22 Ground-Water Composition and Its Relationship to Plutonium Transport Processes JESS M. CLEVELAND, TERRY F. REES, and KENNETH L. NASH
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U.S. Geological Survey, Denver, CO 80225
The degree of plutonium leaching by ground waters from waste glass and its oxidation state distribution in solution were strongly influenced by the chemical composition of the waters. Ground waters from four possible host rock types--basalt, granite, shale, and tuff--as well as deionized water were used in this study. The order of plutonium leachability in the five waters was basalt >> tuff > deionized > granite > shale. High leaching efficiency in the basalt and tuff waters was the result of their high fluoride ion concentrations, whereas the lower leaching ability of granite and shale waters compared to deionized water may result from surface passivation of the glass by dissolved species--possibly sodium or magnesium--present in relatively greater concentrations in these waters. In single-phase speciation experiments with these five waters, plutonium was essentially completely soluble in basalt ground water because of its high fluoride concentration, and almost totally insoluble in shale ground water, probably because of the enhancement of colloid coagulation resulting from the large sulfate concentration in this water. In the single-phase experiments, plutonium(IV) was present in appreciable concentrations only in the basalt ground water, whilst in the leaching studies it was a common component in a l l of the leachates. Trends were more pronounced at 90°C than at 25°C, probably because of kinetic effects. Because of the large influence of ground-water composition on plutonium leachability and subsequent solubility, we propose that it be included as a radioactive waste repository site-selection criterion. T r a n s p o r t o f p l u t o n i u m f r o m a g e o l o g i c r e p o s i t o r y may b e considered t o i n v o l v e three processes : This chapter not subject to U.S. copyright. Published 1983, American Chemical Society
Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
336
PLUTONIUM CHEMISTRY
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1. 2. 3.
L e a c h i n g from the s o l i d - w a s t e form; S i n g l e - p h a s e r e a c t i o n s w i t h i n t h e g r o u n d w a t e r ; and I n t e r a c t i o n s w i t h r o c k and o t h e r m a t t e r a l o n g t h e g r o u n d water f l o w path. T h e s e p r o c e s s e s do n o t o p e r a t e i n d e p e n d e n t l y ; f o r e x a m p l e , t h e b e h a v i o r o f p l u t o n i u m i n s t e p 3 w i l l be g r e a t l y d e p e n d e n t on t h e s p e c i e s f o r m e d as a r e s u l t o f s o l u t i o n - p h a s e r e a c t i o n s i n s t e p 2. However, f r o m a c h e m i c a l s t a n d p o i n t , we h a v e f o u n d t h a t c o n s i d e r a t i o n of these processes i n d i v i d u a l l y i s a u s e f u l a i d to under s t a n d i n g the t r a n s p o r t of p l u t o n i u m i n a ground-water system. Of t h e s e t h r e e p r o c e s s e s , some have r e c e i v e d more s t u d y t h a n others. T h e r e h a v e b e e n a number o f d e t e r m i n a t i o n s o f l e a c h r a t e s ( s t e p 1) p a r t i c u l a r l y f r o m b o r o s i l i c a t e - g l a s s w a s t e f o r m s ( s e e , f o r e x a m p l e , JL, 2_, 3 ) , b u t t h e r e has b e e n m i n i m a l e f f o r t t o d e t e r mine s p e c i e s of d i s s o l v e d p l u t o n i u m . S o r p t i o n of plutonium from g r o u n d w a t e r o n t o r o c k s u r f a c e s ( s t e p 3) has r e c e i v e d e x t e n s i v e i n v e s t i g a t i o n ( 4 , .5, 6) i n an a t t e m p t t o b y p a s s a more f u n d a m e n t a l c h e m i c a l a p p r o a c h . F a i l u r e t o c o n t r o l c o n d i t i o n s and t o d e t e r m i n e the s p e c i a t i o n of plutonium i n the waters i n which the s o r p t i o n s t u d i e s w e r e done r e s u l t e d i n p l u t o n i u m s o r p t i o n c o e f f i c i e n t (K^) v a l u e s w i t h s t a n d a r d i z e d r o c k s a m p l e s t h a t d i f f e r e d by t h r e e o r d e r s o f m a g n i t u d e among v a r i o u s l a b o r a t o r i e s ( 6 ) . By c o n t r a s t , the o x i d a t i o n - s t a t e d i s t r i b u t i o n of p l u t o n i u m i n ground water ( s t e p 2 ) , w h i c h has a s t r o n g i n f l u e n c e on i t s s o l u b i l i t y and s o r p t i o n , has r e c e i v e d v e r y l i t t l e s t u d y . I n an e f f o r t t o f i l l t h e s e gaps i n k n o w l e d g e and d e v e l o p a u n i f i e d c h e m i c a l u n d e r s t a n d i n g o f p l u t o n i u m t r a n s p o r t , we h a v e u n d e r t a k e n an i n v e s t i g a t i o n o f p l u t o n i u m s p e c i a t i o n i n a c t u a l ground waters taken from host r o c k s of p o s s i b l e r e p o s i t o r y s i t e s . The s t u d y i n v o l v e d s o l u t i o n s p r e p a r e d i n two d i f f e r e n t ways: (a) u s i n g e a c h o f t h e g r o u n d w a t e r s f o r l e a c h i n g o f p l u t o n i u m - c o n t a i n i n g b o r o s i l i c a t e g l a s s , and (b) by a d d i t i o n o f a s m a l l v o l u m e o f d i l u t e p l u t o n i u m s o l u t i o n t o e a c h of the ground w a t e r s . We r e p o r t h e r e t h e c u r r e n t s t a t u s o f t h i s dual investigation. Experimental The f o u r g r o u n d w a t e r s u s e d i n t h i s s t u d y were f r o m t h e f o l l o w i n g sources: Grand Ronde b a s a l t f r o m t h e U.S. D e p a r t m e n t o f E n e r g y H a n f o r d R e s e r v a t i o n i n W a s h i n g t o n ; t u f f and C l i m a x s t o c k g r a n i t e f r o m t h e Nevada T e s t S i t e ; and C r e t a c e o u s s h a l e f r o m S o u t h D a k o t a . The f i r s t t h r e e a r e o r have b e e n c o n s i d e r e d as p o s s i b l e h o s t r o c k s f o r a g e o l o g i c r e p o s i t o r y (7); t h e s h a l e , d e s p i t e f a v o r a b l e p r o p e r t i e s ( 8 ) , has been l a r g e l y n e g l e c t e d . C h e m i c a l c o m p o s i t i o n s o f t h e s e f o u r g r o u n d w a t e r s a r e shown i n T a b l e I . I n a d d i t i o n , d e i o n i z e d w a t e r was a l s o e v a l u a t e d . B e f o r e s t a r t i n g the p l u t o n i u m experiments, the i n f l u e n c e of d i s s o l v e d o x y g e n on t h e E^ o f e a c h g r o u n d - w a t e r sample was d e t e r m i n e d by s p a r g i n g s e p a r a t e s a m p l e s w i t h o x y g e n and n i t r o g e n . A f t e r a 1-hour s p a r g e w i t h o x y g e n and s t a n d i n g i n c l o s e d c o n -
Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
22.
CLEVELAND ET
AL.
Table I Solute
Ground-Water Composition and Ground-Water C o m p o s i t i o n s Basait
Alkalinity (as C a C 0 ) Calcium Iron Magnesium Manganese Potassium Sodium Strontium Silica Chloride Fluoride Phosphate Sulfate
146
Pu
Transport 337
(mg/L) Shale
Granite
530
140
Tuff 98
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3
pH
