how to estimate malathion(pesticide) concentration in soil?

Answer 1

Breakdown in soil and groundwater: Malathion is of low persistence in soil with reported field half-lives of 1 to 25 days. Degradation in soil is rapid and related to the degree of soil binding. Breakdown occurs by a combination of biological degradation and nonbiological reaction with water. If released to the atmosphere, malathion will break down rapidly in sunlight, with a reported half-life in air of about 1.5 days. It is moderately bound to soils, and is soluble in water, so it may pose a risk of groundwater or surface water contamination in situations which may be less conducive to breakdown.

Answer 2

Pesticides can volatilize into the atmosphere, which affects the air quality. The ability to predict pesticide volatilization is an essential tool for human risk and environmental assessment. Even though there are several mathematical models to assess and predict the fate of pesticides in different compartments of the environment, there is no reliable model to predict volatilization. The objectives of this study were to evaluate pesticide volatilization under agricultural conditions using malathion [O,O-dimethyl-S-(1,2-dicarbethoxyethyl)-dithiophosphate], ethoprophos (O-ethyl S,S-dipropylphosphorodithioate), and procymidone [N-(3,5-dichlorophenyl)-1,2-dimethylcyclopropane-1,2-dicarboximide] as test compounds and to evaluate the ability of the Pesticide Leaching Model (PELMO) to calculate the predicted environmental concentrations of pesticides in air under field conditions. The volatilization rate of procymidone, malathion, and ethoprophos was determined in a field study during two different periods (December 1998 and September 1999) using the Theoretical Profile Shape (TPS) method. The experiments were performed on bare silty soil in the Bologna province, Italy. Residues in the air were continuously monitored for 2 to 3 wk after the pesticide applications. The amount of pesticide volatilized was 16, 5, and 11% in December 1998 and 41, 23, and 19% in September 1999 for procymidone, malathion, and ethoprophos, respectively. In both these experiments, the PELMO simulations of the concentration of ethoprophos and procymidone were in good agreement with the measured data (factor ± 1.1 on average). The volatilization of malathion was underestimated by a factor of 30 on average. These results suggest that volatilization described by PELMO may be reliable for volatile substances, but PELMO may underpredict volatilization for less-volatile substances.

Pesticide volatilization after soil application is one of the most important processes for the environmental dissipation of xenobiotics and for risk assessment. Results reported in this paper demonstrate that in field conditions, for pesticides with vapor pressures between 5 x 10-3 and 5 x 10-2 Pa, volatilization can represent up to 22.6% of the total fate in the environment. To this extent, volatilization may affect the air quality of the area surrounding the agricultural field, potentially exposing residents and bystanders to the pesticides. Most of these airborne residues are assumed to dissipate quickly in the air due to the photolytic activity of sunlight, but additional studies and measurements are needed.

Mathematical models can greatly help the interpretation of this process and in the assessment of exposure levels at different scales. Based on the experimental data reported in this paper, PELMO can be used for simulating the predicted environmental concentration in air, as required by the EU Directive, but it should be considered that its capacity may be limited, depending on the chemical and physical properties of pesticides. As such it is not meaningful to continue with a version of the model that gives such a poor description of the volatilization processes. The discrepancy between measured and predicted data is evidence that the current version of PELMO allows successful description of pesticide volatilization from soil only for volatile substances (vapor pressure 10-2 Pa) and over an intermediate to long time period (more than a few days). The model could be beneficially amended to take into account the influence of the hourly soil–air temperature variation during the day, a sorption routine including the effect of strong bonds and time-dependant sorption processes, and the temperature dependence of variables such as water solubility, vapor pressure, and Henry's constant. Though not studied here, volatilization from the crop canopy would also be helpful in the model.
The malathion half-lives from laboratory studies suggest that malathion breaks down more rapidly in water or soil than the synthetic pyrethroids (sumithrin, resmethrin and permethrin), but less rapidly than naled and dichlorvos. The very limited amount of degradation data available for the malathion metabolite, malaoxon, suggests its breakdown may also be relatively rapid (at least by hydrolysis in soil) compared to most of the other pesticide active ingredients being compared. Malathion and malaoxon both have soil mobilities and soil particle affinities that are similar to naled and dichlorvos.