Leaching aldicarb

The possibility of aldicarb leaching is recognized by the manufacturer (64) warning on the product label that "...a combination of permeable and acidic soil conditions, moderate to heavy irrigation and/or rainfall, use of 20 or more pounds per acre, and soil temperature below 50°F at application time, tend to reduce degradation and... 

OF APPLIED ALDICARB LEACHING BELCW SPECIFIED DEPTH... 

The Pesticide Root Zone Model (PRZM) is being utilized to calculate a daily concentration of aldicarb leaching into the groundwater. Daily meteorological records and irrigation schedules are input into the model. Other important input parameters include... 

Long term simulations were performed using Identical scenarios as the calibration scenarios. The purpose was to assess trends In aldicarb leaching by examining key PRZM outputs of aldicarb mass and concentration at 2 meters below the soil surface. The results Indicate that there was little to no potential for aldicarb to leach in the sandy loam soils modeled in North Carolina and Wisconsin. 

As a result, 3-5 times as much aldicarb leached below two meters with irrigation. Applications at potato emergence decreased the amount to leach by about one-half as compared to earlier applications at planting. The reasons for this are two-fold avoidance of some spring rain for later applications, and a more rapid rate of decay for later applications. 

According to the U.S. EPA (1986) aldicarb has a high potential to leach to groundwater. 

Aldicarb residues leached deeper than one meter 6-9 months after application to a sandy Long Island soil (39) Based on a comparison of computer simulation modeling and ground water data, at least several percent of the aldicarh applied to certain Long Island fields leached to ground water (37). 

Aldicarb is highly water soluble and soil application of this insecticide has the potential to result in runoff or leaching of the insecticide or active metabolites and contamination of surface or groundwater. Aldicarb is acutely toxic to bees, birds, and fish. Species-specific rates of bioactivation may influence the sensitivity of a particular organism to this insecticide. 

We had a very dramatic example of that in the case of the use of aldicarb on Long Island. Following extensive use of this highly toxic insecticide for the control of the Colorado potato beetle, it was noted that the material readily leached through the coarse sandy soils characteristic of Long Island and finally reached the ground water. Contamination of the only source of drinking water for the area must be classed as a severe environmental crisis. 

Aldicarb. The insecticide/nematicide aldicarb and its mobile degradation products ("aldicarb residues ) reportedly have been found in the groundwaters of 13 states (2). In general, aldicarb residues were highest for wells sampled nearest the field application sites. The influence of soil differences on the pattern of aldicarb movement is well illustrated in a recent study (61-2). Potato fields in northeastern Florida were located on sandy soil underlain at 1-2 m by an impervious clay layer (61). Residues leached downward, then laterally, emerging into drainage ditches at concentrations of ca. 

Recent incidents of chemical contamination of ground-water reservoirs have led to increased concern about the potential for pesticides, such as aldicarb, to leach through the soil profile into the ground water (4, 5, 6, 1, 8, 9, 11). To develop needed... 

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... 

Simulations using the actual parameters for coastal citrus (and other southeastern locations) coupled with experimental data confirm that the central Florida ridge area is more sensitive to leaching of aldicarb residues to groundwater compared to other southeastern agricultural areas. 

Figure 2(A,B). Simulated effect of organic matter (A) and soil hydraulic properties (B) on the leaching of aldicarb residues under Florida Ridge conditions. Continued on next page.

Figure 3. Simulated cumulative probability distributions showing the effect of weather (as a function on depth) and application timing on the leaching of aldicarb residues.

The PRZM model was used to evaluate the potential of aldicarb to leach through soil and contaminate ground water in three use sites tobacco grown on a sandy loam soil in North Carolina and potatoes grown on a sandy loam and a loamy sand in Wisconsin. 

The purpose of this paper is to present an assessment exercise of a leaching pesticide using the PRZM model. The assessment begins with a calibration of PRZM for the pesticide aldicarb applied to tobacco in North Carolina and potatoes in Wisconsin. Following these calibrations, long term simulations are performed using these same calibration scenarios. Examination of key PRZM output indicates the "potential" for aldicarb to contaminate ground water in the scenarios modeled. 

The calibrated half-life for aldicarb is longer than the half-life which was calculated based on field data (Table II). This occurs because runoff loss of aldicarb as well as leaching below the depth of sampling are not accounted for in field-calculated half-lives, which are calculated based only on aldicarb remaining at each sampling date. An additional possible avenue of loss is plant uptake of aldicarb. However, the total amount of uptake was not estimated in the field, nor was it simulated in PRZM. As such, it can be considered that plant uptake loss was "lumped" in the calibrated (and calculated) half-lives. 

Table V summarizes the fate and transport of aldicarb in the calibration scenarios. In North Carolina, the simulations predict that 4.2% of applied aldicarb was lost via runoff and none leached below the depth of sampling. The runoff result cannot be verified since field data of runoff were not taken. However, since the soil was a loam soil, seme water runoff would be expected, and a fraction of the soluble aldicarb present in top zone on the date of runoff would also run off. As a result, the calibrated half-life, 43 days, is higher than the range calculated, 27-39 days.

In the Cameron, Wisconsin simulations, aldicarb did not leach below the depth of sampling for the planting or emergence appli-... 

Table VIII summarizes aldicart) fate and transport results for the 5-year leaching assessments, including the percent of applied aldicart) predicted to leach below 2 meters and soluble concentrations at 2 meters. These soluble concentrations are described as "plume11 and "average". The plume concentration has been arbitrarily defined as an average of a continuous six-month period of high aldicarb concentration at the 2-meter depth, which occurs before the next application of aldicarb. This recognizes that aldicarb does, in fact, move with a plume and the plume will pass the 2-meter mark for approximately a 6 month period. For a May application, the plume usually reached the 2-meter mark by June.

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