Cycling of Mercury across the Sediment-Water Interface in Seepage

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 14, 2016 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0237.ch013

13 Cycling of Mercury across the Sediment-Water Interface in Seepage Lakes James P. Hurley , David P. Krabbenhoft , Christopher L . Babiarz , and Anders W. Andren 1,2

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1Bureau of Research, Wisconsin Department of Natural Resources, 1350 Femrite Drive, Monona, WI 53716 Water Chemistry Program, 660 North Park Street, University of Wisconsin, Madison, WI 53706 U . S . Geological Survey, 6417 Normandy Lane, Madison, WI 53719

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The magnitude and direction of Hg fluxes across the sediment-water interface were estimated by groundwater, dry bulk sediment, sediment pore water, sediment trap, and water-column analyses in two northern Wisconsin seepage lakes. Little Rock Lake (Treatment Basin) received no groundwater discharge during the study period (1988-1990), and Pallette Lake received continuous groundwater discharge. In Little Rock Lake, settling of particulate matter accounted for the major Hg delivery mechanism to the sediment-water interface. Upward diffusion of Hg from sediment pore waters below 2-4-cm sediment depth was apparently a minor source during summer stratification. Time-series comparisons suggested that the observed buildup of Hg in the hypolimnion of Little Rock Lake was attributable to dissolution and diffusion of Hg from recently fallen particulate matter close to the sediment-water interface. Groundwater inflow represented an important source of new Hg, and groundwater outflow accounted for significant removal of Hg from Pallette Lake. Equilibrium speciation calculations revealed that association of Hg with organic matter may control solubility in well-oxygenated waters, whereas in anoxic environments sulfur (polysulfide and bisulfide) complexation governs dissolved total Hg levels.

SOLUTE EXCHANGE across the s e d i m e n t - w a t e r interface serves as a n i m -

portant process i n regulating w a t e r - c o l u m n concentrations o f metals i n nat0065-2393/94/0237-0425$07.25/0 © 1994 American Chemical Society

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

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u r a l waters (1-3). Studies of solid-phase b u l k sediments f r o m lakes i n W i s c o n s i n a n d M i n n e s o t a (4-7) s h o w e d increases i n H g concentrations near the top o f l a k e - s e d i m e n t cores, a n d these increases w e r e a t t r i b u t e d to changes i n a t m o s p h e r i c i n p u t s f o l l o w i n g i n d u s t r i a l i z a t i o n . H o w e v e r , e s t i m a t i n g the a m o u n t of H g r e m i n e r a l i z e d after d e p o s i t i o n at the s e d i m e n t surface has b e e n a difficult task. T h e lack of reliable data o n transport of H g across the s e d i m e n t - w a t e r interface arises f r o m two factors. F i r s t , c o n t a m i n a t i o n m a y o c c u r d u r i n g s a m p l i n g . C l e a n techniques for trace metals that are s i m i l a r to t e c h n i q u e s d e v e l o p e d for s a m p l i n g of l e a d i n the mid-1970s (described i n reference 8) m u s t b e f o l l o w e d d u r i n g H g s a m p l i n g a n d analysis. T h e p o t e n t i a l for c o n t a m i n a t i o n d u r i n g s a m p l i n g is h i g h because of the s m a l l concentrations of H g present i n lake waters (typically 0 . 5 - 1 0 n g / L ) . S e c o n d , the relative i n s e n s i t i v i t y of p r e v i o u s analytical p r o c e d u r e s m a d e it d i f f i c u l t to adequately quantify concentrations of H g i n lake a n d s e d i m e n t p o r e waters. T h e r e f o r e , reasonable profiles o f d i s s o l v e d H g w e r e d i f f i c u l t to o b t a i n , a n d c a l c u l a t i o n of flux rates across this i m p o r t a n t interface w e r e s i m i l a r l y h a m p e r e d . R e c e n t studies d i r e c t e d at assessing the fate a n d transport o f H g i n natural waters u s e d i m p r o v e d analytical m e t h o d s a n d c l e a n t e c h n i q u e s for s a m p l i n g a n d analysis (9, 10). Some lake studies (11, 12) w e r e d i r e c t e d at assessing the effects of point-source H g i n p u t s , s u c h as c h l o r o a l k a l i m a n u facturing plants a n d m i n i n g operations; o t h e r studies (13-16) w e r e d e v e l o p e d i n response to concerns over recent observations of e l e v a t e d H g levels i n fish f r o m lakes r e m o t e f r o m p o i n t sources. P a r t l y because of this c o n c e r n , the W i s c o n s i n D e p a r t m e n t o f N a t u r a l Resources, i n cooperation w i t h the E l e c t r i c P o w e r R e s e a r c h Institute, i n i tiated an extensive study of H g c y c l i n g i n seepage lakes o f n o r t h - c e n t r a l W i s c o n s i n (14). T h e m e r c u r y i n temperate lakes ( M T L ) study u s e d clean s a m p l i n g a n d subnanogram analytical techniques for trace metals (10, 17) to quantify H g i n various lake " c o m p a r t m e n t s " (gaseous phase, d i s s o l v e d lake water, seston, s e d i m e n t , a n d biota) a n d to estimate major H g fluxes (atm o s p h e r i c i n p u t s , v o l a t i l i z a t i o n , i n c o r p o r a t i o n into seston, s e d i m e n t a t i o n , a n d s e d i m e n t release) i n seven seepage lake systems. A p r e l i m i n a r y mass balance r e v e a l e d the f o l l o w i n g i n t e r e s t i n g insights into H g c y c l i n g i n L i t t l e R o c k L a k e (18, 19). 1. A t m o s p h e r i c d e p o s i t i o n was the major external source of H g to the lake. 2. P e r m a n e n t a c c u m u l a t i o n of H g i n the b o t t o m sediments was r o u g h l y balanced b y atmospheric i n p u t s o n an a n n u a l basis. 3. I n p u t f r o m atmospheric d e p o s i t i o n was sufficient to account for a l l of the H g o b s e r v e d i n fish, sediments, a n d water. A l t h o u g h net a c c u m u l a t i o n of H g i n sediments r o u g h l y b a l a n c e d a t m o s p h e r i c i n p u t s , gross s e d i m e n t a t i o n , as m e a s u r e d b y s e d i m e n t a t i o n traps, e x c e e d e d


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net a c c u m u l a t i o n b y a factor o f about 3 . T h i s observation, c o u p l e d w i t h w a t e r - c o l u m n profiles o f H g (20, 21), suggested strong r e c y c l i n g o f H g i n the r e g i o n of the s e d i m e n t - w a t e r interface. R e s e a r c h efforts w e r e therefore d i r e c t e d t o w a r d assessing factors that c o n t r o l H g c y c l i n g near this i m p o r t a n t interface. T h i s chapter summarizes o u r results f r o m t w o n o r t h e r n W i s c o n s i n seep age lakes that w e r e chosen to assess t h e i m p o r t a n c e o f various processes c o n t r o l l i n g transport o f H g across t h e s e d i m e n t - w a t e r interface. T o t a l H g ( H g ) concentrations w e r e d e t e r m i n e d as a f u n c t i o n o f d e p t h i n t h e s o l i d and d i s s o l v e d phases o f t h e water c o l u m n , a n d i n l i t t o r a l a n d p r o f u n d a l sediments. N e w s a m p l i n g a n d analytical p r o c e d u r e s a l l o w e d for the d e t e c t i o n of l o w (picogram) levels o f H g . M e a s u r e m e n t s o b t a i n e d i n this phase o f the study together w i t h those o b t a i n e d f r o m p r e v i o u s l y p u b l i s h e d data o n these lakes w e r e u s e d to make a p r e l i m i n a r y examination of the relative i m p o r t a n c e o f factors i n f l u e n c i n g H g c y c l i n g at t h e s e d i m e n t - w a t e r interface. x

Site Description T w o lakes chosen for this study, L i t t l e R o c k L a k e T r e a t m e n t B a s i n a n d Pallette L a k e , are located i n t h e N o r t h e r n H i g h l a n d s L a k e D i s t r i c t o f W i s c o n s i n . L i t t l e R o c k L a k e (45°50' Ν, 8 9 ° 4 2 ' W ) a n d P a l l e t t e L a k e ( 4 6 ° 0 4 ' Ν, 8 9 ° 3 6 ' W ) are soft-water seepage lakes, d e r i v i n g water f r o m o n l y a t m o s p h e r i c and g r o u n d w a t e r sources. B o t h lakes are r e m o t e f r o m any p o i n t sources o f H g . A l t h o u g h e x p e r i m e n t a l acidification o f one o f the t w o basins o f L i t t l e R o c k L a k e offers a c o m p a r i s o n of the effects of acidification (22, 23), w e w i l l l i m i t o u r H g discussion to t h e treatment p o r t i o n o f t h e lake. T h i s b a s i n , u n l i k e t h e Reference B a s i n , is d e e p e n o u g h to e x h i b i t strong h y p o l i m n e t i c oxygen d e p l e t i o n a n d conditions m o r e c o n d u c i v e to s t u d y i n g t h e release o f r e d o x - c o n t r o l l e d constituents f r o m p r o f u n d a l sediments. F o r t h e r e m a i n d e r of this chapter, w e w i l l refer to t h e T r e a t m e n t B a s i n as L i t t l e R o c k L a k e . A major aspect o f this study was assessment o f the role o f g r o u n d w a t e r transport i n t h e o v e r a l l H g cycle. H o w e v e r , d u r i n g t h e study p e r i o d (1988-1990) L i t t l e R o c k L a k e was m o u n d e d (no g r o u n d w a t e r inflow), b u t Pallette L a k e h a d b o t h g r o u n d w a t e r i n f l o w a n d o u t f l o w . T h e r e f o r e , f o r t h e purposes o f evaluating t h e i m p o r t a n c e o f g r o u n d w a t e r i n f l o w a n d o u t f l o w on H g transport, w e e x t e n d e d o u r study to Pallette L a k e . Water Column. W a t e r - c o l u m n profiles w e r e taken at t h e deepest location (10 m) i n L i t t l e R o c k L a k e . D e t a i l s of the clean s a m p l i n g t e c h n i q u e s (8) that w e r e u s e d d u r i n g s a m p l i n g are g i v e n e l s e w h e r e (18, 20, 21). B y f o l l o w i n g these stringent protocols, o u r g r o u p d e m o n s t r a t e d (18) that t y p i c a l e p i l i m n e t i c H g levels i n seven n o r t h e r n W i s c o n s i n lakes, i n c l u d i n g Pallette and L i t t l e R o c k L a k e ( 0 . 5 - 2 n g o f H g / L for u n f i l t e r e d e p i l i m n e t i c samples), w e r e o f m a g n i t u d e s i m i l a r to those levels o b s e r v e d i n r e m o t e ocean sites


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(24). P r e v i o u s estimates o f u n f i l t e r e d total H g i n n o r t h e r n W i s c o n s i n lakes w e r e 2 orders o f m a g n i t u d e h i g h e r than o u r o b s e r v e d levels (18). O u r w a t e r - c o l u m n s a m p l i n g techniques i n c l u d e i n - l i n e filtration u s i n g an all-Teflon s a m p l i n g d e v i c e w i t h quartz f i b e r filters (0.7-μιτη n o m i n a l size cutoff) to differentiate b e t w e e n d i s s o l v e d a n d particulate phases (21). P a r ticulate concentrations (nanograms p e r gram) a n d subsequent calculations of p a r t i t i o n i n g b e t w e e n particle a n d aqueous phases (log Κ ) are based o n this particle size d i v i s i o n . T h i s fractionation scheme p r e c l u d e s d i r e c t esti mates o f c o l l o i d a l influences o n H g transport. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 14, 2016 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0237.ch013

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Sedimentation Traps. S e d i m e n t traps (25) w e r e installed i n t h e h y p o l i m n i o n o f L i t t l e R o c k L a k e to estimate t h e d o w n w a r d flux o f H g to t h e s e d i m e n t surface. Traps w e r e c o n s t r u c t e d o f a c r y l i c a n d T e f l o n f o l l o w i n g t h e d e s i g n o f Shafer (26). N o m e t a l c o m p o n e n t s w e r e u s e d to a v o i d possible c o n t a m i n a t i o n artifacts. Traps w e r e p l a c e d at 9 m at t h e 10-m-deep h o l e s a m p l i n g site. Traps w e r e s u s p e n d e d f r o m surface floats to p r e v e n t d i s t u r b i n g b o t t o m sediments d u r i n g d e p l o y m e n t a n d r e t r i e v a l . Groundwater. Because sandy l i t t o r a l sediments have greater h y d r a u l i c c o n d u c t i v i t y than silty p r o f u n d a l sediments (27), most of the exchange of w a t e r (and solutes) b e t w e e n g r o u n d w a t e r systems a n d lakes occurs t h r o u g h the l i t t o r a l zone (28). T h e r e f o r e , efforts a i m e d at q u a n t i f y i n g H g transfer b e t w e e n lakes a n d t h e i r contiguous g r o u n d w a t e r systems w e r e focused i n near-shore areas. N u m e r o u s g r o u n d w a t e r s a m p l i n g m e t h o d s a l l o w e d for s a m p l i n g o f different features i n the g r o u n d w a t e r system near the study lake ( F i g u r e 1). T h e m e t h o d s u s e d (piezometers, w e l l s d u g to t h e water table,

Figure 1. Schematic diagram of the sainpling methods used to acquire ground water samples near the aquifer—lake interface.


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acrylic tubes i n s e r t e d into littoral sediments, a n d pore-water extraction f r o m s e d i m e n t cores) a l l o w e d c o m p a r i s o n o f several aspects o f H g c y c l i n g as g r o u n d w a t e r discharges to t h e lake. T h e most c o m m o n m e t h o d for s a m p l i n g c h e m i c a l constituents i n g r o u n d water, p i e z o m e t e r s a m p l i n g (29-31), was u s e d i n t h e i n i t i a l stages. P r e c l e a n e d acrylic piezometers w i t h 0 . 3 - m T e f l o n screens w e r e i n s t a l l e d b y p o w e r a u g u r i n g (hollow-stem auger). A t h r e a d e d f i t t i n g u s e d to j o i n t h e screen a n d casing p r e v e n t e d potential contamination f r o m solvents i n p i e z o m e t e r c o n s t r u c t i o n . W e l l s w e r e nested (several w e l l s at one location w i t h differing depths), w i t h water-table w e l l s at about 3 m a n d d e e p e r w e l l s at 5-15 m . D u g w e l l s w e r e f o r m e d b y t r e n c h i n g a s m a l l hole (about 0.3 m square a n d less than 0.5 m d e e p , 10 c m b e l o w t h e w a t e r table) w i t h a p r e c l e a n e d plastic s h o v e l . N e w w e l l s w e r e d u g for each s a m p l i n g p e r i o d . T h e w e l l s w e r e located w i t h i n 5 m of the shoreline, a n d samples f r o m these w e l l s w e r e u s e d to estimate the b a c k g r o u n d H g content of i n f l o w i n g g r o u n d w a t e r . A f t e r the w e l l s w e r e p u m p e d for about 45 m i n (about t w o to t h r e e t r e n c h volumes), samples w e r e taken w i t h a peristaltic p u m p a n d T e f l o n l i n e . P u r g i n g r e d u c e d the effects o f possible contamination a n d particle suspension d u r i n g w e l l d i g g i n g a n d p r o v i d e d a short h y d r a u l i c residence t i m e i n t h e w e l l p r i o r to sampling. T h e tube m e t h o d i n v o l v e d i n s e r t i o n o f p r e c l e a n e d a c r y l i c tubes (5-cm diameter) into t h e littoral zone sediments at a lake-water d e p t h o f about 1 m (about 15 c m into sediments). L a k e w a t e r w i t h i n t h e t u b e was r e m o v e d b y u s i n g a peristaltic p u m p a n d T e f l o n l i n e . O n c e t h e t u b e was p u r g e d o f lake water, g r o u n d w a t e r was a l l o w e d to fill it to a d e p t h o f about 25 c m . T h e tube was t h e n p u r g e d three times w i t h g r o u n d w a t e r before o b t a i n i n g the sample. Pore-water samples w e r e o b t a i n e d f r o m littoral sediments b y p u s h c o r i n g w i t h p r e c l e a n e d acrylic core barrels (6.7-cm i . d . , 7.6-cm o.d.). A d e v i c e that eliminates a i r contact w h i l e s a m p l i n g (32) was u s e d for p o r e water extraction. T e f l o n p l u n g e r s at either e n d o f the barrels w e r e f o r c e d t o w a r d each other to pressurize t h e b a r r e l . Interstitial w a t e r flowed o u t o f s a m p l i n g ports (2-cm intervals) i n response to the external pressure. Samples w e r e t h e n f i l t e r e d ( 0 . 4 - μ ι η filter; N u c l e p o r e ) a n d p r e s e r v e d w i t h 6 N H C 1 before H g analysis. L a b o r a t o r y studies i n d i c a t e d no c o n t r i b u t i o n o f H g f r o m the f i l t e r i n g u n i t , t u b i n g , o r sample bottle. Because t h e f i l t e r i n g system r e m o v e d interstitial water f r o m the c e n t e r of the core ( > 2 c m f r o m the wall), l o w diffusion constants l e d to n e g l i g i b l e c o n t a m i n a t i o n f r o m t h e core b a r r e l . Solid-phase materials (sands) i n t h e l i t t o r a l zones w e r e n o t a n a l y z e d for particulate H g . Profundal Sediments. S e d i m e n t cores w e r e c o l l e c t e d i n p r e c l e a n e d acrylic tubes b y scuba divers f o l l o w i n g s i m i l a r clean s a m p l i n g p r o c e d u r e s



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for trace metals i n the littoral-zone s e d i m e n t s a m p l i n g . Benefits o f core s a m p l i n g b y scuba d i v i n g o v e r other t r a d i t i o n a l m e t h o d s (such as J e n k i n s c o r i n g a n d p i s t o n coring) i n c l u d e d careful selection o f the s a m p l i n g site a n d the a b i l i t y to observe w h e t h e r m i x i n g or disturbance of the core o c c u r r e d d u r i n g s a m p l i n g . C o r e s w e r e taken w i t h m i n i m a l surface d i s r u p t i o n a n d processed w i t h i n 2 - 4 h of s a m p l i n g . P r o f u n d a l pore waters w e r e s a m p l e d i n a m e t h o d s i m i l a r to that u s e d w i t h littoral p o r e waters. Solid-phase samples w e r e taken f r o m separate cores, w h i c h w e r e sectioned at 1-cm intervals to a d e p t h o f 30 c m . C a r e was taken to d i s c a r d s e d i m e n t i n contact w i t h the core b a r r e l , i n case s m e a r i n g o c c u r r e d d u r i n g extrusion a n d slicing. Because o r g a n i c - r i c h p r o f u n d a l sediments p r e v e n t g r o u n d w a t e r i n f l o w , o t h e r sam p l i n g m e t h o d s u s e d for g r o u n d w a t e r s a m p l i n g i n littoral zones (such as tubes a n d piezometers) w e r e not n e e d e d i n p r o f u n d a l zones. Various ancillary measurements f r o m a c c o m p a n y i n g cores w e r e n e e d e d to calculate a c c u m u l a t i o n rates a n d describe phase associations of H g . Pb and C s profiles i n sediments w e r e u s e d to d e t e r m i n e s e d i m e n t a t i o n rates f r o m w h i c h historical interpretations c o u l d b e m a d e . 2 1 ( )

1 3 7

Laboratory Methods L o w - l e v e l (picogram) H g analysis r e q u i r e d p r e c o n c e n t r a t i o n b y two-stage g o l d amalgamation, f o l l o w e d b y d e t e c t i o n w i t h a c o l d - v a p o r atomic fluores cence d e t e c t i o n system (9, 33). B r i e f l y , aqueous samples are treated w i t h a strong o x i d i z i n g agent ( B r C l ) to destroy o r g a n o - H g b o n d s a n d c o n v e r t a l l H g i n t o the soluble Hg(II) state. Stannous c h l o r i d e is a d d e d to r e d u c e Hg(II) to the e l e m e n t a l (Hg°) state. T h i s volatile f o r m is s t r i p p e d f r o m s o l u t i o n b y n i t r o g e n onto a gold-coated sand trap. T h e H g is t h e n t h e r m a l l y d e s o r b e d onto a second g o l d trap, a n d f r o m this trap into the a t o m i c fluorescence c e l l . Solid-phase s e d i m e n t (about 1 g) r e q u i r e d i n i t i a l d i g e s t i o n i n 5 : 2 H N 0 - H S 0 . I n this study no d i s t i n c t i o n was m a d e b e t w e e n total a n d methyl H g . 3

2

4

Solid-phase s e d i m e n t digestions w e r e a n a l y z e d i n t r i p l i c a t e , w i t h one d u p l i c a t e d i g e s t i o n a n d a spike r e c o v e r y or analysis of reference m a t e r i a l e v e r y 10 samples. Coefficients o f v a r i a t i o n ( C . V . ) for triplicates f e l l w i t h i n 0 . 5 - 1 1 . 5 % [n = 60, m e a n C . V . = 3 . 6 % ± 2 . 4 % (std)], a n d spike recoveries w e r e w i t h i n 9 0 - 1 0 3 % [n = 3, m e a n = 9 6 % ± 7%]. E i g h t replicates o f standard reference m a t e r i a l [ N a t i o n a l Institute of Standards a n d T e c h n o l ogy ( N I S T ) Tennessee R i v e r s e d i m e n t , C a t a l o g N o . 8406] w e r e w i t h i n 10% (0.053 ± 0.004 μg/g, C . V . = 7.2%) of the r e c o m m e n d e d value o f 0.06 μg/g. A standard reference for H g i n n a t u r a l water was not available. T y p i c a l duplicates of s m a l l - v o l u m e p o r e waters ( Ζ ο se κ C/Ï m »

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Cycling of Mercury across the Sediment-Water Interface in Seepage

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