CANADIAN JOURNAL OF FISHERIES AND AQUATIC … present study assessed the vertical distribution (stra tigraphy) and historic accumulation of mercury in recent depositional sediments

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Stratigraphy and historic accumulation of mercury in recent depositional sediments in the Sudbury River Massachusetts USA

Bradley E Frazier James G Wiener Ronald G Rada and Daniel R Engstrom

Abstract The distribution and deposition of sedimentary mercury in (he Sudbury River were linked to an industrial complex (Nyanza site) that operated from 1917 through 1978 In two reservoirs just downstream from the Nyanza site estimated rates of mercury accumulation increased markedly in the 1920s and 1930s were greatest during 1976-1982 decreased within 5 years after industrial operations ceased and have decreased further since capping of contaminated soil at the Nyanza site was completed in 1991 The most contaminated sediments were typically buried yet the 0- to 1-cm stratum remained substantially contaminated in all cores Mercury accumulating in Ihe surficial reservoir sedishyments was probably from continuing albeit much lesser inputs from the Nyanza site whereas recent inputs to downshystream wetland areas were attributed to recycling of sedimentary mercury or to mercury from unidentified local sources In the reservoirs burial of highly contaminated sediments is gradually decreasing the amount of sedimentary mercury available for methylation In downstream wetlands however sedimentary mercury seemed to be more availshyable than that in the reservoirs for physical transport and biogeochemical cycling

Resume Le depot et la distribution de mercure sedimentaire dans la riviere Sudbury etaient associes a un complexe industriel (site Nyanza) exploite de 1917 a 1978 Dans deux reservoirs situes immediatement en aval du site Nyanza les taux estimes de Iaccumulation de mercure ont augmente nettement dans les annees 20 et 30 etaient tres eleves entre 1976 et 1982 puis ont diminue dans les cinq annees qui ont suivi la cessation des activites industrielles et contishynuent de diminuer depuis la fin des travaux de recouvrement du sol contamine de ce site en 1991 En general les sedishyments les plus contamines ont ete enfouis niais la contaniination de la strate de 0 a 1 cm est assez marquee dans toutes les carottes prelevees Laccumulation de mercure dans les sediments de surface du reservoir etait probablement due aux apports continus bien que beaucoup moindres du site Nyanza tandis que les apports recents dans les milieux humides situes en aval etaient attribuables a la remise en circulation du mercure sedimentaire ou au mercure provenant de sources locales non identifiees Dans les reservoirs Ienfouissement de sediments fortement contamines diminue proshygressivement la quantite de mercure sedimentaire disponible pour la methylation Toutefois la disponibilite du mercure sedimentaire pour le transport physique et la remise en circulation dans le cycle biogeochimique semblait etre plus grande dans les milieux humides situes en aval que dans les reservoirs

[Traduit par la Redaction]

Introduction

The Sudbury River in eastem Massachusetts was contamishynated by mercury from the Nyanza chemical waste dump site (Nyanza site) a former industrial complex that operated

Received August 29 1997 Accepted June 8 1999 J14190

BE Frazier and RG Rada River Studies Center University of Wisconsin-La Crosse La Crosse WI 54601 USA JG Wiener^ US Geological Survey Biological Resources Division Upper Midwest Environmental Sciences Center 2630 Fanta Reed Road La Crosse WI 54603 USA DR Engstrom St Croix Watershed Research Station Science Museum of Minnesota Marine on St Croix MN 55047 USA

Current address US Fish and Wildlife Service 11103 E Montgomery Dr Spokane WA 99206 USA

^Author to whom all correspondence should be addressed e-mail james_wienerusgsgov

from 1917 to 1978 in the town of Ashland (Wiener and Shields 2000) Fine-grained sediment in downstream areas of the Sudbury River contain substantial mercury posing potential risks to biota in the river and adjoining wetlands including the Great Meadows National Wildlife Refuge (Fig I) Fish in contaminated reaches of the Sudbury River contained elevated concentrations of mercury prompting adshyvisories waming against consumption of sport fish from the River (NUS Corporation 1992 Wiener and Shields 2000) To decrease further influx of mercury into the River the US Environmental Protection Agency excavated and capped highly contaminated soils at the Nyanza site a remeshydial action completed in 1991

The vast majority of mercury retained in freshwater ecoshysystems eventually becomes associated with bottom sedishyments (Kudo 1989 Rada et al 1993) Sediment can function as a sink for mercury in aquatic ecosystems if sediment-associated mercury is isolated from active biogeochemical cycling (Henry et al 1995) Conversely the sediments can serve as a source of potentially available mercury given that some of the inorganic mercury in the sediment can be meth-

Can J Fish Aquat Sci 57 1062-1072 (2000) copy 2000 NRC Canada

Frazier et al 1063

Fig 1 Map of the Sudbury River basin showing the four contaminated areas downstream from the Nyanza site (Reservoir 2 Reservoir 1 the wetland coring sites within the Great Meadows National Wildlife Refuge and Fairhaven Bay) and the reference area (Whitehall Reservoir) from which sediment cores were taken The river flows in a northeasterly direction The black bars represent dams on the Sudbury River system

Fairhaven 42deg25N Great Meadows Bay 7V37N -National Wildlife

Refuge^ ^ ^

I Wetland coring sites

Whitehall Reservoir -

42deg11 N 7 r 3 7 W

ylated (Winfrey and Rudd 1990 Ramlal et al 1993 Gilmour et al 1998)

The present study assessed the vertical distribution (strashytigraphy) and historic accumulation of mercury in recent depositional sediments in the Sudbury River focusing on depositional areas and palustrine wetlands in the river basin Burial in sediments often creates a stratigraphic record that reflects the time of deposition and relative environmental concentrations of mercury Inorganic mercury binds tightly to organic particles and shows little tendency to diffuse within the sediment profile once buried (Fitzgerald et al 1998 Bloom et al 1999) Thus long-term temporal trends in influxes of mercury to surface waters can be estimated or reconstructed by detailed analyses of sediment cores (Swain

et al 1992 Engstrom and Swain 1997) Our objectives were to (0 determine the vertical distribution of mercury in sedishyments from the river basin and ii) estimate the recent accushymulation of mercury in depositional sediments

Methods

Study area We studied five areas in the Sudbury River basin Four of these

areas (Reservoir 2 Reservoir 1 a wetland area within the Great Meadows National Wildlife Refuge and Fairhaven Bay) spanned a spatial gradient in the intensity of mercury pollution attributed to the upstream Nyanza site (Fig I) Reservoirs 2 and 1 were small reservoirs constructed in the 1870s to provide water to Boston

copy 2000 NRC Canada

1064 Can J Fish Aquat Sci Vol 57 2000

Massachusetts Reservoir 2 was filled in August 1879 and Resershyvoir 1 was filled in December 1879 (Boston Water Works 1882) The fifth study area Whitehall Reservoir was a reference area that had not been directly contaminated by wastes from the Nyanza site The inundation of Whitehall Reservoir in 1900 flooded what was formerly called Whitehall Pond

Field procedures Sediment cores were taken during 14-18 May 1994 with an 88shy

mm inside diameter piston corer (ASTM 1989) This device recovshyers the watery uncompacted sediment surface as well as deeper strata without disturbance displacement or core shortening (Wright 1991) In Reservoirs 2 and 1 cores were taken within the zones of maximum water depth and sediment thickness based on a survey by Izbicki and Parker (1991) In Fairhaven Bay and Whiteshyhall Reservoir cores were taken near the center of the zone of maximum^water depth The core from Whitehall Reservoir was taken within the boundary of the original Whitehall Pond (Boston Waterworks 1882)

Two cores were taken within the contaminated wetland area one near the main channel of the river (wetland near-channel core) and one in a vegetated palustrine wetland about 100 m from the chanshynel (wetland off-channel core) Sampling sites were identified by latitude and longitude with a global positioning system (Magellanreg Nav 1000 Pro) Sampling coordinates water depths and lengths of cores are given in Table 1

All cores were maintained in vertical position and sectioned on site with stainless steel implements that were rinsed with surface water to prevent cross-contamination between strata The uppershymost 6 cm of each core was sectioned into 1-cm strata Sediments at core depths of 6-20 cm were sectioned into 2-cm strata sedishyments at 20-50 cm into 3-cm strata and sediments deeper than 50 cm into 5-cm strata

The sediment from each stratum was placed into a labeled Zip-Locreg freezer bag which was sealed and stored on ice in a cooler while in the field For ^Pb dating we subsampled each stratum from the cores and placed each subsamplc into a 20-mL scintillashytion vial Within 8 h after collection the samples were transferred to a conventional freezer (-20degC) Frozen sediment samples were transported to the laboratory and held at -20degC until analysis Chain-of-custody procedures were followed during the transport and storage of samples (APHA et al 1992)

Laboratory procedures After thawing for 1-2 h at room temperature (20 plusmn 3degC) the

bags with samples were massaged by hand to homogenize the sedishyments Water and volatile solids content were detemiined on a 2- to 5-g wet weight subsample from each stratum (APHA et al 1992) Density of sediment was calculated from the water and volatile solshyids content with equations from Hakanson and Jansson (1983)

The remaining sediment from each stratum was air-dried at 20 plusmn 3degC ground with a porcelain mortar and pestle passed through a 2-mm sieve and homogenized (Rada et al 1993) Total mercury was determined for subsamples (02-05 g) of air-dried sediment from each stratum which were digested and analyzed by cold-vapor atomic absorption spectrophotometry following the methods in Rada et al (1993) Concentrations of total mercury are reported as micrograms per gram dry weight We determined aluminum as a reference element to assess differences in grain size of the sedishyment among strata (Rada et al 1986 Hanson et al 1993) Total acid-extractable aluminum was determined for 1-g subsamples of the dried sediment Samples were digested with 20 mL of a 211 (vvv) solution of 16 M HNO3 12 M HCl and deionized water in ignition tubes heated for 1 h at 50degC and then for 12 h at 120degC in aluminum blocks Samples were analyzed by flame atomic absorpshytion spectrophotometry (APHA et al 1992)

Sediment cores from the Sudbury River basin were analyzed for excess -Pb activity to determine age and sediment accumulation rates for the past 100-150 years -Pb was measured in 15 to 20 strata for each core through its granddaughter product -Po with -Po added as an intemal yield tracer The polonium isotopes were distilled from 05-20 g of dry sediment at 550degC following preshytreatment with concentrated HCl and plated directly onto silver planchets from a 05 N HCl solution (modified from Eakins and Morrison 1978) Activity was measured for 1 x 10 to 6 x 10 s with ion-implanted or silicon-depleted surface barrier detectors and an Ortec alpha spectroscopy system Unsupported -Pb was calcushylated by subtracting supported activity from the total activity meashysured at each level supported -Pb was estimated from the asymptotic activity at depth (the mean of the lowermost samples in a core) Dates and sediment accumulation rates were determined according to the constant rate of supply (CRS) model (Appleby and Oldfield 1978) with confidence intervals calculated by first-order error analysis of counting uncertainty (Binford 1990) The CRS model which assumes a constant flux of -Pb to the core site is the preferred model for systems with variable rates of sedishyment accumulation (Appleby and Oldfield 1983 Oldfield and Appleby 1984) However the assumption of a constant ~degPb flux may not be strictly met in the Sudbury River where sedimentation pattems might have shifted with flow or more specifically imshypoundment This dating uncertainty is greatest for older strata esshypecially in the cores from the two reservoirs Nonetheless the agreement between basal dates from the reservoir cores and the dates of impoundment provides empirical evidence that assumpshytions of the CRS model have not been seriously violated and thus our estimates of the rate of sediment accumulation and mercury acshycumulation at the coring sites are reasonably accurate

The mercury accumulation rate for each dated stratum was calculated as the product of the sediment accumulation rate (kiloshygrams per square metre per year) and the total mercury concentrashytion (micrograms per gram dry weight) in the stratum Sediment enrichment factors (SEP) and mercury flux ratios were estimated for surface and peak strata in each core The SEE was calculated with the equation

(1) SEF = ( Q - C )C

where C is the concentration of mercury in the surface or peak stratum and C( is the estimated natural abundance of mercury (deshyfined as the concentration of mercury in fine-grained sediments atshytributed to natural mercury cycling processes and native soils of watersheds before industrial influences in the basin) In each of the cores from the wetland near-channel area Fairhaven Bay and Whitehall Reservoir C was calculated as the mean concentration of the six deepest strata The grand mean of the estimated natural abundance for these three cores was used as the estimate of C^ for the cores from Reservoir 2 Reservoir I and the wetland off-channel coring sites mercury concentrations in the deepest strata of these latter three cores exceeded our estimate of its natural abundance

To assess the impact of the Nyanza site on recent and maximal mercury enrichment in the sediments from Reservoirs 2 and 1 we estimated what we termed the source-specific SEF (SSEF) for the coring sites in Reservoirs 2 and 1 with the equation

(2) SSEF = C~C pre-Nyanza )Cb

where Cp yanza is the concentration of mercury in the last stratum deposited before 1917 The SSEF is similar to the SEF but the nushymerator was adjusted by subtracting the pre-Nyanza site concentrashytion of mercury in the last stratum deposited before 1917 the year when industrial operations began at the Nyanza site In calculating the SSEF it was assumed that merciiry inputs (into Reservoirs 2


Frazier et al 1065

Table 1 Infomiation on study areas coring locations and sediment cores taken from the Sudbury River basin in May 1994

Coring location Oldest datable stratum

Study area Surface area (ha) Latitude Longitude Water depth (m) Core length (cm) Depth (cm) Date

Reservoir 2 47deg 42deg1642 71deg2630 44 55 23-26 1894 Reservoir 1 49deg 42deg1729 7Ideg2637 39 38 35-38 1888 Wetland (off-channel) na 42deg2304 71deg2259 12 60 32-35 1851 Wetland (near-channel) na 42deg2315 71deg2229 10 80 23-26 1846 Fairhaven Bay 27 42deg2528 7Ideg2112 34 110 35-38 1838

Whitehall Reservoir 233^ 42deg1303 71deg3444 83 125 60-65 1831

Note Sites in the Sudbury River are listed in order from nearest to furthest downstream from the Nyanza site Whitehall Reservoir is a reference area in the basin na not applicable

Izbicki and Parker (1991) US Geological Survey Maynard Mass topographical map 42071-D3-TM-025 Massachusetts Division of Fisheries and Wildlife Westboro Mass map of Whitehall Reservoir Estimated date of sediment deposition at the bottom of the stratum based on Pb dating

and 1) attributable to sources other than the Nyanza site remained constant since the last stratum before 1917 was deposited

The Hg flux ratio is analogous to the SEF but uses the rate of mercury accumulation rather than concentration as follows

(3) Hg flux ratio = A^ - 4b)^b

where A is the rate of mercury accumulation in the surface or peak stratum and Af is the background rate of mercury accumulation The Aj for the wetland near-channel wetland off-channel Fairshyhaven Bay and Whitehall Reservoir cores was estimated from the deepest datable stratum in each core For the Reservoir 2 and 1 cores A^ was estimated as the product of the sediment accumulashytion rate (in the oldest datable stratum) and the estimated grand mean of the natural abundance of mercury The mercury flux ratio for the Reservoir 2 and I coring sites therefore is a theoretical flux ratio based on presumed conditions in the reservoirs had they existed in preindustrial times

Quality assurance To estimate the accuracy of our mercury determinations we dishy

gested and analyzed the following quality assurance samples with each analytical batch of samples procedural blanks procedural calibration standards procedurally (matrix) spiked samples Nashytional Institute of Standards and Technology standard reference materials (NIST river sediment Buffalo River sediment and Montana soil) and a National Research Council of Canada certishyfied reference material (NRCC PACS-1 sediment) Our mean meashysured concentrations of total mercury were within the certified ranges for three reference materials and within 6 of the lower limit of the certified range for the fourth material NIST Montana soil Recovery of mercury from analyses of 70 procedurally spiked sediment samples averaged 97 plusmn 2 (95 confidence interval) Our method precision (relative standard deviation) estimated from analyses of 26 sets of triplicate subsamples averaged 9 and ranged from less than 1 to 24 Mercury concentrations in all core strata analyzed exceeded our method detection limit (07 ng-g dry weight) and our limit of quantitation (18 ng-g) both calculated according to APHA et al (1992)

The error (plusmn1 SD) of estimating the age of sediment strata was less than 2 years for strata up to the following ages 65 years in the Reservoir 2 core 30 years in the Reservoir 1 core 48 years in the wetland off-channel core 40 years in the wetland near-channel core and 67 years in the Fairhaven Bay core For the Whitehall Reservoir core the error ranged from about 3 to 4 years in strata dated up to 76 years old

Results

Variation in mercury concentration in cores taken downshystream of the Nyanza site was not caused by variation in eishyther the grain size or the volatile solids content of sediment The stratigraphic profile for the ratio of mercury to alumishynum in each core for example closely paralleled the profile for mercury concentration Consequently we do not present data on aluminum concentrations or volatile solids in the cores

Dating and sediment accumulation The ^degPb dating of the cores provided fairly robust chroshy

nologies that dated back to the mid- to late 1800s In all cases rates of sediment accumulation appeared conformshyable with little evidence of depositional hiatuses slumping or substantial bioturbation

All cores showed declining levels of unsupported -degPb to a depth of 30-60 cm Supported (background) activity indishycated where ^degPb becomes constant was reached in the deeper sections of cores from Fairhaven Bay (065 pCi-g) (1 pCi = 37 mBq) Whitehall Reservoir (056 pCi-g) and the two wetland areas (029 and 068 pCi-g) (Fig 2) Both Fairhaven Bay and Whitehall Reservoir were lake basins beshyfore impoundment of the Sudbury River (Fairhaven Bay was never dammed) therefore the older sediments in these cores can be used to estimate supported degPb for Reservoirs I and 2 which lack lacustrine sediments older than 1879 The core from Reservoir 1 which terminated on hard clay at 395 cm showed a continuous decline in total -^Pb to a minimum of 108 pCi-g indicating that supported ^Pb values were not reached at this site The core from Reservoir 2 showed a sharp transition from lacustrine sediments to peat below 26 cm -Pb activity continued to decline below this transishytion to values approaching 039 pCi-g Because peat probashybly contains less supported ^degPb than lake sediment (as shown by the wetland off-channel core) the ^degPb activities in the peat cannot be used to estimate supported activity for the overlying deposits Instead supported ^degPb for Resershyvoirs 1 and 2 was estimated from values from the Fairhaven Bay core In both cases the missing inventory of unsupshyported ^degPb estimated by extrapolation was less than 5 of the total inventory for the core indicating that dating should be reliable except for the oldest strata

In all six cores the decline in unsupported degPb activity with depth was nonexponential indicating changes in sedishy



Fig 2 Stratigraphic profiles for total -Pb in sediment cores from the Sudbury River basin Supported -Pb for Reservoirs 1 and 2 was estimated from the Fairhaven Bay core (see text) Error bars represent plusmn1 SD based on counting statistics

(a) Y 0shy

10shy J 2 Q

o ^ ^ 20shy

30shy

40-^

(NJ

agt bull c o Q a Ul

Y

jf^ K

uY^

Y ^

SOshy Reservoir 2 SO- 1 r - yen 4 - n - | 1 1 1 l l l l l l

03 10 15 10 15

0 (C)

10-^ e pound 20-Q 05

bulla 2 30shyc 03

E 40shy

T3 0) SO- Wetland Wetland

(off-channel) (near-channel) SO

01 1 10 10

lo-i (f) _ v ^ ^ ^ 20- x 30-^

^ 40^

bull 50-

60- ]

70-

80- bull

Whitehall 90-1 Reservoir

100 100^ bull 1 1

03 10 30

Total 2i0pb activity (pCi-g)

ment accumulation according to the CRS dating model (Fig 2) In the wetland near-channel core -Pb was nearly constant in the uppermost 10 cm of sediment Although it is possible that this zone of uniforin ^Pb indicates sediment mixing (bioturbation) addition of a mixing term to the CRS model changed the resulting dates only slightly because the putative mixed zone was less than 15 of the length (denshysity corrected) of the ^Pb profile (Oldfield and Appleby 1984)

The oldest date from Reservoir I was 1887 at 38 cm This value is quite reasonable given that the impoundment was created in 1879 and an additional centimetre or so of sedishy

ments lied beneath the deepest dated stratum In Reservoir 2

the shift from lake sediment to preimpoundment peat (26 cm) was dated at 1894 Although this date was 15 years later than the date of impoundment (also 1879) the sediment increment at 26-29 cm was intermediate in volatile solids content compared with shallower and deeper strata and may represent a transition layer cortesponding more closely to the time of reservoir inundation The -degPb chronologies from Whitehall Reservoir Fairhaven Bay and the two wetshyland cores were reasonable and the modeling of these data was straightforward

Plots of sediment accumulation rate versus -Pb dates showed distinctly different trends among the six cores (Fig 3) Reservoir 2 showed a doubling in sediment flux


Frazier et al 1067

Fig 3 Sediment accumulation rates versus -Pb dates for sediment cores from the Sudbury River basin Error bars represent plusmn1 SD based on first-order propagation of countini uncertainty

2000 2000shy

1 mdash mdash I mdash i i

0 02 04 06 08 1 12 14 16 0 02 04 06 08 1 12 14 16

Wetland Wetland (off-channel) (near-channel)

0 02 04 06 08 1 12 14 16 0 02 04 06 08 1 12 14 16

HWhitehall

Reservoir

0 02 04 06 08 1 12 14 16 0 02 04 06 081 mdash mdash i mdash mdash i mdash

1 12 14 16

Sediment accumulation (kg-m^-year)

from about 03 to 06 kg-m--year at the transition from peat to lacustrine sediment after 1880 which was close to the time of impoundment Increased sediment accumulation rate might be expected under conditions where a preexisting wetland was permanently inundated by a larger drainage system Likewise Whitehall Reservoir (formerly Whitehall Pond) showed a major increase in the rate of sediment accushymulation after impoundment in 1900 but declined sharply after 1910 The rale of sediment accumulation varied little at the Reservoir 2 coring site between the time of impoundshyment and 1994 the year of sampling In Reservoir 1 sediment accumulation rate fluctuated around a mean of

1 kg-m-year but was asynchronous with Reservoir 2 and the other coring sites Pattems of sediment accumulation were similar in the wetland off-channel site and Fairhaven Bay with sediment flux remaining relatively steady at 0 2shy04 kg-m~--year until about 1940 and increasing sharply thereafter At the wetland near-channel coring site sediment accumulation rates remained constant at 03 kg-m^-year until about 1980 The increase in the rate of sediment accushymulation at this wetland site after 1980 may have been real or an artifact of bioturbation in either case the increase only slightly influenced the profile of mercury accumulation for this location


1068 Can J Fish Aquat Sci Vol57 2000

Mercury concentrations and SEFs The natural abundance of mercury estimated from conshy

centrations in the six deepest strata was 0049 ng-g dry weight in the wetland near-channel core 0038 ng-g~ in Fairhaven Bay and 0046 )ag-g in Whitehall Reservoir The grand mean of these three cores was 0044 ng-g~ this value is a reasonable estimate of the preindustrial background conshycentration of mercury in fine-grained sediments in the Sudbury River basin

SEFs in contaminated sediments at coring sites downshystream from the Nyanza site ranged from 20 to nearly 1000 (Table 2) Concentrations of mercury and SEFs for the upshypermost I cm of the cores from Reservoir 2 Reservoir 1 the wetland (near-channel core) and Fairhaven Bay were roughly one fourth to one half of those in the deeper most-contaminated strata (Fig 4 Table 2) In the Reservoir 2 core taken about 43 km downstream from the Nyanza site concentrations of mercury were 18 Mg-g~ in the 0- to 1-cm stratum 41-44 ^g-g in the 6- to I2-cm stratum and 009shy014 (Jg-g~ in strata deeper than 32 cm which were probably preimpoundment soils (Fig 4) In the Reservoir 1 core taken 65 km downstream from the Nyanza site the most contaminated sediments (20-32 |ag-g~) were also 6-12 cm deep with concentrations declining from 135 xg-g~ at 5shy6 cm to 72 |ig-g~ at the sediment surface (Fig 4)

In the wetland off-channel core concentrations of mershycury ranged from 007 to 009 ng-g~ below 35 cm moving upward through the core concentrations increased about 10shyfold ranging from 086 to 120 |ig-g~ in the uppermost 32 cm (Fig 4) At this location the surface SEF was about 77 of the peak SEF (Table 2) Mercury concentration in the wetland near-channel core ranged from Il to 17 ng-g~ in the upper 10 cm was about 6 |ig-g~ at 12-16 cm deep and ranged from 003 to 007 lgbullg~ below 29 cm (Fig 4) The sharp peak in mercury concentration at 12-16 cm in this core would not have been preserved if sediments had been substantially mixed indicating that bioturbation had little impact on core stratigraphy In the Fairhaven Bay core taken about 36 km downstream from the Nyanza site concentrashytions were about 2 |tg-g in the uppermost 12 cm and ranged from 34 to 37 |ag-g in strata between 14 and 26 cm and from 003 to 005 |ig-g below 47 cm

Concentrations of mercury in the core from the reference area Whitehall Reservoir ranged from 004 to 006 |ig-g~ in strata below 70 cm Concentrations gradually increased upshyward through the core and were greatest in the uppermost 10 cm ranging from 032 to 038 jug-g (Fig 4) The surface and peak SEFs in this core were 69 and 72 respectively (Table 2)

The SSEFs for the Reservoir 2 coring site were 376 for the surface stratum and 971 for the most contaminated strashytum For the Reservoir 1 core the surface SSEF value was 149 and the maximum value was 703 Comparison of these SSEF values with the calculated SEFs (Table 2) indicated that 91-94 ofthe most recent (ie surficial) anthropogenic mercury was from the Nyanza site When the most contamishynated sediments were deposited an estimated 97-98 of the anthropogenic mercury was from the Nyanza site

Mercury accumulation rates and flux ratios The estimated annual rates of mercury accumulation were

greatest during the late 1970s to early 1980s at the Reservoir 2 and I coring sites and during the mid-1950s to the midshy1960s at the wetland near-channel coring site but have since decreased at these sites (Fig 5) The most recent estimated mercury accumulation rates and Hg flux ratios were 18-42 of the earlier maximal values at these three sites (Fig 5 Tashyble 2) The estimated rate of mercury accumulation at the Reservoir 1 coring site for example was less than 10 mg-m--year from the time of inundation through the 1920s increased greatly to a maxirrium of 29 mg-m-^-year during the late 1970s to early 1980s and thereafter declined to 57 mg-m^-year in the 0- to 1-cm stratum (Fig 5) In contrast the estimated annual rate of mercury accumulation has not changed notably at the Fairhaven Bay site since the mid-1940s or at the wetland off-channel site since the 1960s (Fig 5) The surface (most recent) mercury flux ratios were still estimated to be more than 75 of the peak mercury flux ratios at the Fairhaven Bay and wetland off-channel coring sites At the wetland off-channel coring site the estimated rates of mercury accumulation were less than 05 mg-m-shyyear before the 1930s thereafter the rate gradually inshycreased and has varied little (086-108 mg-m^-year) since about 1969 In contrast the estimated rate of mercury accushymulation at the wetland near-channel coring site has varied greatly over the past 150 years rates of mercury accumulashytion were less than 010 iTig-m-year until the early 1900s increased to nearly 20 mg-m--year in the 1950s and deshycreased to 07 mg-m--year in the 1990s At the Fairhaven Bay coring site estimated rates of mercury accumulation inshycreased in the early 1900s but varied little between 1946 and 1994 (16-24 mg-m^-year) (Fig 5) The estimated rate of mercury accumulation in the uppermost stratum (18 mg-m-shyyear) was similar to rates during about 1930-1970 when the most contaminated strata were deposited

The estimated rates of mercury accumulation in the most recently deposited sediments generally decreased with inshycreasing distance downstream from the Nyanza site (Fig 5) Recent accumulation rates were 11 57 09 07 and 18 mg-m -year at coring sites in Reservoir 2 Reservoir 1 wetland off-channel wetland near-channel and Fairhaven Bay respectively Maximal estimated rates of mercury accushymulation were similar in the Reservoir 2 and 1 coring sites but were much less at the downstream wetland and Fairshyhaven Bay coring sites

Estimated mercury accumulation rates and mercury flux ratios at the coring site in Whitehall Reservoir the reference area were very low compared with those in contaminated reaches of the Sudbury River (Fig 5) In the mid-1800s the estimated rate of mercury accumulation was about 002 mg-m^-year and the rate of sediment accumulation was about 03 kg-m--year (Figs 3 and 5) Estimated rates of both mercury and sediment accumulation began to inshycrease in the late 1800s After impoundment in 1900 the rate of sediment accumulation increased to just over 10 kg-m^-year and the estimated rate of mercury accushymulation increased to about 009 mg-m~^-year From the late 1910s to about 1940 the rate of sediment accumulation decreased to 050 kg-m ^-year but the estimated rate of mercury accumulation remained at about 009 mg-m--year The estimated rate of mercury accumulation was greatest in the 1940s (012 mg-m--year) and cortesponded to a slight


Frazier et al 1069

Table 2 Sediment enrichment factors (SEF) and mercury flux ratios in the most-recent (surface) sedishyment strata and in the most-contaminated (peak) strata of cores from the Sudbury River basin

Surfacedeg Peak

Study area SEF Hg flux ratio SEF Hg flux ratio

Reservoir 2 Reservoir 1 Wetland (off-c Wetland (near-Fairhaven Bay

lannel) channel)

Whitehall Reservoir

401 163 20 28 51

69

456 146

89 18 31

24

997 (6-8) shy717 (8-10) 26 (18-20)

130 (12-14) 97 (18-20)

72 (2-3)

1084 (6-8) 754 (8-10)

11 (6-8) 53 (12-14) 41 (2-3)

41 (16-18)

Note The sediment depth (cm) for the most-contaminated stratum is given in parentheses The surface SEF pertains to the uppermost 1 cm of a core the surface mercury flux ratio pertains to the uppemiost

1 cm of a core except the core from the wetland off-channel site (uppermost 3 cm) and the Whitehall Reservoir (uppennost 2 cm)

The peak SEF pertains to the stratum with the greatest concentration of mercury whereas the peak flux ratio pertains to the stratum with the greatest rate of mercury accumulation

Fig 4 Mercury in sediment cores from the Sudbury River basin Arrows denote the bottom of each sediment core The estimated year of deposition is given for the bottom of the stratum with the greatest mercury concentration delineated in solid black for each core The triangle denotes the last stratum deposited before 1917 the year that industrial operations began al the Nyanza site

Mercury concentration (jjg-g dry weight)

06 09 I

1987

1958

Wetland Wetland Whitehall (off-channel) (near-channel) Reservoir

43 km 65 km 27 km 28 km 36 km Reference area

Distance downstream from the Nyanza site

increase in the rate of sediment accumulation (058 kg-m - and 1 the two coring areas nearest the Nyanza site coinshyyear) Since the early 1950s the estimated rate of mercury cided with industrial operations there The estimated rate of accumulation has ranged from 006 to 010 mg-m~^-year mercury accumulation at-the reservoir coring sites increased correlating with the rate of sediment accumulation (019- markedly in the 1920s and 1930s soon after the industrial 032 kg-m--year) complex opened in 1917 and were greatest during the late

1970s to early 1980s Estimated rates of mercury accumulashytion at the two reservoir coring sites decreased a few years Discussion after industrial operations ended in 1978 and have decreased

Temporal and spatial pattems in mercury contamination of further since excavation and capping of contaminated soil at sediments and analysis of SEFs confirm the Nyanza site as a the Nyanza site was completed in 1991 primary source of the mercury in the Sudbury River ecosysshy Concentrations and accumulation rates of mercury in reshytem The timing of mercury accumulation in Reservoirs 2 cent sediment deposits generally decreased with increasing


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Can J Fish Aquat Sci Vol 57 2000

Fig 5 Estimated annual rates of mercury accumulation in sedishyment cores from the Sudbury River basin Note the two changes in scale on the horizontal axis

-2 -1 Mercury accumulation rate (mg-m -year )

distance downstream from the Nyanza site This pattern in sedimentary mercury concentration was also found by Colman et al (1999) and Naimo et al (2000) In our study the concentration of mercury in sediments was greatest (44 |ig-g dry weight) in Reservoir 2 the first reservoir downstream from the Nyanza site

Our estimates of the natural abundance of mercury in cores from the Sudbury River basin (0038-0049 |ig-g~) were similar to estimates for fine-grained sediments from midcontinental lakes and rivers in North America Rada et al (1989) for example reported natural mercury concentrashytions of 004-007 ng-g in cores from 11 northern Wisconshysin lakes Balogh et al (1997) estimated a value of 0044 ng-g for soils in the Minnesota River basin and JG Wiener (US Geological Survey La Crosse Wis unpubshylished data) estimated a natural abundance of 0041 ^ig-g in a core from Lake Pepin on the Upper Mississippi River

Concentrations of mercury in the most recently deposited sediments (uppennost 1 cm) ranged from 093 to 177 ng-g in our cores taken downstream from the Nyanza site far exshyceeding the natural abundance of mercury in fine-grained sediments from the river basin The high concentrations in surficial sediments were presumably not derived from the upward movement of mercury within the core profiles Upshyward diffusion of mercury from the deeper more contamishynated strata seems improbable given the high organic content of these sediments (Gobeil and Cossa 1993 Hurley et al 1994) Substantial vertical mixing of sediments was not evident from the -degPb profiles Rather the high concenshytrations in surficial sediments indicate that some mercury is still entering or recycling within the ecosystem The continushying recent accumulation of mercury in Reservoirs 2 and I

was probably from the continuing albeit much lesser inputs of mercury from the Nyanza site based on our findings and the mass-balance analysis of Waldron et al (2000)

Further downstream at the wetland off-channel and Fairshyhaven Bay locations estimated rates of mercury accumulashytion and mercury flux ratios in the surficial sediments were more than three fourths of the historic maximal values This temporal pattern is in contrast with the recent substantive deshycreases in mercury accumulation rates at the coring sites in Reservoirs 2 and 1 The continued high rates of mercury acshycumulation at the wetland off-channel and Fairhaven Bay sites are attributable partly to the increased rates of sedimenshytation at these sites during recent decades Moreover some of the mercury accumulating in surficial sediments at the wetland off-channel and Fairhaven Bay sites inay have origishynated from other unidentified point sources downstream of the Nyanza site (NUS Corporation 1992 Waldron et al 2000)

Mercury in surficial sediments at these downstream sites cannot be logically attributed to contemporary inputs of mershycury from the Nyanza site or to mobilization of contamishynated sediments from the reservoirs upstream (Reservoir 2 Reservoir 1 and the Saxonville Impoundment) In 1995 only 32 of the mercury load transported in the river downshystream of the Saxonville Impoundment (15 km downstream) was attributable to contemporary inputs froin the Nyanza site (Waldron et al 2000) Under normal flow conditions the upstream impoundments export considerably less total mercury than they import (Waldron et al 2000) Results of sediment transport modeling similarly indicate that very litshytle contaminated bed sediment is mobilized and transported from Reservoirs 2 and 1 to downstream areas even during record floods (D Abraham US Army Corps of Engineers Waterways Experiment Station Vicksburg Miss unpubshylished data)

A possible source of the continuing sedimentary mercury accumulations at the wetland oft-channel and Fairhaven Bay sites is remobilization and recycling of contaminated sedishyments from wetland areas or the river channel upstream Waldron et al (2000) found that during July 1995 concenshytrations of total mercury in the river increased from about 5 to 25 ng-L after flowing through 5 km of palustrine wetshylands downstream from the Saxonville Impoundment This suggests that mercury was being transported laterally from the adjoining wetlands presumably in association with susshypended sediment or organic particles (Balogh et al 1997 Waldron et al 2000)

The pattern of mercury deposition at our wetland near-channel coring site (taken about 5 m beyond the river bank) was similar to that in a core taken within the river channel at a point about 6 km upstream by Colman et al (1999) These two cores had low mercury concentrations at depths greater than 25 cm maximum concentrations between 5 and 15 cm and decreasing concentrations toward the surface of the sedishyment At our near-channel coring site the estimated mercury accumulation rate declined sharply in recent decades as it did in the two upstream reservoir sites but the decrease preshyceded the closure of the Nyanza site by about 20 years The estimated maximum rate of mercury accumulation at the wetland near-channel location occurred during the tnidshy1950s to the mid-1960s whereas the largest recorded flood


Frazier et al 1071

in the river occurred in 1955 Maximum flow during this flood at the Reservoir 1 dam was 137 m^-s For comparishyson the 110-year average flow at the dam was 33 m^-s (US Geological Survey 1987) The 1955 flood which is considered a 100-year flood could have moved substantial amounts of mercury from the Nyanza site to the wetland arshyeas downstream Concentrations of total mercury for examshyple reached 92 ng-L at a location just downstream from the Nyanza site during peak flow after a brief storm in July 1995 (Waldron et al 2000) Similarly in the Minnesota River concentrations of total mercury ranged from less than 10 ng-L in winter (low-flow months) to greater than 35 ng-L following a spring precipitation event (Balogh et al 1997) The decrease in mercury concentration and mershycury accumulation rate toward the surface of the wetland near-channel core could reflect remobilization or dilution of highly contaminated sediments deposited during the 1955 flood

As expected sedimentary concentrations and estimated recent accumulation rates of mercury in contaminated reaches of the Sudbury River vastly exceeded those in cores from the Whitehall Reservoir as well as several mid-continental North American lakes that received mercury largely via atmospheric deposition (Rada et al 1989 Engstrom and Swain 1997) Surficial sediments in the core from the Whitehall Reservoir were enriched with mercury and the maximum SEF of 72 in the Whitehall Reservoir core was tnuch greater than the maximum SEFs (08-28) reshyported for 11 northern Wisconsin seepage lakes lacking on-site anthropogenic sources of mercury (Rada et al 1989) The enrichment of surficial lacustrine sediments in North America is a widespread phenomenon often attributed to atshymospheric deposition of mercury from anthropogenic sources (Rada et al 1989 Swain et al 1992 Engstrom and Swain 1997)

The cores from Reservoir 2 Reservoir 1 the wetland near-channel site and Fairhaven Bay had subsurface peaks in total mercury concentration a stratigraphic pattern obshyserved in other aquatic systems where mercury loadings from industrial point sources have decreased (Rada et al 1986 Parks and Hamilton 1987 Lodenius 1991 Klein and Jacobs 1995) The gradual burial of the most contaminated sediments is probably decreasing the amount of inorganic mercury available for microbial methylation within the resshyervoirs and Fairhaven Bay given that the net rate of methylation is consistently greatest in the uppermost 5 cm of the sediment profile and that little methyl mercury is proshyduced in deeper sediments (Rudd et al 1983 Korthals and Winfrey 1987 Gilmour et al 1998) Disturbance of the conshytaminated sediments in the Sudbury River could however substantively increase the exposure of resident biota to mershycury Contaminated bottoin sediments can be resuspended into the water column by human disturbance or by natural processes such as bioturbation and wind-induced turbulence (Kristensen et al 1992 Evans 1994) Results from -degPb datshying of our sediment cores however indicated intact sedishyment profiles showing little evidence of bioturbation or past physical disturbance The profiles for mercury in the cores from Reservoirs 1 and 2 also showed no evidence of bioshyturbation or physical disturbance These cores displayed fine-scale trends of decreasing contamination from the most

contaminated strata to the sediment-water interface a pattern evident across 1-cm strata in each of the cores Simishylarly sediment-transport modeling of Reservoirs 2 and 1 (D Abraham US Army Corps of Engineers Waterways Exshyperiment Station Vicksburg Miss unpublished data) showed that scouring andmovement of sediments would be minimal and occur only in small localized areas of the resshyervoirs even during extreme floods Barring human disturshybance or dam failure the probability of substantial resuspension or transport of the contaminated sediments in the reservoirs would seem to be small

Acknowledgments

This work was funded by the US Environmental Protecshytion Agency (Region I) through an interagency agreement with the US Fish and Wildlife Service (Region 5) We thank Pamela Shields Susan Svirsky and Steven Mierzyshykowski for assistance during the initial planning of this study We are grateful to personnel at the Great Meadows National Wildlife Refuge for allowing the use of their facilshyity during sampling trips Teresa Naimo and Elizabeth Allen assisted with the sampling Elizabeth Allen Lorrine Rabuck Chad Hammerschmidt and Kristi Jackson provided technishycal assistance in the laboratory We thank Clare Kirk and Thomas Patriarca of the Massachusetts Water Resource Aushythority for providing access to archived historical records on the reservoirs We are grateful to Teresa Naimo William Orem David Powell Edward Swain Marcus Waldron two anonymous referees and an Associate Editor for construcshytive reviews of an earlier draft of the manuscript

References

APHA et al (American Public Health Association American Water Works Association and Water Environment Federation) 1992 Standard methods for the examination of water and wastewater 18th ed APHA Washington DC

Appleby PG and Oldfield R 1978 The calculation of lead-210 dates assuming a constant rate of supply of unsupported -degPb to the sediment Catena 5 1-8

Appleby RG and Oldfield R 1983 The assessment of -degPb data from sites with varying sediment accumulation rates Hydroshybiologia 103 29-35

ASTM (American Society of Testing and Materials) 1989 Stanshydard guide for core sampling submerged unconsolidated sedishyments In 1989 Annual book of ASTM standards water and environmental technology Vol 1102 Water (II) ASTM Philashydelphia Pa pp 585-597

Balogh SJ Meyer ML and Johnson DK 1997 Mercury and suspended sediment loadings in the lower Minnesota River Enshyviron Sci Technol 31 198-202

Binford MW 1990 Calculation and uncertainty analysis of degPb dates for PIRLA Project lake sediment cores J Paleoliinnol 3 253-267

Bloom NS Gill GA Camppellino S Dobbs C McShea L Driscoll C Mason R and Rudd J 1999 Speciation and cyshycling of mercury in Lavaca Bay Texas sediments Environ Sci Technol 33 7-13

Boston Water Works 1882 Additional supply from Sudbury River Rockwell and Churchill Boston Mass

copy 2000NRC Canada


Colman JA Waldron MC Breault RR and Lent RM 1999 Distribution and transport of total mercury and methylmercury in mercury-contaminated sediments in reservoirs and wetlands of the Sudbury River east-central Massachusetts US Geol Surv Water-Resour Invest Rep 99-4060

Eakins JD and Morrison RT 1978 A new procedure for the determination of lead-210 in lake and marine sediments Int J Appl Radiat Isot 29 531-536

Engstrom DR and Swain EB 1997 Recent declines in atmoshyspheric mercury deposition in the Upper Midwest Environ Sci Technol 31 960-967

Evans RD 1994 Empirical evidence of the importance of sedishyment resuspension in lakes Hydrobiologia 284 5-12

Fitzgerald WR Engstrom DR Mason RP and Nater EA 1998 The case for atmospheric mercury contamination in reshymote areas Environ Sci Technol 32 1-7

Gilmour C C Riedel GS Ederington MC Bell JT Benoit JM Gill GA and Stordal MC 1998 Methylmercury conshycentrations and production rales across a trophic gradient in the northern Everglades Biogeochemistry 40 327-345

Gobeil C and Cossa D 1993 Mercury in sediments and sedishyment pore water in the Laurentian Trough Can J Fish Aquat Sci 50 1794-1800

Hakanson L and Jansson M 1983 Principles of lake sedimentology Springer-Verlag Berlin

Hanson PJ Evans DW Colby DR and Zdanowicz VS 1993 Assessment of elemental contamination in estuarine and coastal environments based on geochemical and statistical modeling of sediments Mar Environ Res 36 237-266

Henry EA Dodge-Murphy LJ Bigham GN Klein SM and Gilmour CC 1995 Total mercury and methylmercury mass balance in an alkaline hypereutrophic urban lake (Onondaga Lake NY) Water Air Soil Pollut 80 509-518

Hurley JP Krabbenhoft DR Babiarz CL and Andren AW 1994 Cycling processes of mercury across sedimentwater intershyfaces in seepage lakes In Environmental chemistry of lakes and reservoirs Edited by LA Baker American Chemical Society Washington DC pp 425^49

Izbicki JA and Parker GW 1991 Water depth and thickness of sediment in Reservoirs 1 and 2 Framingham and Ashland MasshysachuseUs US Geol Surv Open-File Rep 91-508

Klein SM and Jacobs LA 1995 Distribution of mercury in the sediments of Onondaga Lake NY Water Air Soil Pollut 80

1035-1038 Korthals ET and Winfrey MR 1987 Seasonal and spatial variashy

tions in mercury methylation and demethylation in an oligoshytrophic lake Appl Environ Microbiol 53 2397-2404

Kristensen P Sondergaard M and Jeppesen E 1992 Resuspension in a shallow eutrophic lake Hydrobiologia 228 101-109

Kudo A 1989 Mercury in the Ottawa River (Canada) In Aquatic ecotoxicology fundamental concepts and methodologies Vol L Edited by A Boudou and F Ribeyre CRC Press Boca Raton Fla pp 201-217

Lodenius M 1991 Mercury concentrations in an aquatic ecosysshytern during twenty years following abatement of the pollution source Water Air Soil Pollut 56 323-332

Naimo TJ Wiener JG Cope WG and Bloom NS 2000 Bioavailability of sediment-associated mercury to Hexagenia mayflies in a contaminated floodplain river Can J Fish Aquat Sci 57 1092-1102

NUS Corporation 1992 Final remedial investigation report Nyanza Operable Unit III Sudbury River study Middlesex County Masshysachusetts Vol I NUS Rep W92194R NUS Corporation Wil- mington Mass

Oldfield R and Appleby PG 1984 Empirical testing of lOpbshydating models for lake sediments In Lake sediments and envishyronmental history Edited by EY Haworth and JWG Lund University of Minnesota Press Minneapolis Minn pp 93-124

Parks JW and Hamilton AL 1987 Accelerating recovery ofthe mercury-contaminated WabigoonEnglish River system Hydroshybiologia 149 159-188

Rada RG Pindley JE and Wiener JG 1986 Environmental fate of mercury discharged into the upper Wisconsin River Washyter Air Soil Pollut 29 57-76

Rada RG Wiener JG Winfrey MR and Powell DE 1989 Recent increases in atmospheric deposition of mercury to north-central Wisconsin lakes inferred from sediment analyses Arch Environ Contam Toxicol 18 175-181

Rada RG Powell DE and Wiener JG 1993 Whole-lake burshydens and spatial distribution of mercury in surficial sediments in Wisconsin seepage lakes Can J Fish Aquat Sci 50 865-873

Ramlal PS Kelly CA Rudd JWM and Furutani A 1993 Sites of methyl mercury production in remote Canadian Shield lakes Can J Fish Aquat Sci 50 972-979

Rudd JW Turner MA Furutani A Swick AL and Townsend BE 1983 The English-Wabigoon River system I A synthesis of recent research with a view towards mercury amelioration Can J Fish Aquat Sci 40 2206-2217

Swain EB Engstrom DR Brighain ME Henning TA and Brezonik PL 1992 Increasing rates of atmospheric mercury deposition in midcontinental North America Science (Washingshyton DC) 257 784-787

US Geological Survey 1987 Water resources data for Massachushysetts and Rhode Island water year 1985 US Geol Surv Water-Data Rep MA-RL85-1

Waldron MC Colman JA and Breault RR 2000 Distribution hydrologic transport and cycling of total mercury and methyl mercury in a contaminated river-reservoir-wetland system (Sudbury River eastern Massachusetts) Can J Pish Aquat Sci 57 1080-1091

Wiener JG and Shields PJ 2000 Mercury in the Sudbury River (Massachusetts USA) pollution history and a synthesis of reshycent research Can J Pish Aquat Sci 57 1053-1061

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Wright HE Jr 1991 Coring tips J Paleolimnol 6 37^9


Frazier et al 1063

Fig 1 Map of the Sudbury River basin showing the four contaminated areas downstream from the Nyanza site (Reservoir 2 Reservoir 1 the wetland coring sites within the Great Meadows National Wildlife Refuge and Fairhaven Bay) and the reference area (Whitehall Reservoir) from which sediment cores were taken The river flows in a northeasterly direction The black bars represent dams on the Sudbury River system

Fairhaven 42deg25N Great Meadows Bay 7V37N -National Wildlife

Refuge^ ^ ^

I Wetland coring sites

Whitehall Reservoir -

42deg11 N 7 r 3 7 W

ylated (Winfrey and Rudd 1990 Ramlal et al 1993 Gilmour et al 1998)

The present study assessed the vertical distribution (strashytigraphy) and historic accumulation of mercury in recent depositional sediments in the Sudbury River focusing on depositional areas and palustrine wetlands in the river basin Burial in sediments often creates a stratigraphic record that reflects the time of deposition and relative environmental concentrations of mercury Inorganic mercury binds tightly to organic particles and shows little tendency to diffuse within the sediment profile once buried (Fitzgerald et al 1998 Bloom et al 1999) Thus long-term temporal trends in influxes of mercury to surface waters can be estimated or reconstructed by detailed analyses of sediment cores (Swain

et al 1992 Engstrom and Swain 1997) Our objectives were to (0 determine the vertical distribution of mercury in sedishyments from the river basin and ii) estimate the recent accushymulation of mercury in depositional sediments

Methods

Study area We studied five areas in the Sudbury River basin Four of these

areas (Reservoir 2 Reservoir 1 a wetland area within the Great Meadows National Wildlife Refuge and Fairhaven Bay) spanned a spatial gradient in the intensity of mercury pollution attributed to the upstream Nyanza site (Fig I) Reservoirs 2 and 1 were small reservoirs constructed in the 1870s to provide water to Boston














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Documents

CANADIAN JOURNAL OF FISHERIES AND AQUATIC … present study assessed the vertical distribution (stra tigraphy) and historic accumulation of mercury in recent depositional sediments