December 1996


Geological Survey reports improved water quality

The latest study of the Mississippi River by the U.S. Geological Survey, published earlier this year, concludes that the rapid biodegradation of LAS has contributed significantly to improvement in the rivers water quality in recent years, despite a more than 30 percent increase in surfactant consumption.1

Concentrations of anionic surfactants (MBAS)2 in the river have continued to decrease since 1972, and show a substantial drop since 1990, the study reports, despite increased populations and use. Druing the 1991-92 period, concentrations ofanionic surfactants were as low as 20 to 100 parts per billion, well below the drinking water standard of 500 parts per billion.

LAS concentrations in the river also were found to be very low. LAS was detected in only 16% of mainstream grab samples, at concentrations ranging from 0.1 to 10.3 parts per billion.

Although measurable concentrations of LAS were found near the major population centers of Minneapolis, St. Louis and New Orleans, LAS concentrations fell to below detectable levels within 80 km downstream from these sources as a result of dilution and instream biodegradation.

Concentrations of LAS in bed sediments ranged from about 0.1 to 1 part per million of sediment. Biodegradation continued to remove LAS from the sediment, the study found, but rates of biodegradation varied with river conditions.

The study notes that EDTA3, the dissolved organic chemical generally considered to be an indicator of industrial contamination, is found in the Mississippi at about one-fourth of the concentration found in some European rivers.

1. Contaminants in the Mississippi River, 1987-92 / edited Robert H. Meade (U.S. Geological Survey circular: 1133); C.F. Tabor and L.B. Barber, II, Environ.Sci. Technol., 30(1996) 161-171.

2. Methylene-blue-active substances, a measure of anionic surfactant concentrations.

3. Ethylenediaminetetraacetic acid.