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 (NO,) 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-chl oro-4-(ethylamino)-6-(isopropylamino)-s-triazine]; cyanazine [2-chloro-4-(1 -cyano-1-methyl-ethyl-amino)-6-ethylamino-s-triazine]; and alachlor [2-chlo ro-2',6'-diethyl-N-(methoxymethyl) acetanilide] were detected in subsurface drainflow in the first large rainstorm after chemical application, which o ccurred 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, depe nding on year and drain spacing. Losses of all other pesticides were less t han or equal to 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 f or the 5-m spacing and least for the 20-m spacing. Annual NO3-N losses to d rainflow ranged from 14 to 105 kg ha(-1) during the 1988 to 1991 period. Th e data indicate that preferential flow may be the primary mechanism of pest icide leaching to shallow ground water in silt loam soils, but that total a mounts leached are often quite small, Conversely, NO3-N leaching is most si gnificant during the off-season, when most of the drainflow occurs.