Environ. Sci. Technol. 1902, 16, 437-439
NOTES Characterization of Plutonium in Ground Water near the Idaho Chemical Processing Plant Jess M. Cleveland* and Terry F. Rees U S . Geological Survey, MS 412, Lakewood, Colorado 80225
Plutonium is present in very low concentrations in ground water near the disposal well at the Idaho Chemical Processing Plant but was not detected in waters at greater distances. Because of the absence of strong complexing agents, the plutonium is present as an uncomplexed (perhaps hydrolyzed) tetravalent species, which is readily precipitated or sorbed by basalt or sediments along the ground-water flow path.
Table I. Ground-Water Solute Concentrations (mg/L)
a
The mobility of plutonium in ground water is largely influenced by its chemical and physical form-oxidation state, charge, presence or absence of complexes, hydrolytic species, and colloids-which in turn depend on the chemistry of the ground water. In an effort to relate plutonium speciation to ground-water composition, we have sampled and analyzed plutonium-containing waters in and around several low-level radioactive-waste-disposalsites in the US. An earlier report ( I ) describes our results from a study of trench leachates from the Maxey Flats radioactivewaste-disposal site in Kentucky. In this report we present data obtained from ground water in the vicinity of a lowlevel waste-disposal well at the Chemical Processing Plant of the Idaho National Engineering Laboratory. This plant has been in operation since 1952 and is used almost entirely to process highly enriched uranium fuels. It is on the Snake River plain, approximately 80 km west of Idaho Falls, ID, in an area underlain principally by basaltic lava flows to a depth of 600-700 m (2), and receives 250-300 mm of annual precipitation. The site thus contrasts markedly with Maxey Flats in terms of geology, hydrology, and climate. High-level liquid waste from the Chemical Processing Plant is converted to solid in an evaporator and stored as granules. Condensate from the evaporator, containing measurable amounts of radioactivity, is diluted with a large excess of other waste streams such as cooling water and ion-exchange regenerating solutions and discharged into the ground through a disposal well at depths ranging from 135 to 180 m. So that ground-water quality could be monitored, a series of wells has been drilled to similar depths. Selected for sampling in this study were four wells a t varying distances from the disposal well, whose ground-water compositions were known to be especially sensitive to waste discharges. A map of the area is shown in Figure 1, with the sampled wells indicated by number. For indication of the direction of ground-water flow, isopleths of sodium concentration as reported by Barraclough and Jensen (3) are superimposed on this map. Selected solute concentrations in waters from the four wells are shown in Table I. Because there is considerable nitrate in the discharged waste, it is not surprising that this anion is present in relatively high concentrations in the ground water, particularly that from well 40, the closest
well
DOCa
NO,
SO,
alkalinity (asCaC0,)
40 43 67 37
3.9 5.0 5.5 7.6
102 53 66 36
58 33 41 35
150 112 132 135
DOC = Dissolved organic carbon.
to the disposal well. Dissolved organic carbon contents are low because organic species are destroyed in the evaporation process, suggesting the absence of strong organic complexing agents. This conclusion was confirmed by analyses indicating the absence of detectable (25 pg/L) concentrations of ethylenediaminetetraaceticacid (EDTA), the compound shown to be largely responsible for plutonium mobilization in Maxey Flats waters (I). Solute concentrations in both the waste solution and the ground waters vary with time, thus making it difficult to obtain an exact dilution factor. However, on the basis of data presented in ref 2, it is estimated that waste solution has been diluted through hydrodynamic dispersion by a factor of approximately 2 when it reaches well 40.
Experimental Section Sampling and analysis procedures were the same as those employed at Maxey Flats (I). Water from each well was filtered sequentially through 5-, 0.4-, and 0.05-pm Nuclepore (the use of brand names in this report is for information purposes only and does not imply endorsement by the US. Geological Survey) membrane filters, with samples taken of the unfiltered water and each of the filtrates. The samples were collected in bottles containing enough HN03 to render the final volume 1 M in HNO,. In addition, unacidified samples of the 0.05-pm filtrates were collected under argon in Teflon bottles for oxidation-state analyses. Two separate sampling runs, using clean equipment, were made on each well except well 40, which was sampled three times. Analysis for plutonium, described in detail elsewhere ( 4 ) , consisted of anion-exchange separation of plutonium from 8 M HNO, followed by electrodeposition and counting on an (Y spectrometer. Three replicate 300-mL samples of each fraction were analyzed. Within a matter of minutes after collection, separate 100-mL samples of the unacidified 0.05-pm filtrates were subjected, in an inert atmosphere, to a series of carrierprecipitation and solvent-extraction procedures (5) to determine the oxidation-state distribution of the plutonium. After separation, the extracted phases were heated to evaporate the organic solvent, and the residue was dissolved in HNO, and analyzed by the usual anion-ex-
Not subject to US. Copyright. Publlshed 1982 by the American Chemical Society
Environ. Sci. Technol., Vol. 16, No. 7, 1982
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Table 111. Oxidation-State Results of Well 40 (0.05-pm Filtrate)a
separation thenoyltrifluoroacetone (TTA) extraction [PU(IV)I methyl isobutyl ketone (hexone) extraction [PU(IV), (VI11 Zr(I03)4precipitation [Pu( 111), (IV)] PrF, precipitation [Pu(III), (IV)] PrF, (after reduction) [Pu(III), (IV), (V), (VI11 NaUO,(C,H,O,), precipitation [Pu(VI)] a
0
IDAHO CHEMICAL PROCESSING PLANT AREA IDAHO NATIONAL ENGINEERING LABORATORY
Figure 1. Idaho Chemical Processing Plant Area, Idaho National Engineering Laboratory: closed circles identify locations of monitoring wells; those that are numbered refer to wells sampled in this study. Isopleths indicate ground-water sodium concentrations (mg/L) resulting from waste discharges into the dlsposai well.
Table 11. 23sPuConcentrationsa in Ground Water 23sPuconcn, fCi/L unfiltered 0.05-pm filtrate
well 40 43 67 37 a
Values in
59 (13) 35 (3) 54 (20)
