Effect of Conventional vs. No-Tillage on Pesticide Leaching to Shallow Groundwater
1990
Isensee, A. R. | Nash, R. G. | Helling, C. S.
A field site was established at Beltsville, MD, in 1986 to assess the effect of conventional and no-till cultural practices on the movement of pesticides into shallow groundwater. Groundwater samples taken from unconfined (<1.5 m deep) and confined (<3 m deep) monitoring wells in 1986–1988 were analyzed for atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine], deethylatrazine [6-chloro-N-(1-methylethyl)-1,3,5-triazine-2,4-diamine], alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide], cyanazine [2-[[4-chloro-6-(ethylamino)-1,3,5-triazine-2-y1]amino)-2-methylpropanenitrile], and carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarbamate). Atrazine was found in groundwater all year, while cyanazine, alachlor, and carbofuran were present only for a short period (<3 mo) after pesticide application. Fairly constant background levels of <0.5 µg L⁻¹ atrazine were found under fields treated before 1986, while levels under continuously treated fields were <2.0 µg L⁻¹ for 22 of 25 samplings. Pesticide residues in unconfined groundwater were usually higher (ca. 2 to 4×) than in confined groundwater. Rainfall timing relative to pesticide application was critically important to pesticide leaching. A prolonged rain immediately after the 1988 application resulted in peak atrazine and cyanazine levels of ca. 200 µg L⁻¹ in unconfined and ca. 30 to 40 µg L⁻¹ in confined groundwater, which resulted in short-term levels ca. 2 to 50× greater under no-till than conventional till plots. Results of this study suggest that preferential transport occurred. Contributions from the USDA-ARS.
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