Abstract

Twenty-one counties in east Texas show elevated naturally-occurring arsenic levels in the Eocene Weches Formation. Elevated levels occur in both green unweathered rock, and red soils on the Weches Formation. There is no obvious source for the As 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 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 caused by 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. Target areas for future study include the specific occurrences of arsenic in unweathered Weches Formation rock and in soils developed on the Weches Formation.