Pesticide and Nitrate Transport into Subsurface Tile Drains of Different Spacings

Leaching of agrichemicals into subsurface tile drainage water is a concern for water quality. The objective of this 3‐yr study was to determine field‐scale pesticide and nitrate (NO3) losses to instrumented subsurface drains as affected by drain spacing (5, 10, and 20 m). Small amounts of carbofuran [2,3‐dihydro‐2,2‐dimethyl‐7‐benzofuranyl‐methylcarbamate]; atrazine [2‐chloro‐4‐(ethylamino)‐6‐(isopropylamino)‐s‐triazine]; cyanazine [2‐chloro‐4‐(1‐cyano‐1‐methyl‐ethyl‐amino)‐6‐ethylamino‐s‐triazine]; and alachlor [2‐chloro‐2′,6′‐diethyl‐N‐(methoxymethyl) acetanilide] were detected in subsurface drainflow in the first large rainstorm after chemical application, which occurred between 3 to 14 d after applications and produced 0.2 to 1.0 cm net drainflow. Annual carbofuran losses in subsurface drainflow ranged from 0.6 to 28.1 g ha−1, or 0.04 to 1.9% of the amount applied to the soil, depending on year and drain spacing. Losses of all other pesticides were ≤0.1% of the amount applied. Total mass of pesticides, NO3‐N, and water removed by subsurface drains on a per‐area basis was greatest for the 5‐m spacing and least for the 20‐m spacing. Annual NO3‐N losses to drainflow ranged from 14 to 105 kg ha−1 during the 1988 to 1991 period. The data indicate that preferential flow may be the primary mechanism of pesticide leaching to shallow ground water in silt loam soils, but that total amounts leached are often quite small. Conversely, NO3‐N leaching is most significant during the off‐season, when most of the drainflow occurs.