Movement of diuron and hexazinone in clay soil and infiltrated pond water

result in runoff of residues so improved incorporation of pre-emergence herbicides into the soil is recomPre-emergence herbicide residues were detected in domestic wells mended to reduce concentrations in runoff water. sampled near Tracy, CA. This study sought to determine the source Thesetwoscenarios arenotinclusiveof allgeographiof contamination by comparing soil distribution of diuron [N-(3,4cal settings where residues have been detected in Calidichlorophenyl)-N,N-dimethylurea] and hexazinone [3-cyclohexyl6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1H,3H)-dione] in an fornia’s ground water (Troiano et al., 2000). Pre-emeragricultural field where the soil was a cracking clay to infiltration of gence herbicide residues were detected in seven wells residues in water captured by an adjacent holding pond. Diuron and sampled within a 1554-ha area located near the town hexazinone were applied in December to a 3-yr-old alfalfa (Medicago of Tracy, CA: atrazine was detected in five wells at 0.16 sativa L.) crop. Water content of soil taken after major rainfall but to 2.8 gL 1 , diuron in one well at 0.06 gL 1 , hexazibefore irrigation at 106 d after application was elevated at the lowest none in three wells at 0.051 to 0.11 gL 1 , and simazine depth sampled centered at 953 mm, indicating water was available (6-chloro-N,N-diethyl-1,3,5-triazine-2,4-diamine) in one for percolation. Herbicide residues (reporting limit 8 gk g 1 ) were well at 0.098 gL 1 . Tracy is centrally located on the confined above the 152 mm soil depth, even after subsequent applicawestern side of the Central Valley of California (Fig. 1). tion of two border-check surface irrigations. The pattern of distribuThe predominant cropping pattern was a rotation of tion and concentration of residues in the soil were similar to results obtained from the LEACHM model, suggesting that macropore flow alfalfa with corn (Zea mays L.) and bean (Phaseolus was limited to a shallow depth of soil. Herbicide residues were mea- vulgaris L.). The residues were related to agricultural sured in runoff water at the first irrigation at 20 gL 1 for diuron applications, especially since the only reported pestiand 1 gL 1 for hexazinone. Runoff water captured in the pond cidal use of hexazinone was on alfalfa. The affected area rapidly infiltrated into the subsurface soil, causing a concomitant rise was surveyed for any obvious potential pathways to in ground water elevation near the pond. Herbicide residues were ground water. The predominant soil was a clay soil that also detected in the sampled ground water. We concluded that the developed surface cracks on drying. Rapid movement pond was the predominant source for movement to ground water. through soil, termed macropore flow, has been identiSince addition of a surfactant to the spray mixture did not reduce fied as a potential pathway for contamination (Bouma concentrations in runoff water, mitigation methods will focus on min