Mercury
in Humans
Health Risk Assessment
Dr. John Risher
Agency for Toxic Substances and Disease Registry
The Agency for Toxic Substances and Disease Registry (ATSDR) is a component of the U.S. Public Health Service, under the Department of Health and Human Services. Among its legislative mandates, it is charged with determining levels of environmental substances identified at hazardous waste sites that represent minimal risk of adverse health effects to exposed populations. These chemical-specific Minimal Risk Levels (MRLs), which are derived for different routes and durations of exposure, are based upon scientific studies of laboratory animals, controlled human clinical studies, and/or epidemiological data from human populations. The process used for deriving MRLs is essentially the same as that used by the U.S. Environmental Protection Agency (EPA) for calculating oral Reference Doses (RfDs) and inhalation Reference Concentrations (RfCs).
The first step in this process is a critical examination of the overall database for the chemical or substance under investigation. From that analysis, a single study or group of similar studies that represent(s) the single effect that is believed to be the most sensitive toxic endpoint is selected as the "critical study." Typically, a no-observed-adverse-effect-level (NOAEL), representing the highest dosage at which no observable adverse effects were seen in the critical study, or a lowest-observed-adverse-effect-level (LOAEL) then serves as a starting point for MRL or RfD/RfC derivation. After appropriate adjustment for duration of exposure and/or dosimetric airway adjustments (for RfCs) , a number of uncertainty factors are applied to recognize various areas of uncertainty in the MRL or RfD/RfC calculation.
Whether the traditional NOAEL/LOAEL approach or the benchmark dose approach is employed, the NOAEL, LOAEL, or benchmark dose is divided by a composite uncertainty factor to arrive at the MRL or RfC. Such values may then be employed by public health officials to make decisions deemed necessary for the protection of the public health.
Seychelles Study
Drs. Phillip Davidson and Gary Myer
University of Rochester School of Medicine and Dentistry
The Seychelles Child
Development Study: Background, Design, and Results Through 66 Months of Age
Philip W. Davidson
University of Rochester School of Medicine and Dentistry
The Seychelles Child Development Study (SCDS) is testing the hypothesis that there is an association between prenatal exposure to methylmercury (MeHg) from maternal fish consumption and child development. We are longitudinally following a large inception cohort (n=779) of mother-child pairs in the Republic of Seychelles where 85% of the population consumes marine fish daily. The mean prenatal exposure in maternal hair is about 7 ppm while other toxic exposures (e.g., lead, alcohol, PCBs and pesticides) are too low to be confounders. This presentation describes the study design, test batteries, analysis plan, and reviews the results of developmental and neurodevelopmental examinations through 66 months of age. Test results show the expected associations between co-variates and developmental endpoints. No adverse association between prenatal exposure and any developmental endpoint has been found.
The Seychelles Child
Development Study: Testing and Results at 9 years of Age
Gary J. Myers
University of Rochester School of Medicine and Dentistry
The Seychelles Child Development Study (SCDS) is testing the hypothesis that there is an association between prenatal exposure to methylmercury (MeHg) from maternal fish consumption and child development. This longitudinal study enrolled 789 mother-child pairs at six months of age and this talk describes the results of the children's fifth neurodevelopmental evaluation at 9 years of age. The evaluation consisted of two three-hour test batteries given individually. All of the tests have been commonly used in previous developmental neurotoxicological studies. The tests examined global and domain specific abilities and included nearly all of the tests previously reported to show an adverse association with prenatal MeHg exposure. They specifically tested cognition (memory, attention, executive functions) and learning, perceptual, motor, social and behavioral abilities.
A total of 21 primary endpoints were analyzed for their relationship with prenatal MeHg exposure. Test results showed the expected associations between co-variates and developmental endpoints, as have tests from previous evaluations of this cohort. Two out of 21 endpoints showed a significant association with prenatal exposure; one association was adverse (the grooved pegboard, non-dominant hand) and the other was beneficial (Conner's Teacher Rating Scale, ADHD Index). No other significant associations between exposure and outcome were found. These findings do not support an association between prenatal exposure to MeHg from uncontaminated ocean fish consumption and adverse neurodevelopmental consequences in a population not exposed to other neurotoxins.
Health effects of
seafood contamination with methylmercury in the Faroes
Philippe Grandjean
University of Southern Denmark and Boston University
Neurotoxicity due to methylmercury is well documented from unfortunate poisoning incidents. However, the dose-effect relationships have been poorly documented, and the impact of imprecision and potential bias is unclear from past epidemiological studies. We first generated a cohort of 1022 consecutive singleton births during 1986-1987 in the Faroe Islands, where increased methylmercury exposure is mainly due to consumption of pilot whale meat. We measured total mercury concentrations in cord blood and maternal hair collected at parturition. Because prenatal neurotoxic effects would be permanent, assessment of neurobehavioral functions was postponed to the age of 7 years, where the children could undergo detailed testing. A total of 917 of the cohort children underwent thorough examinations. Significant exposure-related dysfunctions were seen in most neuropsychological tests and were most pronounced in the domains of language, attention, and memory. Mercury-associated effects were also seen in delayed latencies for evoked potentials and in blood pressure regulation. The associations remained significant after adjustment for covariates and also after exclusion of children with high maternal hair-mercury concentrations (corresponding to the benchmark dose) or high PCB exposures. Results from examinations conducted of the same children at age 14 years are currently underway. As expected, the cord-blood mercury concentration was the best risk indicator. However, statistical analyses suggested that even this parameter was associated with an error variance that substantially exceeded the analytical imprecision, although it was much less than that associated with the maternal hair-mercury concentration. Such imprecision leads to an underestimation of the true mercury effects.
A second Faroese cohort of 182 singleton term births was generated in 1994 with more detailed exposure documentation. In this cohort, we have documented mercury-associated decreases in the neonatal Neurological Optimality Score and in postnatal growth. Detailed statistical analyses of both prospective cohort studies have failed to identify any covariates that could account for the mercury-associated effects, which remained robust when using different analytical strategies. Although exposure misclassification may be more likely in cross-sectional studies, we have also seen adverse effects in studies of mercury-exposed children from Brazil and Madeira, where developmental exposure levels were determined from current hair-mercury concentrations. In conclusion, we have obtained evidence of subtle adverse effects on neurobehavioral functions, blood pressure, and growth. At age 7 years, a doubling of the mercury exposure corresponds to a developmental delay of up to 2 months. Although IQ tests were not done, such delays would be comparable to a loss of about 1.5 IQ points. These dysfunctions are detectable at exposure levels prevalent in many parts of the world where contaminated seafood or freshwater fish constitutes an important part of the diet.
Development of Methylmercury
Reference Dose
Dr. Kathryn Mahaffey
Office of Prevention, Pesticides and Toxic Substances
U.S. Environmental Protection Agency
Methylmercury: Current Understanding of Health Risks and US Exposures
Methylmercury is widely recognized as a neurotoxin affecting adults, children, and the developing fetus. Until the mid-1990s, peripheral neuropathy was considered to be the critical effect for methylmercury among adults. Paresthesias formed the basis of multiple organizations' evaluations of adverse effects; i.e., EPA, Food and Drug Administration, and the World Health Organization. Data from poisoning episodes during the 1960s and 1970s had shown that neurological problems among children exposed in utero to methylmercury whose mothers themselves demonstrated no or minimal symptoms. The vulnerability of the developing nervous system to a myriad of adverse effects following methylmercury exposures has been described in a broad array of studies in rodents, nonhuman primates, and humans. In 1995 EPA revised it's Reference Dose for methylmercury to be based on fetal protection.
In the 2001 revision of US EPA's RfD for methylmercury, the BMDLs are based data showing adverse effects of methylmercury exposure on multiple tests of child development. The RfD is based on data from the Faroese cohort, with supporting analyses from the New Zealand study, and the integrative analysis of the two preceding studies and the Seychelle Islands study. The UF was 10 and the Modifying Factor (MF) was 1. The BMDL exposure parameters selected are associated with a doubling of the number of children with scores in a range considered clinically subnormal (i.e., the lowest 5% of the distribution) on multiple tests of neurobehavioral function. Multiple endpoints yielded BMDLs in the range of 32 to 79 µg/L in maternal blood for different neuropsychological effects in the offspring at 7 years-of-age corresponding to a range of daily maternal intakes of 0.596 to 1.472 µg/kg. The RfD remains 0.1 µg/kgbw/day associated with a cord blood of approximately 6 µg/L. This is not a "no observed adverse effect level". Within the Faroese cohort's data, effects at exposures less than those associated with a maternal hair mercury concentration of less than 10 ppm have been reported raising questions about whether or not a threshold for methylmercury's effects exists..
Blood mercury data from the first year of the fourth National Health and Nutrition Examination Survey (NHANES IV-99) indicated that the 90th percentile value for women ages16 through 49 years was 6.2 (95% CI 4.7 - 7.9) µg/L. These data are from a survey that is intended to be representative of the United States population as a whole. The 1999 data indicated that approximately 10% of adult women of child-bearing age had blood mercury levels above a level that US EPA considers protective from adverse effects of methylmercury on children's neurological development. Data from various locations in the United States indicate that more elevated exposures exist in some geographic areas. Case reports of blood mercury concentrations considerably in excess of the Reference Dose have been reported from Wisconsin and Massachusetts. The prevalence of these more serious elevations in blood mercury concentrations has not yet been determined.
EPA Fish Consumption Advisories
Mr. Joel Hansel
U.S. Environmental Protection Agency - Region 4
The states have primary responsibility for protecting their residents from the health risks of consuming contaminated noncommercially caught fish. They do this by issuing consumption advisories for the general population, including recreational and subsistence fishers, as well as for sensitive subpopulations (such as pregnant women/fetus, nursing mothers and their infants, and children). These advisories inform the public that high concentrations of chemical contaminants, such as mercury, have been found in local fish. The advisories recommend either limiting or avoiding consumption of certain fish from specific waterbodies or, in some cases, from specific waterbody types (such as lakes or rivers).
As of December 2000, mercury was the chemical contaminant responsible, at least in part, for the issuance of 2,242 fish consumption advisories by 41 states. Almost 79% of all advisories issued in the United States are at least partly due to mercury contamination in fish and shellfish. Advisories for mercury have increased steadily, by 149% from 899 advisories in 1993 to 2,242 advisories in 2000. The number of states that have issued mercury advisories also has risen steadily from 27 states in 1993 to 41 states in 2000. Advisories for mercury increased nearly 8% from 1999 (2,073 advisories) to 2000 (2,242 advisories).
Thirteen states have issued statewide advisories for mercury in their freshwater lakes and/or rivers: Connecticut, Kentucky, Indiana, Maine, Massachusetts, Michigan, Minnesota, New Hampshire, New Jersey, North Carolina, Ohio, Vermont and Wisconsin. Another ten states (Alabama, Florida, Georgia, Louisiana, Maine, Mississippi, North Carolina, South Carolina and Texas) have statewide mercury advisories in effect for their coastal marine waters.
On January 12, 2001, EPA
and FDA jointly issued a press release notifying the public of a national fish
consumption advisory due to mercury contamination. EPA's advice is that if you
are pregnant or could become pregnant, are nursing a baby, or if you are feeding
a young child, limit consumption of freshwater fish caught by family and friends
to one meal per week. For adults one meal is six ounces of cooked fish or eight
ounces uncooked fish; for a young child one meal is two ounces cooked fish or
three ounces uncooked fish. Many states collect data on mercury levels in fish
from local waters. Check with your state or local health department for specific
advice on waters where your family and friends are fishing.
In addition, the Food and Drug Administration (FDA) has issued advice on mercury
in fish bought from stores and restaurants, which includes ocean and coastal
fish as well as other types of commercial fish. FDA advises that women who are
pregnant or could become pregnant, nursing mothers and young children not eat
shark, swordfish, king mackerel, or tilefish. FDA also advises that women of
childbearing age and pregnant women may eat an average of 12 ounces of fish
purchased in stores and restaurants each week. Therefore, if in a given week
you eat 12 ounces of cooked fish from a store or restaurant, then do not eat
fish caught by your family or friends that week. This is important to keep the
total level of methylmercury contributed by all fish at a low level in your
body.
EPA recommends that women who are or could become pregnant, nursing mothers
and young children follow the FDA advice for coastal and ocean fish caught by
family and friends. Check with your local or state health department for specific
advice.
As noted earlier, States have primary responsibility for protecting their residents
from the health risks of consuming contaminated noncommercially caught fish.
Distribution of these state fish consumption advisories/guidelines is typically
done through one of three routes. First, this information is contained within
most state fishing regulations that are distributed at the time that an angler
purchases a fishing license. Second, signs may be posted at common public access
points to inform anglers of the chemical contaminant and the species which are
affected. Third, the public can request such information directly from the appropriate
resource management agency. This information is also compiled in a national
database of fish and wildlife advisories which can be found at http://www.epa.gov/waterscience/fish.
When EPA issues an advisory
as it did related to mercury in January, 2001, the information was directly
distributed to media outlets throughout the Nation. Also, EPA, along with ATSDR,
has initiated an effort to inform medical professionals about the danger posed
to their patients by consuming contaminated fish. All of this information can
be found at the EPA website noted above.
Development of Consistent
Mercury Advisories in the Gulf of Mexico
Dr. Frederick Kopfler
EPA Gulf of Mexico Program
Survey of the Occurrence of Mercury in Fishery Resources of the Gulf of Mexico
An understanding of methylmercury
concentrations in edible fish and shellfish tissues is the foundation for public
health risk assessments. A regional database - the Gulfwide Mercury in Tissue
Database - was created with recent GIS-based tissue monitoring data from the
five Gulf States (Florida, Alabama, Mississippi, Louisiana, Texas), the USEPA
EMAP program, the NOAA NOS National Status and Trends program, and the NMFS
GulfChem study. The study area for database analysis included waters within
the 94 USGS 8-digit hydrologic unit code watersheds that comprise the major
estuarine drainage areas of the Gulf of Mexico, and the nearshore and blue waters
of the Gulf of Mexico. We present the occurrence of mercury in 24 estuarine/marine
species and species groups (and 3 size classes of king mackerel) commonly harvested
in the study area. Species-specific maps show relative mercury concentrations
at tissue sample sites across the Gulf of Mexico. Based on input from the Gulf
of Mexico Program's Mercury Project Advisory/Review Committee, we present recommendations
for Gulf-region tissue monitoring program enhancements.
Mercury
in the Environment
Historical Background of Mercury in the Environment
Mr. Charles Moore
South Carolina Department of Natural Resources
Mercury is a basic chemical element of our solar system. There is a fixed amount on earth that cannot be created or destroyed. Mercury cycles through the earth's biosphere; including the atmosphere, surface waters, aquatic sediments, soils, as well as all plant and animal life. Mercury emissions into the environment can be characterized by three sources: the natural release and cycling of geologically bound mercury, anthropogenic releases, and the re-emission of mercury to the atmosphere from that previously deposited. EPA estimates 50 to 75 percent of the mercury released annually comes from human activities. Of approximately 200,000 tons of mercury emitted to the atmosphere since 1890, about 95 percent resides in terrestrial soils, 3 percent in ocean surface waters and 2 percent in the atmosphere. Mercury is a known toxicant, affecting growth, reproductive success, and development of both plant and animal life. It is a neurotoxin that bioaccumulates through the food chain with its primary pathway to humans being through the consumption of fish.
The natural global bio-geochemical cycling of mercury involves the degassing of mercury from soils and surface waters, atmospheric transport, and the deposition of mercury back to the land and open water. It may then be either re-volatized into the atmosphere or converted to insoluble mercury sulfide that is absorbed to the soil, or bio-converted into more volatile or soluble forms that re-enter the atmosphere or are bioaccumulated in aquatic and terrestrial food chains.
Mercury occurs in three oxidation states. Metallic or elementary mercury has no charge and quickly vaporizes from its liquid form. Over 50% to 95% of the mercury found in the atmosphere is gaseous mercury (HgO) that has a residence time in the atmosphere of between 6 days and 2 years. During this time it is transported great distances, circulating globally. Elementary mercury is not very soluble and atmospheric water (rain and snow) does not serve as a significant means of transfer. Elemental mercury in the atmosphere is oxidized by ozone, hydrogen peroxide, hypochlorite or organoperoxide compounds.
Reactive gaseous mercury (mercuric, with a double electric charge, and mercurous, with a single positive charge) occur at much lower levels than the elementary form representing approximately 3% of the total gaseous mercury in the air. These forms are water-soluble and are removed from the air by gravity (dry deposition), and by rain, snow, dew and humidity (wet deposition). They have an atmospheric residence time of hours to days. Mercury adsorbed onto organic and inorganic microparticulates, may range from less than one percent to 40% of the total ambient mercury level in industrialized areas. Particulate forms are effectively removed by rain and have a relatively short residence time in the atmosphere.
Man's activities that release mercury into the environment are a complex combination of (a) activities that directly emit or inject mercury into the air, soil or water and (b) industrial utilization in products that eventually may be returned to the environment though landfills, combustion, or other means. Estimates of the annual total global input to the atmosphere from all sources including natural, anthropogenic, and oceanic emissions is about 5,500 - 6,000 tons with US sources estimated to have contributed about 3 percent. Mercury today is utilized in the electrical industry (switches, thermostats, batteries etc.), dentistry (dental amalgams, which are 50% mercury), medicinal products including antiseptics (mercurochrome), laxatives, worming medications, teething powers, pharmaceutical preservative products (thimerosal), a red tattoo dye, measuring devices, (thermometers), numerous industrial processes (the production of chlorine and caustic soda), in nuclear reactors, as an anti-fungal agent (wood processing), a solvent for reactive and precious metal, a coloring agent for paint as well as numerous other uses.
Global production of mercury, primarily from cinnabar (mercuric sulfide) mines, has declined 38% from 5,356 tons in 1990 to 3,337 tons in 1996. U.S. mercury production has declined from more than 2,000 metric tons per year in the 1970's, to less than 500 metric tons in 1996, most resulting from secondary sources and industrial recovery. Although the domestic use of mercury has shown a downward trend since the early 1970s mercury imports (277 metric tons in 1995) have escalated in recent years as a result of the suspension of mercury sales from the National Defense Stockpile in 1994, which had formerly been a major supplier of mercury to the domestic market.
Of the estimated 158 tons of mercury emitted annually into the atmosphere by human activities in the United States, approximately 87 percent is from combustion point sources, 10 percent from manufacturing, and 3 percent from all other sources. Of this total, about one-third (52 tons) is deposited within the lower 48 states and two-thirds (107 tons) is transported outside of U.S. borders. An additional 35 tons is deposited within US borders from the global reservoir for a total annual mercury deposition of 87 tons. Four specific source categories (all high temperature waste combustion of fossil fuel processes) account for approximately 80 percent of total mercury emissions in the U.S.: coal-fired utility boilers (33 percent), municipal combustion (19 percent), commercial /industrial boilers (18 percent) and medical waste incinerators (10 percent). In 1994, electric power plants built during the 1940's to 1970's emitted an estimated total of 91,422 pounds of mercury. The vast majority (95%) came from coal-burning plants, and most of that was from plants built prior to 1977 (77%).
According to EPA documents the amount of mercury in the atmosphere is estimated to have increased by 200 % to 500 % since the beginning of the industrial revolution. Others report that there is 3 to 6 times more mercury today vs. pre-industrial times in Atlantic Ocean water, Atlantic bird feathers, peat bogs, soils and lake sediments. Whereas mercury deposition rates have decreased in the vicinity of some localized sources in the western United States during the 1990s, measurements continue to increase in remote sites in northern Canada and Alaska indicating that the global atmospheric burden is continuing to increase.
The production and utilization of mercury are decreasing both on a worldwide and national level. However, based on past mercury releases, it may take decades and perhaps longer, before we observe measurable declines in the environment and affected biological systems. Increasing background levels of mercury increase the potential impact of emissions from local point sources to affected areas.
Chemistry of Mercury
to Methylmercury
Dr. Gary Gill
Texas A&M University - Galveston
Biogeochemical Controls
on Monomethyl Mercury Production in Aquatic systems
It is now well recognized that the chemical form or chemical speciation of an
element in aquatic systems dictates the elements transport, fate, bioavailability,
and toxicity in aquatic systems. The chemical and phase speciation of mercury
in aquatic systems is especially complex. Mercury can be found in the environment
in two oxidation states, Hg (0) and Hg (II), it has an affinity to interact
with natural organic material, it is very particle reactive, and it can be converted
to methylmercury, which bioconcentrates in aquatic food webs, potentially leading
to concentration levels in top fish which exceed safe consumption advisory levels.
From a thermodynamic viewpoint, the inorganic chemical speciation of mercury is fairly well understood. Interactions of mercury with organic material in aquatic systems has long been recognized as potentially important, but currently is poorly understood. Recent investigations in our laboratory have suggested that a substantial portion of what is often considered "dissolved" mercury is actually mercury associated with macromolecular colloidal organic matter. In addition, using competitive ligand equilibration techniques, we have found that a major portion of the mercury present in Galveston Bay is complexed by ~10 pM of a natural organic ligand(s), with a conditional stability constant > 1029. These findings suggest that >99% of the solution forms of mercury in oxic estuarine systems exist as mercury-organic complexes. How broadly representative these findings hold true remains to be investigated.
It is now fairly well accepted that the main pathway for the introduction of methylated mercury forms into aquatic systems is via in situ production, mediated by sulfate-reducing bacteria. There is often a temptation in assessing methyl mercury concentrations and production in aquatic systems to focus predominately on loading or abundance of inorganic Hg as the dominant controlling factor. While this is indeed an important factor, it is by no means the only important factor, nor necessarily the controlling factor. A number of parameters have been identified as important in influencing the production and abundance of methyl mercury in aquatic systems including: mercury loading, the chemical form of mercury (chemical speciation), temperature, the availability of organic substrate for sulfate-reducing bacteria (i.e. a food source), mercury de-methylation activity (by bacteria), in situ reduction-oxidation conditions and in some cases photo-demethylation. To complicate the issue even more, many of these parameters vary temporally and spatially in aquatic systems. Any of these parameters can potentially limit the abundance of methylmercury in an aquatic system.
Benoit, Gilmour, Mason and colleagues have recently proposed that sulfide levels in aquatic systems can be very important in controlling methylmercury production by sulfate-reducing bacteria. This influence arises from the strong interaction between inorganic mercury and sulfide to form mercury-sulfide complexes and the bioavailability of these complexes to sulfate-reducing bacteria. They hypothesize that only neutrally charged mercury complexes (e.g. Hg0, HgS0 or HgCl20) are capable of readily passing bacterial membranes for intra-cellular mercury methylation. Hence, the in situ chemical speciation of mercury is very important in controlling Hg methylation. In anoxic systems, the inorganic speciation of mercury is dominated by sulfide complexes. At low sulfide levels (< 10 µM), neutrally charged HgS0 dominates the Hg-sulfide speciation. Above this concentration level, polysulfide (charged) complexes of Hg dominate.
The role of oil and gas
platform operations and practices in directly or indirectly promoting the formation
of methylmercury and its incorporation into the food webs around the platforms
is currently of great interest. Platforms are frequently characterized as "Oases
for Marine Life in the Gulf" or "Islands or Life", due to the
proliferation of marine life which concentrates around the platforms. It is
possible to speculate that this phenomenon might be promoting environmental
conditions in sediments around the platforms which enhance the production of
methylmercury. If the deposition of organic debris from the marine life around
the platforms is locally elevated, this might provide the "fuel" needed
to drive down oxygen levels, promote sulfate-reduction, and enhance methylmercury
formation. This condition would exist even without the current concern of the
possible augmentation of mercury in sediments from barite drilling mud. Whether
such a localized phenomena is important on a Gulf-wide basis remains to be determined.
Atmospheric Deposition
of Mercury
Dr. Jane Guentzel
Coastal Carolina University
The Importance of Chemical Speciation, Climate, and Meteorology
Mercury exists in many different physical and chemical forms in the environment and it is the interconversions between these species that mediate its distribution patterns and biogeochemical cycling. The most widely known conversion is the biological transformation of inorganic Hg (II) to organic (methyl) Hg and its subsequent biomagnification in piscivorous fish, which poses a risk to higher trophic level organisms and humans who consume these fish. As the atmosphere is considered the dominant pathway for the delivery of inorganic Hg to aquatic ecosystems,1 this presentation will discuss the chemical species of Hg in the atmosphere; the sources of Hg to the atmosphere; and the transport and deposition of inorganic Hg to aquatic ecosystems.
In the atmosphere, mercury
exists predominantly in the zero oxidation state as gaseous elemental Hg (Hgo)
and in the +2 oxidation state as particulate Hg (Hgp) or as reactive gaseous
Hg (Hg(II)). Gaseous elemental Hg comprises 97-99% of the total mercury found
in the atmosphere and has a residence time on the order of 1 year.1,2 The
remaining 1-3% is comprised of Hg(II) and Hgp, with residence times on the
order of days to weeks.2 Reactive gaseous Hg can be formed in the atmosphere
through the oxidation of gaseous elemental Hg by ozone3,4 or halogen radicals
in the marine boundary layer and troposphere.5,6 Hg(II) is incorporated into
cloud droplets or becomes attached to particulate material and is scavenged
from the atmosphere by wet and dry deposition processes. Hgp can dry deposit
to surfaces, be incorporated into cloud droplets, or be scavenged by precipitation.
These various species of
mercury in the atmosphere originate from natural processes (25-30%) and anthropogenic
activities (60-75%).7 Natural or background sources of atmospheric mercury,
mainly in the form of Hgo, include emissions from volcanoes, soils, vegetation,
and the ocean.8 It has been estimated that 20-30% of the current oceanic
emissions originates from mercury mobilized by natural sources, with the remaining
70-80% derived from recycled anthropogenic Hg.7 Forest fires may emit Hgo
and some partially oxidized species.8. Estimates of contributions from natural
sources are limited by our uncertainties regarding the amount of Hg in the pre-industrial
environment as well as uncertainties in estimating the amount of anthropogenic
Hg that is recycled by the ocean and terrestrial environment.9
Modeling calculations estimate that anthropogenic emissions have tripled the concentration of mercury in the atmosphere and surface ocean over the last century.7 Anthropogenic sources of mercury include fossil fuel combustion (coal, oil, gas), waste incineration, chloro-alkali production, metal extraction processes, and cement production. These sources emit Hgo, Hgp, and Hg(II), which can cycle within the atmosphere and be deposited to ecosystems mainly as Hg(II)and Hgp. The distance that anthropogenically mobilized Hg is transported prior to deposition is determined largely by the speciation of Hg that is emitted.8 Hg(II) and Hgp deposit locally (50 km), while a significant fraction of Hgo can be transported over long distances ( 10,000 km) and enter the global mercury cycle.8 This mercury is subsequently available for oxidation to Hg(II) in the troposphere and marine boundary layer, resulting in a global or "background" contribution of Hg(II) to mercury deposition.5,10
Chemical speciation, climate, and meteorology influence the extent to which local and or global sources contribute to Hg deposition. The sub-tropical climate and complex meteorology of Southern Florida provide us with a rather unique environment to investigate Hg deposition. The annual rainfall volumes across Southern Florida range from 128-150 cm, with greater than 70% of the rainfall occurring during the rainy season (May-Oct.).10 The summertime wet season in Southern Florida is characterized by the almost daily occurrence of tall convective thunderstorms (12-16 km) and daily ventilation of background air by the strong synoptic southeasterly winds associated with the North Atlantic trade winds. Findings from the Florida Atmospheric Mercury Study (FAMS) suggest that the annual deposition of Hg in Southern Florida is mediated by long range transport of Hg (mainly Hg(II) resulting from the oxidation of Hgo in the global atmosphere) coupled with strong convective thunderstorm activity during the wet season. Model calculations indicate that long range transport accounts for 54-70% of the summertime rainfall Hg deposition, with the remaining 30-46% attributable to local anthropogenic Hgp and Hg(II) emissions.10 It is important to recall that 60-70% of the Hgo in the modern atmosphere results from industrial activity7 and reductions in Hg deposition will likely require reductions in local and global Hg emissions.
References
1 Fitzgerald, W.F., Mason, R.P., and Vandal, G.M. (1991) Water Air Soil Poll.
56, 745-768.
2 Lindqvist, O., Johansson, M., Aastrup, A., Andersson, L., Iverfeldt, A.,
Meili, M., and Timm, B. (1991) Water Air Soil Poll. 55, 1-262.
3 Hall, B. 1995 Water Air Soil Pollut. 80, 301-315.
4 Munthe, J. (1992) Atmos. Environ. 26A, 1461-1468.
5 Mason, R.P., Guey-Rong, S., and Lawson, Nichole. (2001) Abstract submitted
to the Symposium on Methylmercury: Impacts on Wildlife and Human Health. Charleston,
South Carolina, 2001.
6 Lindberg, S.E., Brooks, S., Lin, C.J., Scott, K.J., Landis, M.S., Stevens,
R.K., Goodsite, M., and Richter, A. (2002) Environ. Sci. Technol. 36, 1245-1256.
7 Mason, R.P., Fitzgerald, W.F., Morel, F.M.M. (1994) Geochim. Cosmochim.
Acta. 58, 3191.
8 Porcella, D.B., Chu, P., and Allan, M.A. (1996) Inventory of North American
Hg Emissions to the Atmosphere; Relationship to the Global Hg Cycle. In; Global
and Regional Mercury Cycles: Sources, Fluxes, and Mass Balances, 179-190.
9 EPMAP (Expert Panel on Mercury Atmospheric Processes) (1994) Mercury Atmospheric
Processes: A Synthesis Report. EPRI/TR-104214. EPRI, Palo Alto, CA 94304, 23p.
10 Guentzel, J.L., Landing, W.M., Gill, G.A., and Pollman, C.D. (2001) Environ.
Sci. Technol.35, 863-873.
Offshore Oil and Gas
Sources
Dr. Jerry Neff
Battele-American Petroleum Institute
Mercury is a metal that is present naturally at very low concentrations in the atmosphere, water, sediments, and tissues of all plants and animals. Mercury is released into the environment from a wide variety of natural and human activities. It is present in the oceans as mercury metal and as inorganic and organic mercury compounds. An organic form of mercury, methylmercury, is the most toxic to animals. Living animals can absorb into their tissues some inorganic and organic mercury compounds from food and water. Methylmercury is toxic to terrestrial, freshwater, and marine organisms, if a large amount accumulates in their tissues. The main pathway for human exposure to methylmercury is through consumption of freshwater and marine fish.
Recent newspaper articles suggested that offshore oil and gas operations might be a secondary source of the more toxic, organic form of mercury in marine fish and shellfish in the Gulf of Mexico. This presentation reviews the scientific literature on the sources of mercury in the Gulf of Mexico environment and the potential contribution of offshore oil and gas operations to mercury levels in fish and shellfish consumed by man. The scientific literature shows that:
The Gulf of Mexico environment receives inputs of mercury from natural and human sources via rainfall, river inflows, runoff from land, and commercial activities in the coastal zone and offshore. Annually, an estimated 55,000 pounds of mercury is deposited from the atmosphere to the surface waters of the entire Gulf of Mexico. An additional 48,000 pounds of mercury enters the northern Gulf of Mexico in the freshwater inflow from the Mississippi River. By comparison, approximately 420 pounds of mercury enters the Gulf in drilling and production discharges under EPA permits. The amount of mercury entering the Gulf from offshore oil and gas operations is less than one-half of one percent of the amount entering the Gulf from the air and in Mississippi River water.
Drilling muds, used to drill wells, contain a natural mineral called barite that contains traces of mercury. During drilling, rock chips, called cuttings, are produced by the drill bit as it penetrates the earth. Drill cuttings may contain traces of mercury. Water may come to the surface with the oil and gas from a production well. This produced water also may contain traces of mercury. The Environmental Protection Agency permits drilling muds to be discharged to the ocean more than 3 miles from shore if the barite in the mud contains less than 1 part per million (ppm) mercury. Drilling muds used offshore in the Gulf of Mexico since imposition of the EPA limit on mercury in barite in 1993 have contained an average of about 0.5 ppm mercury, well below EPA's standard. Drilling muds and cuttings discharged to offshore waters of the Gulf during drilling of approximately 900 wells in 2001 contained about 340 pounds of mercury. An additional 80 pounds of mercury was discharged to the Gulf in treated produced water discharges from offshore platforms.
Some of the mercury entering the Gulf from all sources binds to suspended particles in the water column and settles with them to the bottom. Most nearshore sediments in the Gulf contain less than 0.1 ppm total mercury. Deep water, offshore sediments usually contain less than 0.05 ppm mercury. Most concentrations of mercury in sediments near offshore platforms are 0.2 ppm or less. Sediments at only one platform site, of more than 30 platforms surveyed, contained more than 1 ppm mercury, and elevated mercury concentrations were restricted to sediments within about 100 ft of the platform. In this atypical drilling operation, drilling muds and cuttings were discharged directly to the sea floor to prevent any possibility of damage to nearby coral reefs. With one exception, this was the only site, among the more than 30 surveyed, where surface sediments near the discharge contained more than the sediment quality guideline, the Effects Range Median (ERM), of 0.7 ppm mercury. A few sediment samples collected near a platform off Galveston, TX in 1978-79 contained slightly more than the ERM concentration of mercury. These samples were analyzed by less reliable methods than those used today and reported concentrations may be higher than true values. These results show that ocean discharge of drilling muds and cuttings does not result in environmentally significant mercury contamination of sediments near platforms.
Most of the mercury entering the Gulf of Mexico is inorganic mercury and metallic mercury (the silvery liquid in a mercury thermometer). Inorganic and metallic mercury are only moderately toxic and they are not passed efficiently through the marine food chain to man. However, some species of bacteria that live in sediments and ocean water containing very low concentrations of oxygen are able to convert some of the inorganic mercury dissolved in water to toxic methylmercury through a process called methylation. Mercury methylation also occurs in the organic-rich sediments of coastal salt marshes and wetlands, such as the Everglades in south Florida.
The mercury in drilling mud is in a solid, insoluble form. Bacteria have only a very limited ability to absorb the mercury found in barite; therefore, it is not likely to be methylated. Marine plants and animals cannot accumulate the insoluble mercury from barite in in their tissues. Therefore, little or none of the mercury from drilling mud and cuttings is methylated and it does not bioaccumulate in the marine food chain.
Marine animals are not able to convert inorganic mercury to methylmercury. However, they are able to accumulate inorganic mercury and methylmercury directly from the water or by ingestion of food or sediments. Most of the methylmercury in the edible muscle tissues of fish comes from their food. Concentrations of mercury in tissues of marine shellfish and finfish from the Gulf of Mexico vary widely in different species. However, concentrations in edible tissues of fish and shellfish from the Gulf of Mexico are comparable to those in the same or similar species from other marine environments in the U.S. and abroad that do not have offshore oil and gas operations. Mercury concentrations are highest in muscle tissues of large, predatory ocean fish, such as swordfish, sharks, and king mackerel. Mercury concentrations usually are low in soft tissues of shellfish, such as oysters, crabs, and shrimp, and bottom fish, such as flounder, red snapper, and mullet.
There have been several studies to measure the concentrations of mercury in tissues of marine animals in the vicinity of offshore platforms. Fish collected near the Gulf platform where mercury concentrations in sediments were above 1 ppm contained slightly elevated mercury concentrations in their livers. Whole soft tissues of edible shrimp from the vicinity of this platform contained low concentrations of mercury, similar to concentrations in shrimp from throughout the Gulf. In all cases where measurements were made, mercury concentrations in edible muscle tissues of fish near platforms were similar to concentrations in muscle tissue of the same or similar species well away from and out of the influence of the platforms. Shellfish from the vicinity of platforms contained low concentrations of mercury, similar to those in shellfish collected away from offshore platforms. This distribution of mercury in marine fish and shellfish populations throughout the Gulf of Mexico indicates that the mercury in the edible tissues of these seafoods is not derived from offshore platform discharges.
Current
and Proposed Mercury Science and Education Projects
Fish Advisories in Alabama
Dr. Neil Sass
Alabama Department of Public Health
A Healthy, Informed
Choice
Contaminants in Fish
There is variability in the health value of eating some fish. Part of this is due to the chemical composition of the fish itself, the protein, fat, vitamin, and/or mineral content of the fish. There is another aspect of fish composition that should be examined to determine its value as a healthy or unhealthy source of nutrition. This other aspect is the level of contaminants that may be present in the fish.
If fish live in a clean body of water, their health should be good and they are good to eat. If fish live in a body of water that has higher than normal contaminants, their health might not be good and they should not be eaten. The longer fish live in contaminated water, the more likely it is that they are contaminated. Since fish cannot get rid of all the poisons or toxins from their bodies, their bodies end up storing them. The highest concentrations of toxins, like some pesticides, mercury and polychorinated biphenyls (PCBs), are found in the fat and liver of fish.
Contaminants in the environment can be natural or man-made. Some of the materials we call contaminants are actually beneficial chemicals that have been distributed in the environment intentionally, like pesticides that have been applied on crops. Others are by-products of processes which have escaped into the environment, e.g., dioxins and furans from the production of bleach kraft paper. A substance like mercury, although a natural product, escapes into the environment in large amounts through the burning of coal in commercial power plants and factories. Other materials, like polychlorinated biphenyls (PCBs) enter the environment as waste materials put into discard piles at factories. The PCBs then are subject to the effects of wind and rain in becoming disbursed throughout the environment. These contaminants can find their way into waters in the State. Contaminants can be taken up by plants or fish or other organisms (e.g., crayfish) in the water. Fish develop measurable levels of contaminants depending on the content of contaminant in the materials on which the fish feed. Some of these can accumulate in the fish over time.
As a rule, eating fish is a healthy choice. Fish should be included in every balanced diet. However, while most Americans need to include more fish in their diet, care must be taken in choosing and preparing fish. Some fish may actually cause harm. Alabama Department of Public Health, in conjunction with the Alabama Department of Environmental Management, surveys fish in varying waterbodies across the state to determine whether or not conditions in the waters of the state are changing. Increases of specific contaminants in specific waterbodies will result in the issuance of consumption advisories to inform the public of dangers involved in consuming certain species of fish from specific waterbodies. To make sure you and your family get all the health benefits from fish, you should know:
Through awareness of these elements, individuals can increase the health content of their diet without significantly increasing their exposure to potentially hazardous contaminants in fish.
Fish Monitoring Programs
in Alabama
Mr. Fred Leslie
Alabama Department of Environmental Management
Regular monitoring of fish in Alabama for mercury contamination was initiated in 1970 by ADEM's predecessor, the Alabama Water Improvement Commission (AWIC). The monitoring was conducted in Cold Creek Swamp and adjacent Mobile River in response to concerns of mercury contamination from area industries. Monitoring of fish by the AWIC/ADEM continued in this area from 1970 to the present.
In 1991, the ADEM expanded its Fish Tissue Monitoring Program to provide statewide monitoring. The Program expansion was in response to concerns regarding mercury and other bioaccumulative contaminants in fish, and national emphasis by the U.S. Environmental Protection Agency (USEPA). The expanded program exists as a cooperative arrangement between the ADEM, the Alabama Department of Public Health (ADPH), the Alabama Department of Conservation and Natural Resources (ADCNR), and the Tennessee Valley Authority (TVA). With increasing awareness during the 1990's of mercury contamination in fish, the ADEM also joined other states in this region as a member of the Southern States Mercury Task Force to share information and expertise in determining necessary action.
Through the Program, fish are collected from all major reservoirs and streams in Alabama over five (5) year periods in a basin rotation cycle. Each year, sites in waterbodies outside the scheduled basin are also included for monitoring as needed and as resources allow. Since 1991, 596 composite samples comprised of several thousand fish have been collected from over 230 sites and analyzed for mercury. In addition, individual analyses of many fish have also been conducted. All samples are analyzed by the ADEM Environmental Laboratory for mercury and other contaminants with the potential to bioaccumulate. Analytical data is provided to the Alabama Department of Public Health (ADPH) for review, with the ADPH using FDA action levels of 1.0 parts per million (ppm) mercury in fish tissue for issuance of advisories.
Recently, analyses of mercury concentrations in fish samples from 30 Alabama coastal estuary sites was conducted by ADEM, USEPA, and ADCNR for the Coastal 2000 Program. No fish collected for this Program exceeded FDA levels for mercury. During 2001, fish tissue samples were collected by ADEM from 34 sites in the state and analyzed for mercury concentrations, with a total of 407 fish collected. Eighteen of these sites were in the Mobile Bay area, where the greatest number of ADPH consumption advisories in Alabama currently exist. Bass collected from the Escatawpa River and Styx River in 2001 exceeded FDA guideline levels, though bass collected from these locations in 1995 did not. As in previous years, bass collected during 2001 from Fowl River and Fish River exceeded FDA guidelines for mercury. In addition, two fish in a sample of six largemouth bass from Chickasaw Creek and one fish in a sample of six largemouth bass from the Tensaw River exceeded the guideline levels. Bass collected from Bay Minette Creek in 2001 did not exceed FDA levels, while those collected in 1997 and in 1998 did.
Future monitoring activities include sample collection in the Warrior and Cahaba River basins in 2002, resampling locations where fish exceeded FDA levels for the first time in 2001, resampling of south Alabama locations not sampled in several years, as well as targeting the remaining areas in south Alabama where fish have not been collected for the ADEM Program. Future sampling of marine and estuarine species depends in part on the outcome of ongoing negotiations at the state and federal level.
Survey of the Occurrence
of Mercury in Fishery Resources of the Gulf of Mexico
Dr. Frederick Kopfler
EPA Gulf of Mexico Program
An understanding of methylmercury concentrations in edible fish and shellfish tissues is the foundation for public health risk assessments. A regional database - the Gulfwide Mercury in Tissue Database - was created with recent GIS-based tissue monitoring data from the five Gulf States (Florida, Alabama, Mississippi, Louisiana, Texas), the USEPA EMAP program, the NOAA NOS National Status and Trends program, and the NMFS GulfChem study. The study area for database analysis included waters within the 94 USGS 8-digit hydrologic unit code watersheds that comprise the major estuarine drainage areas of the Gulf of Mexico, and the nearshore and blue waters of the Gulf of Mexico. We present the occurrence of mercury in 24 estuarine/marine species and species groups (and 3 size classes of king mackerel) commonly harvested in the study area. Species-specific maps show relative mercury concentrations at tissue sample sites across the Gulf of Mexico. Based on input from the Gulf of Mexico Program's Mercury Project Advisory/Review Committee, we present recommendations for Gulf-region tissue monitoring program enhancements.
Methylmercury in Marine
Fish: A Gulf-Wide Initiative
Mr. Ron Lukens
Gulf States marine Fisheries Commission
The Gulf States Marine Fisheries Commission, during its Annual Fall Meeting in October 2001, began an initiative to investigate the need for a Gulf-wide survey to collect fish tissue for analysis of mercury content, and to determine the need for convening appropriate federal and state agency and industry representatives to discuss developing compatible fish consumption advisory levels and advisory language.
A report was provided to the Commission during its Annual Spring Meeting in March 2002. That report provide the Commissioners with background information on sources of mercury, biological processes and implications, public health concerns, and federal and state actions regarding mercury in fish. That report also contains seven recommendations, listed below, which were presented to the Commissioners.
The Commissioners tabled definitive action on the report, pending additional staff work to develop more detail for the recommendations. Included will be appropriate agency roles, effort involved, and associated costs if the recommendations were to be implemented. The Commission staff and the Steering Committee are currently compiling the necessary information regarding each recommendation and will be providing the information to the Commissioners at the Annual Fall Meeting in October 2002.
Synoptic Survey of
Total Mercury in Recreational Finfish in the Gulf of Mexico
E. Spencer Garrett and Tony Lowery
National Marine Fisheries Service
The public health ramifications of mercury (Hg) in fish is a complex issue of numerous dimensions. There are professional differences of opinion on what the allowable tolerance (guideline) in fishery products should be. Methylmercury is an ecotoxicant that bio-accumulates in marine seafood species. There are natural sources and anthropogenic sources of mercury released into the marine environment that through bacterial processes becomes the bio-accumulating ecotoxic Methylmercury. Methylmercury binds to proteins in living organisms and is passed up the food chain where the Methylmercury can reach dangerous levels in certain seafood species.
The strategy to protect the public against ingesting unsafe amounts of Methylmercury has been premised on the following:
Various consumption advisories have been issued by FDA and EPA encouraging the public, especially women of child bearing age and children to consume seafood species that are low in Methylmercury, and to avoid eating Swordfish, Shark, Tile Fish, and King Mackerel. As Methylmercury can adversely affect the neurological development of fetuses and small children at low doses, women of child bearing age and children are of particular concern. EPA and FDA recognize and have so indicated in 1996, that "...FDA's action levels ensure a safe food supply for consumers of commercial fish, they may not be appropriate levels for ensuring the safety of those who consume locally caught fish...." and therefore various fish consumption advisories have been issued by states to take into account local conditions and local consumption patterns.
There may be an unrecognized
portion of the public that consumes seafood in excess of 15 pounds per year,
and they also consume large quantities of seafood that are harvested for personal
consumption. In particular, subsistence, commercial, and marine recreational
fishermen and their families may be at increased risk of exceeding the FDA Methylmercury
consumption guidelines as they may be consuming seafood well in excess of 15
pounds per year, and they may be consuming non-commercially harvested seafood
that exceeds the FDA's 1.0 ppm
Methylmercury monitoring and restrictions. Therefore, subsistence, commercial,
and marine recreational fishermen and their families represent a new sub-population
of the seafood consuming public that could likely require additional informational
safeguards in order to protect them against excessive Methylmercury ingestion
via seafood.
The Methylmercury levels in commonly available commercially harvested seafood species are fairly well known. However, the Methylmercury levels in seafood species not commonly available through commercial sources are less well known. Since the development of consumption advisories for the subsistence, commercial, and marine recreational fishermen and their families should be based on sound science, data on the mercury levels in the seafood species this sub-populations consumes will be collected. NMFS' National Seafood Inspection Laboratory and EPA's Gulf of Mexico Program plan to carry out a synoptic survey analyzing 2,500 samples in 2002-2003 to collect preliminary data on the mercury level for selected popular marine recreational seafood finfish, and to provide data for later more extensive Gulf-wide mercury in seafood survey designs if needed.
The synoptic survey will be carried out in three parts:
Migratory Species Sampling: Selected highly migratory finfish species will be collected from off the Florida Gulf and Texas Coasts. The samples will be tested to determine if a statistical difference exists between the fishes taken from these geographic regions of the Gulf. If no difference can be determined, then a generic Gulf-wide modeling of the mercury levels in these species could be possible. Conversely, if a difference is observed, then additional surveys would be required.
NMFS anticipates that the synoptic survey will, at a minimum, provide valuable data that will allow for an assessment of the scope of sampling required to adequately cover the marine recreational finfishes of the Gulf of Mexico. Such data is needed to support the development of consumption advisories for the general public, and especially for the subsistence, commercial, and marine recreational fishermen sub-population that is believed to be at the highest risk presently.
It should be understood that this NMFS Synoptic Survey deals, in a limited manner, with only one-half of the information needs that address the exposure component of a mercury risk assessment. The other necessary component of the exposure risk assessment is the need for consumption studies for the recreational fisheries and/or commercial fishers (who consume portions of their catch) relative to the species that may be identified as containing elevated mercury levels.
Selected
Mercury Related Research
Current
Research into Mercury Control from Coal-Fired Power Plants
Dr. Larry Monroe
Southern Company Services, Inc.
In December 2000, EPA announced their intention to require coal-fired utility boilers to control mercury emissions. Since mercury concentrations in flue gas are one million times lower than that for the other pollutants of interest, control of these emissions will be quite a technical challenge. Various approaches to controlling mercury emissions will be discussed, and the commercial potential of each technology will be noted. Several exciting new processes have been proposed and these will also be discussed. Finally, the probable time table and several proposed emission levels will be presented.
Distribution of Mercury
in the Mobile River Basin in Relation to Land Use
Dr. Kimberley Warner
University of Alabama Department of Biological Sciences
Co-authors: Jean-Claude Bonzongo, Eric Roden, W. Berry Lyons, Milt Ward, Indrajeet
Chaubey, and Hobson Bryan.
In the past decade, mercury
(Hg) concentrations above levels that could pose human health risks have been
measured in predatory fish from many rivers and reservoirs in the southeastern
region of the United States. This region, and mainly its Coastal Plain portion,
may be particularly vulnerable to Hg contamination in aquatic food chains, due
to the coexistence of both natural and human-imposed conditions which favor
the production and accumulation of methyl-Hg. Several specific factors are hypothesized
to contribute to the development of such conditions, including: (1) nutrient
loading from certain land-use activities and increased sedimentation above water
impoundments develop conditions favorable for methyl-Hg production. (2) Increased
sulfate loading from energy resource extraction operations result in increased
methyl-Hg production. (3) Abundant wetlands within the Mobile-Alabama River
Basin (MARB) contribute to methyl-Hg loads downstream and in fish. (4) Fish
tissue levels of methyl-Hg are related to levels of Hg in river waters and to
net rates of methyl-Hg production in sediments.
The objectives of this
study were to: (1) to determine levels and speciation of Hg in different compartments
of various aquatic systems in the MARB; (2) to investigate the linkage between
land use types or the presence of wetlands and microbial processes associated
with methyl-Hg production; (3) to use GIS to represent spatially arranged data
and ultimately to predict Hg levels in fish; and (4) to use a participatory
approach to environmental decision-making to ameliorate conflict, and achieve
an effective public understanding and support for Hg policy. This report will
address preliminary results and progress on the science aspects of this interdisciplinary
research project conducted by the University of Alabama Center for Freshwater
Studies.
The first phase of the
project involved a wide survey of Hg distributions in largemouth bass, water
and sediment at 52 sites representing various hypothesized impact factors (e.g.
wetlands, agriculture, dams). Various sediment chemistry and water quality parameters
were measured in conjunction. A total of 96 fish samples were taken from 51
out of 52 sites. While we attempted to collect 2 fish at each site, at least
two fish were sampled at 43 sites and only one fish at the remaining 8 sites.
The concentrations of Hg in these 96 fish spanned over 2 orders of magnitude,
from 0.02 to 2.8 ppm (mg kg-1 wet tissue). Mean and median concentrations were
0.45 and 0.32 ppm, respectively, with the mean close to that of the National
Mercury Survey (0.39 ppm) reported for AL largemouth bass. Twelve percent (12%)
of fish had Hg concentrations 1 ppm, the level at which consumption advisories
are posted in Alabama. Average fish Hg concentrations were 0.5 ppm at 21 (41%)
of the sites. Coefficients of variation in fish Hg concentrations from any one
site were usually high, averaging 68%. In many cases, Hg concentration was higher
in the smaller of two fish. Total-Hg and methyl-Hg concentrations in water were
low and ranged from 0.2-3.8 and 0.01-1.5ng L-1, respectively. This results in
an average -log bioconcentration factor of 5.6 for Hg between fish and water.
Hg speciation determinations in sediments are still underway. The annual flux
of aqueous total-Hg from the MARB to Mobile Bay is estimated at 138Kg yr-1.
No obvious trends were
apparent between fish Hg concentrations and projected impact factors, likely
due to the large within-site variability noted above. However, some variability
within certain impact factors could be explained by other factors. For example,
fish Hg concentrations in wetland sites were positively related to watershed
area and inversely related to water depth above dams, for dam impacts. Fish
Hg concentrations were also found to be a weak negative function of water pH.
Aqueous total-Hg and many sediment and water quality parameters were found to
be a significant function of land use.
The second phase of the
project involved in-depth investigations examining controls on Hg transformation
and bioaccumulation. We focused on one pool (Demopolis) containing 4 sites with
different impact factors: (i) dam, (ii) agricultural, (iii) wetland, and (iv)
open river. Potential rates of microbial Hg methylation and methyl-Hg demethylation
were determined in sediments. The ratio of methylation to demethylation rates
was positively related to the percent of methyl-Hg formed in native sediments.
Concentrations of Hg (127-393ng g dry sediment-1) and methyl-Hg (0.13-2ng g-1)
in native sediments increased in the following order of impacts: open river
< dam < agriculture < wetland. The percentage of methyl-Hg produced
in the sediments was a positive function of sediment iron, organic matter, and
porosity. Aqueous total-Hg increased with aqueous total suspended solids and
iron concentrations. Aqueous methyl-Hg was a positive function of dissolved
organic carbon in water and of methyl-Hg and total iron concentration in sediments.
Despite differences in
net production of methyl-Hg in sediments from the different sites, the average
concentration of Hg in fish tissues among the 4 sites was consistently rather
high. The average concentrations of Hg (ppm±1SD) in 6 fish from each
site were: open river: 0.77±0.45; dam: 0.85±0.35; agriculture:
0.88±0.35; wetland: 1.7±0.80. Comparing mean fish Hg concentrations
among sites, only the wetland site was found to be significantly different from
the other three sites.
These preliminary results suggest net methyl-Hg production in sediments and flux to water is greater in environments with organic-rich, slower moving turbid waters and fine-grained sediments. These, in turn, are related to land use and hydrological variables, consistent with our hypotheses. However, we found that fish collected from rivers/stream sites with differing impacts have highly variable Hg burdens, which may be explained, at least in part, by their mobility or the mobility of their prey. Therefore, it may not be possible to draw direct quantitative links between land use types and Hg concentration in fish in physically dynamic riverine ecosystems.
Social Impact Assessment
of Mercury Contamination in Mobile River Basin
Dr. Hobson Bryan
University of Alabama Department of Geography
Coauthors: Misty Samya, Hendrik Snow, Kimberly Warner, Jean-Claude Bonzongo,
Eric Roden, W. Berry Lyons, Milton Ward, and Indrajeet Chaubey
This reports the social assessment and public involvement dimension of a three-year research project concerning mercury in largemouth bass. An interdisciplinary research team in the Departments of Biology, Geology, and Geography at the University of Alabama is currently just completing an investigation of factors that control the movement of methyl mercury through the aquatic system of the primary rivers of the Mobile-Alabama River Basin. These factors include inputs from agricultural and urban land uses, impoundment of rivers, and wetland abundance. Findings of high mercury levels in some largemouth bass prompt concerns about human exposure to mercury, primarily through consuming contaminated fish.
The social assessment phase of the research focuses on identifying a full range of stakeholder groups to solicit prominent issues and suggest public policy approaches to controlling methyl mercury exposure. This project took a dual approach to public involvement through the social assessment process. As stakeholders, recreational fishermen (i.e., tournament bass anglers) participated in the data collection, a resident expert fisherman, knowledgeable about access sites and area land use along the waterways, advised project management. In addition, meetings on the topic were conducted with representatives of the power generation industry, environmental groups, the coal bed methane industry, and various state government agency officials. Thus, the process informed stakeholders about the research, solicited expert advice, and generated suggestions for policy response through meetings with a number of groups representing a range of interests and concerns about mercury levels.
As final project results are obtained, stakeholder groups will be informed and solicited for their advice and feedback. A particular challenge will be to reach such at-risk groups as minorities and the poor--who may not be in the information mainstream, yet who may be heavy consumers of fish from public waters--women of child-bearing age, those who are pregnant, and children. Information strategies will have to be addressed that inform the public on reasonable actions they can take to avoid harm from eating fish, while being cognizant of the concerns of various segments of the recreational fishing industry and other areas of the economy dependent on a healthy fishery.
Economic Realities of
Mercury in the Environment
Seafood Industry Perspective
Mr. Bob Collette
National Fisheries Institute
There is not much about mercury that is simple. The science is complicated and the issues surrounding public policy are complex and multifaceted. It goes without saying that fishermen, processors and purveyors of seafood do not add mercury to fish or to the environment. Nevertheless, we find ourselves in the middle of a debate that intermingles environmental and public health concerns. We certainly believe that protecting people, such as pregnant women, who are especially vulnerable to the potential effects of mercury is a paramount concern. Because fish is such an important part of a healthy diet for many consumers, government agencies must have sound scientific justification when they tell people to limit consumption of fish or place limits on which fish can be sold. Decisions about protecting consumers, therefore, must be based on a thorough assessment of scientific data and the public health impacts, both positive and negative, associated with various risk management approaches.
I will focus my comments on how various approaches to managing and communicating about the relative risks and benefits associated with fish consumption may impact consumer habits, the nutrition and health status of consumers and the viability of the seafood industry.
Recreational Fishing
Perspective
Dr. Bob Shipp
University of South Alabama
Recreational fishermen do not share quite the level of concern over mercury in finfish as one might expect from commercial harvesters. This is because there has been much more sentiment toward tag and release in the recreational sector in recent years, and much less toward "meat fishing." Nevertheless, recreational fishermen are anxious to learn the methyl mercury levels at various life stages of popular species.
This is because many species
are still consumed by family members, and regulations regarding slot limits,
bag limits, seasonal closures, etc. may be impacted by this information.
Additionally, recreational tournaments often donate catches to local charitable
groups, orphanages, and other worthwhile recipients. This practice may be sharply
curtailed or modified should mercury levels dictate.
Of additional concern and interest to recreationals is the suggestion that oil
platforms may contribute to mercury levels in reef species. Should this prove
true, fishing preferences would likely be greatly modified.
Environmental Perspective
Ms. Felice Stadler
National Wildlife Federation
Minamata Plus 50: Where
Are We?
Dr. Leonard Levin
Electric Power Research Institute
Some 50 years after the direct discharge of methylmercury into Minamata Bay, and the "cats of Minamata," our understanding of environmental mercury has increased substantially, but mercury management methods remain less developed. Inventories of atmospheric sources show that not only do Asian industrial sources contribute half of the global burden, but that they contribute substantially to mercury additions to U.S. waterways by atmospheric deposition. Background sources, both natural sources such as hot springs and legacy sources such as abandoned mill sites, are roughly equal to industrial sources as emitters, but may play a lesser role in local and regional deposition.
Coal-fired electric power plants make up about one-third of current U.S. industrial emissions. These plants already remove about 40% of the mercury in the coal fuel before it is released from the stack, due to coal cleaning and current emissions controls. Studies show that additional levels of current controls, such as sulfur scrubbers or precipitators, become substantially more expensive when dedicated to mercury removal, and that advanced systems such as activated carbon are essentially unproven. This leaves unresolved the basic management question: will there be a substantial drop in fish mercury if there is a substantial cut in utility mercury emissions? This source-receptor relationship is not only unresolved, but is faced with increasing questions as scientific issues continue to be addressed. The possibility that emissions plumes chemically reduce the soluble divalent form of mercury to the globally-cycling elemental form has now been demonstrated initially in both field trials and laboratory measurements, but remains to be clarified. Modeling studies by EPA and others have recently shown a small contribution of utility emissions to deposition patterns at U.S. locations. These and other issues remain to be clarified.