Simulation of concentration effects on mobility and persistence of alachlor

Abstract

The effect of concentration on mobility and persistence of alachlor was investigated. The investigation was divided into two components: soil column study and model study. In the soil column study, PVC pipe was hand packed to a depth of 30 cm with Catlin silt loam soil. The surface of the soil columns was contaminated with alachlor at rates of 3, 30 and 3000 kg a.i./ha. Columns were then subjected to a simulated rainfall of constant or variable intensity three times during a two month period. Leachate from the columns was collected twice during each rainfall event. Six times during the two months, columns were analyzed to determine pesticide persistence and mobility.

The mass of alachlor leached increased with an increasing application rate. However, the mass leached as a percentage of the alachlor present in the columns was either approximately the same for all rates or less for the 3000 kg a.i./ha rate. The mobility of alachlor was not increased by an increasing application rate. The majority of the alachlor remained within 10 cm of the soil surface for all application rates. Persistence of alachlor in the soil columns increased with an increasing application rate.

In the model study, several currently available simulation models were reviewed for the potential to represent the mobility and persistence of high concentrations of pesticides. The simulation model LEACHMP was selected as the model for detailed study.

The simulation model LEACHMP over-estimated the center of pesticide mass for the 3 and 30 kg a.i./ha rates and under-estimated the center of pesticide mass for the 3000 kg a.i./ha rate. Based on the normalized objective function values for pesticide distribution, the estimated concentration for each layer was off by 100-200 percent of the average measured concentration. The simulation model under-estimated the mass of pesticide in the profile for the 3 and 30 kg a.i./ha rate columns and over-estimated the mass of pesticide in the soil profile for the 3000 kg a.i./ha rate. The simulation model under-estimated the quantity of alachlor leached from the soil columns.