Arsenic trace-element enrichment

Splits of coal samples collected near affected villages analyzed for trace elements revealed high concentrations of As and other deleterious elements (Belkin et al., 1997). Subsequent field work and analysis not only confirms the high concentration of arsenic and other trace elements, but also provides evidence that bears on the paragenesis of these enrichments (Belkin et al., 1998). 

Trace mineralization including sporadic arsenic enrichment is present within the Illinois Basin, particularly in the southern portion and along its western edge (Erickson et al., 1987). Follow on studies failed to detect elevated trace-element contents in rocks of the central Illinois Basin (Goldhaber, unpubhshed data). However, data from further north suggests enrichment in elements related to MVT deposits including arsenic extending into southeast Minnesota, based on chemical analysis of acid insoluble residues from that area (Lively et al., 1997). 

The regional fluid-flow described above produced widespread diffuse trace mineralization over large areas of the U.S. Midcontinent. In addition to this diffuse mineralization, specific paleohydrologic processes can lead to somewhat more intense element enrichment, ranging from economic ore deposits down to sub-economic but nonetheless significant concentrations of ore and gangue minerals. Because arsenic can accompany MVT mineralization, these more intensely mineralized areas are important to identify because they may be particularly susceptible to enviromnental impacts. 

For both surface waters and groundwaters, input of or mixing with geothermal waters can have dramatic impacts on both major ion and trace element composition because of the enrichment of geothermal waters in many chemical constituents. In particular, geothermal waters are often enriched in arsenic (11) and contribute significant amounts of arsenic to surface waters, for example, in the eastern Sierra Nevada in California (19,20). 

The deposition and accumulation of fly ash downwind from coal-combustion sites is a concern because it may be significantly enriched in potentially toxic trace elements, including lead (Pb) and arsenic (As), compared to the burned coal (Coles et al, 1979 Eary et al, 1990 Hower et al, 1999 Kaakinen et al, 1975). Other elements such as zinc (Zn) and germanium (Ge), of less environmental concern, may also be enriched in fly ash. The relatively high concentrations of As in fly ash reflect partly its presence in pyrite in coal from the Appalachian Basin (Goldhaber et al, 2002). More importantly, the concentration of these metals and metalloids occurs during the combustion process itself. A suite of elements including As, Ca, Cr, Cu, Ga, Mo, Ni, Pb, Sb, Se, V, and Zn is enriched in the fine fraction of coal fly ash (Coles et al, 1979), because of vaporization in the furnace and subsequent condensation or absorption onto ash particles (Kaakinen et al, 1975). 

The average trace-element concentrations of seven xtl MMs from the Cap Prudhomme collection are listed in Table 18.14 based on data published by Kurat et al. (1994). The enrichment or depletion of trace elements in the MMs relative to average rocks of the continental crust (Taylor and McLennan 1985) was expressed as the logarithm to the base 10 of the MM crustal-rock ratios. The resulting distribution of data points in Fig. 18.31 indicates that the MMs are strongly enriched in iridium and gold as well as in nickel, chromium, selenium, iron, cobalt, arsenic, and antimony relative to crustal rocks. 

The sedimentary section of the U.S. southern midcontinent consists in large part of Paleozoic platform carbonate rocks interbedded with shale and subordinate sandstone. As documented below, a large volume of the Paleozoic carbonate rocks in this region is enriched in arsenic, as well as lead, zinc, copper, cobalt, molybdenum, and nickel compared to surrounding carbonate strata (Erickson et al., 1981 Lee and Goldhaber, 2001). In plan view this trace-mineralized area extends over thousands of square kilometers. The widespread metal enrichment process was related to the formation of much more spatially localized ore districts. These districts consist of economic concentrations of the ore elements zinc and lead, and to a lesser extent copper, cobalt, and barium that occur in the platform... 

As is the case with isotopic labeling of molecules, enriched levels of stable isotopes of elements can be used as tracers. Isotopes of elements can be used as nutritional supplements for plants or animals to trace absorption, assimilation, and metabolism of elements (Allen and Georgitis). Processes such as biomethylation of elements like mercury and arsenic in the environment can be studied using isotopically enriched elements. In some cases, methylated metals are more toxic than the inorganic species, and generally accumulate up the food chain. 

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