a vacuum pump to remove the remaining suspended material. The residue 

 was analyzed as a separate component called seston, which included primarily 

 phytoplankton, detritus, and other suspended soHds. Seston as defined here does include 

 some small zooplankton. The filtration process was usually carried out within 24 to 48 hr 

 after the collecfion. Water samples were kept in darkness to inhibit growth until filtration 

 could be accomplished. 



The term "zooplankton" was used collectively for all small planktonic animals 

 trapped in a number 10 plankton net (160-ium mesh size). These samples contained seston 

 as well as large aquatic insects. The insects were separated from the samples. It was 

 assumed that most of the sestonic material was probably smaller than the 160-/im mesh 

 size and would pass through the net since the zooplankton sample was rinsed in the 

 collecting net by repeated dunkings in the pond. Zooplankton were identified to species, 

 but biomass estimates were not determined. A 12. 7 -cm-diameter Clark— Bumpus plankton 

 sampler was towed behind a boat in an attempt to sample organisms. 



Bass (Ictiobus bubalus) and carp (Cyprinus carpio) were collected by seining and 

 angling. No fish were present in the B-series ponds, but minnows (Hybosis sp.) were 

 collected in pond Ci and pond Aj with a large collecting net. The fish were too small for 

 angling, and seining would have disturbed the bottom sediments. The maximum fish 

 length observed in the ponds was approximately 6 cm. Vegetation (primarily Juncus 

 balticus, Rumex crispus, and Typha latifolid) was collected in and around the ponds, 

 streams, and reservoirs throughout the study period. Generally, the aerial portions of the 

 plant samples were clipped with grass shears, and the roots were extracted separately 

 owing to excessive sediment— soil contamination. 



