Atmospheric Mercury Deposition to Lakes and ... - ACS Publications

2 Atmospheric Mercury Deposition to Lakes and Watersheds

Downloaded by FUDAN UNIV on March 2, 2017 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0237.ch002

A Quantitative Reconstruction from Multiple Sediment Cores Daniel R. Engstrom , Edward B. Swain , Thomas A. Henning , Mark E . Brigham , and Patrick L . Brezonik 1

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Limnological Research Center, University of Minnesota, Minneapolis, M N 55455 Minnesota Pollution Control Agency, St. Paul, M N 55155 3Department of Civil and Mineral Engineering, University of Minnesota, Minneapolis, M N 55455 1

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Historic increases in atmospheric mercury loadings caused by anthropogenic emissions are documented from sediment cores from seven remote headwater lakes in Minnesota and northern Wisconsin. Whole-basin changes in Hg accumulation, determined from lakewide arrays of Pb-dated cores, show that regional atmospheric Hg deposition has increased by a factor of 3.7 since preindustrial times. The relative increase is consistent among lakes, although preindustrial Hg accumulation rates range from 4.5 to 9 µg/m per year, and modern rates range from 16 to 32 µg/m per year. The distribution of these rates is highly correlated with the ratio of catchment area to the lake surface area. Modern and preindustrial atmospheric deposition rates of 12.5 and 3.7 µg/m per year are calculated from this relationship, along with the relative contribution of Hg from the terrestrial catchment. Release of atmospheric Hg from catchment soils accounts for 20-40% of the Hg loading to the lakes, depending on 210

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'Current address: James M . Montgomery Consulting Engineers, 545 Indian Mound, Wayzata, M N 55391 Current address: U.S. Geological Survey, District Office, 2280 Woodale Drive, Mounds View, M N 55112 5

0065-2393/94/0237-0033$09.50/0 © 1994 American Chemical Society

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

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ENVIRONMENTAL CHEMISTRY O F L A K E S A N D RESERVOIRS

catchment size. This export represents about 25% of the atmospheric Hg falling on the catchments under both modern and preindustrial conditions. The absence of geographic trends in either Hg deposition rates or their increase implies regional if not global sources for the Hg entering these lakes.


LAKE SEDM I ENTS ARE EXCELLENT ARCHV I ES

forh i s t o r i c changes i n t h e i n d u s t r i a l discharge o f p o t e n t i a l l y toxic metals s u c h as H g , Z n , P b , C u , N i , C d , a n d C o (1-3). M o s t heavy metals have short residence times i n t h e water c o l u m n a n d are quantitatively r e t a i n e d i n t h e sediments, so stratigraphic interpretations are r e l a t i v e l y straightforward (but see refs. 4 a n d 5). S e d i m e n t records have p r o v i d e d c o m p e l l i n g e v i d e n c e for t h e role o f atm o s p h e r i c d e p o s i t i o n i n c o n t a m i n a t i n g aquatic e n v i r o n m e n t s far r e m o v e d f r o m d i r e c t (point-source) i n d u s t r i a l i n f l u e n c e (6, 7). T h e s e studies have b e e n u s e d to d o c u m e n t t h e history o f emissions, t h e effect o f abatement, a n d t h e g e o c h e m i c a l c y c l i n g o f trace metals i n t h e aquatic e n v i r o n m e n t (8-10). I n most cases stratigraphie interpretations are l i m i t e d to d e s c r i p t i o n o f relative changes i n m e t a l l o a d i n g ; m o r e (or less) o f s o m e t h i n g is d e p o s i t e d each year at a g i v e n site o n t h e lake b o t t o m . H i g h e r concentrations o f a m e t a l i n surface sediments relative to d e e p e r (uncontaminated) strata—often c o r r e c t e d for matrix effects b y ratio o f t h e m e t a l to a silicate p r o x y s u c h as T i 0 — r e p r e s e n t t h e relative e n r i c h m e n t attributable to i n d u s t r i a l discharge. W h e r e fine-scale d a t i n g (e.g., P b ) is available, actual d e p o s i t i o n rates m a y b e calculated. B u t because s e d i m e n t d e p o s i t i o n a n d c o m p o s i t i o n are spatially variable, a c c u m u l a t i o n rates at a single core site cannot b e automatically extrapolated to t h e e n t i r e lake b o t t o m (11-14) a n d actual fluxes to t h e lake i n a mass-balance sense cannot b e calculated. 2

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F e w studies have e x p l o i t e d t h e f u l l p o t e n t i a l o f lake sediments to o b t a i n quantitative estimates o f w h o l e - l a k e m e t a l fluxes, despite t h e fact that sedi m e n t s often represent t h e o n l y available r e c o r d o f past d e p o s i t i o n . M o r e o v e r , w h o l e - b a s i n s e d i m e n t r e t e n t i o n m a y b e c r i t i c a l to mass-balance c a l culations w h e r e c o n t e m p o r a r y fluxes are difficult to measure because of t h e i r spatial a n d t e m p o r a l v a r i a b i l i t y (15,16). T h e p r i m a r y l i m i t a t i o n o n o b t a i n i n g w h o l e - b a s i n fluxes is the large effort r e q u i r e d to analyze a n d date m u l t i p l e cores r e p r e s e n t i n g t h e various d e p o s i t i o n a l e n v i r o n m e n t s w i t h i n a single basin. M o s t m u l t i p l e - c o r e studies o f m e t a l p o l l u t i o n have b e e n l i m i t e d to one lake (17) a n d d o not p r o v i d e a regional p i c t u r e o f d e p o s i t i o n rates, transport pathways, o r geographic trends. I n t h e f e w studies w h e r e m u l t i p l e cores w e r e taken f r o m several lakes, total a n t h r o p o g e n i c s e d i m e n t b u r d e n s w e r e calculated w i t h o u t recourse to d a t i n g o r s e d i m e n t a t i o n rates (6, 18), and actual m e t a l fluxes c o u l d n o t b e calculated. I n this study, h o w e v e r , w e demonstrate that i t is feasible to o b t a i n w h o l e basin m e t a l f l u x e s — i n this case H g — f r o m a suite o f lakes w i t h an e c o n o m y


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ENGSTROM ET AL.

Atmospheric Mercury

35

Deposition


of d a t i n g a n d stratigraphie analysis. T h e p r i n c i p a l o u t c o m e o f this i n v e s t i gation is t h e r e c o n s t r u c t i o n of p r e i n d u s t r i a l a n d a n t h r o p o g e n i c H g i n p u t s to a suite o f r e l a t i v e l y u n d i s t u r b e d lake catchments i n M i n n e s o t a a n d n o r t h central W i s c o n s i n . M e r c u r y c o n t a m i n a t i o n has b e e n a serious c o n c e r n for lakes i n this r e g i o n a n d elsewhere for m o r e than 20 years. E l e v a t e d H g levels i n fish from sites r e m o t e f r o m point-source discharge (19-21), together w i t h stratigraphie e v i d e n c e for recent increases i n H g s e d i m e n t a t i o n (7, 2 2 - 2 5 ) , indicate that anthropogenic emissions to t h e atmosphere are to b l a m e . T w o mechanisms for t h e o b s e r v e d increase i n H g i n p u t s have b e e n advanced: 1. increased H g emissions, p r i n c i p a l l y f r o m coal c o m b u s t i o n a n d waste i n c i n e r a t i o n (26), o r 2. changes i n a t m o s p h e r i c c h e m i s t r y (higher levels o f S 0 , ozone, a n d other oxidants) that c o u l d enhance a t m o s p h e r i c r e m o v a l o r p r o m o t e l e a c h i n g f r o m w a t e r s h e d soils (27). 4

Results f r o m this study s h e d l i g h t o n t h e relative i m p o r t a n c e o f these p r o cesses. B y c o m p a r i n g w h o l e - l a k e H g fluxes a m o n g a g r o u p of sites of d i f f e r i n g h y d r o l o g y , w e are able to assess the relative c o n t r i b u t i o n of w a t e r s h e d i n p u t s versus d i r e c t atmospheric d e p o s i t i o n to t h e lake surface. F i n a l l y , t h e calc u l a t e d H g a c c u m u l a t i o n rates p r o v i d e a r e g i o n a l p i c t u r e o f H g l o a d i n g f r o m w h i c h w e are able to d e r i v e a t m o s p h e r i c d e p o s i t i o n rates a n d infer possible geographic sources a n d m e c h a n i s m s for e n h a n c e d d e p o s i t i o n .

Study Sites D u r i n g the course o f the study seven lakes w e r e a n a l y z e d for w h o l e - b a s i n sedimentary H g : an i n i t i a l set of four lakes f r o m t h e S u p e r i o r N a t i o n a l F o r e s t i n northeastern M i n n e s o t a ( T h r u s h , D u n n i g a n , M e a n d e r , a n d Kjostad), t w o additional sites that e x p a n d e d the geographic coverage to central a n d w e s t e r n Minnesota (Cedar and Mountain), and Little Rock Lake i n northern W i s consin, c u r r e n t l y t h e site o f a split-lake acidification e x p e r i m e n t ( F i g u r e 1). T h e four sites i n northeastern M i n n e s o t a are u n d e r l a i n b y P r e c a m b r i a n crystalline b e d r o c k a n d a t h i n v e n e e r of noncalcareous glacial drift. I n n o r t h w e s t e r n W i s c o n s i n ( L i t t l e R o c k Lake) t h e glacial deposits are substantially t h i c k e r , a n d i n w e s t e r n M i n n e s o t a ( C e d a r a n d M o u n t a i n lakes) t h e drift is h i g h l y calcareous. A l l o f the sites except M o u n t a i n L a k e l i e i n m i x e d d e c i d uous-conifer forest. T h i s vegetation b e c o m e s progressively m o r e b o r e a l t o w a r d the M i n n e s o t a - C a n a d i a n b o r d e r . T h e w a t e r s h e d o f M o u n t a i n L a k e is c o v e r e d w i t h a mosaic o f native tall-grass p r a i r i e a n d oak w o o d l a n d . T h e study lakes span a c l i m a t i c gradient that b e c o m e s a p p r e c i a b l y d r i e r t o w a r d t h e southwest. M e a n a n n u a l p r e c i p i t a t i o n ranges f r o m 7 5 - 8 0 c m i n northeastern M i n n e s o t a a n d n o r t h w e s t e r n W i s c o n s i n to 60 c m near C e d a r


36


ENVIRONMENTAL CHEMISTRY O F L A K E S A N D RESERVOIRS

Figure 1. Location of study lakes, bathymétrie maps of their basins, and approximate position of core sites; shaded contours represent nondepositional areas for fine-grained sediment. Key: star, stratigraphically detailed cores; ·, coarse-interval cores; o, cores from nondepositional sites; and •, supplemental Pb-dated cores from Little Rock. Depth contours are given in meters. 2W

L a k e a n d M o u n t a i n L a k e ; evaporative losses e x c e e d p r e c i p i t a t i o n i n t h e west (Cedar a n d M o u n t a i n ) , b u t represent o n l y about 6 0 % o f t h e p r e c i p i t a t i o n falling i n n o r t h e r n W i s c o n s i n ( L i t t l e Rock). P r e c i p i t a t i o n c h e m i s t r y varies along t h e same gradient, w i t h p H increasing f r o m 4 . 6 i n n o r t h e r n W i s c o n s i n to 4.8 i n northeastern M i n n e s o t a a n d 5 . 2 i n w e s t e r n M i n n e s o t a . T h e corr e s p o n d i n g values for w e t sulfate d e p o s i t i o n decrease from 15 kg/ha p e r


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ΕNGSTROM

E T AL.

Atmosphenc Mercury

Deposition

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year i n n o r t h e r n W i s c o n s i n to 5 - 8 kg/ha p e r year i n northeastern a n d westcentral M i n n e s o t a (28, 29). A l l seven study lakes are located i n p r i m a r y watersheds a n d are f e d p r i n c i p a l l y b y g r o u n d w a t e r seepage a n d d i r e c t p r e c i p i t a t i o n to the lake sur face; t w o o f the lakes ( M e a n d e r a n d Kjostad) r e c e i v e drainage f r o m i n t e r m i t t e n t streams a n d possess p e r m a n e n t surface outlets. T h e lakes are r e l a t i v e l y s m a l l ( 7 - 4 0 ha), except for Kjostad (168 ha), a n d s h a l l o w ( Z = 4-16 m, Z = 3 - 7 m) (Table I). W a t e r residence times are 5 - 1 0 years, except for the t w o w e s t e r n sites (~30 years) w h e r e m u c h of the i n f l o w is lost t h r o u g h evaporation. T h e w e s t e r n M i n n e s o t a lakes are d i s t i n c t l y h i g h e r i n d i s s o l v e d solids (e.g., a l k a l i n i t y is 2600 a n d 4200 μ β ς υ ί ν / ^ because o f evaporative concentration a n d drainage f r o m calcareous soils. T h e o t h e r five sites are d i l u t e (alkalinity is 2 5 - 1 8 0 μequiv/L), a n d o n l y K j o s t a d is a p p r e c i a b l y c o l o r e d b y organic acids (30 P t - C o units). m a x


m c a n

A p r i m a r y selection c r i t e r i o n for a l l of these sites was the absence o f significant land-use disturbance i n the catchment, w h i c h m i g h t o t h e r w i s e accelerate soil erosion a n d s e d i m e n t flux d u r i n g this c e n t u r y . F i v e o f the sites ( T h r u s h , D u n n i g a n , K j o s t a d , C e d a r , a n d L i t t l e Rock) w e r e l o g g e d to some extent i n the early 1900s; the forests w e r e a l l o w e d to r e g r o w a n d have not b e e n a p p r e c i a b l y d i s t u r b e d since t h e n . E x c e p t for a f e w s h o r e l i n e cabins o n K j o s t a d a n d one o n C e d a r , n o n e o f the watersheds are p r e s e n t l y i n h a b i t e d . T h e M e a n d e r w a t e r s h e d was partially b u r n e d i n the L i t t l e Sioux fire o f 1971, a n d a s m a l l p o r t i o n of the M o u n t a i n L a k e c a t c h m e n t was f a r m e d for a b r i e f p e r i o d f o l l o w i n g E u r o p e a n settlement. Today the w a t e r s h e d of M o u n t a i n L a k e is c o n t a i n e d e n t i r e l y w i t h i n G l a c i a l L a k e s State P a r k , a n d it is one of the most p r i s t i n e lakes r e m a i n i n g i n the a g r i c u l t u r a l regions of w e s t e r n Minnesota.

Experimental Methods Coring Strategy. Sediment cores were collected w i t h a thin-walled poly carbonate tube fitted w i t h a piston and operated from the lake surface by rigid drive rods (30). This device recovers the very loose uncompacted sediment surface as w e l l as deeper strata without disturbance or displacement (core-short ening, cf. refs. 31 and 32). C o r e sections were extruded vertically from the top of the tube into polypropylene collection jars, transported on ice to the labo ratory, and stored at 4 °C until analysis. Because a large n u m b e r of cores was required for this study, we chose to economize on the n u m b e r of samples for P b and H g analysis by sectioning most of the cores at coarse intervals. Historic trends i n H g deposition were provided by a few cores analyzed i n stratigraphie detail, whereas the coarsely sectioned cores p r o v i d e d the spatial pattern i n H g accumulation across each basin at a few discrete time intervals. The samples from the coarse-interval cores were homogenized and analyzed for H g and P b content i n the same man ner as the detailed cores. D e t a i l e d cores were collected w i t h a 10-cm-diameter corer, and the coarse-interval cores were obtained w i t h a 5-cm-diameter corer. T h e n u m b e r of cores collected from each lake is shown i n Table II. 2 1 0

2 i 0



Cook Lake St. Louis St. Louis Hubbard Pope Vilas

Thrush Dunnigan Meander Kjostad Cedar Mountain Little Rock

MN MN MN MN MN MN WI

State

60 66 70 180 2600 4200 25

Alkalinity (p£quiv/h)

"Catchment area excludes lake surface. ''Calculated from inflow estimates.

County

hake 6.6 32.9 39.6 167.7 39.1 15.7 18.2

Surface Area (ha) 5.3 28.6 28.7 117.6 27.9 12.9 11.5

Deposition Area (ha) 14.5 4.3 7.6 15.6 8.4 4.2 10.3

Maximum Depth (m) 6.9 2.3 4.8 6.5 3.8 2.7 3.5

Mean Depth (m) 4.52 7.42 19.01 108.79 14.76 4.27 6.24

1

Volume (10" m )

23.8 46.0 126.6 985.3 87.9 81.5 34.8

Catchment Area (ha)"

Table I. Lake Location and Certain Morphometries, Hydrologie, and Chemical Characteristics


5.7 4.6 6.1 5.0 34.4 28.5 10.0

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ENGSTROM

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Atmospheric Mercury

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E N V I R O N M E N T A L CHEMISTRY

O F L A K E S

A N D RESERVOIRS

The exact sampling strategy varied somewhat among the study lakes. F o r the four lakes i n northeastern Minnesota, sediment cores were collected from three locations within each hasin (representing deep, shallow, and intermediate water depths) and analyzed at fine intervals (1-2 cm). These detailed sediment profiles guided the collection and sectioning of subsequent cores, w h i c h were extruded i n three coarse intervals (—0-2, 2-45, and 4 5 - 6 0 c m , although the exact intervals varied from lake to lake). T h e topmost samples gave modern H g concentrations, and the bottom intervals provided preindustrial values (>150 years ago for the Midwest). T h e long middle section was used to calculate wholecore P b burdens required for dating. This approach provided sediment ac cumulation rates for only the topmost interval. H o w e v e r , these values were used to calculate H g flux for the lower sections, assuming a constant sediment ac cumulation rate for the entire core. D a t i n g results from the detailed cores showed this to be a valid assumption for these relatively pristine lakes. F o r C e d a r and M o u n t a i n lakes, detailed sediment cores collected from the deeper regions of each basin showed increasing sedimentation rates up-core. Thus a slightly different approach was used to provide additional temporal detail from the coarsely sectioned cores. These subsequent cores were extruded into five intervals 5 - 2 0 c m long so that dates and sediment accumulation rates could be explicitly calculated for the deeper strata. F o r L i t t l e Rock L a k e , a single core from each of its two basins was analyzed and dated i n stratigraphie detail. The remaining cores were analyzed for H g content i n three coarse intervals as described, but none of these profiles was actually dated. Instead the sedimentation rates were inferred from a series of five nearby cores that had been dated by P b for other purposes (16). T h e mean sedimentation rates from dated cores collected at similar depth i n the same basin were used to calculate H g accumulation for each undated profile.


2 1 0

2IW

Analytical M e t h o d s . Loss on Ignition. W e t density, d r y mass density, organic content, and carbonate content were determined by the method of D e a n (33). Volumetric samples (1.0 em ) were d r i e d overnight at 100 °C and ignited at 550 and 1000 °C for 1 h. Mass measurements were made on the wet sample and after each heating on an electronic analytical balance. 3

Lead-210 Dating* Sediment cores were analyzed at appropriate depth intervals for P b to determine age and sediment-accumulation rates for the past 100-150 years. Lead-210 was measured through its granddaughter product P o , w i t h P o added as an internal yield tracer. The polonium isotopes were distilled from 1-10 g of dry sediment at 550 °C following pretreatment w i t h concentrated H C l . They were plated directly (without H N 0 oxidation) onto silver planchets from a 0.5 Ν H C l solution (modified from ref. 34). Activity was measured for 1-5 X 10 s w i t h Si-depleted surface barrier detectors and an a spectroscopy system (Ortec Adcam). Unsupported P b was calculated by sub tracting supported P b (estimated from constant activity at depth) from total activity at each level. Dates and sedimentation rates were determined according to the constant rate of supply (c.r.s.) model (35), with confidence intervals cal culated by first-order error analysis of counting uncertainty (36). 2 1 0

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Mercury. Sediment samples were digested with a strong manganate-persulfate digestion technique. Total H g analyses were ed by cold-vapor atomic absorption spectrophotometry (37). W e t samples (2-8 g) were treated with 10 m L of H S 0 (cone), 5 m L 2

4


acid-perconduct sediment of H N O

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ENGSTROM

Atmospheric Mercury

E T AL.

41

Deposition

(70%), and 2 m L of H C l (36%), and then heated for 2 - 3 h at 8 0 - 1 1 0 °C o n a sand bath. After cooling the samples were treated w i t h 15 m L of saturated K M n 0 ( - 6 % w/v), followed by 5 m L of K S 0 (5% w/v). T h e next day excess oxidant was reduced w i t h 10 m L of hydroxylamine hydrochloride (6% w/v) i n 6% N a C l . T w o hours later the digests were poured into 2 5 0 - m L polypropylene jars and connected to a H g purging system. F i v e milliliters of S n C l (10% w/v i n 2 M H S 0 ) was transferred b y syringe to each jar, the contents of w h i c h was then vigorously stirred for 2 m i n . T h e headspace gas was swept through a d r y i n g tube ( C a C l desiccant) with prepurified carrier-grade N and through a 10-cm absorption cell. Absorbance readings were based o n peak height. Three distilled, deionized reagent blanks ( d - H 0 ) and National Institute of Standards and Technology (NIST) standards ( N B S 1646; estuarine sediment), and two sets of H g standards (four or five different dilutions of 1000-ppm A l p h a H g standard) were digested a n d analyzed each day that samples were r u n . Reagent blanks were quite low i n H g , generally yielding absorbance values ^0.002 above the N baseline. If at least two of the three N I S T standards were not within the certified range of acceptable concentration (63 ± 12 ng/g), the entire sample r u n was redigested and reanalyzed. N o absorbance due to matrix effects—determined periodically b y standard additions—was observed. O u r detection limit ( I U P A C method, ref. 38) for total H g was about 6 n g , and the coefficient of variation on replicate samples was about 8%. 4

2

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Calculations. T o calculate whole-basin H g accumulation, a portion of the lake bottom was assigned to each core according to two methods based on either spatial proximity (polygon method) or lake depth (contour method). I n both approaches core sites were located spatially o n bathymétrie lake-basin maps b y visual approximation to shoreline features and other landmarks. F o r the polygon method the lake map was d i v i d e d into tiles or Theissen polygons, w h i c h were generated b y the perpendicular bisectors between each core site and its nearest neighbors. I n the second method 1-m depth contours were used to subdivide the lake bottom, and core sites were assigned to these bathymétrie regions according to their depths. Nondepositional regions of the lake bottom—shallow areas where finegrained sediments do not accumulate conformably—were excluded from these calculations. T h e nondepositional region of each lake was d e l i m i t e d b y the depth contour above w h i c h silty or sandy sediments were found at or near the m u d surface. Because the exact location of this boundary was uncertain i n some lakes, depth contours 1 m above and below were used to estimate a range of values for the depositional region. T h e area of each polygon or contour was measured from bathymétrie maps on a microcomputer-digitizer. T h e location of coring sites, depositional zones, and the bathymetry of each study lake are shown i n Figure 1. T h e terrestrial drainage basin for each lake was digitized from 1:24,000 U . S . Geological Survey topographical maps. T h e reported values (which exclude lake surface area) represent the average of two estimates, one that includes and one that excludes small wetlands w i t h internal drainage and areas where watershed boundaries were uncertain.

Results Sediment Lithology. T h e organic content of sediments at most c o r i n g sites, b e t w e e n 20 a n d 6 0 % o f the d r y mass, is t y p i c a l for s m a l l n o r t h - t e m -


42

E N V I R O N M E N T A L

CHEMISTRY

O F LAKES

A N D RESERVOIRS

perate lakes i n forested catchments. D u n n i g a n , C e d a r , a n d L i t t l e R o c k s e d i ments are consistently above 5 0 % organic matter, whereas M e a n d e r , T h r u s h , Kjostad, a n d M o u n t a i n are consistently less organic (20-40%). C a r b o n a t e , w h i c h accounts for 3 0 - 5 0 % o f M o u n t a i n L a k e sediments a n d 5 - 1 0 % o f most C e d a r L a k e cores, is v i r t u a l l y absent i n t h e other five lakes ( F i g u r e 2).


I n most o f the study lakes organic content is p o o r l y c o r r e l a t e d w i t h lake d e p t h , a feature that may b e partially a t t r i b u t e d to the fact that f i n e - g r a i n e d sediments are o n l y w e a k l y focused i n s m a l l shallow lakes (6, 18). I n d e e d , sediments do b e c o m e somewhat m o r e organic w i t h d e p t h i n t h e two deepest lakes, T h r u s h a n d Kjostad. T h e p e r c e p t i o n o f u n i f o r m s e d i m e n t c o m p o s i t i o n also results f r o m s a m p l i n g bias, because f e w littoral cores w e r e actually retained for analysis. V i s i b l y inorganic sediments w e r e assumed to r e p r e s e n t n o n d e p o s i t i o n a l sites for purposes of calculating a H g flux a n d w e r e s i m p l y mapped and discarded. F o r t h e most part, sediments are also stratigraphically u n i f o r m , s h o w i n g o n l y a f e w percentage variation i n lithologie c o m p o s i t i o n . C o r e s f r o m M o u n tain L a k e , w h i c h consistently show up-core decreases i n carbonate content (to about 6 0 % that at depth), are t h e o n l y exception. A n u m b e r o f shallowwater cores that contain a t h i n v e n e e r o f o r g a n i c - r i c h sediments o v e r l y i n g silt a n d sand w e r e also e x c l u d e d f r o m analysis. I n most locations t h e spatial b o u n d a r y b e t w e e n o r g a n i c - r i c h p r o f u n d a l - t y p e sediments a n d littoral d e posits o f coarse detritus or massive silt was clearly d e f i n e d .

Dating and Sediment Accumulation. Stratigraphie Patterns. L e a d - 2 1 0 profiles f r o m p r o f u n d a l cores f r o m each study lake show c o n f o r m able declines i n u n s u p p o r t e d activity to an asymptote o f s u p p o r t e d Pb t y p i c a l l y b e l o w 4 0 - 6 0 c m d e e p ( F i g u r e 3). S u p p o r t e d activity, attained at shallower depths i n T h r u s h (28 cm) a n d Kjostad (36 cm), indicates slower linear rates o f s e d i m e n t a t i o n at these sites. T h e activity profiles for several lakes, most notably T h r u s h a n d Kjostad, are almost perfectly exponential a n d thus indicate nearly constant s e d i m e n t a c c u m u l a t i o n rates. O t h e r s , such as C e d a r a n d M o u n t a i n , show flat spots a n d k i n k s that p r o b a b l y represent shifts i n s e d i m e n t flux. 2 1 0

S e d i m e n t a c c u m u l a t i o n curves d e r i v e d b y c . r . s . calculations f r o m these activity profiles show variable s e d i m e n t a t i o n rates i n D u n n i g a n , M e a n d e r , a n d L i t t l e R o c k ( F i g u r e 4). A l t h o u g h the s e d i m e n t flux varies b y m o r e than a factor o f 2 w i t h i n each o f these profiles, the changes are asynchronous a m o n g core sites i n a g i v e n lake (other d e t a i l e d cores n o t shown) a n d m a y therefore represent shifts i n s e d i m e n t d e p o s i t i o n patterns w i t h i n t h e basin, as o p p o s e d to changes i n w h o l e - l a k e s e d i m e n t l o a d i n g (16, 39). I n C e d a r a n d M o u n t a i n lakes, the systematic increase i n s e d i m e n t a c c u m u l a t i o n since 1930 is synchronous a m o n g cores w i t h i n each lake, a n d thus constitutes a


ENGSTROM

E T AL.

Atmospheric Mercury

Deposition

43


2.

Organic Content (%Dry Sediment)

40

Lake Depth at Core Site (m) Figure 2. Organic content of surface sediments from all cores arrayed hy lake depth. Significant carbonate is present only in Cedar Lake and Mountain Lake sediments.


44

E N V I R O N M E N T A L CHEMISTRY O FLAKES

A N D RESERVOIRS

Οη 102030405060-

Meander

7080-


100

f

I

10

100

1000

Mtn. 0.1 210

10

100

Pb Activity (pCi. g" dry sediment) 1

Figure 3. Lead-210 activity profiles for the deep-water cores from each basin. Counting errors are generally smaller than the plotted symbols and are not shown. l a k e w i d e increase i n s e d i m e n t d e p o s i t i o n . T h e h i g h e r a c c u m u l a t i o n i n M o u n tain L a k e is p r i m a r i l y a n erosion signal (greater inorganic sedimentation) that is p r o b a b l y related to w a t e r s h e d disturbance f o l l o w i n g t h e d e v e l o p m e n t of G l a c i a l L a k e s State Park. O r g a n i c a c c u m u l a t i o n increases i n C e d a r L a k e i m p l y h i g h e r biological p r o d u c t i v i t y . T h e s e results indicate that o u r use o f a single average s e d i m e n t a c c u m u l a t i o n rate for t h e cores f r o m D u n n i g a n , M e a n d e r , T h r u s h , K j o s t a d ,


2.

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ET AL.

Atmospheric Mercury

45

Deposition


1990

0

100 200 300 400

200 400 600

800

100 200 300

Sediment Accumulation (g.m"

2

400

y"

Figure 4. Plots of sediment-accumulation rate versus age for the cores in Figure 3. Error bars represent one standard deviation propagated from counting uncertainty. The apparent increase in recent sediment accumulation in Little Rock Lake is atypical of other cores from this site.

a n d L i t t l e R o c k w i l l not result i n a systematic bias i n calculation of H g fluxes. H o w e v e r , the estimates w i l l b e less reliable than i f w e h a d d o n e d e t a i l e d d a t i n g o n a l l cores. O n the other h a n d , recent increases i n s e d i m e n t flux to C e d a r a n d M o u n t a i n lakes are factored i n t o o u r estimates of H g d e p o s i t i o n to compensate for greater d i l u t i o n of H g b y the s e d i m e n t matrix i n m o d e r n times. Spatial Variability.

A s u m m a r y of u n s u p p o r t e d

2 1 0

P b b u r d e n s for b o t h

coarse- a n d fine-interval cores ( F i g u r e 5) shows fairly s i m i l a r s e d i m e n t a r y conditions w i t h i n some basins (e.g., D u n n i g a n a n d M o u n t a i n ) a n d substantial v a r i a b i l i t y i n others (e.g., Kjostad, C e d a r , a n d M e a n d e r ) . T h e least variable lakes are the smallest a n d shallowest; this result is to be e x p e c t e d because s e d i m e n t d e p o s i t i o n patterns are least accentuated b y w a v e a n d c u r r e n t action i n s m a l l basins of u n i f o r m d e p t h . U n i f o r m s e d i m e n t d e p o s i t i o n i n



46

ENVIRONMENTAL

Cumulative Unsupported °Pb (pCi cm' ) 21

2

35302520-I 15 10H 5 0

CHEMISTRY

O F LAKES

A N DRESERVOIRS

Little Rock S. Basin

N. Basin

Mill &

§

ê

&

£

§§

§

Lake Depth at Core Site (m)

Figure 5. Cumulative unsupported Pb activity for all cores arrayed by lake depth. 210

y

D u n n i g a n a n d M o u n t a i n lakes is also i n d i c a t e d b y the fact that most w h o l e core P b inventories are 1 5 - 2 0 p C i / c m . T h e s e values c o r r e s p o n d to a calculated P b flux o f 0 . 4 6 - 0 . 6 2 p C i / c m p e r year, w h i c h is a reasonable estimate for m e a n a t m o s p h e r i c P b d e p o s i t i o n for this r e g i o n (40). C o r e sites w i t h inventories substantially greater (less) than these values are l i k e l y 2 1 0

2

2 1 0

2

2 1 0


2.

E N G S T R O M

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47

Deposition

to overestimate (underestimate) t h e average l a k e w i d e s e d i m e n t a c c u m u l a t i o n because o f resuspension a n d focusing of sediments f r o m shallow to d e e p water. T h i s pattern is clearly s h o w n b y C e d a r L a k e , w h e r e s e d i m e n t acc u m u l a t i o n rates are highest i n cores w i t h t h e largest P b inventories. 2 1 0

Kjostad L a k e , t h e largest basin, shows increasing P b burdens with greater lake d e p t h . T h e d i s t r i b u t i o n o f l e a d a n d organic c o n t e n t indicate that fine-grained P b - b e a r i n g sediments are h i g h l y focused i n this lake. H o w ever, t h e p o o r correlation o f P b b u r d e n s w i t h lake d e p t h i n t h e other study lakes indicates that s i m p l e bathymétrie m o d e l s cannot r e l i a b l y p r e d i c t whole-lake P b d e p o s i t i o n i n most s m a l l basins. T w o cores, notable b y t h e i r v e r y l o w P b inventories a n d l o w organic content (Kjostad cores at 2 . 2 0 a n d 2 . 9 9 m) w e r e subsequently e x c l u d e d from calculations o f w h o l e - b a s i n s e d i m e n t a c c u m u l a t i o n because they w e r e c o n s i d e r e d to r e p r e s e n t n o n d e positional sites. 2 1 0

210

2 1 0

2 1 0


2 1 0

D r y - m a s s s e d i m e n t a c c u m u l a t i o n rates calculated b y t h e c . r . s . d a t i n g m o d e l ( F i g u r e 6) are most u n i f o r m across t h e s m a l l flat-bottomed basins o f D u n n i g a n a n d M o u n t a i n lakes, a n d most variable i n K j o s t a d , M e a n d e r , a n d C e d a r . M e a n a c c u m u l a t i o n rates (Table II) are lowest i n T h r u s h a n d D u n nigan (~100 g / m p e r year) a n d o n l y slightly h i g h e r i n K j o s t a d , L i t t l e R o c k , M e a n d e r (excluding cores at 4 . 6 3 a n d 5 . 7 7 m), a n d p r e s e t t l e m e n t C e d a r (—140 g / m p e r year). M o u n t a i n L a k e is t h e one site that stands out w i t h substantially h i g h e r sedimentation rates than the other basins. B o t h m o d e r n a n d presettlement rates (550 a n d 360 g / m p e r year, respectively) a r e strongly e n h a n c e d b y carbonate p r e c i p i t a t i o n , w h i c h does n o t c o n t r i b u t e to s e d i m e n t l o a d i n g i n t h e other six lakes. M o u n t a i n L a k e i s , nonetheless, a relatively p r i s t i n e site. Its s e d i m e n t a t i o n rates are a n o r d e r o f m a g n i t u d e l o w e r than that m e a s u r e d i n cores f r o m a g r i c u l t u r a l l y affected lakes i n southe r n M i n n e s o t a ( u n p u b l i s h e d data). 2

2

2

C o n t r a r y to expectations of s e d i m e n t focusing, o n l y T h r u s h L a k e s h o w e d any relationship b e t w e e n lake d e p t h a n d s e d i m e n t a c c u m u l a t i o n , a n d i n this case d e p o s i t i o n rates w e r e lowest i n p r o f u n d a l regions o f t h e basin a n d highest t o w a r d t h e margins. A n d e r s o n (41) m a d e s i m i l a r observations f r o m m u l t i p l e cores f r o m a n e q u a l l y s m a l l lake i n N o r t h e r n I r e l a n d a n d c o n c l u d e d that s e d i m e n t t r a p p i n g b y macrophytes a n d h i g h e r organic loads i n shallow water c o u l d account for h i g h l i t t o r a l d e p o s i t i o n rates, p a r t i c u l a r l y i f w i n d i n d u c e d currents w e r e insufficient to m o v e s e d i m e n t offshore. A l t h o u g h macrophytes are n o t abundant i n T h r u s h L a k e , extensive beds o f aquatic mosses (Drepanocladus a n d Sphagnum) e x t e n d to considerable d e p t h i n t h e lake's clear water a n d c o u l d i n h i b i t s e d i m e n t resuspension a n d act as a local source o f organic detritus. Problematic Cores. T w o cores f r o m M e a n d e r L a k e e x h i b i t s e d i m e n t a c c u m u l a t i o n rates c o n s i d e r a b l y h i g h e r than t h e basin average (cores at 4 . 6 3 a n d 5.77 m). T h e core at 4 . 6 3 m is h i g h l y inorganic (5% organic matter) a n d

American Chemical Society Library 1155 IBthChemistry St., N.W. Baker; Environmental of Lakes and Reservoirs Advances in Chemistry; AmericanDC Chemical Washington, 2003Society: 6 Washington, DC, 1994.

48

ENVIRONMENTAL CHEMISTRY O F LAKES

τ /ego Meander

A N D RESERVOIRS

335

^^to^tocnco'-P*io.

U

CO

CO CO Ο

Ο -t».

Α CO -vl

— k

rorororvjcjcncncnooco—^—'—'—' rohocotDioÎ3^^r>o"-«-a Ρ r* S** Ρ 9 ο ϋ ΐ ω δ ό ο ο υ ι ο - ' ω ο ^ ω ω ω » * . ο -» en σ> ο

Γ Γ° Ç* .-^ 0

en

en

1

3

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cr>

ro σ> co

ro co

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co co σ>

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1

co co ro

250200· Sediment Accumulation

150-

(g-nrT yr" )

100-

2

1

Little Rock N. Basin

S. Basin

•

I

5.80

4.88

4.39

9.85

6.97

II

5.26


â


Figure 6. Average sediment accumuhtion rates for cores from Dunnigan, Meander, Thrush, Kjostad, and Little Rock. Modern (—post-1980) and preindustrial (—pre-1850) rates are shown for Cedar and Mountain.

is located v e r y near t h e d e p o s i t i o n a l l i m i t f o r fine-grained sediments. Its a c c u m u l a t i o n rate o f m o r e than 1 k g / m p e r year c o u l d b e a n artifact o f d o w n w a r d m i x i n g o f P b - b e a r i n g s e d i m e n t i n t o an o t h e r w i s e erosional sand and silt deposit (42). T h e l o w surface activity i n this core relative to o t h e r sites (6 p C i / g versus 3 5 - 5 0 p C i / g ) tends to support this c o n t e n t i o n . O n t h e other h a n d , t h e core site is located near a small i n l e t stream, a n d t h e h i g h 2

2 1 0


2.

ΕNGSTROM

ET AL.

Atmospheric Mercury

49

Deposition

a c c u m u l a t i o n rate c o u l d represent offshore d e p o s i t i o n of stream-borne clastic materials. H o w e v e r , for purposes o f estimating l a k e w i d e H g d e p o s i t i o n , i t makes little difference w h e t h e r this core is i n c l u d e d o r e x c l u d e d f r o m c a l culations. T h e H g concentrations are q u i t e l o w , a n d h e n c e t h e H g flux is similar to the b a s i n w i d e average. T h e other atypical core at 5 . 7 7 m has a calculated s e d i m e n t a c c u m u l a t i o n rate o f 335 g / m p e r year, b u t i n this case a h i g h P b i n v e n t o r y (56 p C i / c m ) a n d surface activity (47 pCi/g) indicates that fine-grained sediments are c u r r e n t l y a c c u m u l a t i n g at this site. H o w e v e r , t h e presence o f h i g h l y inorganic silts ( 2 - 6 % organic matter) b e n e a t h t h e P b - r i c h surface v e n e e r indicates that u n t i l recently the site was n o n d e p o s i t i o n a l . Because s u c h conditions clearly violate t h e assumption o f a constant P b flux r e q u i r e d b y the c . r . s . m o d e l , t h e d a t i n g is c o n s i d e r e d to b e u n r e l i a b l e a n d t h e core is e x c l u d e d f r o m f u r t h e r analysis. 2

2 1 0

2

2 1 0


2 1 0

Mercury Concentration. Stratigraphie Trends. D e t a i l e d profiles of H g concentration f r o m t h e deep-water cores f r o m each o f the study lakes are s h o w n i n F i g u r e 7. D e f i n i t e e n r i c h m e n t is seen i n the u p p e r sediments over t h e d e e p e r strata i n all cases. I n D u n n i g a n , M e a n d e r , T h r u s h , Kjostad, a n d L i t t l e R o c k , surficial H g concentrations range f r o m 200 to 400 ng/g o f d r y sediment, a n d b a c k g r o u n d concentrations range f r o m about 5 0 to 100 ng/g. Surface concentrations i n C e d a r L a k e d o not exceed 150 ng/g a n d i n M o u n t a i n L a k e 100 ng/g; b a c k g r o u n d concentrations i n M o u n t a i n L a k e are about 20 ng/g. T h e d i s t i n c t l y l o w e r concentrations i n M o u n t a i n L a k e c a n be a t t r i b u t e d to d i l u t i o n b y h i g h s e d i m e n t i n p u t s , p a r t i c u l a r l y carbonates. I n C e d a r L a k e , t h e relatively modest up-core increase i n H g concentration also results f r o m d i l u t i o n o f H g inputs b y recent increases i n l a k e w i d e sed i m e n t flux. T h e data suggest two distinct periods of increase i n H g d e p o s i t i o n to lakes i n this r e g i o n . T h e first c a m e b e t w e e n 1860 a n d 1890 a n d t h e second b e t w e e n 1920 a n d 1950. T h e other d e t a i l e d H g profiles (not s h o w n here) are o f generally s i m i l a r shape a n d m a g n i t u d e . S i m i l a r trends a n d concentrations have b e e n r e p o r t e d f r o m s e d i m e n t cores c o l l e c t e d f r o m other lakes i n t h e r e g i o n . M e g e r (24) f o u n d recent (core top) a n d b a c k g r o u n d concentrations o f about 110 a n d 40 ng/g for cores f r o m two large lakes (Crane a n d Kabetogama, respectively) i n n e a r b y Voyageurs N a t i o n a l Park. R a d a et a l . (Τ) o b s e r v e d surficial s e d i m e n t concentrations b e t w e e n 90 a n d 190 ng/g a n d values b e t w e e n 40 a n d 70 ng/g i n d e e p e r strata i n 11 seepage lakes i n n o r t h - c e n t r a l W i s c o n s i n . S o m e w h a t h i g h e r concentrations ( 1 0 0 - 2 0 0 ng/g b a c k g r o u n d a n d 2 0 0 - 5 0 0 ng/g surface) w e r e f o u n d i n cores f r o m southern O n t a r i o b y E v a n s (6) a n d Johnson et a l . (23). M o s t of these w o r k e r s consider atmospheric d e p o s i t i o n of H g from i n d u s t r i a l sources to b e t h e l i k e l y cause o f increasing H g concentrations i n lake s e d i -



50

ENVIRONMENTAL

CHEMISTRY

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Figure 7. Plots of Hg concentration versus Pb age for deep-water cores. Dates older than about 1800 are extrapolations based on mean dry-mass sediment accumulation rates. m

ments that are remote f r o m point-source discharge. H e n c e for discussion purposes, H g levels f r o m strata d e p o s i t e d before the mid-1800s are c o n s i d e r e d to represent p r e i n d u s t r i a l conditions, a l t h o u g h a n t h r o p o g e n i c emissions of H g began earlier than this i n some regions. Enrichment Factors. M o d e r n a n d p r e i n d u s t r i a l (background) H g c o n centrations for b o t h coarse- a n d f i n e - i n t e r v a l cores are i l l u s t r a t e d i n F i g u r e 8. A s s h o w n b y the d e t a i l e d core stratigraphy, H g concentrations i n surficial sediments are significantly elevated above those i n d e e p e r strata i n n e a r l y e v e r y case. Surface H g concentrations i n T h r u s h L a k e average 370 ng/g, whereas b a c k g r o u n d values are ~ 1 0 0 ng/g (Table II). I n Kjostad a n d L i t t l e R o c k recent H g concentrations are 255 ng/g, a n d b a c k g r o u n d levels average a r o u n d 80 a n d 60 ng/g, respectively. H g levels are slightly l o w e r i n M e a n d e r and D u n n i g a n , w h e r e surface concentrations are about 200 ng/g a n d backg r o u n d values are —60 ng/g. M o u n t a i n a n d C e d a r lake sediments are c o n -


2.

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Atmospheric Mercury

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51

Deposition

500

Dunnigan

400 300 200

g

500·

Thrush


4.32

4 09 q ce

3.48

3.03

2.74

2.34

1.98

2.02

1.91

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Mountain

jl ι fc L _ l

L_L

500

Hg Concentration (ng · g" dry sediment)

Little Rock

400

N. Basin

1

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Modern

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Preindustrial

300

S. Basin

200-I 100-) 0

i l 11 ill! 111 ωαΐϋΐσ)^οοοοΝ3*.ϋΐϋΐαΐϋΐσι


Figure 8. Hg concentration in modern (core-top) and preindustrial 1850) sediments from all cores, arrayed by lake depth.

(~pre-

sistently lowest i n H g ; recent values are 100 a n d 60 ng/g a n d b a c k g r o u n d concentrations are 5 5 a n d 20 ng/g, r e s p e c t i v e l y . I n most o f the study lakes H g concentrations are spatially less variable than s e d i m e n t a c c u m u l a t i o n rates, a l t h o u g h t h e large range o f m o d e r n H g values for T h r u s h L a k e is a notable e x c e p t i o n . A l t h o u g h t h e r e are n o o b v i o u s


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trends o f i n c r e a s i n g H g c o n c e n t r a t i o n w i t h lake d e p t h , i t is e v i d e n t from l o w H g values i n a f e w shallow-water cores from M e a n d e r a n d K j o s t a d that H g is closely associated w i t h t h e fine-grained organic s e d i m e n t s that are p r e f e r e n t i a l l y transported offshore (6, 43). A s p r e v i o u s l y m e n t i o n e d , l i t t o r a l cores o f l o w organic content w e r e generally n o t c o l l e c t e d . A c o m p a r i s o n o f m o d e r n a n d b a c k g r o u n d (preindustrial) H g c o n c e n t r a tions b y s e d i m e n t e n r i c h m e n t factors ( S E F ) , w h e r e Hg( fe ) mot

H§(background)

m

^


Hg^ckground) indicates fairly s i m i l a r increases a m o n g t h e s t u d y lakes (Table II). M e a n S E F s f o r M o u n t a i n , T h r u s h , D u n n i g a n , a n d K j o s t a d range f r o m 2 . 3 to 2 . 7 ; i n L i t t l e R o c k a n d M e a n d e r average S E F s are 3 . 2 ; a n d i n C e d a r L a k e t h e S E F is 1.3. T h e l o w e r e n r i c h m e n t for t h e latter site results from d i l u t i o n o f recent H g inputs b y a h i g h e r l a k e w i d e flux o f the organic s e d i m e n t . T h i s fact illustrates the difficulty o f r e l y i n g solely o n c o n c e n t r a t i o n data to evaluate m e t a l c o n t a m i n a t i o n f r o m s e d i m e n t a r y records. S E F s are somewhat m o r e variable a m o n g t h e i n d i v i d u a l cores w i t h i n each lake, b u t t h e range is still m o d e s t (0-7), whereas 8 0 % o f a l l cores have a n e n r i c h m e n t o f 1 - 4 . A fairly n a r r o w range o f H g e n r i c h m e n t ( S E F = 0 . 8 - 2 . 8 ) was also n o t e d i n surface sediments from o t h e r r e m o t e lakes i n n o r t h e r n M i n n e s o t a a n d W i s c o n s i n (7, 24). Lakewide H g Accumulation. M e r c u r y a c c u m u l a t i o n rates c a n b e calculated for i n d i v i d u a l core sites as the p r o d u c t of the P b - b a s e d s e d i m e n t a c c u m u l a t i o n rate a n d H g concentration i n different strata. I f e n o u g h cores are a n a l y z e d , w h o l e - l a k e H g inputs c a n b e calculated b y w e i g h t i n g t h e H g flux o f each core b y t h e p o r t i o n o f the d e p o s i t i o n a l b a s i n i t represents. I n this study w e calculate H g l o a d i n g for each lake o n a n areal basis f o r t w o time-stratigraphic u n i t s — m o d e r n (roughly the last decade) a n d p r e i n d u s t r i a l (before 1 8 5 0 ) — a c c o r d i n g to e q 2: 2 1 0

(2)

w h e r e Q , is t h e H g flux i n m i c r o g r a m s p e r square m e t e r p e r year for t i m e stratigraphic i n t e r v a l i , R is s e d i m e n t a c c u m u l a t i o n i n grams p e r square m e t e r p e r year for i n t e r v a l i i n core j, H g is H g c o n c e n t r a t i o n i n m i c r o g r a m s p e r g r a m for s e d i m e n t i n t e r v a l i i n core j, Aj is d e p o s i t i o n a l zone i n square meters r e p r e s e n t e d b y core j , A is total lake surface area i n square m e t e r s , and η is n u m b e r o f cores. 0

0

0


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D e p o s i t i o n a l areas for each core w e r e a p p r o x i m a t e d b y b o t h T h e i s s e n polygons a n d d e p t h contours, a l t h o u g h the r e p o r t e d fluxes are those o n l y from the p o l y g o n m e t h o d . I n practice the t w o approaches gave v i r t u a l l y the same results. H o w e v e r , w e favor the p o l y g o n m e t h o d because p r o x i m i t y to the core site s h o u l d be a better p r e d i c t o r o f H g a c c u m u l a t i o n i n basins s u c h as these, w h e r e s e d i m e n t d e p o s i t i o n is not c o r r e l a t e d w i t h lake d e p t h . T h e b a s i n w i d e flux calculations f r o m the seven lakes show that p r e i n d u s t r i a l H g a c c u m u l a t i o n rates i n the sediments r a n g e d b e t w e e n 4 . 5 a n d 9.0 μ g / m p e r year, a n d the m o d e r n rates range b e t w e e n 16 a n d 32 μ g / m p e r year ( F i g u r e 9). M o r e s t r i k i n g is the observation that the range i n these rates is a f u n c t i o n of the relative size of the terrestrial c a t c h m e n t s u r r o u n d i n g each lake basin. O v e r 9 0 % of the variation i n m o d e r n H g ac c u m u l a t i o n can be accounted for b y the ratio of a l a k e s c a t c h m e n t area to its surface area (A :A ). T h e correlation b e t w e e n p r e i n d u s t r i a l H g a c c u 2


2

d

0

40Modem

35·

Pre-lndustrial

Mountain

CM*

Ε

ό> 25 y = 12.5 +3.27 x (^ = 0.91)

•Β 20·

Ι

Little Rock • Dunnigan

15-

Ο ο

< 10·

5^

y = 3.7 +0.83 x (^ = 0.70) I

I

I

I

I

1

1

'

1

I

2

3

4

5

6

Catchment Area/Lake Area Figure 9. Lake-wide Hg accumulation rates as a function of the ratio of ter restrial catchment area to lake area. The error bars propagate maximum and minimum estimates of the depositional region of each lake and that of its functional catchment.


7

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m u l a t i o n a n d A :A is also strong ( r = 0.7), b u t t h e range o f H g fluxes is substantially smaller, a c c o u n t i n g f o r t h e w e a k e r regression. N o n e o f t h e other h y d r o l o g i e o r l i m n o l o g i e variables can account for this d i s t r i b u t i o n o f H g a c c u m u l a t i o n (Table I). W a t e r residence t i m e a n d i o n i c strength, f o r example, are highest i n M o u n t a i n a n d C e d a r lakes, y e t C e d a r has H g acc u m u l a t i o n rates s i m i l a r to that of L i t t l e R o c k , a d i l u t e seepage lake. M o u n tain a n d Kjostad lakes have s i m i l a r H g fluxes, t h o u g h one is s u r r o u n d e d b y u p l a n d p r a i r i e a n d t h e other b y conifer forest a n d m u s k e g .


d

0

2

T h e s e results strongly i m p l y that a p o r t i o n o f t h e H g inputs to t h e sediments i n these lakes comes f r o m t h e i r watersheds, a n d that t h e m a g n i t u d e of this terrestrial c o m p o n e n t increases w i t h catchment size. H o w e v e r , the relative increase i n H g a c c u m u l a t i o n from p r e i n d u s t r i a l rates is n e a r l y constant a m o n g sites (Table II). T h e ratio o f m o d e r n to b a c k g r o u n d accum u l a t i o n ranges from 3 . 2 to 4 . 9 . T h i s u n i f o r m i t y indicates that t h e change i n H g i n p u t s since p r e i n d u s t r i a l times has b e e n regionally s i m i l a r for r u r a l or r e m o t e areas o f the u p p e r M i d w e s t .

Discussion Mercury Fluxes and Sediment Records. O u r calculations of lakew i d e H g fluxes f r o m m o r e than 8 0 dated s e d i m e n t cores from seven s m a l l headwater lakes reveal a regionally consistent increase i n H g i n p u t s f r o m p r e i n d u s t r i a l times to the present; the m o d e r n H g flux to each of these lakes is about 3 . 7 times that o f the early 1800s. S u c h increases are t y p i c a l o f that r e p o r t e d i n other investigations of lake sediments f r o m r e m o t e o r r u r a l sites i n eastern N o r t h A m e r i c a (3, 6, 7, 10, 23). M o s t researchers have c o n c l u d e d that t h e increase is anthropogenic a n d that t h e H g m u s t b e t r a n s p o r t e d t h r o u g h t h e atmosphere a n d d e p o s i t e d o n t h e lake a n d its terrestrial catchment. Precise estimates o f t h e anthropogenic c o m p o n e n t o f t h e global H g b u d g e t have b e e n elusive (27, 44). Values f o r anthropogenic c o n t r i b u t i o n s range f r o m 2 0 % to 7 5 % o f total (natural a n d industrial) H g emissions, b u t recent estimates seem to converge o n the u p p e r e n d of this scale (26, 45-49). T h e d o m i n a n t i n d u s t r i a l sources today are coal c o m b u s t i o n (—65%) a n d waste i n c i n e r a t i o n (25%), whereas v o l c a n i s m , m a r i n e degassing, a n d terrestrial volatilization are the most i m p o r t a n t natural sources (44, 50). H o w e v e r , t h e r e is exceedingly little i n f o r m a t i o n o n actual rates of H g d e p o s i t i o n at p o t e n t i a l l y sensitive sites r e m o t e from point-source discharge. R e l i a b l e m e a s u r e m e n t s of H g i n wet d e p o s i t i o n are available f r o m o n l y a f e w locations, a n d these are t e m p o r a l l y l i m i t e d to t h e past f e w years (51-54). T h u s a l t h o u g h t h e relative change i n H g inputs to lakes a n d landscapes is reasonably w e l l d o c u m e n t e d , t h e actual m a g n i t u d e o f the increase is p o o r l y k n o w n . Results f r o m this study c a n p r o v i d e quantitative estimates o f H g d e position rates n o w a n d i n the past. Because of the strong relationship b e t w e e n



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H g flux a n d w a t e r s h e d area, t h e relative i m p o r t a n c e of d i r e c t d e p o s i t i o n a n d catchment c o n t r i b u t i o n s c a n b e assessed. S u c h interpretations assume that lake sediments are stratigraphically a n d q u a n t i t a t i v e l y r e l i a b l e archives o f H g inputs to aquatic systems. E x p e r i m e n t a l observations o n the g e o c h e m i c a l b e h a v i o r o f H g i n l a custrine e n v i r o n m e n t s indicate that sediments s h o u l d r e t a i n most o f t h e H g e n t e r i n g a lake. M e r c u r y i n the water c o l u m n is r a p i d l y s o r b e d b y particulates a n d efficiently r e m o v e d to t h e sediments w h e r e t h e b u l k of H g i n t h e aquatic e n v i r o n m e n t is i m m o b i l i z e d (55, 56). Mass-balance studies o n o n e o f o u r sites, L i t t l e R o c k L a k e , indicate that about 9 0 % of t h e i n c o m i n g H g is d e p o s i t e d i n the sediments; t h e r e m a i n i n g i n p u t s are lost to gaseous evasion of H g ° (52, 57). I n h u m i c - s t a i n e d lakes w i t h v e r y short r e s i d e n c e t i m e s (one to a f e w months) outflow losses o f H g m a y also b e significant (58). W i t h i n the s e d i m e n t c o l u m n , H g s o r p t i o n kinetics strongly favor t h e particulate phase so that postdepositional m o v e m e n t o f H g appears to b e v e r y l i m i t e d (59). W e l l - p r e s e r v e d H g stratigraphy w i t h subsurface peaks has b e e n n o t e d i n s e d i m e n t cores f r o m r i v e r i n e systems after p o i n t - s o u r c e H g inputs w e r e c u r t a i l e d (60-62). Direct Atmospheric Deposition. O u r calculation o f w h o l e - b a s i n s e d i m e n t d e p o s i t i o n shows that H g a c c u m u l a t i o n i n a lake is d i r e c t l y p r o p o r t i o n a l to t h e ratio o f the catchment area to t h e lake surface area ( F i g u r e 9). T h e i n t e r c e p t o f the regression l i n e i n this r e l a t i o n s h i p p r e d i c t s t h e H g a c c u m u l a t i o n rate f o r a lake w i t h n o terrestrial catchment. I n o t h e r w o r d s , it shows t h e n e t atmospheric d e p o s i t i o n rate a c c o u n t i n g for w e t a n d d r y d e p o s i t i o n a n d losses d u e to gaseous evasion a n d o u t f l o w (groundwater o r surface water) after d e p o s i t i o n . T h e p r e i n d u s t r i a l a t m o s p h e r i c flux o f H g estimated f r o m t h e i n t e r c e p t is 3 . 7 μ g / m p e r year, a n d t h e m o d e r n rate is 12.5 μ g / m p e r year. T h e s e values have a r e l a t i v e l y h i g h u n c e r t a i n t y because they are extrapolations b e y o n d the data set. H o w e v e r , t h e m o d e r n rate is i n g o o d agreement w i t h c u r r e n t measurements o f H g d e p o s i t i o n i n n o r t h c e n t r a l N o r t h A m e r i c a . Glass et a l . (54) f o u n d a 3-year average o f 15 μ g / m p e r year for three M i n n e s o t a d e p o s i t i o n sites, M i e r l e (63) m e a s u r e d 10.2 μ g / m p e r year for a 1-year study at a catchment i n c e n t r a l O n t a r i o , a n d F i t z g e r a l d et a l . (52) r e p o r t e d a m e a n o f about 10 μ g / m p e r year for L i t t l e R o c k L a k e . T h e first t w o studies are for w e t d e p o s i t i o n o n l y ; d r y d e p o s i t i o n is thought to b e as large as 5 0 % o f w e t d e p o s i t i o n (49). F i t z g e r a l d et a l . estimate 6.8 a n d 3 . 5 μ g / m p e r year for w e t a n d d r y d e p o s i t i o n , r e s p e c t i v e l y . 2

2

2

2

2

2

O u r estimates o f a t m o s p h e r i c d e p o s i t i o n i n p r e i n d u s t r i a l a n d m o d e r n times indicate that H g inputs have increased b y a factor o f 3.4 i n 130 years (3.7 to 12.5 μ g / m p e r year). A l t e r n a t i v e l y , a factor o f 3 . 7 is o b t a i n e d b y averaging the increase factor f r o m each lake (Table II). T h e 3 . 7 - f o l d increase translates to an average increase o f about 2 . 2 % p e r year, c o m p a r e d to a n annual increase of 1 . 5 % m e a s u r e d i n a i r o v e r t h e n o r t h A t l a n t i c O c e a n for the p e r i o d 1 9 7 7 - 1 9 9 0 (26). 2


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O u r results show that for the geographic r e g i o n r e p r e s e n t e d b y the seven lakes, atmospheric H g l o a d i n g has increased b y a r e l a t i v e l y constant factor a n d that m o d e r n d e p o s i t i o n rates are s i m i l a r a m o n g sites (accounting for catchment size). T h e H g b u r d e n of u n d i s t u r b e d forest soils is also r e l a t i v e l y u n i f o r m across this r e g i o n b u t becomes significantly h i g h e r to the east (64), p r e s u m a b l y because o f greater p r o x i m i t y to i n d u s t r i a l i z e d regions. Scand i n a v i a n researchers have d o c u m e n t e d a s i m i l a r gradient of i n c r e a s i n g H g d e p o s i t i o n t o w a r d i n d u s t r i a l i z e d areas (22, 53, 65, 66). C a t c h m e n t C o n t r i b u t i o n s . T h e slope of the regression lines i n F i g ure 9 is the rate at w h i c h H g is transported f r o m the terrestrial c a t c h m e n t to the lake sediments (in units of micrograms of H g p e r square m e t e r of catchment p e r year). If one assumes that a l l o f the H g i n the c a t c h m e n t is d e r i v e d f r o m the atmosphere, t h e n the slope d i v i d e d b y the a t m o s p h e r i c d e p o s i t i o n rate (the intercept) is the p r o p o r t i o n of terrestrial H g d e p o s i t i o n that is transported to the lake. T h e slope w i l l e q u a l the rate of a t m o s p h e r i c d e p o s i t i o n i f the e n t i r e flux to the catchment is transported to the lake. O n the other h a n d , the slope w i l l e q u a l z e r o — a s o b s e r v e d for P b b y D i l l o n a n d E v a n s (18)—if d i r e c t d e p o s i t i o n to the lake surface is the o n l y significant source. T h i s s i m p l e m o d e l of H g a c c u m u l a t i o n assumes that t h e r e are m i n i m a l losses of H g f r o m the lake b y evasion or outflow, that d r y d e p o s i t i o n rates are s i m i l a r for lake surfaces a n d terrestrial catchments, a n d that t h e r e are no significant m i n e r a l sources of H g i n the catchment. T h e good fit of the data to straight lines ( r = 0.91 for m o d e r n a n d 0.70 for preindustrial) indicates that none of these processes exerts a large effect. F u r t h e r m o r e , the gabbro a n d granite b e d r o c k of n o r t h e r n M i n n e s o t a is p o o r i n H g , averaging 10 ng/g (67). T h e d e e p e r m i n e r a l - s o i l horizons ( 7 5 - 1 0 0 cm) i n this r e g i o n contain an average of 14 ng/g (64) c o m p a r e d to an average p r e i n dustrial s e d i m e n t concentration of 80 ng/g. T h e lake sediments c o n t a i n 3 0 - 7 0 % m i n e r a l matter, w h i c h i n c l u d e s d i a t o m silica a n d a u t h i g e n i c i r o n as w e l l as d e t r i t a l silts a n d clays. T h e r e f o r e , erosion of m i n e r a l soil c o n t r i b u t e d at most 5 - 1 2 % of the p r e i n d u s t r i a l s e d i m e n t a r y H g a c c u m u l a t i o n a n d 2 - 4 % of the m o d e r n a c c u m u l a t i o n . 2

B y u s i n g this m o d e l , w e find that r o u g h l y the same p r o p o r t i o n of atm o s p h e r i c H g has b e e n transported f r o m the catchments to the various lakes i n m o d e r n a n d p r e i n d u s t r i a l times (26% a n d 22%, respectively). T h e balance of the H g is e i t h e r v o l a t i l i z e d back to the atmosphere o r r e t a i n e d b y soils i n the catchment. Because H g has a h i g h affinity for soil organic matter, it is not a p p r e c i a b l y l e a c h e d f r o m soils e v e n u n d e r acidic c o n d i t i o n s , i n contrast to other metals (68, 69). H o w e v e r , v o l a t i l i z a t i o n to the atmosphere f r o m soils can be significant. I n one e x p e r i m e n t w i t h u n d i s t u r b e d soil profiles, none of the H g a p p l i e d at the surface m o v e d d e e p e r than 20 c m after 19 weeks o f i r r i g a t i o n a n d i n c u b a t i o n , a l t h o u g h 7 - 3 1 % o f the a p p l i e d H g was


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lost t h r o u g h v o l a t i l i z a t i o n d u r i n g this t i m e (70). S i m i l a r l y , N a t e r a n d G r i g a l (64) f o u n d that forest soils i n M i n n e s o t a , W i s c o n s i n , a n d M i c h i g a n have r e t a i n e d o n l y a s m a l l p r o p o r t i o n of the total H g d e p o s i t e d f r o m the atmosphere since déglaciation 10,000 years ago (165 years w o r t h of d e p o s i t i o n , or about 2%). If about 2 5 % of a t m o s p h e r i c d e p o s i t i o n is t r a n s p o r t e d to lakes and ponds i n the catchment, about 7 0 - 7 5 % of d e p o s i t e d H g m u s t b e rev o l a t i l i z e d to the atmosphere. T h e r e appears to be v e r y little net r e t e n t i o n of H g i n these soils. A a s t r u p et a l . (71) f o u n d i n a S w e d i s h study that about 8 0 % of the H g d e p o s i t e d i n a catchment was r e t a i n e d i n the m o r (organic surface soil), b u t n o t e d that an u n k n o w n p r o p o r t i o n of the r e t a i n e d H g m i g h t be lost to the atmosphere. S i m i l a r l y , i n a mass-balance study of three catchments a r o u n d H a r p L a k e i n central O n t a r i o , M i e r l e (63) estimated that 8 4 - 9 2 % of H g d e p o s i t i o n was r e t a i n e d . A l t h o u g h the r e t e n t i o n of H g i n soils m a y b e l o w , it is still i m p o r t a n t to use r e l a t i v e l y u n d i s t u r b e d lake catchments to assess a t m o s p h e r i c H g l o a d i n g . Sites strongly affected b y land-use changes (such as f a r m i n g or urbanization) w i l l e x h i b i t erosion rates m a n y times that of u n d i s t u r b e d systems, a n d greater l o a d i n g of s o i l - b o u n d H g w i l l m u l t i p l y the H g a c c u m u l a t i o n i n the sediments. T h u s the modest increase i n soil erosion to M o u n t a i n L a k e i n the mid-1900s m a y be responsible for an increase i n H g d e p o s i t i o n — a 4 . 9 - f o l d increase over p r e i n d u s t r i a l rates (Table II)—that is notably larger than that at the other sites. M o r e s t r i k i n g are p r e l i m i n a r y results f r o m an a g r i c u l t u r a l l y affected lake i n s o u t h e r n M i n n e s o t a , w h i c h s h o w m o d e r n H g a c c u m u l a t i o n rates i n a single s e d i m e n t core that are m o r e than an o r d e r of m a g n i t u d e greater than the highest values r e p o r t e d h e r e (72). S i m o l a a n d L o d e n i u s (73) reached s i m i l a r conclusions r e g a r d i n g the large increase i n s e d i m e n t a r y H g a c c u m u l a t i o n that a c c o m p a n i e d peatland d i t c h i n g a n d afforestation of a lake catchment i n n o r t h e r n F i n l a n d , a l t h o u g h m o b i l i z a t i o n of H g b y soil h u m i c substances rather than particulate e r o s i o n was p r o b a b l y responsible. M i e r l e (63) a n d others (58, 74, 75) suggested that, because d i s s o l v e d organic matter ( D O M ) strongly complexes H g , the export of H g to lake basins f r o m t h e i r terrestrial watersheds m a y be c o n t r o l l e d b y the nature of catchment soils a n d the m o v e m e n t of h u m i c a n d f u l v i e acids. T h e s e observations offer a possible m e c h a n i s m for the o b s e r v e d r e l a t i o n s h i p b e t w e e n H g a c c u m u l a t i o n a n d catchment area i n o u r study lakes. If most c a t c h m e n t d e r i v e d H g e n t e r e d lakes b y g r o u n d w a t e r , these i n p u t s s h o u l d b e o n l y w e a k l y related to the size of the topographic w a t e r s h e d , especially for seepage lakes. O n the other h a n d , i f H g export is l i n k e d to D O M , c a t c h m e n t area is a logical correlate of H g l o a d i n g . T h e export of D O M f r o m c a t c h m e n t soils, w h i c h occurs largely i n surficial drainage f r o m u p p e r soil h o r i z o n s (76), can be expected to increase w i t h size of c a t c h m e n t area. T h u s for a g i v e n biogeographic r e g i o n , the h u m i c content of a lake is strongly related to the


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relative size o f its catchment (77, 78). T h e close c o r r e l a t i o n b e t w e e n H g concentration a n d h u m i c matter i n surface waters (74, 75), t h e observation that peak concentrations o f b o t h H g and D O M t e n d to o c c u r d u r i n g p e r i o d s of h i g h r u n o f f (71, 74), a n d t h e e x p e r i m e n t a l d e t e r m i n a t i o n that H g transport occurs p r i m a r i l y i n t h e u p p e r soil horizons (71) a l l s u p p o r t the c o n c l u s i o n that H g export m a y b e e x p l a i n e d b y factors r e g u l a t i n g t h e export o f f u l v i c and h u m i c matter a n d b y catchment area i n particular. Mercury in Fish. O u r results show that w a t e r s h e d c o n t r i b u t i o n s are n e i t h e r d o m i n a n t n o r t r i v i a l i n t h e total H g budgets o f s m a l l m i d w e s t e r n lakes, a l t h o u g h t h e terrestrial i n p u t s are u l t i m a t e l y a t m o s p h e r i c . E v a n s (6) r e a c h e d s i m i l a r conclusions f o l l o w i n g m u c h t h e same a p p r o a c h u s e d i n this study. H e o b s e r v e d a strong positive relationship b e t w e e n w h o l e - l a k e H g b u r d e n s calculated f r o m m u l t i p l e cores a n d A ;A f r o m o n e o f t h r e e lake districts i n south-central O n t a r i o . T h e study l a c k e d s e d i m e n t d a t i n g o r flux calculations, w h i c h m a y account for weak correlations from t h e o t h e r t w o lake groups. L i k e w i s e , Suns a n d H i t c h i n (79) n o t e d a strong c o r r e l a t i o n b e t w e e n H g residues i n y e l l o w p e r c h a n d t h e ratio o f c a t c h m e n t area to lake v o l u m e . M e r c u r y levels i n fish are several steps r e m o v e d f r o m H g d e p o s i t i o n rates, a n d y e t b o t h t h e intercept a n d slope o f this regression are significant positive terms. E v i d e n t l y d i r e c t H g d e p o s i t i o n to the lake surface a n d w a s h out f r o m the catchment are i m p o r t a n t i n p u t s to t h e lake. d

0

C l e a r l y H g l o a d i n g rates are n o t t h e o n l y factor l i m i t i n g H g residues i n fish. T h e p r o d u c t i o n o f m e t h y l m e r c u r y is p r o b a b l y t h e c r i t i c a l process c o n t r o l l i n g H g b i o a c c u m u l a t i o n , b u t factors affecting m e t h y l a t i o n are not w e l l u n d e r s t o o d . E m p i r i c a l observations have i d e n t i f i e d p H , a l k a l i n i t y , a n d d i s s o l v e d organic c a r b o n ( D O C ) as significant correlates o f H g i n fish (19, 80-82). Possible p H effects i n c l u d e controls o n H g s o l u b i l i t y , m e t h y l a t i o n rates, a n d t h e p r o d u c t i o n o f volatile species s u c h as H g ° (52, 83, 84). H u m i c and f u l v i c c o m p o n e n t s o f D O C m a y e n h a n c e H g l o a d i n g a n d s o l u b i l i t y , catalyze H g m e t h y l a t i o n , o r d i r e c t l y m e t h y l a t e H g (85, 86). A l k a l i n i t y m a y be a p r o x y for c a l c i u m , w h i c h itself m a y i n h i b i t H g uptake b y fish (87). F u r t h e r m o r e , recent e v i d e n c e for t h e role o f sulfate r e d u c t i o n i n H g m e t h y l a t i o n (88) i m p l i e s that increased S 0 l o a d i n g m a y i n some systems c o n t r i b u t e to h i g h e r H g levels i n t h e biota. 4

N o n e t h e l e s s , m e t h y l m e r c u r y p r o d u c t i o n c a n b e p r o p o r t i o n a l to H g c o n centration (83, 89) a n d may b e l i m i t e d i n lakes b y t h e flux o f reactive Hg(II) species across t h e s e d i m e n t - w a t e r interface (52). T h u s a n increase i n total H g d e p o s i t i o n c o u l d p r o d u c e a n e q u i v a l e n t response i n H g b i o a c c u m u l a t i o n , all other factors b e i n g e q u a l . It m a y b e significant i n this r e g a r d that t h e average rate o f increase i n H g residues i n fish i n M i n n e s o t a is o f t h e same m a g n i t u d e as that calculated h e r e for atmospheric l o a d i n g , r o u g h l y 3 % a n n u a l l y since 1930 (19).


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Beyond Mercury T h e bulk of

Other Applications of the Multiple-Core Approach.

this chapter has dealt w i t h t h e specific a p p l i c a t i o n o f m u l t i p l e - c o r e m e t h odology to questions o f atmospheric H g d e p o s i t i o n . W h o l e - b a s i n H g accum u l a t i o n rates for seven lakes, calculated f r o m m u l t i p l e s e d i m e n t

cores,

w e r e u s e d i n a s i m p l e mass-balance m o d e l to estimate a t m o s p h e r i c fluxes a n d H g transport f r o m catchment soils. T h i s approach can b e u s e d to answer other l i m n o l o g i c a l questions, a n d the m o d e l is not r e s t r i c t e d to H g o r atmospheric deposition. M u l t i p l e - c o r e methods a p p l i e d to i n d i v i d u a l lakes y i e l d i n f o r m a t i o n o n Downloaded by FUDAN UNIV on March 2, 2017 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0237.ch002

w h o l e - b a s i n fluxes, i n t e r n a l b i o g e o c h e m i c a l c y c l i n g , a n d variations i n d e positional processes i n space a n d t i m e . Specific applications i n c l u d e t h e calc u l a t i o n o f catchment erosion rates (90, 91), studies o f n u t r i e n t l o a d i n g a n d r e t e n t i o n (15), interpretations o f m i n e r a l c y c l i n g (92) a n d t h e diagenesis o f magnetic minerals (93), studies o f sulfur a c c u m u l a t i o n a n d c y c l i n g (5, 16), reconstructions o f p r o d u c t i v i t y f r o m fossil diatoms (14, 94, 95), a n d i n v e s tigations o f p o l l e n r e c r u i t m e n t a n d d e p o s i t i o n (96, 97). M u l t i p l e cores taken f r o m a suite o f lakes c a n b e u s e d to generate a regional p i c t u r e o f m a t e r i a l fluxes, transport m e c h a n i s m s , a n d geographic trends. T h e a p p l i c a t i o n o f m u l t i p l e cores o n m u l t i p l e lakes was p i o n e e r e d b y E v a n s a n d his co-workers to assess the atmospheric d e p o s i t i o n o f heavy metals i n c l u d i n g H g , P b , Z n , a n d C d (6, 18, 98). T h e s e p u b l i c a t i o n s reco g n i z e d t h e value of p l o t t i n g w h o l e - b a s i n m e t a l a c c u m u l a t i o n against t h e ratio o f catchment to lake areas (A :A ), d

0

b u t d i d not describe t h e general

m e a n i n g o f t h e slope a n d i n t e r c e p t o f this r e l a t i o n s h i p . A s w e have s h o w n p r e v i o u s l y (99), t h e i n t e r c e p t is t h e a c c u m u l a t i o n rate o f a g i v e n m a t e r i a l for a lake w i t h n o catchment; for H g it is t h e atm o s p h e r i c d e p o s i t i o n rate. F o r parameters w i t h n o appreciable a t m o s p h e r i c source, such as soil erosion, o n e w o u l d expect an i n t e r c e p t n o t significantly different from zero. T h e slope o f the regression l i n e is t h e rate at w h i c h t h e m a t e r i a l is transported from t h e catchment to t h e lake sediments (grams p e r square m e t e r of catchment p e r year). If the m a t e r i a l has n o significant source w i t h i n t h e catchment except atmospheric d e p o s i t i o n , t h e n t h e slope d i v i d e d b y t h e i n t e r c e p t is the p r o p o r t i o n o f the a t m o s p h e r i c flux to t h e c a t c h m e n t that is transported to the lake. I f the slope is zero, t h e n there is n o significant transport to the lake f r o m t h e catchment. T h i s s i m p l e m o d e l assumes that the u l t i m a t e fate o f the substance is the sediments of t h e lake. Specifically, t h e r e must b e n o significant losses t h r o u g h degradation, diagenesis, evasion, or o u t f l o w i n surface water o r g r o u n d w a t e r . M o r e o v e r , i f catchment o r atm o s p h e r i c fluxes are regionally variable, this feature s h o u l d b e e v i d e n t as a weak relationship b e t w e e n w h o l e - b a s i n a c c u m u l a t i o n a n d

A :A . d

0


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H o w M a n y Cores? A l t h o u g h the heterogeneity of l a k e - s e d i m e n t a c c u m u l a t i o n is w e l l k n o w n , most workers t e n d to r e l y o n a single core to represent l a k e w i d e conditions. T h e single-core approach can reveal a good deal about lake history i f uncertainties arising f r o m spatial heterogeneity are taken into account. C h a n g e s i n n u t r i e n t l o a d i n g , erosion, o r a t m o s p h e r i c d e p o s i t i o n that alter the c h e m i c a l c o m p o s i t i o n of the sediments s h o u l d r e g -


ister the same stratigraphie trends at different core sites (as s h o w n i n this study), although the actual flux a n d c o m p o s i t i o n of the sediments may differ greatly a m o n g sites. E v e n t h o u g h single cores can d o c u m e n t stratigraphie trends a n d trajectories, m u l t i p l e cores are r e q u i r e d to obtain quantitative data o n w h o l e basin fluxes. M u l t i p l e - c o r e studies m i g h t be m o r e c o m m o n w e r e it not for the large effort r e q u i r e d to collect, date, a n d stratigraphically analyze sediments f r o m the various d e p o s i t i o n regions of a lake b a s i n . T h i s study i l l u s trates h o w , b y r e d u c i n g the n u m b e r of stratigraphie units i n each core, it is possible to e c o n o m i z e o n historical detail a n d increase the n u m b e r of cores that can be a n a l y z e d . T h e actual n u m b e r of cores n e e d e d d e p e n d s on the size a n d m o r p h o m etry of the basin, the nature of the e n v i r o n m e n t a l signal u n d e r investigation, and the l e v e l of accuracy r e q u i r e d b y the study. S e d i m e n t d e p o s i t i o n is spatially m o r e variable i n large d e e p lakes w i t h i r r e g u l a r m o r p h o m e t r y than i n small basins of u n i f o r m d e p t h . M o r e cores are r e q u i r e d f r o m the f o r m e r to attain the accuracy that a f e w cores w o u l d p r o v i d e i n the latter. I m p o u n d ments a n d lakes w i t h major r i v e r inputs t y p i c a l l y e x h i b i t strong d e p o s i t i o n a l gradients (J00) a n d r e q u i r e a h i g h e r d e n s i t y of cores to accurately characterize s e d i m e n t a c c u m u l a t i o n . F i n a l l y , o n l y a few cores m i g h t be necessary to c o n f i r m that a c c u m u l a t i o n changes i n one core are q u a l i t a t i v e l y r e p r e sentative of the entire lake, whereas a quantitative estimate of l a k e w i d e flux c o u l d r e q u i r e a d o z e n cores or m o r e . Results f r o m o u r study can be u s e d to address i n a statistical sense the q u e s t i o n of accuracy a n d core n u m b e r s . If w e assume that o u r existing core data p r o v i d e the true m e a n a n d variance for l a k e w i d e H g a c c u m u l a t i o n , w e can ask h o w l i k e l y w e are to o b t a i n , w i t h a g i v e n n u m b e r of cores, an estimate of H g a c c u m u l a t i o n that is close to the true m e a n . W e restrict this analysis to those sites w i t h at least 12 dated cores ( D u n n i g a n , M e a n d e r , T h r u s h , a n d Kjostad). M e a n s a n d standard deviations are calculated w i t h e q u a l w e i g h t i n g of cores (as o p p o s e d to u n e q u a l w e i g h t i n g b y d e p o s i t i o n a l area). T h e p r o b ability of o b t a i n i n g results that are w i t h i n ± 1 0 % a n d ± 2 5 % of the true m e a n for l a k e w i d e H g a c c u m u l a t i o n are calculated f r o m a n o r m a l d i s t r i b u t i o n for 2, 4, 8, a n d 16 cores ( F i g u r e 10). T h e s e calculations show that the l i k e l i h o o d of e s t i m a t i n g the true lakew i d e H g a c c u m u l a t i o n rate f r o m t w o cores is not v e r y good i f o u r c r i t e r i o n is ± 1 0 % of the true value (p = 0 . 2 - 0 . 3 5 ) , b u t is substantially b e t t e r i f w e ^ower o u r standards to ± 2 5 % (p = 0 . 4 5 - 0 . 7 5 ) . If w e increase the n u m b e r


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2.

Figure 10. The probability of estimating mean lakewide Hg accumulation rates within ±10% and ±25% of the true mean as a function of the number of analyzed cores. The population mean and variance for each lake are approximated from the existing core data by equal weighting of cores, and probabilities are drawn from a normal distribution. of cores to 16, o u r chances of g e t t i n g w i t h i n ± 10% of the t r u e v a l u e are as h i g h as 0 . 7 5 - 0 . 8 for some sites ( D u n n i g a n a n d M e a n d e r ) , b u t n o b e t t e r t h a n 0 . 5 - 0 . 6 for others ( T h r u s h a n d Kjostad). O n the other h a n d , 16 cores v i r t u a l l y guarantees a correct answer (p = 0 . 9 - 0 . 9 9 ) i f an estimate of ± 2 5 % of the true l a k e w i d e m e a n is acceptable. F o r some sites ( D u n n i g a n a n d M e a n d e r ) h a l f that n u m b e r o f cores does n e a r l y as w e l l .


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T h e conclusions that may be d r a w n f r o m this exercise are 1. M a n y cores (>16) are r e q u i r e d to obtain h i g h l y accurate estimates of l a k e w i d e a c c u m u l a t i o n rates, e v e n i n lakes w i t h relatively u n i f o r m s e d i m e n t d e p o s i t i o n . 2. I n certain lakes as few as four cores w i l l give the d e s i r e d result if somewhat l o w e r accuracy is acceptable.


3. T o attain the same l e v e l of accuracy, large or steep-sided basins w i t h heterogeneous sediments may r e q u i r e 4 times the n u m b e r of cores n e e d e d for small shallow lakes w i t h m o r e u n i f o r m sediments. I n practice, most m u l t i p l e - c o r e studies s h o u l d y i e l d greater accuracy than is suggested b y this analysis. Because a c c u m u l a t i o n rates v a r y greatly a m o n g different depositional regions, a large n u m b e r of cores w o u l d be r e q u i r e d to estimate the l a k e w i d e m e a n i f core c o l l e c t i o n w e r e actually r a n d o m (as assumed i n o u r statistical exercise). H o w e v e r , c o r i n g strategies are usually based o n k n o w l e d g e of basin m o r p h o m e t r y a n d s e d i m e n t a r y processes, a n d sites are selected to encompass a range of d e p o s i t i o n a l e n v i r o n m e n t s . A s s e d i m e n t d e p o s i t i o n is n o n r a n d o m a n d n e i g h b o r i n g cores f r o m the same e n v i r o n m e n t are generally similar, a s m a l l selection of representative cores f r o m specific depositional regions, each w e i g h t e d b y the area of that r e g i o n , s h o u l d p r o v i d e a m o r e accurate estimate of l a k e w i d e a c c u m u l a t i o n than a strategy of r a n d o m core c o l l e c t i o n .

Summary T h i s study demonstrates the use of m u l t i p l e - c o r e m e t h o d s to obtain w h o l e basin s e d i m e n t fluxes f r o m a suite of lakes a n d the a p p l i c a t i o n of these data to questions of atmospheric m e t a l d e p o s i t i o n . M u l t i p l e - c o r e data can be e c o n o m i c a l l y p r o d u c e d b y integrating longer core sections a n d r e d u c i n g the n u m b e r stratigraphie units for analysis. A s few as three P b analyses p e r core can y i e l d a m o d e r n a c c u m u l a t i o n rate; a d d i t i o n a l samples p r o v i d e m o r e historical detail. 2 1 0

T h e n u m b e r of cores n e e d e d to characterize a c c u m u l a t i o n i n a lake basin d e p e n d s on h y d r o l o g y , b a t h y m e t r y , a n d degree of accuracy d e s i r e d . T h e fewest cores w i l l be n e e d e d i n small lakes of u n i f o r m d e p t h that have no significant i n f l o w i n g streams. If w h o l e - b a s i n a c c u m u l a t i o n rates for a substance are p r o d u c e d for m u l t i p l e lakes i n a geographic r e g i o n , it is possible to use a s i m p l e mass-balance m o d e l to estimate b o t h the atmospheric d e p o s i t i o n rate a n d transport from the terrestrial catchment. T h e m o d e l was a p p l i e d to b o t h m o d e r n a n d p r e i n dustrial H g a c c u m u l a t i o n i n seven u n d i s t u r b e d lakes i n the u p p e r m i d w e s t


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of the U n i t e d States. M o s t of the variation i n H g fluxes a m o n g the lakes can b e e x p l a i n e d b y the m o d e l , w h i c h incorporates the ratio of c a t c h m e n t to lake area. L o c a l geological sources of H g r e p r e s e n t o n l y a m i n o r c o m p o n e n t of the total budget.


T h e atmospheric d e p o s i t i o n rate i n this m i d c o n t i n e n t a l area, w h i c h has increased b y a factor of about 3.7, suggests that natural H g concentrations w e r e o n l y about 2 5 % of m o d e r n levels. C u r r e n t estimates of recent increases i n global atmospheric H g support this c o n c l u s i o n , a n d indicate that increased anthropogenic H g emissions, rather than e n h a n c e d r e m o v a l b y a t m o s p h e r i c oxidants, are responsible for elevated H g d e p o s i t i o n . M o r e o v e r , the increase appears to be relatively u n i f o r m across o u r study area, i m p l y i n g r e g i o n a l i f not global sources for the H g f a l l i n g on these r e m o t e sites. A b o u t 2 5 % of the H g d e p o s i t e d to the terrestrial c a t c h m e n t is trans p o r t e d to the various lakes, p r o b a b l y i n association w i t h organic acids. L a k e s w i t h larger catchments (relative to t h e i r surface area) receive p r o p o r t i o n a l l y h i g h e r H g l o a d i n g . A m o n g the seven lakes s t u d i e d , b e t w e e n 40 a n d 8 0 % of H g inputs w e r e made d i r e c t l y to the lake surface.

Acknowledgments W e thank M . H o r a of the M i n n e s o t a P o l l u t i o n C o n t r o l A g e n c y for i n i t i a t i n g this project, D . H e l w i g for h e l p i n d e s i g n i n g parts of the study, a n d H . W i e g n e r , W . P o p p , a n d D . V e r s c h u r e n for assistance i n s e d i m e n t c o r i n g . S u p p o r t was p r o v i d e d b y the L e g i s l a t i v e C o m m i s s i o n o n M i n n e s o t a R e sources, the A c i d D e p o s i t i o n P r o g r a m of the M i n n e s o t a P o l l u t i o n C o n t r o l A g e n c y , a n d the U n i v e r s i t y of M i n n e s o t a W a t e r Resources Research C e n t e r . T h i s chapter is C o n t r i b u t i o n N o . 438 of the L i m n o l o g i c a l Research C e n t e r .

References 1. Goldberg, E. D.; Hodge, V. F.; Griffin, J. J.; Koide, M.; Edgington, D. N. Environ. Sci. Technol. 1981, 15, 466-471. 2. Rippey, B.; Murphy, R. J.; Kyle, S. W. Environ. Sci. Technol. 1982, 16, 23-30. 3. Norton, S. Α.; Dillon, P. J.; Evans, R. D.; Mierle, G.; Kahl, J. S. In Acidic Precipitation: Sources, Deposition, and Canopy Interactions; Lindberg, S. E.; Page, A. L.; Norton, S. Α., Eds.; Springer-Verlag: New York, 1990; Vol. 3, pp 73-102. 4. Carignan, R.; Tessier, A. Science (Washington, D.C.) 1985, 228, 1524-1526. 5. White, J. R.; Gubala, C. P.; Fry, B.; Owen, J.; Mitchell, M. J. Geochim. Cosmochim. Acta 1989, 53, 2547-2559. 6. Evans, R. D. Arch. Environ. Contain. Toxicol. 1986, 15, 505-512. 7. Rada, R. G.; Wiener, J. G.; Winfrey, M. R.; Powell, D. E. Arch. Environ. Contain. Toxicol. 1989, 18, 175-181. 8. Carignan, R.; Nriagu, J. O. Geochim. Cosmochim. Acta 1985, 49, 1753-1764. 9. Renberg, I. Hydrobiologia 1986, 143, 379-385. 10. Johnson, M. G. Can. J. Fish. Aquat. Sci. 1987, 44, 3-13. 11. Dearing, J. A. Hydrobiologia 1983, 103, 59-64.


64 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.


22. 23. 24. 25. 26. 27. 28.

29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46.


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