Environ. Sci. Technol. 1996, 30, 161-171
Fate of Linear Alkylbenzene Sulfonate in the Mississippi River CHARLES F. TABOR AND LARRY B. BARBER, II* U.S. Geological Survey, 3215 Marine Street, Boulder, Colorado 80303
The 2 800-km reach of the Mississippi River between Minneapolis, MN, and New Orleans, LA, was examined for the occurrence and fate of linear alkylbenzene sulfonate (LAS), a common anionic surfactant found in municipal sewage effluents. River water and bottom sediment were sampled in the summer and fall of 1991 and in the spring of 1992. LAS was analyzed using solid-phase extraction/derivatization/ gas chromatography/mass spectrometry. LAS was present on all bottom sediments at concentrations ranging from 0.01 to 20 mg/kg and was identified in 21% of the water samples at concentrations ranging from 0.1 to 28.2 µg/L. The results indicate that LAS is a ubiquitous contaminant on Mississippi River bottom sediments and that dissolved LAS is present mainly downstream from the sewage outfalls of major cities. The removal of the higher LAS homologs and external isomers indicates that sorption and biodegradation are the principal processes affecting dissolved LAS. Sorbed LAS appears to degrade slowly.
Introduction Linear alkylbenzene sulfonate (LAS) is the most commonly used anionic surfactant in the United States, accounting for about 34% of the total surfactant use in 1990 (1). LAS use in the United States during 1991 was 390 × 106 kg, of which 88% was in laundry detergent and household cleaners (2). Based on an estimated population of 67 371 000 (3) and a per capita LAS consumption of 2.6 g/day (4), the daily LAS use in the Mississippi River basin in 1991 was 1.8 × 105 kg. Nearly all of the LAS consumed is disposed of through wastewater and treated at municipal sewage treatment plants (STP) or on-site septic systems. In 1987, 55% of STP in the United States used the trickling filter process for treating sewage, 39% used activated sludge, and 6% used primary clarification (5). These methods remove an average of 80%, 98%, and 27% of influent LAS by physical, chemical, and biological processes (4-6). Assuming the above distributions of treatment processes and their relative removal rates, the estimated 1991 LAS loading from sewage effluents in the Mississippi River basin was 2.8 × 104 kg/day. Many STP discharge their treated effluents directly into the Mississippi River and its tributaries, and significant amounts of LAS may enter the river.
This article not subject to U.S. Copyright. Published 1995 by the American Chemical Society.
There is an extensive body of research on LAS, including sorption and biodegradation experiments (7-13), development of predictive models for the environmental behavior of LAS (14-16), sampling of STP influents and effluents (4, 17-19), sampling of small streams and selected reaches of rivers (20-22), and sampling of groundwater (23, 24). However, until the present study, determination of the fate of LAS in a chemically and hydrologically complex surface water system such as the Mississippi River has not been done. The Mississippi River is the largest river in North America and the seventh largest river in the world in terms of water discharge (25). Its drainage basin covers 3.3 × 106 km2 and drains 41% of the 48 conterminous United States (25, 26). The northern boundary of the basin ranges from Alberta, Canada, to southern New York state. The basin is bounded on the west by the Rocky Mountains, on the east by the Appalachian Mountains, and on the south by the Gulf of Mexico. The present study sampled the 2 800-km reach of the Mississippi River between Minneapolis, MN, and New Orleans, LA, including 13 tributaries (Figure 1). The river can be divided into two hydrologically distinct reaches. The upper river between Minneapolis and St. Louis consists of a series of navigation pools separated by 29 locks and dams. The lower river is free flowing from St. Louis to the Gulf of Mexico. LAS is a mixture of homologs having alkyl chain lengths ranging from C10 to C14 with isomers having phenyl positions ranging from 2 to 7, resulting in 26 isomers. Due to this structural variability, LAS has an aqueous solubility that ranges from 0.2 to 160 mg/L (20) and sediment/water distribution coefficients (Kd) that range from
