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Simulation of Pesticide Leaching in the Field and in Zero-Tension Lysimeters

Alterra, Wageningen Univ. and Research Centre, P.O. Box 47, 6700 AA Wageningen, The Netherlands

View larger version (11K): [in this window] [in a new window]   FIG. 1. The lognormal probability density function of the dispersion length as used in the Monte Carlo simulations considering coincidental favorable combinations of unknown pesticide–soil parameters. The median is 5 cm, and the standard deviation of the logarithm of the dispersion length is 0.86 (–).

View larger version (24K): [in this window] [in a new window]   FIG. 2. (A) The groundwater level in the field soil and (B) the fraction of the pore volume filled with water ("Percentage saturation") at 97 cm depth in the field and lysimeter soils as a function of time as simulated with the SWAP model. The year numbers indicate 1 January of the corresponding year.

View larger version (26K): [in this window] [in a new window]   FIG. 3. Cumulative probability density of the ratio of leaching concentrations for the lysimeter divided by leaching concentrations for the field. Ratios are for 80th percentiles of a series of 20 2- or 3-yr average flux concentrations for application every 2 or 3 yr, respectively, and for average flux concentration over 40 and 60 yr for application every 2 or 3 yr, respectively. The probability density was based on 200 hypothetical pesticides with randomly generated organic-matter/water distribution coefficient, KOM, values between 0 and 100 L kg–1 and DegT50 values between 1 and 100 d, but results were included only if the 80th percentile concentration of the pesticide was above 0.1 µg L–1.

View larger version (21K): [in this window] [in a new window]   FIG. 4. Flux concentrations of a tracer (i.e., a substance that is not sorbed, not transformed, nor taken up by plants) at 1 m depth in the soil as a function of time as calculated for the field and lysimeter systems. Year numbers indicate 1 January of the corresponding year. Tracer (1 kg ha–1) was applied on 31 March in 1911, 1913, and 1915.

View larger version (20K): [in this window] [in a new window]   FIG. 5. Cumulative probability density of the ratio of the maximum annual leaching concentration for the lysimeter divided by the 80th percentile leaching concentration for the field. The maximum is the maximum of two annual average leaching concentrations following a single application. The 80th percentile is derived from a series of 20 2- or 3-yr average flux concentrations for application every 2 or 3 yr, respectively. The probability density was based on 200 hypothetical pesticides with randomly generated organic-matter/water distribution coefficient, KOM, values between 0 and 100 L kg–1 and DegT50 values between 1 and 100 d, but results were included only if the 80th percentile concentration of the pesticide was above 0.1 µg L–1.

View larger version (19K): [in this window] [in a new window]   FIG. 6. The ratio of the maximum annual leaching concentration for the lysimeter divided by the 80th percentile leaching concentration for the field as a function of the organic-matter/water distribution coefficient, KOM. The maximum is the maximum of two annual average leaching concentrations following a single application. The 80th percentile was derived from a series of 20 2- or 3-yr average flux concentrations for application every 2 or 3 yr, respectively. The points are simulations for 200 hypothetical pesticides with DegT50 values ranging from 1 to 100 d, but results were included only if the 80th percentile concentration of the pesticide was above 0.1 µg L–1.

View larger version (15K): [in this window] [in a new window]   FIG. 7. Probability that the lysimeter study generates a maximum annual leaching concentration below 0.1 µg L–1 as a function of the 80th percentile leaching concentration in the field calculated for application every 3 yr. Points are averages of at least three Monte Carlo simulations (of 500 runs each), and bars indicate standard deviations. The 80th percentile leaching concentration is the 80th percentile of a series of 20 3-yr average flux concentrations. Closed symbols are for an average organic-matter/water distribution coefficient, KOM, of 35 L kg–1 and average DegT50 values ranging from 20 to 100 d; open symbols are for average KOM values ranging from 5 to 35 L kg–1 and an average DegT50 of 20 d.

View larger version (21K): [in this window] [in a new window]   FIG. 8. Probability that the lysimeter study generates a maximum annual leaching concentration below 0.1 µg L–1 as a function of the 80th percentile leaching concentration in the field calculated for application every 3 yr. Points are averages of at least three Monte Carlo simulations (of 500 runs each), and bars indicate the standard deviations. The 80th percentile leaching concentration is the 80th percentile of a series of 20 3-yr average flux concentrations. Each line was calculated using an average organic-matter/water distribution coefficient, KOM, of 35 L kg–1 and average DegT50 values ranging from 20 to 100 d. Upper line is for stochastic DegT50, KOM, Freundlich exponent, and dispersion length. Other lines are simulations in which all parameters were kept fixed except the parameter indicated in the legend.