Transported arsenic in the sediments of Ayish Bayou, San Augustine County, Texas

Ayish Bayou in San Augustine County, Texas has elevated levels of arsenic in the stream sediment. The arsenic appears to come from outcrops of the Eocene Reklaw and Weches Formations. The Weches Formation is a marine mudstone with abundant fossils of shallow-water organisms. These types of formations typically contain appreciable arsenic (Turekian and Wedepohl, 1961). Chemical data show that when Ayish Bayou crosses the Reklaw and Weches Formations, average sediment arsenic levels rise to 48 ppm for the fine fraction and 37 ppm in the coarse fraction. Maxima are 71 ppm in the fine fraction and 74 ppm in the coarse fraction. This is compared to 13 ppm in Ayish Bayou fine sediment (4 ppm in the coarse fraction) as it crosses the Carrizo Sandstone before entering the Reklaw and Weches outcrops Amorphous Fe(OH)3 and other Fe oxides occur as red colloidal precipitates which adsorb arsenic oxyanions. The Texas Commission on Environmental Quality (TCEQ) has found Lake Sam Rayburn sediment arsenic levels to 57 ppm (TCEQ, 2003; Judy, et al., 2004). Ayish Bayou is the link between outcrops of the Reklaw and Weches Formations and the elevated levels of arsenic in Lake Sam Rayburn sediment. The sample below the dam shows background levels of arsenic. Thus Lake Sam Rayburn is a sink for arsenic-bearing sediments.