Surface grab and core sediment samples were collected from Lakes Michigan, Superior, and Huron from 2010 to 2012, and concentrations of herbicides atrazine, simazine, and alachlor, as well as desethylatrazine (DEA), were determined. Concentrations of atrazine in surface grabs ranged from 0.01 to 1.7 ng/g dry weight and are significantly higher in the southern basin of Lake Michigan (latitude <44°) than other parts of the three lakes. The highest concentration of alachlor was found in sediments of Saginaw Bay in Lake Huron. The inventory and net fluxes of these herbicides were found to decline exponentially from the south to the north. The concentration ratio of DEA to atrazine (DEA/ATZ) increased with latitude, suggesting degradation of atrazine to DEA during atmospheric transport. DEA/ATZ also increased with sediment depth in the sediment cores. Diffusion of deposited herbicides from the upper sediment into deeper sediments has occurred, on the basis of the observed patterns of concentrations in dated sediment cores. Concentrations of atrazine in pore water were estimated and were higher than those reported for the bulk waters, suggesting the occurrence of solid-phase deposition of atrazine through the water column and that contaminated sediments act as a source releasing atrazine to the overlying water.
■ INTRODUCTIONCurrent agricultural practices often involve extensive use of herbicides. 1 Atrazine [1-chloro-3-ethylamino-5-isopropylamino-2,4,6-triazine], simazine [6-chloro-N,N′-diethyl-1,3,5-triazine-2,4-diamine], and alachlor [2-chloro-2,6-diethyl-N-(methoxymethyl)acetanilide] are among those used in the United States. Atrazine and simazine are both triazines and were initially registered as herbicides in 1958 and 1956, respectively. 2,3 Alachlor, a chloroacetanilide, was first registered in 1969. 4 From 1992 to 2011, reported annual usages on crops were 32 000−36 000 tons for atrazine, 1814−2722 tons for simazine, and 454−22 680 tons for alachlor. 5 These herbicides are primarily used in the "Corn Belt" near the Great Lakes of North America, including the states of Iowa, Illinois, and Indiana and parts of others. 1,5 Their use has been banned by the European Union because of ubiquitous and unpreventable water contamination. Atrazine is classified as a Restricted Use Pesticide in the United States. 6 According to the U.S. Environmental Protection Agency (EPA), atrazine is an endocrine disruptor, 6 simazine may have reproductive or developmental toxicity, 7 and alachlor was classified as a "likely" human carcinogen at high dose. 4 Compared with organochlorine pesticides, atrazine and related compounds are more soluble in water. A major concern is their potential to leach from soils to surface and ground waters that are used as sources of drinking water; 3 thus, most monitoring efforts to date have been conducted for water. Atrazine was found in waters of all five Laurentian Great Lakes. 8−10 In rivers of the Midwestern U.S., elevated concentrations of atrazine, simazine, and alachlor were fou...
