Saturated zone aldicarb residues

Saturated Zone Movement. The movement of aldicarb residues in the saturated zone is in the same direction as groundwater movement. In most areas, this means that if aldicarb residues reach the saturated zone, they will move primarily in a horizontal direction. Since there is usually little organic matter in the saturated zone, aldicarb residues travel at the same speed as the groundwater, generally 0.03 to 0.5 m per day. The residues tend to remain near the top of the saturated zone and vertical movement is slow. 

Research conducted on the movement and degradation of aldicarb residues in the unsaturated and saturated zones has shown that it is a complex process affected by soil and hydrogeological properties, climatic conditions, and agricultural practices. This paper presents the results of unsaturated and saturated zone field studies conducted in 16 states over a period of six years in which approximately 20,000 soil and water samples have been collected. Results from laboratory degradation studies are also included. 

Many factors influence the degradation of aldicarb residues to biologically inactive compounds. Some of the more important include temperature and pH, and the presence of moisture and microbial populations. The following paragraphs discuss these in more detail for degradation occurring in both the unsaturated and saturated zones. 

Saturated Zone Degradation. Because of the rapid oxidation process in the root zone, parent aldicarb is rarely detected in the saturated zone. In rare instances where transport from the soil surface is rapid, aldicarb may be present at less than five percent of the total residues found. In the saturated zone, residues are usually a mixture of aldicarb sulfoxide and aldicarb sulfone in an average ratio of 3 2. 

If aldicarb residues leach into the saturated zone, or groundwater, degradation of the residues continues, mainly by chemical hydrolysis in cold areas and by both chemical hydrolysis and microbial degradation in warm areas. Factors which tend to increase the degradation rate are high temperature and high pH. Microbial populations may provide a significant contribution to... 

The driving force for movement of aldicarb residues is the movement of water. Therefore, the movement of water must be defined before the movement of aldicarb residues can be quantified. Factors influencing the movement of water include rainfall, irrigation, evapotranspiration and soil field capacity in the unsaturated zone. The amount of organic matter determines the rate of aldicarb movement relative to water movement in both the unsaturated and saturated zones. 

TABLE II. Results of Laboratory Studies Measuring Degradation Rates of Aldicarb Residues in Saturated Zone Samples... 

Saturated Zone Models. Results from unsaturated zone simulations can be used as inputs to saturated zone models to predict concentrations of aldicarb residues in groundwater. The saturated zone model used by the author takes the pesticide inputs into groundwater, as predicted by PRZM, and calculates the concentration and movement of aldicarb residues in the upper portion of the saturated zone. The core of the saturated zone model is a finite element solute transport calculation procedure developed at the University of Wisconsin (25). The accuracy of this model in estimating pesticide movement in groundwater is (as with other... 

The movement of aldicarb residues is quite complex, depending on a number of interacting factors. In most aldicarb use areas, residues degrade completely before moving through the unsaturated zone and into the saturated zone. In the few areas Where aldicarb residues have entered the saturated zone, residues are usually located in shallow groundwater near treated fields. 

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