Environmental Chemistry
The fate of mercury in the environment depends on the chemical form of mercury released and the environmental conditions. Elemental mercury, inorganic mercury, and methylmercury are the three most important forms of mercury in natural aquatic environments. Most mercury is released into the environment as inorganic mercury, which is primarily bound to particulates and organic substances and may not be available for direct uptake by aquatic organisms. The process of methylation of inorganic mercury to methylmercury, which is highly bioavailable, is thus an important key to the fate of mercury in the environment.
Elemental mercury has a high vapor pressure, a low solubility, does not combine with inorganic or organic ligands, and is not available for methylation. The mercurous ion (Hg[I]) combines with inorganic compounds only and cannot be methylated. The mercuric ion (Hg[II]) combines with both inorganic and organic ligands, and can be methylated. Methylation in aquatic habitats is primarily a biological process. Mono- and dimethylmercury are formed by microorganisms in both sediment and water through the methylation of inorganic mercuric ions (Hg[II]). Dimethylmercury, which is highly volatile, is generally not persistent in aquatic environments.
Methylation is influenced by environmental variables that affect both the availability of mercuric ions for methylation and the growth of the methylating microbial populations. Methylation rates are higher under anoxic conditions, in freshwater compared to saltwater, and in low pH environments. The presence of organic matter can stimulate growth of microbial populations (and reduce oxygen levels), thereby enhancing the formation of methylmercury. Sulfide can bind mercury and limit methylation. Methylmercury production can vary due to seasonal changes in nutrients, oxygen, temperature, and hydrodynamics. In most studies, methylation increased during the summer months when biological productivity was high, and decreased during the winter months.
Measurements of total mercury concentrations in the sediment do not provide information on the form of mercury present, methylation potential, or availability to organisms locally and downstream. If environmental conditions are conducive for methylation, methylmercury concentrations may be high in proportion to the supply and distribution of total mercury.
Bioaccumulation
Mercury is accumulated by fish, invertebrates, mammals, and aquatic plants and the concentration tends to increase with increasing trophic level (mercury biomagnifies). Although inorganic mercury is the dominant form of mercury in the environment and is easily taken up, it is also depurated relatively quickly. Methylmercury accumulates quickly, depurates very slowly, and therefore biomagnifies in higher trophic species. The percentage of methylmercury, as compared to total mercury, also increases with age in both fish and invertebrates.
Uptake and depuration rates vary between tissues within an organism. Partitioning of mercury between tissues within aquatic organisms is influenced by the chemical form of mercury and route of exposure (ingestion or via the gills). Due to its preferential uptake, ability to be transferred among tissues, and slow depuration, most of the mercury in fish muscle tissue (Å99%) is methylmercury.
Marine mammal tissues have some of the highest concentrations of mercury found in all marine organisms, with the liver generally having the highest total mercury concentration. Although many juvenile and adult marine mammals primarily feed on fish, which contain high percentages of methylmercury, high concentrations of inorganic mercury are found in adult specimens. Apparently, adult marine mammals can mineralize methylmercury into inorganic mercury. Juvenile marine mammals have lower concentrations of total mercury than adults; but unlike fish and invertebrates, the percentage of methylmercury is higher in juvenile mammals.
Invertebrates generally have a lower percentage of methylmercury, as compared to total mercury, in their tissues than do fish and marine mammals. The percentage of methylmercury in invertebrates varies greatly and can range from one percent in deposit-feeding polychaetes, to close to 100% in crab.
Bioconcentration factors (BCFs) reflect uptake from water in laboratory experiments. BCFs for mercury are variable, with the highest factors determined for methylmercury. BCFs for methylmercury in brook trout range from 69,000 to 630,000, depending on the tissue analyzed. BCFs for inorganic mercury (mercuric chloride) in saltwater species range from 129 for adult lobster (Homarus americanus) to 10,000 for oysters (Crassostrea virginica).
While sediment is usually the primary source of mercury in most aquatic systems, the food web is the main pathway for accumulation. High trophic level species tend to accumulate the highest concentrations of mercury, with concentrations highest in fish-eating predators. Mercury concentrations in higher trophic species often do not correlate with concentrations in environmental media. Correlations have been made between sediment and lower trophic species that typically have a high percentage of inorganic mercury, and between mercury concentrations in higher trophic species and their prey items. The best measure of bioavailability of mercury in any system can be obtained by analyzing mercury concentrations in the biota at the specific site.
Toxicity
Toxicity is influenced by the form of mercury, the environmental media, environmental conditions, the sensitivity or tolerance of the organism, and the life history stage. Inorganic mercury is less acutely toxic to aquatic organisms than methylmercury, but the range in sensitivity among individual species for either compound is large. Toxicity was found to be greater at elevated temperatures, lower oxygen content, reduced salinities in marine environments, and in the presence of metals such as zinc and lead.
In general, toxic effects occur because mercury binds to proteins and alters protein production or synthesis. Toxicological effects include reproductive impairment, growth inhibition, developmental abnormalities, and altered behavioral responses. Reproductive endpoints are generally more sensitive than growth or survival, with embryos and the early developmental stages the most sensitive. Mercury can be transferred from tissues of the adult female to developing eggs. Exposure to low concentrations of mercury may not result in mortality directly, but may retard growth thereby increasing the risk of predation.
Data available on the effects of mercury-contaminated sediment on aquatic organisms reviewed by Long and MacDonald (1992) resulted in effects range-low (ERL) and effects range-median (ERM) concentrations of 0.15 mg/kg and 0.71 mg/kg, respectively. However, these numbers were less accurate than other metals in predicting adverse effects, highlighting the need for site-specific effects data to determine concentrations of mercury in sediment that pose a threat to aquatic biota.
Few studies report both tissue residues and effects in long-term exposure to low concentrations of mercury. However, results from studies on different freshwater species indicate that reproductive effects could be expected to occur in sensitive fish species at tissue concentrations close to the FDA action level of 1 mg/kg (ppm).
The interaction of mercury and other trace elements (e.g., cadmium, copper, selenium, and zinc) can be both antagonistic and synergistic, primarily depending on exposure concentrations and form of mercury. Effects were generally less than additive (antagonistic) at lower exposure levels and greater than additive (synergistic) at higher levels. Zinc and cadmium were reported to reduce the teratogenic effects of methylmercury to killifish while selenium reduced mercury's toxic effects on development in medaka embryos.
Applications
Ecological assessments of waste sites with elevated concentrations of mercury in the aquatic environment are particularly challenging due to the complexity of the factors that affect the availability of mercury to aquatic organisms. Depending on the magnitude of the problem (local versus system-wide), the level of effort necessary to evaluate mercury contamination may range from simple monitoring of chemical concentrations to more complex programs including monitoring of numerous physical, chemical and biological parameters. The distribution of total mercury in sediment, which in most cases is predominantly inorganic mercury, may not by itself provide useful information about the bioavailability of mercury to aquatic species. Concentrations of total mercury in sediment that decrease with increasing distance from the source may still pose a threat to organisms if the bioavailability of the mercury increases (i.e., environmental conditions are more conducive for methylation). Mercury concentrations in aquatic organisms, particularly higher trophic-level organisms, may provide the best measure of the availability of mercury in a particular area.
In sites where a whole system has been affected, evaluation of remedial alternatives may need to be based on an understanding of the system-specific processes that lead to increased methylation and the pathways to resources of concern. An assessment of environmental parameters that affect the activity of methylating microbes (e.g., nutrients, temperature, pH, and dissolved oxygen) and the factors affecting the availability of inorganic mercury for methylation (e.g., the resuspension of sediment, total organic carbon, and sulfides) may be warranted when designing sampling plans for a remedial investigation.
To establish protective sediment target cleanup concentrations and remedial options for mercury-contaminated sites, we must understand the extent of contamination, the major pathways of transport, and bioavailability. Therefore, data on the accumulation of mercury in tissues of aquatic organisms should be included in assessment studies because it addresses potential human health concerns and availability to aquatic receptors. In addition, studies should assess toxicity to aquatic organisms, focusing on early life stages and reproductive effects.
Detection limits should reflect the program objectives. Contract Lab Program (CLP) methodology may be appropriate for screening level assessments; however, biologically relevant detection limits are often required and not available at CLP laboratories. Thus, analytical laboratories that can achieve lower detection limits may need to be used. Quality control is an important aspect of any testing program but is particularly important when analyzing mercury in environmental matrices. In water, very low concentrations need to be measured; the separation of the different forms of mercury requires special analytical techniques. Matrix effects in the extraction of mercury from tissue may interfere with accurate analyses for methyl and total mercury. When analyzing mercury in water, sediment, and tissue, analysis of certified standards for the appropriate matrix must be included as part of the quality control plan.
