NATURAL OCCURRENCE OF ELEVATED SELENIUM AND ARSENIC IN THE GEORGIA PIEDMONT: IMPLICATIONS FOR THEIR RELATIVE MOBILITY IN SOILS

Pyrite and arsenopyrite occur in a mafic schist of the Piedmont in Gwinnett County, Georgia and is associated with a fossil hydrothermal system of the Brevard Zone. Pyrite is texturally associated with late stage retrograde metamorphic laumontite, which occurs along brittle fractures and within foliations. Bedrock analysis shows Se and Ar resides in the arsenopyrite giving bulk concentrations above regional baselines. Analysis of the Se and Ar in overlying saprolite and soil indicates they are weathered from the arsenopyrite but have different fates. Se is presumed to migrate out of the weathering profile and carried off by the rivers. Ar is conserved most likely as arsenate complexes adsorbed to abundant Fe-oxyhydroxide surfaces in the saprolite/soil. This scenario is directly supported by the high As/Se ratios measured in the saprolite of this study and indirectly by the very low As/Se ratios measured in Asiatic clam (Corbicula fluminea), which serve as biomonitor proxies of river waters draining the Brevard Zone (Peltier et al., 2008). Their specific interest is the contributions of trace elements associated with different point sources and land uses in a large river. In particular they studied the tributaries of the Chattahoochee River, whose main channel in the Georgia Piedmont is largely controlled by the trace of the Brevard Zone. Realizing that the Chattahoochee integrates a watershed larger than the Brevard Zone, analysis of Se and Ar in nine samples from each of fifteen river sites reveals an As/Se ratio of 0.6, which is much smaller than the ratios of about ~40 for the bedrock and ~2000 for the saprolite in this study site. If the primary Se and Ar signature of the Georgia Piedmont is controlled by arsenopyrite similar to here, then both the saprolite and the river waters (as proxied by a biomonitor) provide a good model for the partitioning of the two elements in the Piedmont of SE United States.

Peltier et al., 2008 Environmental Pollution, 154(2): 283- 290.