WATER-QUALITY - LEACHING OF NITRATE, ATRAZINE, AND METRIBUZIN FROM SUGARCANE IN SOUTHERN LOUISIANA

We have studied the leaching losses of NO3, atrazine yl-N'-(1-methylet hyl)-1,3,5-triazine-2.4-diamine], and metribuzin hylethyl)-3-(methylth io)-1,2,4-triazine-5(4H)-one] applied to sugarcane (Saccharum officina rum L.) planted in Mississippi River alluvial soil in southern Louisia na. Nitrogen (122 kg/ha) and atrazine (2.24 kg/ha) were applied in Jun e, and atrazine (2.24 kg/ha) and metribuzin (1.12 kg/ha) were applied in December; losses through a Sharkey clay (very fine, montmorilloniti c, nonacid, thermic Vertic Haplaquepts) into subsurface drains (5.5- a nd 10.9-m spacing) were measured for about 100 d in both seasons. Five days after application NO3-N appeared in its highest concentrations ( 5-11 mg/L) in the drain water; after this first event, concentrations remained below 10 mg/L throughout the summer season. After application in the summer atrazine appeared in the subsurface drains at its highe st seasonal concentrations (114-144 mu g/L) on the day of application; after 4 to 7 wk these concentrations remained below 3.0 mu g/L. Total losses in the summer amounted to 3 to 8% of the NO3 application and 0 .6 to 1.2% of the atrazine application. Almost 50% of the NO3 leaching into the subsurface drains occurred after Day 76, whereas 82% of the atrazine leached into the drains by Day 8. After the winter applicatio n, high concentrations of atrazine (67-81 mu g/L) and metribuzin (52-9 4 mu g/L) were measured within 8 d. Similarly, large concentrations of atrazine occurred in the drain water throughout the winter season. Th e much higher concentrations of atrazine during the winter study, comp ared with the summer, coincided with soil surface concentrations that were 3 to 10 times those of the summer. Total losses during the winter were 0.4 to 2.0% (atrazine) and 0.4 to 1.7% (metribuzin) of the appli cations. Evidence for preferential Row into the drains of the NO3 and the herbicides is presented.