Trends in pesticide concentrations in 38 major rivers in the U.S. during 1992-2010 reflect large-scale trends in pesticide use and regulatory changes, according to a new study by the U.S. Geological Survey.
The study, the first to rigorously compare riverine pesticide concentrations with trends in pesticide use at the national scale, examined 11 pesticides that have sufficient historical data for trend analyses and that are among the top 20 most frequently detected in rivers and streams in the United States. Most of the 11 long-used chemicals had primarily downward trends in concentrations in most regions over the study period. Focusing on this group of 11 pesticides with the most extensive concentration data affords a unique opportunity to study the relations between river concentrations and use or other factors that may influence trends.
Trends in pesticide concentrations followed agricultural usage patterns and regulatory restrictions on use for pesticides used primarily on agricultural crops — cyanazine, alachlor, atrazine (and its degradate, deethylatrazine), metolachlor, and carbofuran.
"In major river basins, the overall influence of agricultural pesticide use is so strong," said Karen Ryberg, USGS statistician and lead of the study, "that any changes in other causes of trends in pesticide concentrations in the water — changes that might be traced to enhanced agricultural management practices — are difficult to discern, especially without improved data on both the use of specific pesticides and the timing, location, and extent of management practices."
Alachlor concentration trends in major rivers, for example, declined nationwide from 1992-2010 as the use of alachlor, a herbicide most commonly applied to corn, dropped from about 20,000 to 2,500 metric tons. The introduction of a new herbicide (acetochlor) and the increase in use of glyphosate-resistant corn and soybeans contributed to the nationwide decline in alachlor use.
For pesticides with substantial use in both agricultural and urban areas — simazine, chlorpyrifos, malathion, diazinon, and carbaryl — pesticide concentration trends in major rivers reflect both agricultural and nonagricultural usage patterns.
Urban contributions of pesticides have marked effects on concentration trends of some pesticides in major rivers, despite there being a much smaller area of urban land compared to agriculture in most river basins.
More than 400 pesticides are used in agriculture each year. Regulatory changes, market forces, and introduction of new pesticides continually alter the use of these pesticides over time. The USGS National Water-Quality Assessment Program currently monitors less than half of the pesticides currently being used for agriculture because of resource constraints. However, USGS is working to fill these gaps by monitoring new pesticides that come into use, such as the neonicotinoid and pyrethroid insecticides.
The article, "Trends in Pesticide Concentrations and Use for Major Rivers of the United States" by Karen Ryberg and Robert Gilliom, has recently been published in the journal Science of the Total Environment.
National maps and trend graphs that show the distribution of the agricultural use of 459 pesticides for each year during 1992-2012 in the conterminous U.S. are available online.