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Regional Distribution of Arsenic in the Weches Formation of East Texas

E. B. Ledger
Department of Geology, Stephen F. Austin State University, Nacogdoches, Texas 75962

Twenty counties in east Texas show elevated naturally-occurring arsenic levels in the Eocene Weches Formation. Elevated levels occur in both green unweathered rock, and in red soils on the Weches Formation. There is no obvious source for the arsenic in the Weches Formation. Counties with the highest arsenic concentrations are Anderson, Cherokee, Nacogdoches, Sabine and San Augustine. Some counties average more than ten times the global average for mudrocks (13 ppm). Anderson County averages 151 ppm with one sample containing 178 ppm. Cherokee County (55 samples) averages 210 ppm with one sample containing 337 ppm. Nacogdoches County (37 samples) averages 101 ppm with one sample containing 141 ppm. San Augustine County averages 323 ppm while one unweathered sample contains 475 ppm, 36 times the global average. No adverse health effects are known to be due to naturally-occurring arsenic in east Texas. However, these extremely elevated levels occurring over a large area may be cause for concern. Further study on the nature of arsenic occurrence in the Weches Formation, and the behavior of arsenic during weathering and soil formation is critical. Possible target areas for future study include the specific occurrences of arsenic in unweathered Weches Formation rock, water in shallow domestic wells from the Weches Formation, mode of arsenic transport in east Texas streams, stability of arsenic known to occur at high levels in some east Texas lake sediment, and the geographical extent of the arsenic anomaly into south Texas, Louisiana, and Arkansas where similar rocks occur.


AAPG Search and Discovery Article #90080©2005 GCAGS 55th Annual Convention, New Orleans, Louisiana