Leaching, soil-applied pesticides

The pesticide component of SWRRB takes into account the fate of the chemical applied under field conditions For example, the amount of pesticide actually reaching the ground after application over a plant canopy is calculated. Further, field dissipation of the chemical by photolysis on leaf surfaces as well as degradation in the soil is accounted for with the pesticide component of SWRRB. Leaching of the pesticide below the top 1cm of soil is also computed and runoff corrected for such losses. Further, adsorption of the pesticide to soil surfaces and sediment is taken into account by SWRRB. 

The second class of pesticide transport processes is that of movement of dissolved or particulate-sorbed pesticides in water. Leaching of pesticides hais been recognized as a critical process, if not in the sense of the absolute percentage of applied pesticide leached into the soil profile, then with respect to the contamination of groundwater by trace quantities of pesticide. Several pathways of dissolved pesticide movement with leaching water are recognized (18). 

Mass balance cannot be strictly obtained in any open field study however, in field-scale groundwater studies, accounting for as much of the applied material as possible in order to interpret the results is particularly important. With the pesticide diluted over a large mass of soil and groundwater, concentrations in some samples may be low and hard to detect, but the total mass leaching may be large. 

To which phase is the substance likely to migrate will a pesticide applied to soil leach or be volatile will a chemical accumulate in the biotic compartment and so on. 

PRZM was applied to a hypothetical situation of a pesticide In a Georgia agricultural environment. An overall, pseudo-first-order degradation rate coefficient of 0.001 day was used, along with a series of values. A cover crop of peanuts was assumed. The simulation was done for a 900 g/ha application to a class A soil (well drained) and a class D soil (poorly drained). Movement through the root zone was simulated using rainfall records. In the hypothetical 1-ha plot, 800 g and 550 g of the pesticide leached past 60 cm In the class A and D soils, respectively, when a Kj value of 0.06 was used 40 g and 5 g leached past 60 cm In the class A and D soils, respectively, when a Kj value of 1.5 was used. These computational results support the conclusion on Kj values stated at the end of this paper. 

Water pollution may be caused by agricultural activities and has increased continually over the last few years. A wide variety of pesticides reach the soil in the agricultural areas where they are applied, but they can also be carried off by leaching as a consequence of water displacement in the soil environment or runoff (4-6). Some of this surface and groundwater is supplied as drinking water because of this, it is necessary to screen them for pesticide contamination. 

In most column-leaching studies, the bulk of triazines remain near the soil surface. For instance, Kruger et al. (1993) found that in a 60-cm column of Iowa soil taken from a held with no previous pesticide history, approximately 1.2% of the 14C-atrazine was recovered in leachate over a 12-week period. By the end of the experiment, 77% of the 14C applied remained in the upper 10 cm of soil, and bound residue was the primary component. Both atrazine and degradation products (DIA > HA > DEA > DEHA > DIHA) were found in the top 10 cm of surface soil. 

An environmental application of liquid extraction is the removal of trace organics from water. Examples are the separation of acetic acid-water mixtures and removal of solvents, insecticides, pesticides, etc., from water. It can also be applied to the separation of liquids with close boiling points or those that form azeotropes, such that distillation is not useful. In addition, zero- or low-volatility compounds, such as metals and organometallic derivatives, can be separated by liquid extraction as can mixtures of water-hydrogen bonded compounds, such as formaldehyde. Solid extraction (leaching) can be used to remove organics or heavy metals from contaminated soils, sludges or contaminated equipment. 

Number and Location of Observation Wells. The precise amount of pesticide which was applied to the soil surface and is available for leaching is known as the source term. For pesticides which are applied directly to the soil, such as granules or liquid formulations sprayed only onto the soil surface, the source term can be assumed to be equal to the total amount of application. 

Two field experiments investigating the mobility of pesticides leached by sprinkler irrigation are discussed. The first experiment applied a pulse of chloride and napropamide, a moderately adsorbed herbicide, to a 0.6 ha field which was subsequently leached with 25 cm water low in chloride over a two-week period. Soil core samples were taken at 36 locations to 300 cm and were analyzed in 10 cm increments for chloride and pesticide. Oistribution coefficient measurements were made at each sampling location by hatch equilibrium and column flow through methods. 

An acidic pesticide of pK 5 is being applied to soils of a given region. Would leaching be greatest for soils of... 

Pesticides that modify water quality include chlorinated hydrocarbons and their related compounds, persistent heibicides, and insecticides directly applied to the soil, which could easily leach into the ground water, and pesticides used for vector s control at water surfaces. 

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