Microbial degradation of linear alkylbenzenesulfonates in sewage contaminated groundwater

Surfactants, or surface active agents, are the active ingredients in detergents used for\ud domestic cleaning applications, such as dish washing and laundering. They are effective cleaning\ud agents by concentrating at interfacial regions, lowering the surface tension of water, and forming\ud micelles that solubilize oils (West, 1992). Surfactants are amphiphilic, because they are composed\ud of both hydrophilic and hydrophobic moieties. The hydrophobic moiety interacts with non-polar\ud phases, such as oils, while the hydrophilic moiety interacts with polar phases, such as water. Oils\ud are removed from surfaces by partitioning into the hydrophobic interior of the surfactant micelle.\ud Upon rinsing with water, the micelles carrying the oils are washed away to waste water treatment\ud facilities, septic tanks, or drain fields.\ud Linear alkylbenzenesulfonates (LAS) are the main anionic surfactants used in detergent\ud formulations (Figure 1). LAS is a mixture of homologues that range between 10 and 14 carbon\ud atoms on the aliphatic chain (Swisher, 1987). Each LAS homologue consists of different isomers,\ud defined by the position of the phenyl ring on the aliphatic chain. The homologues and isomers\ud have different sorption and biodegradation characteristics. For example, SchOberl (1989) found\ud that longer-chain LAS homologues and isomers biodegrade faster than shorter-chain LAS\ud homologues and isomers. Hand and Williams (1987) found that longer LAS alkyl-chain\ud homologues and isomers have higher sediment partition coefficients than their shorter chain\ud 1\ud counterparts. LAS also is generally considered to be biodegradable in aerobic environments and\ud persistent in anaerobic environments (Leisinger et al., 1981). The reason LAS biodegradation is\ud limited to aerobic conditions is because molecular oxygen is required for the first step in its\ud biodegradation pathway.\ud As LAS is a constituent of detergents used in everyday cleaning, it is found in most\ud municipal sewage effluents. Because LAS is an anthropogenic chemical and does not occur\ud naturally, it is a good indicator of groundwater contamination (LeBlanc, 1984). An excellent site\ud for the study of LAS behavior in groundwater is the U.S. Geological Survey's (USGS) Cape Cod\ud Toxic Water Research Site, located on Cape Cod, Massachusetts, where sewage effluent from a\ud waste water treatment facility is discharged onto a shallow groundwater aquifer (LeBlanc, 1984)\ud (Figure 2). The groundwater moves at a rate of 0.2 to 0.7 m/day and has a temperature range of\ud 9.5 to 14°C (Harvey et al., 1994). The waste water treatment facility, located on the Otis Air\ud National Guard Base, has discharged secondary sewage effluent onto 4.86 hectares of rectangular\ud sand beds since 1936 (LeBlanc, 1984). The result of this long-term sewage discharge is a large\ud cigar-shaped that is currently 4000 m long, 762 to 914 m wide and 23 m thick. The plume\ud contains nitrate, ammonia, LAS, chloride, boron, sulfate, and phosphate (LeBlanc, 1984). In\ud addition, alkylbenzenesulfonate (ABS), a branched-chain non-biodegradable analog of LAS, was\ud discharged in the sewage effluent until 1964 and is found in the farthest down-gradient (2-3 km)\ud portion of the plume (LeBlanc, 1984). After 1964, LAS replaced ABS because LAS is more\ud readily biodegradable.\ud Although metabolites of LAS have been detected in the plume, Field et al, (1992)\ud estimated the rate at which LAS is biodegraded to be slow as LAS was detected after a 2.7 to 4.6\ud 2\ud year residence time in the sewage contaminated groundwater. Low temperature (9.5 to 14°C)\ud and oxygen conditions (