Pesticide Degradation Rate Research Articles

Comparison of PRZM computer model predictions with field lysimeter data for dichlorprop and bentazon leaching

Abstract The objective of this project was to optimize the agreement between the Pesticide Root Zone Model (PRZM) simulations for two experiments and then use those input parameters to predict herbicide leaching in a third experiment. The actual numbers and the simulated results all pertain to lysimeter data which has previously been described. The agreement between hydrology simulation results obtained using PRZM and actual field data was optimized by (1) altering the initial soil‐water content in the soil profile and (2) adjusting the water holding capacity for each soil horizon by changing the field capacity water content to permanent wilting point water content relationship. Once hydrology parameters were adjusted for best fit, pesticide degradation rates and adsorption coefficients were adjusted to maximize agreement between observed dichlorprop and bentazon leaching. Good agreement was obtained in hydrology simulations and bentazon movement, and poor agreement was obtained with dichlorprop movement. A “blind simulation”; was then performed on bentazon movement in Lanna clay and Mellby sand soils, which indicated greater water discharge from the Lanna soil, and minimal bentazon loss (less than 0.015 mg/m2) from each soil.

Estimating the pollution potential of pesticides to ground water

A screening model available in the literature has been used to evaluate the ground water pollution potential of a number of commonly used pesticides under irrigated horticulture in Bassendean sand of the Swan Coastal Plain of Western Australia. The original model assumed a decreasing rate of pesticide degradation but a constant organic matter content with depth in the soil profile. A modified version of the model was developed to take into account the generally decreasing organic matter contents with depth in the soil profile. Residual masses and travel times of 40 pesticides were calculated by the model using sorption and degradation data available from the literature. The calculations based on the constant OM mode predicted that for a recharge rate of 0-5 m/yr, some 14 of the pesticides were likely to reach ground water at appreciable levels of the residue (>0.1% of applied mass). The number increased to 21 and was accompanied by a decrease in the travel times required for the pesticides to reach ground water when the decreasing organic matter contents of the profile with depth were taken into consideration. To assess the validity of using sorption and degradation data from the literature for the local soil, comparisons of model calculations were made for five pesticides whose sorption coefficients and degradation half-lives were measured on the local soil. For some pesticides, the predictions based on literature values were significantly different from those based on measured parameters indicating, as expected, that overseas data may not always represent local conditions. However, they may still provide valuable first approximations of the likely relative pollution potentials of different pesticides.

Testing the modified CREAMS/GLEAMS model for pesticide concentration in soil

The accuracy of simulating the trifluralin concentrations in a clay soil and in a loamy sand soil with the modified CREAMS/GLEAMS model has been tested by comparing them with observed values. The simulated concentrations in the soils were in good agreement with those observed in the first weeks after application. In the long run the simulated concentrations decreased faster than the observed ones. In addition, the sensitivity of the model to variations of two pesticide parameters has been analyzed: the pesticide adsorption coefficient for organic carbon and the pesticide degradation rate expressed as half-life in soil. The variation in the two pesticide parameters had a considerable effect on the model output. Especially large were the effects of the adsorption coefficient on the pesticide concentration in the percolated water leaving the root zone.

Aerobic and Anaerobic Degradation of Alachlor in Samples from a Surface‐to‐Groundwater Profile

AbstractEstimates of pesticide degradation rates in subsoils are needed to improve models predicting pesticide movement to groundwater. Biodegradation rates of the herbicide alachlor [2‐chloro‐(2,6‐diethylphenyl)‐N‐(methoxymethyl)acetamide] in surface soil, vadose zone, and aquifer samples collected from a single site near Plains, GA were determined in the laboratory under aerobic and anaerobic conditions. Degradation was described by first‐order kinetics during 126 d of incubation. Under aerobic conditions the halflife (t1/2) of alachlor in the surface soil (t1/2 = 23 d) was less than in the vadose zone (t1/2 = 73 to 285 d) and aquifer samples (t1/2 = 320 to 324 d). Alachlor in anaerobic samples degraded less rapidly in the surface (0 to 0.6 m) and the next deepest (0.6 to 2.4 m) subsoil than under aerobic conditions (t1/2 = 100 and 144 d, respectively). Degradation in anaerobic aquifer samples was very slow (t1/2 = 337 to 553 d). Addition of organic nutrients enhanced aerobic degradation in subsurface soils and one aquifer sample, indicating that nutrient availability limits biodegradation. Total aerobic microbial populations ranged from 6.6 × 103 to 2.5 × 106 cells per gram of soil in the subsoils and aquifer samples, but were not correlated with aerobic or anaerobic degradation rates. The lower degradation rates in vadose zone and aquifer materials may be due to less microbial activity or the absence of alachlor degraders.