WATER RESOURCES RESEARCH GRANT PROPOSAL
Title: Cycling and Speciation of Mercury in the Soil of Acadia National Park
Focus Categories: HYDGEO, TS, NPP
Keywords: Geochemistry, Toxic Substances
Federal Funds Requested: $20,200.
Non-Federal Funds Pledged: $29,346.
Principal Investigators: Aria Amirbahman (Department of Civil and Environmental Engineering), Terry A. Haines (U.S. Geological Survey and the Department of Biological Sciences) and Jeffrey S. Kahl (Water Research Institute).
Congressional District: Second
Statement of critical Regional Problem
Even though the waters surrounding the Acadia National Park are largely unaffected by direct discharge of mercury, fish from these waters have some of the highest values of mercury in the nation with concentrations in some fish exceeding 3 ppm. This has led the state health agencies to issue more fish consumption advisories for mercury than for all other contaminants combined. The high mercury concentrations also pose a threat to fish-eating birds and mammals. The impending deregulation of the electric utility industry raises the additional concern that deposition of mercury may increase in the future. The possible future operation of coal-fired power plants in the coastal eastern and midwestern USA will also result in more input of pollutants such as mercury in Maine. Given the importance of the preservation of a pristine environment in the Acadia National Park, the threat to the wildlife, and the recreational implications of the mercury advisories, a better understanding of the role of the watersheds in the park in cycling and transport of mercury is needed.
Statement of Results and Benefits
The soils of the forested watersheds are known as important sources for the methylation of Hg and the export of both Hg and monomethyl mercury (CH3Hg). The mercury burden in higher organisms consists almost entirely of CH3Hg, with a biomagnification factor of about 10 between trophic levels. Successful completion of this study will contribute to the currently limited mechanistic knowledge of the role of forest soils in speciation and transport of Hg and CH3Hg. When combined with other ongoing projects at Acadia, the information collected in this study will also assist us obtain mass balance on mercury species in the entire watershed. This information can be used to define aquatic resources at risk from high mercury concentrations in biota.
Nature, Scope and Objectives of the Research
Mercury is an important contaminant of major concern in the northeastern United States. The biota in Acadia National Park (ANP) possesses some of the highest Hg concentrations in the world for the sites with no point source. This is primarily due to the atmospheric input of Hg averaging a wet-only deposition of 7.9 mg/m2-yr into ANP. Dry deposition is expected to be a major source of Hg as well. The University of Maine and the National Park Service are presently conducting a ‘paired watershed’ study to gain more insight into the biogeochemistry of Hg in two watersheds at ANP, one recently burned, with thin soils and deciduous vegetation, and one unburned, with thicker soil and coniferous vegetation. As part of this study, the concentrations of Hg and methylmercury (CH3Hg) is measured in the litterfall, throughfall and stream water. Wet deposition is monitored by the Mercury Deposition Network (MDN). As such, a general mass balance may be established that accounts for the main sources and sinks of Hg in the watershed.
In ANP, however, the role of soil in the accumulation and possible methylation of Hg is unknown and requires further study. Depending upon the presence of organic carbon in soil and in groundwater and physical factors such as soil drainage, the distribution and speciation of Hg will vary. In the forested watersheds, production of CH3Hg is expected to take place primarily in the subsurface under anaerobic conditions.
The objective of this study is to study cycling and transport of Hg and CH3Hg in the soil of ANP. This study includes sampling and analysis of the physical and chemical parameters that control the fate of mercury species in soil. Similar to the ongoing study, a ‘paired watershed’ approach is followed, where a recently burned watershed and an unburned watershed in Acadia are studied. The contrasting characteristics of these two watersheds will allow us to better understand the factors that control the cycling and speciation of Hg in forested watersheds. The data collected from this study and the ongoing study at ANP will be incorporated into watershed models to provide a complete regional mass balance for Hg.
The different vegetative covers, organic matter concentrations and solution chemistry will influence the distribution of Hg and CH3Hg in the recently burned and the unburned watersheds in Acadia. The differences in chemical parameters would be the greatest in the upper soil horizons, and therefore, sampling and analysis are focused on the upper increments.
So far, we have selected several sampling sites in both watersheds. Selection of sampling sites has been limited to areas in each watershed that have the greatest potential of having quantifiable levels of methylmercury. More specifically, methylation tends to be increased under anaerobic conditions, and therefore most of the sampling has been conducted in poorly-drained soils. These are areas in which ponding might occur with a higher potential for anoxia. Several well-drained sites were also sampled. Attempts have been made to distribute the sampling sites evenly throughout each watershed, so as to determine a more accurate distribution of methylmercury. We are currently in the process of determining the distribution of Hg and CH3Hg in soil. The soil samples will also be characterized with respect to their organic matter content and mineralogy. Speciation of Hg will be correlated to the relevant physical and chemical parameters in the aqueous and solid phases.
Based on the samples analyzed to date, the concentration of methylmercury in the soils from the Canon Brook watershed ranges from 0 to 1.0 ng/g methylmercury, whereas for the Hadlock Brook watershed the range is 0 to 1.2 ng/g methylmercury, with highest concentrations in both watersheds belonging to the poorly-drained soils and wetlands.
Methods, Procedures and Facilities
This proposed study aims at establishing the distribution of Hg and CH3Hg in the soil of ANP. We will attempt to understand the processes that lead to methylation of Hg to CH3Hg and factors that facilitate release and transport of different mercury species into the streams and lakes of forested watersheds. To this end, we propose to use a paired-watershed approach to compare in each watershed the distribution of Hg and CH3Hg in the soil and the processes involved in their cycling. These two watersheds provide contrasting cases, since one has been burned by an intense fire in 1947, whereas the other has been left intact. Due to their proximity, atmospheric deposition to both is expected to be the same.
The two watersheds chosen for this study are currently subjects of another ecological research study by several researchers at the University of Maine (Demonstration of Intensive Sites Project (DISPro), US EPA Environmental Assessment and Monitoring Program). The objective of DISPro is to establish long-term sites that can serve to identify changes in environmental parameters in the ecosystem. DISPro addresses N cycling and saturation, and characterization of Hg input in paired watersheds with different forest types. The Hg study involves using input/output measurements at the watershed scale to define the unknowns of Hg input and to determine locations and processes of Hg speciation. The information on atmospheric wet deposition is provided by MDN. Specifically, Hg and CH3Hg in the throughfall, litterfall and streams are determined as part of DISPro. As such, DISPro will not address the distribution of mercury species in soil. The role of soil in speciation of mercury is of great importance in cycling, bioavailability and transport and should be thoroughly investigated. This need is all the more important since process-level research on the patterns and mechanisms of Hg biogeochemistry in ANP is lacking.
We are especially interested in the role of the organic carbon pool in the speciation and transport of mercury in forested soils. In the upper soil horizons of the burned watershed, ecosystem pools of C, Hg and other elements such as N have been depleted. Therefore, studying the two watersheds allows us to characterize the accumulation and cycling processes in each, to make comparisons between the mercury speciation and to correlate distribution of elemental C with Hg and CH3Hg in forested soils.
We propose to perform a methodical study that includes sampling and analyzing the soil at different horizons and at different times of the year in both watersheds. Soil samples are collected routinely from the pre-designated sites and analyzed for Hg, CH3Hg, pH, metals such as Fe, Mn and Zn, major cations and anions, and total C, N and S.
The distribution and type of organic matter in soil may influence the speciation and transport of Hg and CH3Hg in different ways, as discussed below. This study will attempt to determine the importance of the different effects of organic carbon on cycling of mercury species in forested watersheds.
Effect of organic matter on methylation of Hg: Catchments that contain wetlands and forested watersheds have shown consistently higher export of CH3Hg (Rudd, 1995). This is partly attributed to the methylation of Hg that is catalyzed by the sulfate reducing bacteria (Gilmour et al., 1992). The high concentrations of organic carbon in such watersheds may result in the production of the anaerobic environment and specifically sulfate reducing conditions.
The differences between burned and unburned ecosystems are significant. Our basis for selecting such ecosystems is that in the regions that have been burned, ecosystem pools of C and Hg are largely lost. Therefore, we expect a more active methylation process in and a consequently higher export of CH3Hg from the unburned watersheds.
Morrison et al. (1994) have shown that flooding the soil may lead to the mobilization of mercury species. This may be related partly to the onset of anoxia and specifically sulfate reducing conditions as flooding blocks the transport of oxygen in the unsaturated zone. Temporal variations in the distribution of mercury species has indeed been observed before (Rudd, 1995; Krabbenhoft et al., 1995). To address such variations in the export of Hg and CH3Hg from the watershed, we undertake sampling and analysis at wet and dry seasons.
Effect of organic matter on transport of Hg and CH3Hg: A typical unburned forest floor has a large accumulation of acidic soil organic horizons, whereas burned forest floors contain less organic matter in soils. Dissolved organic matter is known to complex Hg and CH3Hg strongly (Hintelmann, et al. 1997; Reid, Amirbahman and Haines, unpublished results). Seasonal release of mercury species from wetlands as mentioned above might partly be explained by the higher export rate of the dissolved organic carbon (DOC) during the wet seasons. Periodic analysis of the soil together with the data collected by DISPro and MDN will shed more light on the effect of DOC levels on release of mercury in the ANP.
The leaching of organic acids from the O-horizon has an important implication so far as the mobilization of metals in the lower horizons are concerned. Natural organic acids with carboxylic or phenolic functional groups, which form chelate rings, are known to strongly complex and transport metals such as Fe and Mn, resulting in the dissolution of their oxide surfaces (Stumm and Morgan, 1996). Therefore, due to their relatively higher concentrations of organic acids at the forest floor, the unburned watersheds are expected to possess lower concentrations of Fe- and Mn- hydroxides below the O-horizon than the burned watersheds. Given that metal oxides are strong adsorbents for Hg and CH3Hg (Tiffereau et al., 1995; Yin et al., 1996), this phenomenon may have important implications on the transport of mercury species in the subsurface. We expect to find more Hg adsorbed to the soil that contains higher concentrations of metal hydroxides. These soils would limit the vertical transport of Hg and CH3Hg to the groundwater.
Special attention is given to the uppermost layers of the forest floor (i.e., the O-horizon) that contain very high concentrations of organic matter. The O-horizon is of great importance because highest concentrations of Hg and CH3Hg as well as methylation of Hg have been observed primarily in this horizon. O-horizon is involved as the major exporter of CH3Hg in the forested watersheds (Krabbenhoft et al., 1995). The total organic C budget of this horizon in a forested watershed is relatively independent of the composition of the wet and dry deposition. The chemistry of throughfall may have a significant effect on soil chemistry, resulting in increased exchangeable cations and pH in mineral soil horizons, but little effect on the O-horizon (Fernandez, 1987).
To study the different effects of organic matter on speciation and transport of Hg and CH3Hg, several parameters in soil and in soil water will be analyzed. These include, soil and water pH and the concentrations of the exchangeable cations such as H, Ca, Al, Mg, K and Na. Extractable metals such as Fe, Mn and Zn will be measured and total C, N and S and the water-soluble sulfate in the soil will be determined. We will attempt to correlate factors such as variations in the concentration of DOC and metal (hydr)oxides to the speciation and transport patterns of mercury. We expect to observe the most rapid changes in chemical gradient at the uppermost layers of the forest floor. It is at the top horizon that we are likely to detect differences between the burned and the unburned watersheds.
Acadia National Park is located in mid-coastal Maine in Mount Desert Island. Nearly half of the 25,000 hectare island in the Gulf of Maine is occupied by ANP. Due to its location and topography, ANP receives significant atmospheric inputs of mercury and other compounds from air masses from Boston and the eastern coast of the USA.
In 1947, ANP experienced an intense wildfire that burned approximately one third of the park. As discussed above, to take advantage of this contrasting natural setting to understand Hg cycling in soil, we propose to locate the study sites at the representative burned and unburned watersheds. To this end we have chosen Canon Brook watershed that will serve as the burned ecosystem and Hadlock Brook watershed that will serve as the unburned reference ecosystem. These two sites have also been selected for a related study by DISPro.
The specific locations of gauging stations that determine the upstream watershed size have been established as part of DISPro. The burned watershed consists mostly of northern hardwoods and the unburned watershed is dominated by fir and spruce. We expect that burned watershed has previously had the same forest type as the unburned watershed.
As mentioned above, sampling sites have been selected in areas with the highest potential of having quantifiable levels of methylmercury. These are primarily wetland and riparian areas with poorly-drained soils. Several well-drained sites have also been sampled. A total of six sites at the Hadlock Brook watershed and eight sites at the Canon Brook watershed have been selected. Six 15×15m plots were previously located in both Canon Brook and Hadlock Brook watersheds to collect litterfall and throughfall for both Hg and N as part of DISPro. Some of our sampling sites are adjacent to these plots. Special attention is given to the upper increment of the forest floor because of the high concentration of organic matter that may strongly bind Hg and CH3Hg species and lead to methylation of Hg under anoxic conditions.
Samples have been taken in August, September and October, representing a variety of conditions. August and early September samples were taken under very dry conditions due to the lack of rain over the summer months. However, the late September and October samples represented the other extreme, due to the heavy rains received during that period. Multiple cores of the organic layer were taken from each site for analysis.
Soils are sampled using a stainless steel core sampler of 2.5 cm diameter and 20 cm length. A minimum of three samples is taken at each site so that a representative estimate of the content at that site can be obtained. So far, soils have been sampled primarily at the top layer at the Oa and Oe+i horizons. In selected sites where relatively high methylation rates are observed, samples will also be taken from the mineral soil horizon (E) and the B horizon consisting of Bh, Bs and Bc.
Samples collected in the field are kept on ice and transferred to the laboratory for analysis. All samples are split. Chemical extraction, as described in the following section, is performed on a portion of the samples to dissolve Hg, CH3Hg and other metals. Another portion of the samples are air-dried to measure C, N, pH and exchangeable cations including Ca, Mg, K, Na, Fe, Mn and Zn extracted in 1 M NH4Cl, 0.5 M HCl and hot 5 M HCl.
We are aware of the possible environmental impact of sample taking in a National Park, and our activities are coordinated with other ongoing projects at the Park under the supervision of the National Park Service. We are careful that the sites sampled are filled with local soil and then covered with leaf litter.
Total Hg measurements in water will be performed following the EPA method 1631 (USEPA, 1997). This method involves the addition of 0.2 M BrCl to the sample and subsequent reduction of Hg with 20% SnCl2. The concentration of CH3Hg in water will be measured using ethylation, purge and trap, desorption, and cold-vapor atomic fluorescence detection (Horvat et al., 1993; Bloom and von der Geest, 1995). For both Hg and CH3Hg measurements, a Brooks-Rand cold vapor atomic fluorescence spectrometer will be used.
Chemical techniques will be used to extract Hg and CH3Hg from the soil samples. Total Hg will be extracted from soil samples by the addition of concentrated HNO3 and 30% H2O2 to the samples followed by microwave treatment. Methylmercury extraction from soil generally follows the procedure of Liang et al. (1996). The soil samples will be suspended in 25% KOH in methanol and then placed in a 75°C oven. These extraction methods have been applied successfully previously in our laboratories under the supervision of Dr. Haines.
Organic carbon will be determined by loss-on-ignition. For each soil sample, total C and N will be measured using a CHN analyzer and total S will be measured on a sulfur analyzer.
Soil pH will be measured in deionized water and in 0.01 M CaCl2. Extraction of major metals and anions are discussed by Robarge and Fernandez (1986). Metals such as Fe, Mn and Zn that exist in significant concentrations at lower B horizons will be extracted sequentially using 0.5 M and hot 5 M HCl solutions. Other major cations will be extracted by suspension in 1 M NH4Cl solution. All metals will be measured using the graphite furnace atomic absorption spectrometer. Anions such as sulfate will be extracted in deionized water (“water soluble SO4”) and 0.016 M NaH2PO4 measured (“adsorbed SO4”) using the ion chromatograph.
Laboratory extractions are performed at the lab facilities of the Water Research Institute (WRI) and the Civil and Environmental Engineering Department (CIE). Laminar flow hoods and clean environments suitable for trace concentration levels of Hg are available at both of the facilities.
The analysis of samples is conducted at the laboratories of WRI, CIE and the Department of Biological Sciences. Measurements of CH3Hg are performed at the WRI laboratory. This laboratory is equipped with a clean room with a laminar flow hood, a Brooks-Rand cold-vapor atomic fluorescence spectrometer, model 2, a GC oven with cryogenic GC column plus other necessary equipment such as drying tubes, transformers, a pyrolytic organo-mercury breakdown column, and gold-coated quartz sand traps. A Dell Dimension p75t Pentium computer is solely dedicated to the data collection and analysis for CH3Hg measurements.
In addition, the WRI laboratory has a total organic carbon analyzer (OI), flame and furnace atomic absorption spectrophotometers (Perkin-Elmer) for the measurement of metals, an ion chromatography system (Dionex) for the measurement of inorganic anions and two automated titrating systems (Radiometer) for the determination of alkalinity. A Carlo Erba CHN analyzer and a Leco Sulfur Determinator (SC 132) are available on campus for a nominal fee.
Permanent laboratory apparatus, glassware and water purification systems required for any experimental work are available in all of the three laboratories. Machine shop capabilities are located on campus.
Recent advances in accurate low-level Hg and CH3Hg measurement have made possible the study of cycling and transport of these compounds in natural waters such as lakes and rivers (Matilainen et al., 1991; Hurley et al., 1991; Meuleman et al., 1995). Several studies have also shed light on the uptake of Hg and CH3Hg by biota (Watras et al., 1998; Mason et al., 1996). In recent times, research has been performed on the watershed scale (Krabbenhoft et al., 1995; Aastrup et al., 1991), in which attempts have been made to reach mass balance on the different species. These and many other related investigations have elucidated the major pathways for cycling and bioaccumulation.
The primary non-point sources of Hg to most watersheds are the wet and dry atmospheric deposition. Within a watershed, however, various transport pathways that influence the distribution in different watershed components may be operative. In forested watersheds, similar to the one studied here, throughfall, stemflow and litterfall should be considered as significant sources of Hg to the soil. In general, little is known about such processes and their typical rates (Lindberg et. al., 1994). The extant knowledge of the distribution of Hg in the soils of the upper horizons is also very limited.
In forested catchments, the upper soil layer such as the O-horizon, plays a very important role in Hg cycling due to its high content of organic matter. This horizon may act as a significant storage for Hg especially during the dry seasons (Aastrup et al., 1991). During the wet seasons, the Hg may be mobilized along with the DOC released from the O-horizon (Krabbenhoft et al., 1995). Deeper soil layers such as B-horizons are expected to contain less Hg due to the smaller concentrations of DOC. This has been suggested in a forested soil even though the reported data does not show a strong correlation between the DOC and Hg concentrations (Aastrup et al., 1991). This discrepancy has largely been attributed to the difficulty in collecting pore water samples consistently in the unsaturated zone using lysimeters.
The presence of Hg in the groundwater may also be correlated to the concentration of DOC. Krabbenhoft et al. (1995) have observed substantial increases in Hg and CH3Hg as groundwater discharged through peat layers in a forested watershed.
Rate of release of Hg from forested watersheds into streams and lakes varies temporally. Increase in the concentration of Hg in the streams of a forested watershed has been observed during fall when the concentration of the released DOC was the highest (Krabbenhoft et al., 1995). The existing data does not show a strong correlation between soil acidity and Hg release rate. Atmospheric acid deposition has been shown to increase the solubility of most metals and increase their uptake by plants. However, this seems not to be the case for Hg, presumably due to its strong association with organic matter even at low pH values (Lodenius, 1994).
Due to the importance of the bioaccumulation of CH3Hg in fish, cycling of this species in freshwater lakes has been of great interest. The mercury content of biota consists virtually entirely of CH3Hg (Haines et al., 1994). In freshwater systems, a strong correlation has been found between the aqueous concentration of different mercury species and DOC (Haines et al., 1995; Lee and Hultberg, 1990). The extremely strong complexation of Hg and CH3Hg by humic acids has been attributed to the interactions involving the thiol groups of the latter (Hintelmann et al., 1997; Wallschläger et al., 1996). However, the extant data do not show a consistent correlation between the concentrations of DOC and CH3Hg in biota and a universal relationship between the concentration of DOC in water and the concentration of CH3Hg in biota has not been established.
The processes that lead to the formation of CH3Hg have been studied and bacterially-mediated methylation of Hg has been proposed as an important mechanism in most systems (Rudd, 1995; Porvari, 1995; Matilainen, 1995; Winfrey and Rudd, 1990). Certain species of sulfate reducing bacteria are known to catalyze methylation of Hg (Gilmour et al., 1992). Weber et al. (1985) and Nagase et al. (1984) have described the abiotic pathway for methylation of Hg in the presence of humic acids. However, the abiotic pathway appears to be relatively slow to be of any great significance (Wallschläger et al., 1996). The relative dominance of these two pathways especially in forested watersheds has not been addressed and requires further research.
The major sources of CH3Hg to a freshwater ecosystem are atmospheric deposition, internal production in lakes and terrestrial runoff (Rudd, 1995). Depending upon the type and the geographical location of the watershed, the level of contribution of each source may differ.
Atmospheric deposition of CH3Hg may be related to industrial activity. Rudd (1995) has presented several case studies where areas that are far away from any industries have lower deposition rates than sites near industrialized areas. Atmospheric deposition has also been observed to be an important source for CH3Hg in some seepage lakes (Hultberg et al., 1994). CH3Hg from litterfall in forested catchments may be from the atmospheric deposition, newly produced on leaves or recycled from soil emission. Depending on the origin of CH3Hg in the litterfall, the measured rates of atmospheric deposition should be adjusted (Rudd, 1995). However, the amount of the atmospheric inorganic Hg entering a watershed is generally higher than that of CH3Hg. An increased deposition of Hg does not necessarily lead to higher CH3Hg concentrations (Kelly et al., 1995).
Terrestrial catchments and in particular wetlands, may be very important sources of CH3Hg to lakes. At an experimental lake in northwestern Ontario, CH3Hg distribution was found to be 26-79 times higher than per unit area in wetland areas of the catchment than in non-wetland areas (Rudd, 1995). The importance of the effect of O-horizon on the concentration of CH3Hg in forested watersheds has been discussed by (Krabbenhoft et al., 1995). Their observation of relatively high concentrations of CH3Hg in the spring waters compared to the groundwater at a watershed in northern Wisconsin has led them to identify the top peat layer as an important site for the production and export of CH3Hg to springs.
The high concentration of CH3Hg exported from forested catchments into streams and lakes is due to the bacterial activity that lead to methylation of Hg, and the presence of high levels of DOC in the top soil layers that leads to transport of CH3Hg from the catchment. The top horizons in forested watersheds are largely devoid of solid minerals. Dissolved organic matter is known to strongly bind to clay and oxide minerals (Stumm and Morgan, 1996). Association with solid surfaces generally inhibits the transport of DOC. Therefore, at these horizons, the DOC may be readily mobilized.
In forested watersheds, it is expected that the waterlogged soils with high organic carbon content (e.g., flooding of O-horizon) lead to the methylation of Hg. Flooding the soil may result in the depletion of dissolved oxygen due to microbial activity and the onset of anaerobic processes. Under anoxic conditions and in the presence of sulfate, microbially-catalyzed sulfate reduction and consequently production of CH3Hg can take place.
Due to the concern over the bioaccumulation of CH3Hg in sport-fish, its production in lakes has been the subject of numerous studies. However, our knowledge of the cycling and transport of Hg and CH3Hg in terrestrial ecosystems is relatively scant. Research in this area is needed as wetlands contribute extensively to the total mercury budget in the lakes to its availability to the biota.
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