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Trends of Nitrate and Pesticide Concentrations Along Flowpaths Located in California, Georgia, North Carolina, and Wisconsin

The trends of concentrations and transformations of agricultural chemicals along flow paths at study sites in California, Georgia, North Carolina, and Wisconsin (Tesoriero and others, 2007) were investigated using estimated groundwater ages along with the concentrations of pesticides, nitrogen species, and other redox-active constituents. The California study site is located in the San Joaquin Valley, Fresno County, California. The Georgia study site is located in the Lime Creek subbasin of the Flint River in Sumter County, Georgia. The North Carolina study site is located in the Contentnea Creek subbasin of the Neuse River in Greene County, North Carolina. The Wisconsin study site is located in the Tomorrow River Watershed in Portage County, Wisconsin.

Map showing the locations of study sites in California, Georgia, North Carolina, and Wisconsin.

Trends of nitrate concentrations were determined at each of the four study sites by reconstructing concentrations of nitrate at the time of recharge using the results from groundwater age dating (using chlorofluorocarbons) and water-quality sampling. There was a two-to five-fold increase in nitrate concentrations in groundwater in these aquifers since the 1960s. Trends of nitrate concentrations are related to increases in fertilizer (inorganic and/or manure) applications. The fraction of applied fertilizer that reached the water table was estimated by dividing the amount of fertilizer from both inorganic and manure sources in each study site by the volume of water into which this fertilizer is "dissolved." In Wisconsin, about 20 percent of the fertilizer applied to the land reaches the water table. In North Carolina, about 5 to 15 percent of the applied fertilizer reached groundwater. In California, about 30 to 50 percent of the fertilizer reached the water table. The percent of applied fertilizer reaching the groundwater in Georgia could not be estimated.

Deethylatrazine (DEA), a transformation product of atrazine, was typically present at higher concentrations than atrazine at study sites with thick unsaturated zones but not at sites with thin unsaturated zones. Furthermore, the fraction of atrazine plus DEA that was present as DEA did not increase as a function of groundwater age. These findings suggest that atrazine degradation occurs primarily in the unsaturated zone with little or no degradation taking place in the saturated zone at these study sites. Similar patterns also were observed for alachlor and metolachlor.

Trends in the concentrations of atrazine and DEA are largely consistent with the years in which use of these herbicides was first initiated. At the Wisconsin study site, atrazine and DEA concentrations increased from the early 1960s to the mid to late 1970s. From the late-1970s to the early 1990s, concentrations of DEA declined slightly, while atrazine concentrations were stable. Use of atrazine was prohibited in this site beginning in 1995. The only sample that has a recharge date after 1995 had relatively high concentrations of both atrazine and DEA. It is likely that this well represents a mixture of water from both before and after the ban on atrazine use in this site.

Atrazine and DEA concentrations at the Georgia study site increased in the late 1970s and then declined to nondetectable levels in the late 1980s. Atrazine was commonly used on corn and soybeans, the production of which increased sharply in the early 1970s and decreased in the mid-1980s in this site. Atrazine and DEA concentrations began to rise about 5 years after the increase in corn and soybean production and declined to nondetectable levels about 5 years after production of these crops had peaked.

While trends in the concentrations of atrazine and DEA were less apparent at the California and North Carolina study sites, higher frequencies of detection and higher concentrations were found in water recharged after 1980 in both systems.

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