EDTA, NTA, Alkylphenol Ethoxylate and DOC Attenuation during Soil Aquifer Treatment

Removals of dissolved organic carbon (DOC), ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA) and carboxylated alkylphenol polyethoxylate metabolites (APECs) were studied at the Sweetwater (Tucson, Ariz.) soil aquifer treatment site that treated chlorinated secondary effluent. The site was operated in a first phase by flooding irregularly for weeks interrupted by days of drying and in a second phase by a regular schedule of flooding for 3 days and drying for 4 days. The average hydraulic loading rates were 0.13 and 0.17 m/day in the first and second phases, respectively. During drying, oxygen intruded at least 3 m deep into the unsaturated subsurface causing nitrification of the ammonium that was retained in the top layer during flooding. Nitrification increased nitrate concentrations to >200 mg/L but most was removed to <10 mg/L during transport to 38 m depth. At 38 m depth, removals of DOC, EDTA, NTA, and APECs during the first phase were 85, 80, 90, and 98%, slightly higher (<7%) than during the second phase. Most of the DOC removal occurred during transport to 3 m and most of the trace organics removal occurred during transport from 3 to 38 m depth.