Chapter 7

The Distribution of Mercury in Sediment from
Long Island Sound and Surrounding Marshes


By
Johan C. Varekamp1, Marilyn R. Buchholtz ten Brink2, Ellen L. Mecray2,
and B. Kreulen1
 
 

Table of Contents
Introduction
Methods
Mercury Distribution in LIS Sediments
Acknowledgments
References
Figure Captions
Table Captions
Digital Data
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INTRODUCTION

Researchers from the Coastal and Marine Geology Program of the U.S. Geological Survey  and from the Department of Earth &  Environmental Sciences  at Wesleyan University have studied the sources and distribution of Mercury (Hg) in surface sediment and cores from Long Island Sound (LIS), as described in detail in Varekamp and others, 2000. In addition, cores from the surrounding  coastal salt marshes and several freshwater marshes in Connecticut were analyzed (Kreulen, 1999). Most sediment samples have been analyzed for a variety of other chemical and physical parameters in addition to Hg (Buchholtz ten Brink and Mecray, 1998; Buchholtz ten Brink and others, 2000; Mecray and Buchholtz ten Brink, 2000). The maps and graphs presented here show the variations of the Hg concentrations in space and time, and we evaluate the impact of sediment parameters on the distribution of Hg concentrations in LIS surface sediment.
 

METHODS

Figure 1 - Sample locationsSurficial sediments (0-2cm depth) were collected in LIS with a grab sampler which was deployed with on-board video control. Three cruises were made in 1996 and 1997 and a total of 405 surface samples were selected for Hg analyses (Fig. 1). Several core transects were established in LIS and 11 cores were selected for Hg analyses (Fig. 1).  Cores from coastal marshes were taken with a 10 cm diameter metal corer and 16 cores were analyzed to a depth where background values were obtained (Kreulen, 1999).

Aliquots of dried sediment samples were leached with a mixture of acids and oxidizing substances (HNO3-H2SO4-K2S2O8-KMnO4) to extract the Hg into solution. The resulting solution was first treated with hydroxyl-amine-hydrochloride to reduce excess oxidants and then reduced with stannous chloride in an extraction cell into a stream of Hg-free Nitrogen gas. The gas stream with the extracted Hg was then carried into an Atomic Absorption Spectrometer designed for Hg analyses. This double beam instrument (LDC Hg Monitor 3200) has an absolute detection limit of about1 ng Hg  and the signal output was transmitted to an integrator. The instrument was calibrated with Hg vapor injections and standards made from a HgCl2 solution. The instrument was standardized every day and Hg concentrations in the blank and in standard samples (NIST 2704) were monitored. Overall precision in the marsh core samples and LIS surface sediment samples is about 12 %, whereas the LIS core samples have a precision of about 5 %. The latter were run with a modified gas flow path through the extraction cell of the instrument. The analytical data presented here are not total Hg concentrations but acid-leachable Hg.  The Hg recovery from standard sample NIST 2704 with our method was about 88 %. We use below the analytical data as well as "excess Hg concentrations", which are the Hg concentrations minus the natural background Hg concentration.
 

MERCURY DISTRIBUTION IN LIS SEDIMENTS

The analytical data for surface sediment from LIS (Fig. 2, Table 1) show  low values in the Eastern Sound, with many of them close to the natural background values of about 50 ppb Hg. Sediment samples from the central basins have values from100-400 ppb Hg, whereas sediment from the westernmost Sound has values up to 650 ppb Hg. The Hg distribution map was contoured with an averaging routine and a Hg concentration contour map shows the Hg hot spots for LIS (Fig. 3). The high Hg concentration zones are generally found in the depositional areas with an abundance of fine-grained sediment and high concentrations of organic matter. The low Hg areas are found in transportational or erosional sedimentary regimes (Knebel and Poppe, 2000; Poppe and others., 2000), predominantly in the eastern Sound. The observed east-west trend in Hg concentrations in LIS (Fig. 4) may be caused by changes in sediment characteristics in LIS. Normalization of the Hg data on grain size parameters (% fines or mean grain size) or on a chemical proxy for grain size parameters (e.g., % Fe) creates a spatial pattern with a less steep east-west gradient (Fig. 5). Part of the east-west increase of Hg concentrations is probably the result of focussing of fine-grained, contaminated sediment in the western Sound, but the residual trend indicates the presence of local Hg sources along the densely populated western shores of LIS.

The core studies in LIS (Table 2) show in many cores a traditional contamination pattern for Hg: background values at depth of 25-50 cm of 30-80 ppb Hg, higher Hg concentrations in the core top parts, with again lower concentrations in the upper 5-10 cm (Fig. 6). Many other LIS cores show apparently homogenized Hg contamination profiles, with Hg concentrations that are consistently above the natural background but which show no trend with depth. Many of the other parameters measured on these cores (e.g., C. perfringens) also show evidence for sediment reworking.

Analytical data from the salt marsh cores (Table 3; locations in Table 4.) show similar Hg concentrations as found in LIS, with the exception of the Housatonic River estuary, which has much higher Hg concentrations (up to 1.5 ppm Hg). Concentrations do not show a significant pattern from east to west, but the total Hg burdens (total Figure 7 - Hg burdensamount of excess Hg deposited per cm2) increases from east to west (Fig. 7). The age information from the marsh cores indicates that Hg contamination started around AD 1850 + 20 in the east and possibly 20-30 years earlier in western Connecticut. The contamination peaked between 1960 and 1975 and then decreased by up to 50% over the last three decades of the 20th century (Fig. 8), conform findings by Engstrom and Swain (1997). Mercury data from freshwater marsh cores taken along the Connecticut River indicate similar concentrations, but large Hg burdens prevail in these mud-rich sediments. Marsh cores taken on table-mountain tops where the only Hg input occurs from local atmospheric deposition have similar Hg concentrations as the river and salt marsh core samples.

The main Hg sources in Connecticut and the wider LIS region are atmospheric
deposition, effluents from waste water treatment plants (WWTP) and local point sources such as those presumably present in the Housatonic River area. Integration of our data with the C. perfringens counts (Buchholtz ten Brink and others, 2000) suggests Figure 9 - Schematic view of Hg contaminationvariable WWTP contributions to the LIS Hg budget (~10-50 %), with higher values in westernmost LIS (50-70%), and in some local spots up to 100 % WWTP contributions. The Hg contamination in southern New England probably stems from atmospheric Hg deposition in the watersheds (Mason and others, 1994; USEPA, 1997), with additional contributions from local Hg sources. The enrichment of Hg in western LIS surface sediments has ultimately three causes: 1) Transport of Hg-contaminated fine-grained sediment to the western part of the Sound, 2) Input of Hg-rich sediment from the Housatonic River Basin, and 3) Mercury inputs associated with WWTP effluents in western LIS.  The spatial pattern of Hg contamination in LIS and the potential Hg sources are schematically shown in Figure 9.
 

ACKNOWLEDGMENTS

The work has been carried out with funding from the Connecticut Department of Environmental Protection, the Connecticut Sea Grant College Program and the Coastal and Marine Geology Program at the U.S. Geological Survey in Woods Hole, MA. Many Wesleyan University students participated in this research project.
 

REFERENCES

Buchholtz Ten Brink, M.R. and Mecray, E.L., 1998, Contaminant distribution and accumulation in sediments of Long Island Sound: Field work and initial results. In: L.J. Poppe and C. Polloni (eds.), Long Island Sound Environmental Studies. U.S. Geological Survey Open-File Report 98-502, 1 CD-ROM.

Buchholtz Ten Brink, M.R.; Mecray, E.L., and Galvin, E.L., 2000, Clostridium perfringens in Long Island Sound sediments: An urban sedimentary record: Thematic section, Journal of Coastal Research.

Engstrom, D.R. and Swain, E.B., 1997, Recent declines in atmospheric mercury deposition in the upper Midwest: Environmental Science and Technology, v. 31, p. 960-967.

Knebel. H.J. and Poppe, L. J.,2000, Sea-Floor environments within Long Island Sound: A regional overview: Thematic Section. Journal of Coastal Research.

Kreulen, B., 1999, Mercury Pollution in and Around Long Island Sound: M.A. Thesis, Wesleyan University, Middletown, CT, 176 p.

Mason, R.P.; Fitzgerald, W.F., and Morel, F.M., 1994, The biochemical cycling of elemental mercury: Anthropogenic influences: Geochimica Cosmochimica Acta, v. 58, p. 3191-3198.

Mecray, E.L. and Buchholtz Ten Brink, M.R., 2000, Contaminant distribution and accumulation in the surface sediments of Long Island Sound:  Thematic Section, Journal of Coastal Research.

Poppe, L.J.; Knebel, H.J.; Mlodzinska, Z.J.; Hastings, M.E., and Seekins, B.A., 2000, Distribution of surficial sediment in Long Island Sound and adjacent waters: texture and total organic carbon. Thematic Section, Journal of Coastal Research.

U.S. Environmental Protection Agency (USEPA), 1997, Mercury Study: Report to Congress. EPA report # 452/R-97-0003. World Wide Web: www.epa.gov/ttnuatw1/112nmerc/mercury.html., 8 volumes.

Varekamp, J.C.; Buchholtz Ten Brink, M.R.,; Mecray, E.L, and Kreulen, B. 2000, Mercury in Long Island Sound sediments: Thematic Section, Journal of Coastal Research.
 

FIGURE CAPTIONS

Figure 1. Sampling locations in Long Island Sound: surface sediment samples and core locations are indicated. The studied salt marsh locations are indicated with a thumbnail.

Figure 2. Analytical data for Mercury in Long Island Sound surface sediments. Note the increase in Hg concentrations from east to west.

Figure 3. Contour map of Hg concentrations in Long Island Sound.

Figure 4. East-west cross section through Long Island Sound, showing the regional trend in Hg concentration.

Figure 5. Grain size normalized data for the east-west profile through Long Island Sound: (a) Hg normalized on mean grain size and (b) Hg normalized on Fe concentrations. In both profiles, the scatter is reduced compared to Figure 4 and the east-west trend is less steep. The residual trend in both graphs is still strong, however, indicating Hg sources at the western side of the Sound.

Figure 6. Concentration profiles for Hg and C. perfringens in two Long Island Sound cores  (cores A1C1 and A4C1).

Figure 7. Excess Hg burdens in marsh cores from coastal salt marshes along Long Island Sound, and from fresh water marshes in Connecticut. Note the general increase westward in the salt marshes, culminating in the high burden in the Housatonic River (KI). The high Hg burdens in the Connecticut River freshwater marshes CP3 and DM stems from their mud-rich character.

Figure 8. Mercury profiles from the Barn Island marshes (BI) and the Farm River  (BF) marshes, both in Connecticut. Note the onset of Hg contamination between 1800 and 1850 AD and the decrease in Hg concentrations in the upper core sections.

Figure 9. Schematic view of Hg contamination in Connecticut and Long Island Sound. Mercury deposition in the drainage basins by direct atmospheric deposition (AD) is the main source of Hg for the marshes and the Sound. Additional sources are local Hg point sources, as presumably found along the Housatonic River, and waste water treatment plant effluents.
 

TABLE CAPTIONS

Table 1.  Sample locations and corresponding analytical data for Fe, Hg, and mean grain size for grab samples collected in Long Island Sound. A copy of this table (c7tab1.xls) may also be downloaded in Microsoft Excel format format.

Table 2.  Analytical data for Fe and Hg and depths below the sediment/water interface in cores collected in Long Island Sound. A copy of this table (c7tab2.xls) may also be downloaded in Microsoft Excel format format.

Table 3.  Analytical data for Hg, Fe, Al, Mn, Cu, Zn, and Pb in cores collected in salt marshes around Long Island Sound. A copy of this table (c7tab3.xls) may also be downloaded in Microsoft Excel format format.

Table 4.  List of salt marshes sampled and the station locations for the cores collected from salt marshes around Long Island Sound. A copy of this table (c7tab4.xls) may also be downloaded in Microsoft Excel format format.


1 Department of Earth & Environmental Sciences, Wesleyan University, Middletown CT 06459-0139
2 U.S. Geological Survey, Coastal and Marine Geology Team, Woods Hole, Massachusetts 02543 [an error occurred while processing this directive]