Mercury in the atmospheric and coastal environments of Mexico

In Mexico, published studies relating to the occurrence of Hg in the environment are limited. Among the main sources of Hg in Mexico are mining and refining of Auand Hg, chloralkali plants, Cu smelting, residential combustion of wood, carbo electric plants, and oil refineries. Hg levels are highly variable in the atmospheric compartment because of the atmospheric dynamics and ongoing metal exchange with the terrestrial surface. In atmospheric studies, Hg levels are usually reported as total gaseous Hg (TGM). In Mexico, TGM values ranged from 1.32 ng m-3 in Hidalgo state (a rural agricultural area) to 71.82 ng m-3 in Zacatecas state (an area where brick manufacturers use mining wastes as a raw material).Published information on mercury levels in the coastal environment comprise 21 studies, representing 21 areas, in which sediments constituted the substrate that was analyzed for Hg. In addition, water samples were analyzed for Hg in nine studies.Few studies exist on Hg levels in the Caribbean and in the southwest of the country where tourism is rapidly increasing. Hence, there is a need for establishing baseline levels of mercury in these increasingly visited areas. In regions where studies have been undertaken, Hg levels in sediments were highly variable. Variations in Hg sediment levels mainly result from geological factors and the varying degree of anthropogenic impacts in the studied areas. In areas that still have pristine or nearly pristine environments (e.g., coast, Baja California, Todos Santos Bay, and La Paz lagoon), sediment Hg levels ranged from <0.006 to 0.35 j.lg g-1 on a dry wt basis.When higher levels exist (0.34-57.94 j.lg g-1 on a dry wt basis), the environment generally shows the influence of inputs from mining, oil processing, agriculture,geothermal events, or harmful algal bloom events (e.g., Guaymas Bay and Coatzacoalcos estuary). From chronological studies performed in selected coastal lagoons in NW Mexico, it is clear that Hg fluxes to sediments have increased from2- to 15-fold in recent years. Since the 1940s, historical increases of Hg fluxes have resulted from higher agricultural waste releases and exhaust from the thermo electric plants. The levels of Hg in water reveal a moderate to elevated contamination of some Mexican coastal sites. In Urias lagoon (NW Mexico), moderate to high levels were found in the dissolved and suspended fraction, and these are related to shipping, the fishing industry, domestic effluents, and the presence of a thermoelectric plant. In Coatzacoalcos (SE Mexico), extremely elevated Hg levels were found during the decade of the 1970s. Low to moderate levels of Hg were measured in waters from the Alvarado lagoon (SE Mexico); those concentrations appear to be associated with river waters that became enriched with organic matter and suspended solids inthe brackish mixing zone.Regarding the Hg content in invertebrates, the use of bivalves (oysters and mussels)as biomonitors must be established along the coastal zones of Mexico, because some coastal lagoons have not been previously monitored. In addition, more research is needed to investigate shrimp farms that are associated with agricultural basins and receive effluents from several anthropogenic sources (e.g., mining activity and urban discharges). Hg residues in several vertebrate groups collected in Mexico have been studied.These include mammals, birds, reptiles, and fish. In elasmobranch species, the highest Hg concentration (27.2 flg g-1 dry wt) was found in the muscle of the smooth hammer head shark (Sphyrna zygaena). Teleost fish are the vertebrate group that has been most studied, with regard to Hg residue content; the highest value (5.67 11g g-1dry wt) was detected in the striped marlin (T. audax). Among reptiles, only marine turtles were studied; Hg levels found ranged from 0.795 in the liver to 0.0006flg g-1dry wt in the blood of L. olivacea. In birds, the highest Hg concentration (5.08 flg g-1dry wt) detected was in the liver of the olivaceous cormorant (P. olivaceous).Specimens from stranded marine mammals were also analyzed; levels of Hg ranged from 70.35 flg g-1 dry wt in the liver of stranded spinner dolphin (S. longirostris ), to0.145 flg g-1 dry wt in the muscle of gray whale (E. robustus). The presence of Hgin these marine animals is not thought to have caused the stranding of the animals.Other organisms like macroalgae and vestimentiferan tube worms were used to monitor the occurrence of Hg in the aquatic environment; levels were comparable to data reported on similar organisms from other areas of the world. Few investigation shave been carried out concerning the mercury content in human organs/tissues in Mexico. Considering the potential deleterious effects of Hg on kidney, lung, and the central nervous system, more information about human exposure to organic and inorganic forms of mercury and their effects is needed, both in Mexico and elsewhere.