Column Leaching Study of Six Pesticides, Nitrate, and Chloride Through Four Wisconsin Soils

Abstract: The leaching of pesticides to groundwater from agricultural practices has been of increasing concern in Wisconsin as well as throughout the United States. Differences in chemical solubility, adsorptive characteristics, volatility, and degradability as well as soil properties that effect water movement, biological activity and chemical retention all effect the amount of a pesticide that will leach to groundwater. This project was designed to determine the relative leaching that could be expected for a group of commonly used pesticides on four soil types representing a range of soil properties common to Wisconsin agriculture. The soils studied included two sandy outwash soils (Sparta and Plainfield sands), one silt loam over sand (Antigo), and one sandy loam (Burkart). The pesticides in this study included Aldicarb, Oxymyl, Carbofuran, Carbaryl, Atrazine, and Alachlor. In addition, ammonium nitrate and potassium chloride were added to compare the pesticide leaching to nitrate and chloride movement. Two rates of chemical addition were used representing the normal range of chemical applications for each pesticide.

Results showed more rapid leaching of pesticides, nitrate, and chloride through the sandy Sparta and Plainfield soils. Only small amounts of pesticides were recovered in the leachate from the Antigo of Burkart soils, however, only about 70 percent of the Field Moisture Capacity of these columns was collected as leachate, compared to 200 percent from the sandy soils. All soils had the equivalent of 11.6 inches of water added over a seven week leaching period. Thirty to 48 percent of the highly soluble chemicals (Aldicarb, Oxamyl, and Carbofuan) were found in the leachate from the sandy soils. Carbaryl was not found in detectable amounts in any leachate. Atrazine and Alachlor amounts in leachate from the sands were 2 percent and 0.2 percent, respectively. Aldicarb sulfoxide was the primary form of aldicarb formed and leached at about the same rate as Oxamyl and chloride. Carbofuran appeared to move slightly slower, as did nitrate.

Soil residues in the sands showed fairly complete leaching of the soluble pesticides, with Atrazine residues the highest of any pesticide in the Plainfield soil. Residues in the Antigo and Burkart soil showed relatively large amounts of Aldicarb, Carbofuran, and Atrazine remaining at the end of the seven week study. Carbaryl and Alachlor were in very low amounts, as was Oxamyl in the Burkart soil. Significant Oxamyl was found in the more acid Antigo soil.

One can conclude from this study that Aldicarb, Oxamyl, and Carbofuran are highly leachable in Wisconsin’s sandy soils and that these chemicals move fairly easily to lower depths in finer textured soils. Carbaryl did not appear to move significantly in any of the soils studied. Atrazine, while moving slower than other chemicals, was found in the leachate from the sands at concentrations above current standards. Alachlor data was less conclusive. Some leaching was found to occur, but in generally small and erratic amounts.