Transport and biodegradation of creosote compounds in a large, intact, fractured clayey till column

An experiment was conducted using a large, intact column of fractured clayey till to study the transport and biodegradation of 25 organic compounds typical of creosote. The column (0.5 m in height and 0.5 m in diameter) was collected from a depth of 2.5-3 m at an experimental site on the island of Funen, Denmark. For the first 82 days of the experiment, the column was infiltrated with water containing nitrate, but no organic compounds. During this period, significant nitrate removal and nitrite production were observed indicating that denitrification occurred in the clayey till. After 82 days, a mixture of 25 organic compounds with a total concentration of approximately 70 mg 1(-1) was added to the influent water together with a conservative tracer (92 mg bromide 1(-1)). Most of the organic compounds were transported as rapidly as bromide, and only carbazole, dibenzofuran, fluorene, dibenzothiophene, and phenanthrene were significantly retarded. No extensive loss of organic compounds was observed during this period, which was attributed to the high concentration of applicated organic compounds. After 40 days, the influent concentration of organic compounds was lowered by a factor of 5; subsequently, significant biodegradation of phenol, ethylbenzene, toluene, quinoline, indole, p-xylene, and o-cresol was observed. Additionally, o-xylene, naphthalene, l-methylnaphthalene, phenanthrene, fluorene, 2-methylquinoline, carbazole, acridine, benzothiophene, dibenzothiophene, benzofuran, dibenzofuran, pyrrole, 1-methylpyrrole, and benzene were biodegraded to some degree when oxygen was added concomitantly with nitrate (92 days after the addition of organic compounds). Pyrrole, 1-methylpyrrole, and benzene were only slightly biodegraded. The biodegradation of benzene was likely inhibited by the presence of pyrrole and/or 1-methylpyrrole. The study has shown that the transport of low-molecular-weight organic compounds through fractured clayey till may occur as rapid as the transport of bromide. Consequently, there is a high risk of groundwater contamination if aquifers are overlain with fractured clayey till with properties similar to the till used in this column study. The study has also shown that the till provides an environment when biodegradation of some organic compounds may occur when oxygen is provided. However, the concentration of oxygen present in water will often not be sufficient for complete biodegradation of the organic compounds at the concentrations known to be typical for creosote sites. (C) 1999 Elsevier Science B.V. All rights reserved.