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Contaminant Mobilization Resulting from Redox Pumping in a Metal-Contaminated River-Reservoir System Johnnie N. Moore Department of Geology, University of Montana, Missoula, MT 59812

Both large- and small-scale extraction of metals in the northern Rocky Mountains of Montana has left a legacy of contaminated soil and river and reservoir sediments. The important processes that affect the fate of metals and metalloids involve the often-complicated oxidation-reduction reactions of sulfides and oxygen. Combined with dissolution-solution reactions, such redox reactions result in transferring contaminants from contaminated floodplain sediments to rivers in particulate and solute phases. Reservoirs intercept these contaminants and store them as a major secondary source of contamination. The seasonal change in oxidation state in reservoirs releases some components while fixing others—here the process is termed redox pumping—and leads to tertiary contamination of groundwater adjacent to reservoirs. The effect of this complex interplay between dissolution and redox reactions has extended contamination over 500 km from the primary sources.

EXTRACTION OF METALS FOR 125 YEARS

generated extensive hazardous waste i n t h e area a r o u n d t h e M o n t a n a R o c k y M o u n t a i n s . A b o u t 5 - 7 % o f a l l rivers i n M o n t a n a — m o r e than 2000 k m o f s t r e a m s — a r e c o n t a m i n a t e d b y m i n i n g wastes at a l e v e l that i m p a i r s beneficial use o f w a t e r ( M o n t a n a D e p a r t m e n t o f H e a l t h a n d E n v i r o n m e n t a l Sciences, personal c o m m u n i c a t i o n ) . C e n t r a l to this contaminant b u r d e n is t h e wastes g e n e r a t e d b y m i n i n g a n d

0065-2393/94/0237-0451$06.25/0 © 1994 American Chemical Society

Baker; Environmental Chemistry of Lakes and Reservoirs Advances in Chemistry; American Chemical Society: Washington, DC, 1994.


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s m e l t i n g i n the B u t t e - A n a c o n d a area, w h e r e m o r e than 1 b i l l i o n m e t r i c tons ( M T ) of ore a n d waste rock w e r e p r o d u c e d . T h e wastes w e r e d i s c a r d e d i n t o the headwaters of the C l a r k F o r k R i v e r , the largest t r i b u t a r y of the C o l u m b i a R i v e r (I). E x t r a c t i o n of metals f r o m B u t t e , the major m i n i n g area, began i n 1864. B y 1896 m o r e than 4500 M T of ore was processed e v e r y day, w i t h the wastes d i s c a r d e d d i r e c t l y o n the l a n d surface or into streams d r a i n i n g the area. A t the t u r n of the c e n t u r y one of the largest smelters i n the w o r l d h a d b e e n c o n s t r u c t e d i n A n a c o n d a , a n d w i t h i n 15 years it was p r o c e s s i n g m o r e t h a n 11,500 M T of ore p e r day. D e p r e s s e d c o p p e r prices f o r c e d closure o f that smelter i n 1980, a n d large-scale m i n i n g e n d e d 3 years later. A f t e r a b r i e f hiatus, m i n i n g has r e s u m e d o n a r e d u c e d scale. C a l l e d the " r i c h e s t h i l l o n e a r t h " , B u t t e p r o d u c e d m o r e metals t h a n e i t h e r the L e a d v i l l e district i n C o l o r a d o or the C o m s t o c k L o d e i n N e v a d a (2) a n d left a legacy of e q u a l l y g r a n d c o n t a m i n a t i o n . M i n i n g a n d s m e l t i n g operations left b e h i n d extensive waste deposits. Possibly as m u c h as 1500 k m of l a n d was c o n t a m i n a t e d w i t h i n the C l a r k F o r k R i v e r basin (J), i n c l u d i n g 35 k m of tailings p o n d s ; 300 k m of soil c o n t a m i n a t e d b y air p o l l u t i o n ; tailings deposits along h u n d r e d s of k i l o m e t e r s of r i v e r i n e habitat; m o r e t h a n 50 k m o f c o n t a m i n a t e d , o n c e - p r o d u c t i v e a g r i c u l t u r a l l a n d ; a l l u v i a l a n d b e d rock aquifers c o n t a m i n a t e d w i t h metals, sulfate, a n d arsenic; a n d d o w n s t r e a m reservoirs c o n t a i n i n g thousands of m e t r i c tons of m e t a l - c o n t a m i n a t e d s e d i m e n t . T h i s basin encompasses the largest c o m p l e x of S u p e r f u n d sites i n the c o u n t r y , w h e r e fluvial a n d g e o c h e m i c a l processes transport contaminants h u n d r e d s of k i l o m e t e r s f r o m the source a n d affect the e n t i r e C l a r k F o r k R i v e r system (J). 2

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Contamination Process E a r l y i n the history of m i n i n g a n d s m e l t i n g i n the C l a r k F o r k basin, reservoirs w e r e the first recipients of contaminants. T h e s e reservoirs w e r e b u i l t to retain m i l l i n g wastes for secondary r e c o v e r y of metals, to l i m i t effluent m o v i n g d o w n s t r e a m into the C l a r k F o r k R i v e r , a n d to serve as h y d r o e l e c t r i c i m p o u n d m e n t s . T h e y n o w make u p a vast array of tailings p o n d s i n the headwaters of the C l a r k F o r k R i v e r a n d large d o w n s t r e a m lakes that act as sinks a n d sources for contaminants to surface a n d g r o u n d w a t e r i n the b a s i n . T h e first h y d r o e l e c t r i c r e s e r v o i r ( M i l l t o w n Reservoir) was b u i l t i n 1 9 0 6 - 1 9 0 7 at a site a p p r o x i m a t e l y 200 r i v e r k m d o w n s t r e a m f r o m the major m i n i n g a n d s m e l t i n g operations s u p p l y i n g m e t a l - c o n t a m i n a t e d s e d i m e n t to the r i v e r system ( F i g u r e 1). T h i s m o s t - u p s t r e a m h y d r o e l e c t r i c r e s e r v o i r was the p r i m a r y catch basin for wastes transported b y the r i v e r before tailings ponds w e r e b u i l t i n the headwaters i n the mid-1900s. C o n t i n u i n g d o w n stream, three a d d i t i o n a l reservoirs w e r e b u i l t at 452, 516, a n d 556 k m i n



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Figure 1. Map of the Clark Fork River basin showing the location of the operable units in the main designated Superfund sites and other features mentioned in the text.

1915, 1959, a n d 1952, respectively. T h e s e i m p o u n d m e n t s also have t r a p p e d wastes f r o m u p s t r e a m , but to a lesser extent than M i l l t o w n R e s e r v o i r (I, 3). Metal Sulfide Wastes. T h e c o n t a m i n a t i o n processes associated w i t h this r i v e r - r e s e r v o i r system are c o n t r o l l e d b y the characteristics o f m e t al sulfide wastes left w i t h i n the basin. H i g h - g r a d e veins i n B u t t e u n d e r g r o u n d m i n e s c o n t a i n e d a variety of m e t a l sulfides, i n c l u d i n g chalcocite ( C u S ) , b o r n i t e ( C u F e S ) , c h a l c o p y r i t e ( C u F e S ) , enargite ( C u A s S ) , t e n n a n t i t e - t e t r a h e d r i t e ( C u [ A s , S b ] S ) , sphalerite (ZnS), p y r i t e ( F e S ) , acanthite ( A g S ) , galena (PbS), arsenopyrite ( F e A s S ) , a n d greenockite (CdS). 2

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L o w e r grade o r e a n d waste rock c o n t a i n e d a significant a m o u n t o f p y r i t e ( F e S ) , as w e l l as other m e t a l sulfide m i n e r a l s . Wastes generated b y m i n i n g a n d m i l l i n g released these m e t a l sulfides as particulate substances (J). M i x e d w i t h u n c o n t a m i n a t e d s e d i m e n t i n t h e r i v e r system, these particulate wastes m o v e d h u n d r e d s o f k i l o m e t e r s d o w n stream f r o m t h e i r o r i g i n a l source (4-7). C h a n n e l , floodplains, a n d r e s e r v o i r sediments t h r o u g h o u t the r i v e r n o w contain m u c h o f this c o n t a m i n a t i o n .


2

A n d r e w s (4) s h o w e d that f i n e - g r a i n e d particulate contaminants (arsenic, c a d m i u m , c o p p e r , l e a d , a n d z i n c i n s e d i m e n t less than 0.016 m m i n diameter) decreased d o w n s t r e a m f r o m t h e source a n d that t h e d i s t r i b u t i o n c o u l d b e e x p l a i n e d solely b y m i x i n g o f m i l l tailings w i t h u n c o n t a m i n a t e d f l o o d p l a i n s e d i m e n t . W o r k b y B r o o k a n d M o o r e (5) a n d M o o r e et a l . (6) s h o w e d that the sediments w e r e e n r i c h e d i n arsenic, c a d m i u m , c o p p e r , manganese, l e a d , a n d z i n c . T h e y also f o u n d that t h e contaminants w e r e c a r r i e d m o s t l y i n t h e r e d u c i b l e a n d o x i d i z a b l e phases (operationally defined). I n size-fractionated samples, they f o u n d that concentrations o f some metals generally increased w i t h decreasing particle size. H o w e v e r , m o r e u p s t r e a m samples (nearer to t h e source) c o n t a i n e d anomalously h i g h c o n centrations i n t h e coarsest fractions. Because C l a r k F o r k R i v e r s e d i m e n t is p r e d o m i n a n t l y coarse-grained, coarse fractions significantly a d d to t h e b u l k contaminant content o f the system. Distribution of Contaminants. M o s t r e c e n t l y , A x t m a n n a n d L u o m a (7) s h o w e d that m e t a l contaminants i n b e d sediments decreased i n a n exp o n e n t i a l t r e n d away from t h e source a n d p r e d i c t e d that e l e v a t e d m e t a l concentrations s h o u l d o c c u r m o r e than 550 k m d o w n s t r e a m f r o m t h e c o n taminant source. T h e y ascribed t h e d o w n s t r e a m t r e n d to d i l u t i o n f r o m u n c o n t a m i n a t e d s e d i m e n t m i x e d w i t h m i l l tailings. Johns a n d M o o r e (3, 8) f o u n d that contaminants h a d m o v e d t h r o u g h tailings p o n d s a n d u p s t r e a m reservoirs to accumulate i n d o w n s t r e a m reservoirs; c o p p e r a n d z i n c w e r e e n r i c h e d o v e r b a c k g r o u n d tributaries i n reservoirs m o r e than 556 k m f r o m the source. M e t a l concentrations o f surface s e d i m e n t f r o m these reservoirs lay o n t h e d o w n s t r e a m exponential t r e n d o f b e d s e d i m e n t i n t h e r i v e r ( F i g u r e 2). T h e s e data s h o w e d that fine-grained surficial sediments i n t h e r i v e r basin w e r e h i g h l y c o n t a m i n a t e d w i t h particulate wastes from t h e m i n i n g a n d m i l l i n g operations u p s t r e a m , a n d that reservoirs n o w actively bypass c o n t a m i nated s e d i m e n t d o w n s t r e a m . T h i s d i s t r i b u t i o n has significant effects o n t h e storage a n d r e m o b i l i z a t i o n of contaminants f r o m reservoirs i n t h e drainage. T h e farthest u p s t r e a m rese r v o i r i n t h e C l a r k F o r k drainage ( M i l l t o w n ) is n e a r l y 200 k m f r o m t h e sources o f c o n t a m i n a t i o n ( F i g u r e 2), y e t it contains s e d i m e n t contaminants m a n y times o v e r b a c k g r o u n d values. S e d i m e n t s i n t h e r e s e r v o i r c o n t a i n significant concentrations (depth-averaged values) of arsenic (32 times backg r o u n d values), manganese (7 times), c o p p e r (62 times), z i n c (67 times), l e a d


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200 400 600 DISTANCE DOWNSTREAM, IN KILOMETERS Figure 2. Plot of total copper concentration of surface sediments in the Clark Fork River versus distance downstream from the major sources of contami nants. Sediment came from the bed of the Clark Fork River channel and surface sediment from reservoirs. (Data are taken from references 5 and 7.) (11 times), a n d c a d m i u m (37 times). Tens o f thousands o f m e t r i c tons o f these metals are stored i n t h e reservoir s e d i m e n t (9, 10). T h e reservoir is efficient at t r a p p i n g coarse-grained s e d i m e n t (bed load) a n d is filled w i t h a c o m p l e x assemblage o f sand a n d m u d w i t h a b u n d a n t organic interlayers (9, JO). B u t f i n e - g r a i n e d s e d i m e n t (suspended load) n o w flows t h r o u g h the nearly f i l l e d reservoir d u r i n g s p r i n g r u n o f f w h e n t h e r i v e r contaminant b u r d e n is greatest (9). T h i s b y p a s s i n g has l e d to t h e r e d i s t r i b u t i o n o f fine-grained s e d i m e n t to t h e d o w n s t r e a m reservoirs; t h e farther f r o m the source, t h e m o r e t h e contaminant b u r d e n is transferred to t h e fine fraction o f t h e s e d i m e n t . T h i s situation results i n M i l l t o w n R e s e r v o i r c o n t a i n i n g the most c o m p l e x d i s t r i b u t i o n of contaminants i n the reservoirs o f the drainage basin.

Milltown Reservoir as a Model System M i l l t o w n r e s e r v o i r i m p o u n d s about 180 acres o f w a t e r at t h e confluence o f the Blackfoot a n d C l a r k F o r k rivers ( F i g u r e 3). T h e C l a r k F o r k R i v e r at that



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Figure 3. Map ofMilltown Reservoir showing main tributaries and the location of some of the wells used for hydrogeologic and geochemical monitoring. (Data are taken from references 9 and 12.) site drains about 15,000 k m . W h e n the d a m was b u i l t , the s e d i m e n t transport of the Blackfoot a n d C l a r k F o r k rivers was d i s r u p t e d . B o t h b e d l o a d a n d s u s p e n d e d load p r e v i o u s l y transported w e s t w a r d to the C o l u m b i a R i v e r w e r e i m p o u n d e d i n the reservoir, along w i t h wastes f r o m u p s t r e a m . T h i s s e d i m e n t a t i o n has nearly filled the reservoir, so that most o f t h e present susp e n d e d l o a d of the t w o rivers passes t h r o u g h the system. 2

D a t a c o l l e c t e d b y the M o n t a n a P o w e r C o m p a n y (the operators o f t h e dam) show that the reservoir is e x t r e m e l y inefficient at t r a p p i n g f i n e - g r a i n e d s e d i m e n t . D u r i n g d r a w d o w n s of the reservoir stage, s e d i m e n t is r e m o v e d f r o m the reservoir a n d transported d o w n s t r e a m ; d u r i n g 1 day o f d r a w d o w n i n 1980 approximately 12,000 M T of s e d i m e n t was r e m o v e d . T h e r e s e r v o i r contains about 1.9 m i l l i o n m (or approximately 3.8 m i l l i o n M T ) o f s e d i m e n t , at a m a x i m u m thickness of about 8 m , a n d has a c c u m u l a t e d contaminants for m o r e than 80 years. T h e c o m p l e x i n t e r p l a y o f channels, f l o o d p l a i n , a n d t h a l w e g e n v i r o n m e n t s of d e p o s i t i o n as the reservoir filled has created a c o m p l e x , i n t e r d i g i t i z i n g mosaic o f s e d i m e n t types a n d grain sizes. P r e s e n t m a i n channels contain the coarsest surface sediment, a n d t h e s w a m p y t h a l wegs a n d floodplains are r i c h i n organic substances a n d m u d . Stratigraphie 3


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cores of the reservoir s e d i m e n t show a c o m p l e x v e r t i c a l i n t e r m i x i n g of sandy,


m u d d y , a n d o r g a n i c - r i c h s e d i m e n t throughout t h e r e s e r v o i r (9). S e d i m e n t C o n t a m i n a t i o n . C o n t a m i n a t e d s e d i m e n t i n the r e s e r v o i r was first i d e n t i f i e d b y B a i l e y ( I I ) , w h e n she f o u n d that t h e r e s e r v o i r c o n tained h i g h concentrations of C u a n d Z n . F i v e years later, i n N o v e m b e r 1981, t h e M o n t a n a D e p a r t m e n t o f H e a l t h a n d E n v i r o n m e n t a l Sciences d e t e r m i n e d that four c o m m u n i t y wells adjacent to the r e s e r v o i r c o n t a i n e d arsenic at levels above the d r i n k i n g water standards r e c o m m e n d e d b y t h e Environmental Protection Agency (EPA). W o r k funded b y the E P A Superf u n d (9) subsequently i d e n t i f i e d the reservoir sediments i m p o u n d e d above the o r i g i n a l alluvial valley aquifer ( F i g u r e 4) as t h e source o f this c o n t a m i nation. S t r o n g v e r t i c a l h y d r a u l i c gradients i n t h e r e s e r v o i r d r i v e g r o u n d w a t e r i n t h e c o n t a m i n a t e d sediments into the u n d e r l y i n g , coarse-grained, a l l u v i a l aquifer. T h i s system has r e s u l t e d i n t h e c o n t a m i n a t i o n of the d o m e s t i c water s u p p l y i n adjacent M i l l t o w n , M o n t a n a . Because t h e r e s e r v o i r has a s m a l l storage capacity, significant stage fluctuations are c o m m o n . T h i s situation results i n sediments b e i n g i n u n d a t e d f o r m u c h o f the year a n d exposed for a f e w weeks at most each year, d e p e n d i n g o n flow conditions a n d m a i n t e nance needs. Because t h e reservoir is almost c o m p l e t e l y f i l l e d w i t h s e d i m e n t , at l o w stage o n l y channels are f i l l e d w i t h water. A t h i g h stage t h e b r o a d floodplain flat is partially c o v e r e d b y water. D u r i n g the f u l l stage t h e g r o u n d w a t e r system connects t h e reservoir s e d i m e n t to the r i v e r t h r o u g h c o m p l e x flow paths ( F i g u r e 5 A ) . A t the l o w stage g r o u n d w a t e r flow changes, w i t h some i n p u t back into the r i v e r c h a n n e l a n d c o n t i n u e d flow t h r o u g h t h e sediments into t h e adjacent alluvial aquifer ( F i g u r e 5 B ) . T h e processes c o n t r o l l i n g t h e m o b i l i z a t i o n o f arsenic i n this system result f r o m a c o m p l e x i n t e r a c t i o n b e t w e e n this g r o u n d w a t e r flow system a n d g e o c h e m i s t r y of the c o n t a m i n a t e d sediments (10, 12). W i t h i n this c o m p l e x i t y a m o d e l c a n b e d e v e l o p e d that is generally applicable to reservoirs c o n t a m i n a t e d b y the m i n i n g a n d s m e l t i n g of base-metal, s u l f i d e - r i c h ores. P r e l i m i n a r y w o r k (10) o n t h e transition f r o m o x i d i z e d surface s e d i m e n t to r e d u c e d subsurface s e d i m e n t i n M i l l t o w n R e s e r v o i r s h o w e d that t h e redox transition occurs i n the u p p e r f e w tens o f centimeters. S t r o n g c h e m i c a l gradients occur across this b o u n d a r y . F e r r o u s i r o n i n s e d i m e n t p o r e w a t e r (groundwater a n d vadose water) is c o m m o n l y b e l o w d e t e c t i o n i n t h e o x i d i z i n g surface zone a n d increases w i t h d e p t h . A r s e n i c is also l o w i n p o r e water o f the o x i d i z e d zone, b u t increases across t h e redox b o u n d a r y , w i t h As(III) as t h e d o m i n a n t oxidation state i n t h e r e d u c e d zone. C o p p e r a n d z i n c show the opposite t r e n d , w i t h relatively h i g h concentrations i n p o r e water of the o x i d i z e d surface s e d i m e n t decreasing across t h e redox b o u n d a r y . M o o r e et a l . (10) c o n c l u d e d that the f o r m a t i o n o f diagenetic sulfides p r o v i d e d an i m p o r t a n t c o n t r o l o n metals a n d arsenic m o b i l i t y i n t h e s e d i -



Figure 4. Cross section of Milltown Reservoir site showing the migration of the arsenic plume from reservoir sediments into the adjacent alluvial aquifer.


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Figure 5. Generalized model of groundwater flow when Milltown Reservoir is at full stage (A) and at low stage (B). (Modified from reference 12.)


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m e n t . A r s e n i c a n d metals are c a r r i e d into t h e r e s e r v o i r as p r i m a r y sulfides (likely w i t h o x i d i z e d crusts) a n d as coprecipitates a n d coatings o n o t h e r grains (5). W h e n b u r i e d a n d r e d u c e d , these o x y h y d r o x i d e s o f i r o n a n d manganese dissolve, t h e n arsenic a n d m e t a l sulfides p r e c i p i t a t e . A r s e n i c is r eleas ed to the g r o u n d w a t e r system d o m i n a n t l y as As(III). T h e system is c o n t r o l l e d b y m e t a l sulfide p r e c i p i t a t i o n , w h i c h i n t u r n is c o n t r o l l e d b y sulfate a v a i l a b i l i t y . Sulfate is s u p p l i e d b y o x i d i z i n g sulfides i n t h e surface s e d i m e n t s . A l t h o u g h this early w o r k e x a m i n e d t h e v e r t i c a l changes across t h e redox b o u n d a r y , no t e m p o r a l u n d e r s t a n d i n g was g a i n e d . Stage Changes.

A n o p p o r t u n i t y t o study t h e t e m p o r a l changes i n


d e t a i l arose i n 1986 w h e n t h e M i l l t o w n D a m was severely d a m a g e d b y h i g h , i c e - l a d e n flows f r o m a n early F e b r u a r y t h a w . R e p a i r s t o t h e d a m r e q u i r e d that t h e stage b e d r a w n d o w n a p p r o x i m a t e l y 2 . 5 m a n d h e l d l o w for several m o n t h s . I n M a y 1986 t h e r e s e r v o i r stage was l o w e r e d c o n t i n u o u s l y , a n d after a p p r o x i m a t e l y 100 days i t r e m a i n e d at a constant l o w stage f o r a p p r o x i m a t e l y 2 3 0 days ( F i g u r e 6). I n t h e s u m m e r , t h e r e s e r v o i r stage was r a p i d l y e l e v a t e d onto t h e n e w l y r e p a i r e d d a m , r e s u b m e r g i n g

sediments.

T h e stage was h e l d constant at this h i g h e r elevation t h r o u g h t h e w i n t e r , u n t i l l o w e r e d i n p r e l u d e to t h e s p r i n g runoff.

Sampling Period

3264 3262 3260 3258

·+-* —

φ

3256

Ο) cd •+—»

3254

ω

3252 3250 3248 0

50

100

150

200

250

300

350

400

450 500

Days After May 1 1986 Figure 6. Hydrograph of Milltown Reservoir stage during the temporal geo chemistry and hydrogeology study (12). (Data are from Montana Power Com pany records.)


14.

MOORE



461

T h i s r a p i d change i n stage h a d strong effects o n t h e redox b o u n d a r y a n d h e n c e o n contaminant m o b i l i t y . T h e c h e m i s t r y a n d h y d r o g e o l o g y o f this system w e r e e x a m i n e d tetraweekly, o v e r a 370-day p e r i o d f r o m A u g u s t 1986 to A u g u s t 1987, to d e t e r m i n e t h e role o f redox fluctuations o n t h e m o b i l i zation of arsenic into the u n d e r l y i n g alluvial aquifer (12). W e l l s w e r e i n s t a l l e d i n t h e c o n t a m i n a t e d s e d i m e n t at different elevations to m o n i t o r t h e v e r t i c a l a n d lateral c h e m i c a l changes before a n d d u r i n g r e f i l l i n g .


The Redox Pump M o b i l i z a t i o n o f contaminants i n M i l l t o w n R e s e r v o i r c a n b e e x p l a i n e d b y a m o d e l i n w h i c h g r o u n d w a t e r c o m p o s i t i o n is c o n t r o l l e d b y successive diagen e t i c reactions d u r i n g the transition to a n d f r o m oxic a n d anoxic e n v i r o n ments as t h e reservoir stage changes (12). S e v e r a l i m p o r t a n t reactions g o v e r n the m o b i l i t y o f contaminants i n this system: 1. those i n v o l v i n g m e t a l sulfides; 2. those u t i l i z i n g organic matter i n t h e sediments, a n d 3. those p r o d u c i n g o r d i s s o l v i n g oxide a n d h y d r o x i d e coatings o n grains. A l l o f these reactions are partially o r strongly c o n t r o l l e d b y bacterial i n t e r action. T h e s e reactions d e v e l o p a general v e r t i c a l z o n a t i o n o f oxic, anoxic sulfidic, a n d anoxic m e t h a n i c e n v i r o n m e n t s (13) w i t h i n t h e r e s e r v o i r s e d i m e n t ( F i g u r e 7) that migrates w i t h the rise a n d fall o f t h e r e s e r v o i r stage. T h i s fluctuation develops a " r e d o x p u m p " that m o b i l i z e s contaminants w i t h each successive stage cycle. A t f u l l stage t h e sediments are saturated a n d r e d u c e d , w i t h a t h i n oxic zone (variably b e t w e e n 0.1 a n d 0.5 m) c o r r e s p o n d i n g to t h e vadose z o n e a n d u p p e r m o s t g r o u n d w a t e r ( F i g u r e 7A). W h e n t h e r e s e r v o i r stage drops a n d sediments are d r a i n e d , o x y g e n - r i c h vadose water gains access to t h e u n d e r l y i n g r e d u c e d sediments. T h e redox z o n a t i o n m o v e s d o w n w a r d a l o n g w i t h t h e falling water table ( F i g u r e 7 B ) . T h i s system is d r i v e n b y t h e m i n e r a l o g y - c h e m i s t r y o f the c o n t a m i n a t e d s e d i m e n t f i l l i n g t h e reservoir. R e sidual sulfides transported d o w n s t r e a m a n d authigenic sulfides f o r m e d i n place (10) u n d e r g o oxidation i n t h e vadose zone, releasing metals, sulfate, a n d h y d r o g e n ions, a c c o r d i n g to the f o l l o w i n g general reaction (14-17). 2FeS

2

+ 70

2

+ 2H 0 2

>2Fe

2 +

+ 4S0

4

2

" + 4 H

+

(1)

I n M i l l t o w n R e s e r v o i r sediments a n d o t h e r m e t a l sulfide c o n t a m i n a t e d systems (18), F e S (pyrite) i n this reaction c a n b e r e p l a c e d b y any n u m b e r of other m e t a l sulfides (for example, arsenopyrite, chalcocite, galena, a n d 2




462

Figure 7. Geochemical model showing development of redox environments as the reservoir stage falls: A, reservoir at full stage; B, low reservoir stage. The scale of this system is highly variable. The oxic zone, depicted by the lightest stippled pattern, is generally 0.1-0.5 m thick at high stage and 0.5-1.5 m thick at low stage. The reducing zone, depicted by the medium stipples, extends to depths of approximately 5-8 m. The presence of the methanic zone (heavy stipples) is probably extremely variable.


14.

MOORE


463

Resulting froin Redox Pumping

sphalerite) so that other m e t a l s — A s , C d , C u , M n , P b , a n d Z n — a r e released to the g r o u n d w a t e r along w i t h F e (17, 18). T h i s g e n e r a l reaction is catal y z e d b y bacteria (e.g., Thiobacillus ferrooxidans) (16) a n d continues as l o n g as the p H remains near n e u t r a l (19, 20). T h i s reaction can be seen i n the highest elevation w e l l s at M i l l t o w n , the water-table w e l l s . W h i l e the w a t e r table is b e l o w these w e l l s , sulfides are o x i d i z e d i n the vadose z o n e . A s g r o u n d w a t e r moves into this zone, sulfate c o n c e n t r a t i o n increases dramatically at the water table b u t remains b e l o w d e t e c t i o n i n d e e p e r w e l l s ( F i g u r e 8A). A r s e n i c , i r o n , a n d manganese concentrations also increase r a p i d l y as the w a t e r table rises into w e l l s (Figures 8 B , 8 C , a n d 8 D ) , w h i l e r e m a i n i n g relatively h i g h a n d constant i n d e e p e r zones that r e m a i n saturated.


2 +

T h e oxidation of organic matter v i a reactions s i m i l a r to e q 2 is associated w i t h m e t a l sulfide oxidation a n d the release of metals (21). (CH O) 2

1 0 6

(NH,) (H PO ) + 1380 1 6

3

4

228HCCV

2

+ 122CaC0

+ 1 6 N 0 - + 122Ca 3

>

3

+ 16H 0 + H P 0

2 +

2

3

4

(2)

M e t a l s b o u n d to organic substances are released as the organic m a t e r i a l is o x i d i z e d . T h e c a r b o n a t e - b i c a r b o n a t e c o m p o n e n t of the system buffers the acid p r o d u c e d f r o m sulfide oxidation reactions. T h e s e reactions can be seen readily i n M i l l t o w n sediments as soon as the water-table w e l l s r e c e i v e g r o u n d w a t e r d u r i n g the r i s i n g stage. Bicarbonate a n d c a l c i u m concentrations rise i m m e d i a t e l y w h i l e p H remains fairly constant ( F i g u r e s 9 A , 9 B , a n d 9 C ) ; bicarbonate t h e n increases as c a l c i u m decreases. I n the c o n t i n u o u s l y saturated zones at d e p t h , bicarbonate a n d c a l c i u m r e m a i n r e l a t i v e l y u n r e s p o n s i v e to water-table fluctuations. S u l f i d e a n d organic o x i d a t i o n processes s e e m to be i n t i m a t e l y j o i n e d b y bacterially catalyzed reactions. A strong c o r r e lation b e t w e e n c a l c i u m a n d sulfate ( r = 0.903) suggests that the d i s s o l u t i o n - p r e c i p i t a t i o n of c a l c i u m sulfates is a cross p r o d u c t of these t w o r e d o x - c o n t r o l l e d reactions. 2

O x i d e s a n d h y d r o x i d e s (oxyhydroxides) of i r o n a n d manganese also f o r m d u r i n g oxidation of s e d i m e n t at the l o w r e s e r v o i r stage. T h e u p p e r oxic zone Contains m o t t l e d , " r u s t y " s e d i m e n t after o n l y a f e w days of exposure. F e r r o u s i r o n a n d manganese released d u r i n g the o x i d a t i o n of p y r i t e a n d other sulfides (Figures 8 C a n d 8 D ) w i l l react w i t h oxygen i n the vadose z o n e to f o r m i r o n a n d manganese o x y h y d r o x i d e s v i a reactions s i m i l a r to eqs 3 a n d 4 (15, 22). 4Fe

2 +

2Mn

2 +

+ 10H O + O — - > 4Fe(OH) 2

+ 0

2

É

+ 40H-

>2Mn0

2

3

+ 8H

+

+ 2H 0 2

(3) (4)

T h e s e o x y h y d r o x i d e c o m p o u n d s e o p r e e i p i t a t e - a d s o r b metals a n d arsenic released f r o m the sulfide a n d organic reactions, fixing some metals i n the o x i d i z e d zone (10, 23). T h e s e reactions can be seen i n changes i n i r o n




464

Figure 8. Bubble diagram of concentrations (milligrams per liter) of chemical components in well 138 (Figure 3) showing changes with time in wells of different depths; the upper scale bar (bubble diameter) is the concentration of the species as depicted by bubble, the vertical axis is the elevation of the screened interval of the well, and the horizontal axis is the days since drawdown: A, sulfate; and B, arsenic.


MOORE

Contaminant Mobilization Resulting from Redox Pumping


14.

Figure 8. Continued. C , iron; and D , manganese.


465

466


a n d manganese concentration before stage rise (Figures 8 C a n d 8 D ) . S o m e manganese oxides can also c o n t r o l the oxidation state of arsenic, o x i d i z i n g it f r o m III to V (24-26). I n d e e p w e l l s i n the reservoir the d o m i n a n t arsenic species is usually As(III), a n d total arsenic is strongly c o n t r o l l e d b y As(III) ( r = 0.910). H o w e v e r , shortly after the reservoir was f i l l e d , w e l l s at several sites s h o w e d t e m p o r a r y increases i n As(V) : As(III) ratios. T h i s response p r o b ably results f r o m the oxidation of arsenate i n the oxic zone a n d transport of As(V) i n t o the g r o u n d w a t e r system as anoxic g r o u n d w a t e r rose into the oxic zone. A l l these reactions are representative of the oxic e n v i r o n m e n t d e s c r i b e d b y B e r n e r (13), w i t h one m a i n difference. L a r g e amounts of organic matter r e m a i n to p o w e r anoxic reactions w h e n the w a t e r table rises. Because of the f l u c t u a t i n g reservoir stage, organic material does not o x i d i z e c o m p l e t e l y . T h u s , it remains to p o w e r anoxic reactions w h e n the stage rises once again. A s the stage rises, the oxic reactions d e s c r i b e d are r e p l a c e d b y anoxic reactions as oxygen is c o n s u m e d . I n the u p p e r levels of the r e d u c i n g g r o u n d water system, w h e r e sulfate is available f r o m the oxic reactions, anoxic s u l f i d i c reactions d o m i n a t e (12, 13). A t greater depths, w h e r e sulfate is u s e d u p i n r e d u c t i o n reactions, a transition occurs t h r o u g h postoxic to sulfidic m e t h a n i c e n v i r o n m e n t s (12, 13). I n one d e e p w e l l m e t h a n e was p r o d u c e d i n h i g h e n o u g h quantities to b l o w off the s l i p - o n polyvinyl chloride) w e l l cap.


2

A s the water table rises w i t h increasing r e s e r v o i r stage, these processes c o n t r o l the u p w a r d shift of the redox zonation; anoxic reactions reestablish themselves i n the once-oxic zone. Several m a i n processes are i m p o r t a n t for m o b i l i z i n g contaminants d u r i n g this transition. M a n g a n e s e oxyhydroxides f o r m e d i n the vadose zone u n d e r g o r e d u c t i o n b y organic r e d u c t i o n s i m i l a r to the general e q 5 (21). (CH O) 2

1 0 6

(NH ) (H PO ) + 236Mn0 3

1 6

3

4

366CaC0

3

+ 366Ca

2

+ 236Mn

2 +

+ 8N

+ 260HCCV

2 +

2

+ H P0 3

4

•

+ 260H O

(5)

2

S i m i l a r reactions o c c u r w i t h i r o n o x y h y d r o x i d e s (21). (CH O) 2

1 0 6

(NH ) (H PO ) + 424FeOOH + 758Ca 3

1 6

3

758CaC0

4

3

+ 424Fe

2 +

2 +

+ 652HC0 ~

+ 16NH " + H P0 4

»

3

3

4

+ 636H 0 2

(6)

T h e s e processes are catalyzed b y bacteria a n d p r o b a b l y i n v o l v e b o t h inorganic a n d organic i r o n a n d manganese species (22), T h e y m a y also b e strongly c o n t r o l l e d b y m i c r o b i a l c o m p e t i t i o n b e t w e e n Fe(III) a n d sulfater e d u c i n g bacteria (27). Associated w i t h these r e d u c t i o n reactions is the red u c t i o n of residual sulfate ( p r o d u c e d i n the oxic zone b y bacterially catalyzed reactions) s i m i l a r to e q 7 (21).


14.

MOORE

(CH O) 2

1 0 6


(NH ) (H PO ) + 53S0 " 3

1 6

3

4

4

106CO

2

467

Resulting from Redox Pumping >

2

+ 16NH

3

+ 53S " + H P 0 2

3

+ 106H O

5

2

(7)

T h e s e r e d u c t i o n reactions result i n free m e t a l ions a n d sulfide ions that f o r m diagenetic m e t a l sulfide phases (10) v i a bacterially m e d i a t e d reactions s i m i l a r to e q 8 (16).


Fe

2 +

+ H S

> FeS + 2H+

2

(8)

F e r r o u s i r o n i n this reaction can b e r e p l a c e d b y any o f the m e t a l ions f r e e d i n t h e oxic reactions to f o r m a n u m b e r o f m e t a l sulfides. T h i s system is h i g h l y d e p e n d e n t o n t h e availability o f sulfate. W h e n sulfate is exhausted b y p r e c i p i t a t i n g m e t a l sulfides, processes m o v e i n t o t h e anoxic m e t h a n i c state (13, 27). M e t h a n e p r o d u c t i o n is strongly bacterially m e d i a t e d f o l l o w i n g a general reaction s i m i l a r to e q 9 (21, 28). (CH O) 2

1 0 6

(NH ) (H PO ) 3

1 6

3

4

>53C0

2

+ 53CH

4

+ 16NH

3

+ H P0 3

4

(9)

T h e presence o f these reactions i n M i l l t o w n sediments is i n d i c a t e d b y the v e r t i c a l trends i n g r o u n d w a t e r c h e m i s t r y . E v e n d u r i n g times o f recent stage rise, w h e n t h e sulfate concentrations are highest i n water-table w e l l s , sulfate concentrations decrease r a p i d l y w i t h d e p t h ( F i g u r e 8 A ) . S o m e d e e p e r w e l l s m a i n t a i n relatively constant c h e m i c a l c o m p o s i t i o n for i r o n a n d arsenic, b u t show fluctuations i n response to stage rise. O t h e r w e l l s show that e v e n at t h e deepest levels sulfate is transported into t h e g r o u n d w a t e r as t h e stage rises. T h e concentration o f s o d i u m ( F i g u r e 9 D ) , p r e s u m a b l y a conservative tracer, shows little response to stage rise o r d e p t h . A p p a r e n t l y g r o u n d w a t e r flow does n o t affect concentration, b u t changes i n nonconservative species are t h e result o f r a p i d diagenetic reactions. O n c e t h e h i g h stage has s t a b i l i z e d , t h e anoxic e n v i r o n m e n t is r a p i d l y reestablished at t h e h i g h e r elevations i n t h e s e d i m e n t a r y c o l u m n . W h e n t h e r e s e r v o i r is l o w e r e d again, t h e process is repeated. T h i s fluctuation o c c u r r e d over a p p r o x i m a t e l y 2 - 3 m w i t h t h e stage changes seen i n 1986. D u r i n g e v e r y transition, metals b o u n d to sulfides a n d organic substances are released i n the oxic vadose zone. T h e strong d o w n w a r d g r o u n d w a t e r flow gradient transports m o b i l i z e d metals a n d arsenic i n t o t h e u n d e r l y i n g anoxic e n v i r o n ments, w h e r e they are r e p r e c i p i t a t e d as sulfides u n t i l sulfate is c o n s u m e d . W h e n t h e sulfate is gone, m e t h a n i c r e d u c t i o n takes o v e r . E l e m e n t s that w e r e not efficiently r e m o v e d i n t h e sulfidic zone (i.e., d i d n o t react q u i c k l y enough) are free to m o v e into t h e u n d e r l y i n g a l l u v i a l aquifer. C h e m i s t r y o f w e l l s adjacent to M i l l t o w n R e s e r v o i r show that these elements are arsenic, i r o n , a n d manganese. A l t h o u g h this system is n o w h e r e near e q u i l i b r i u m , c o n t r o l o f what escapes t h e sulfidic zone seems related to t h e s o l u b i l i t y




468

Figure 9. Bubble diagram of concentrations (milligrams per liter) of chemical components and pH in well 138 (Figure 3) showing changes with time in wells of different depths; the upper scale bar (bubble diameter) is the concentration of the species as depicted by bubble, the vertical axis is the elevation of the screened interval of the well, and the horizontal axis is the days since drawdown: A, bicarbonate; and B, calcium.


MOORE

Contaminant Mobilization Resulting from Redox Pumping


14.

Figure 9. Continued. C, pH; and D , sodium.


469

470


p r o d u c t s of the diagenetic sulfides i n v o l v e d a n d the b a c t e r i a l m e d i a t i o n rates of the reactions (10). T h e s o l u b i l i t y p r o d u c t s ( K ) for n a t u r a l , p o o r l y crys talline, m i x e d c h e m i c a l phases are not c h a r a c t e r i z e d , a n d the kinetics of c o m p l e x , c o m p e t i t i v e m i c r o b i a l reactions are not k n o w n . H o w e v e r , the ther m o d y n a m i c s a n d kinetics of sulfide precipitates a n d o x y h y d r o x i d e coatings p r o b a b l y have a strong c o n t r o l o v e r w h a t elements are most m o b i l e d u r i n g redox p u m p i n g (JO). sp


Conclusions M o b i l i z a t i o n o f contaminants i n m e t a l - c o n t a m i n a t e d reservoirs can b e ex p l a i n e d b y a m o d e l i n w h i c h pore water (i.e., groundwater) c o m p o s i t i o n is c o n t r o l l e d b y diagenetic reactions i n v o l v i n g the successive transition f r o m oxic to anoxic e n v i r o n m e n t s as r e s e r v o i r stage changes seasonally. A l t h o u g h the d i s t r i b u t i o n of processes is not p e r f e c t l y u n i f o r m , w h e n the r e s e r v o i r stage falls a n d r e d u c e d sediments are d r a i n e d , a z o n a t i o n t y p i c a l of d e p t h succession (13) becomes time-successive (12). T h e c h a n g i n g r e s e r v o i r stage acts l i k e a redox p u m p to displace contaminants from the surface s e d i m e n t s into u n d e r l y i n g a n d adjacent sediments a n d a l l u v i a l aquifers. T h e late of this h i g h b u r d e n of contaminants to the g r o u n d w a t e r system at M i l l t o w n Res e r v o i r is not k n o w n . H o w e v e r , solute a n d particulate contaminants are rem o b i l i z e d f r o m the r e s e r v o i r s e d i m e n t d u r i n g d r a w - d o w n events at the r e s e r v o i r a n d a d d e d to the o v e r a l l c o n t a m i n a n t b u r d e n o f the C l a r k F o r k R i v e r system a n d p o s s i b l y to d o w n s t r e a m reservoirs. C o n t a m i n a n t s t r a p p e d i n reservoirs are not necessarily f i x e d , e v e n t h o u g h s e d i m e n t is not r e m o v e d f r o m storage. A f l u c t u a t i n g stage m a y b e all that is n e e d e d to m o b i l i z e contaminants l i k e arsenic i n t o the adjacent g r o u n d w a t e r a n d e v e n t u a l l y into the d o w n s t r e a m surface-water system. T h e redox p u m p tends to cleanse the surface s e d i m e n t of contaminants as a result of a strong d o w n w a r d flow gradient that is n e e d e d to p o w e r the p u m p . I n t i m e , c o n t a m i n a t e d surface s e d i m e n t s h o u l d e q u i l i b r a t e w i t h the surfacew a t e r system m o v i n g contaminants out of reach o f surface processes a n d aquatic organisms, a s s u m i n g that the source of contaminants to the r e s e r v o i r is stopped. I n any case, the d y n a m i c , seasonal aspects o f c o n t a m i n a t e d res ervoirs must be taken into account i n any l o n g - t e r m m o n i t o r i n g or successful remediation program.

References 1. Moore, J. N.; Luoma, S. N. Environ. Sci. Technol. 1990, 24(9), 1278-1285. 2. Lang, W. L. In The Last Best Place; Kitteredge, W.; Smith Α., E d s . ; Mont. Hist. Soc. Press, 1988; p 130. 3. Johns, C.; Moore, J. N. Trace Metals in Reservoir Sediments of the Lower Clark Fork River, Montana; Montana Water Resources Research Center: Bozeman, MT, 1986.


14.


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MOORE

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from

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4. Andrews, E. D. In Chemical Quality of Water and the Hydrologic Cycle; Averett, R. C.; McKnight, D. M., Eds; Lewis: Chelsea, MI, 1987. 5. Brook, E. J.; Moore, J. N. Sci. Total Environ. 1988, 76, 247-266. 6. Moore, J. N.; Brook, E. J.; Johns, C. Environ. Geol. Water Sci. 1989, 14(1), 107-115. 7. Axtmann, E.; Luoma S. N. Appl. Geochem. 1990, 6, 75-88. 8. Johns, C.; Moore, J. N. In Proceedings, Clark Fork River Symposium; Carlson, E.; Bahls, L. L., Eds; Montana Academy of Science: Butte, MT, 1985; p 74. 9. Woessner, W.; Moore, J.; Johns, C.; PopofF, M.; Sartor, L.; Sullivan, M. Arsenic Source and Water Supply Remedial Action Study, Milltown, Montana; Solid Waste Bureau, Montana Department of Health and Environmental Science: Helena, MT, 1984. 10. Moore, J. N.; Ficklin, W. H.; Johns, C. Environ. Sci. Technol. 1988, 22(4), 432-437. 11. Bailey, A. K. Proc. Mont. Acad. Sci. 1976, 36, 165-170. 12. Udaloy, A. G. M.S. Thesis, University of Montana, Missoula, M T , 1988. 13. Berner, R. A. J. Sediment. Petrol. 1981, 51(2), 359-365. 14. Singer, P. C.; Stumm, W. Science 1970, 167, 1121-1123. 15. Nordstrom, D. K. Acid Sulfate Weathering (Special Publication N o . 10); Soil Science Society of America: Madison, W I , 1982; p 37. 16. Jørgensen, Β. B. In Microbial Geochemistry; Krumbein, W. E., Ed.; Blackwell Scientific: Boston, MA, 1983; pp 91-124. 17. Moses, C. O.; Nordstrom, D. K.; Herman, J. S.; Mills, A. L. Geochem. Cos mochim. Acta 1987, 51, 1561-1571. 18. Nimick, D. A.; Moore, J. N. Appl. Geochem. 1991, 6, 635-646. 19. Ludgren, D. G.; Stark, M. Ann. Rev. Microbiol. 1980, 34, 263-283. 20. Arkesteyn, G. J. Antonie van Leeuwenhoek 1979, 45, 423-435. 21. Froelich, P. N.; Klinkhamer, G. P.; Bender, M. L.; Luedtke, Ν. Α.; Heath, G. R.; Cullen, D.; Dauphin, P.; Hammond, D.; Hartman, B.; Maynard, V. Geochim. Cosmochim. Acta 1979, 43, 1075-1090. 22. Nealson, Κ. H. In Microbial Geochemistry; Krumbein, W. E., Ed.; Blackwell Scientific: Boston, MA, 1983; pp 191-221. 23. Peterson, M. L.; Carpenter, R. Mar. Chem. 1983, 12, 295-321. 24. Oscarson, D.; Huang, P.; Liaw, W. J. Environ. Qual. 1980, 9(4), 700-703. 25. Oscarson, D.; Huang, P.; Liaw, W. Clays Clay Miner. 1981, 29(3), 219-225 26. Moore, J. N.; Walker, J. R.; Hayse, T. H. Clays Clay Miner. 1990, 38(5), 549-555. 27. Chapelle, F. H.; Lovely, D. R. Ground Water 1992, 30(1), 29-36. 28. Krumbein, W. Ε.; Swart, P. K. In Microbial Geochemistry; Krumbein, W. E., E d . ; Blackwell Scientific: Boston, MA, 1983; pp 5-62. RECEIVED 1992.

for review September 26, 1991.

ACCEPTED

revised manuscript A p r i l 7,


Contaminant Mobilization Resulting from Redox Pumping in a Metal

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