EFFECT OF CONVENTIONAL VS NO-TILLAGE ON PESTICIDE LEACHING TO SHALLOW GROUNDWATER

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-yl]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 4X) 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 50X greater under no-till than conventional till plots. Results of this study suggest that preferential transport occurred.