Pesticides leaching

Eor pesticides to leach to groundwater, it may be necessary for preferential flow through macropores to dominate the sorption processes that control pesticide leaching to groundwater. Several studies have demonstrated that large continuous macropores exist in soil and provide pathways for rapid movement of water solutes. Increased permeabiUty, percolation, and solute transport can result from increased porosity, especially in no-tiUage systems where pore stmcture is stiU intact at the soil surface (70). Plant roots are important in creation and stabilization of soil macropores (71). 

Carsel RF, Mulkey LA, Lorber MN, et al. 1985. The pesticide root zone model (PRZM) A procedure for evaluating pesticide leaching threats to groundwater. Ecological Modeling 30 49-69. 

As more sensitive analytical methods for pesticides are developed, greater care must be taken to avoid sample contamination and misidentification of residues. For example, in pesticide leaching or field dissipation studies, small amounts of surface soil coming in contact with soil core or soil pore water samples taken from further below the ground surface can sometimes lead to wildly inaccurate analytical results. This is probably the cause of isolated, high-level detections of pesticides in the lower part of the vadose zone or in groundwater in samples taken soon after application when other data (weather, soil permeability determinations and other pesticide or tracer analytical results) imply that such results are highly improbable. 

In this section, two types of data are briefly summarized 1) environmental transport and persistence, and (2) monitoring. The discussion Is chemical specific. Some Interesting concepts relevant to some of the chemical characteristics are developed later In the Discussion section. Results from this section are also used In the Conclusion section to derive some generalizations about pesticides leaching to ground water. 

Soil core data are available for only six of the pesticides discussed in this paper. The six pesticides are aldlcarb atrazlne 1,2-dibromo-3-chloropropane (DBCP) 1,2-dlchloropro-pane (DCP) 1,2-dlbromoethane (EDB) and slmazlne. Cores were always sampled at depths greater than one meter and the soil was characterized physically and chemically. The importance of soil core sampling in pesticide leaching assessments is presented in the Discussion section. 

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. 

Brooke, D. and P. Matthiessen (1991). Development and validation of a modified fugacity model of pesticide leaching from farmland. Pestic. Sci., 31 349-361. 

Isensee, A.R., R.G. Nash, and C.S. Helling (1990). Effect of conventional vs. no-tillage on pesticide leaching to shallow groundwater. J. Environ. Qual., 19 434-440. 

Chatupote, W., Panapitukku, N., 2005. Regional assessment of nutrient and pesticide leaching in the vegetable production area of Rattaphum catchment, Thailand. Water, Air Soil Pollut. Focus 5, 165-173. 

Almost all the evaluation of pesticides leaching into groundwater is performed at 1 m depth. The assumption is that groundwater is unlikely to be affected by pesticides at concentrations exceeding 0.1 pg/L if those concentrations are not encountered at a shallow depth. Little research has been conducted on the fate process of pesticides once they have leached through the soil and below the root zone. 

Armstrong AC, Matthews AM, Portwood AM, Leeds-Harrison PB, Jarvis NJ. CRACK-NP A pesticide leaching model for cracking clay soils. Agr Water Manage 2000 44 183-99. 

Boesten JJTI, van der Linden AM A. Modeling the influence of sorption and transformation on pesticide leaching and persistence. J Environ Qual 1991 20 425-35. 

Klein M. PELMO Pesticide leaching model. Schmallenberg, Germany Fraunhofer-Institut fiir Umweltchemie und Okotoxikologie, 1991. 

Nicholls PH, Harris GL, Brockie D. Simulation of pesticide leaching at Vredepeel and Brimstone farm using the macropore model PLM. Agr Water Manage 2000 44 307-15. 

Will the pesticide leach away from the application site ... 

Solid-phase microextraction (SPME) is a technique that was first reported by Louch et al. in 1991 (35). This is a sample preparation technique that has been applied to trace analysis methods such as the analysis of flavor components, residual solvents, pesticides, leaching packaging components, or any other volatile organic compounds. It is limited to gas chromatography methods because the sample must be desorbed by thermal means. A fused silica fiber that was previously coated with a liquid polymer film is exposed to an aqueous sample. After adsorption of the analyte onto the coated fiber is allowed to come to equilibrium, the fiber is withdrawn from the sample and placed directly into the heated injection port of a gas chromatograph. The heat causes desorption of the analyte and other components from the fiber and the mixture is quantitatively or qualitatively analyzed by GC. This preparation technique allows for selective and solventless GC injections. Selectivity and time to equilibration can be altered by changing the characteristics of the film coat. 

The major emphasis of this discussion centered on the importance of surface and subsurface soil characteristics in influencing deep pesticide leaching. Some factors, such as the depth to groundwater and the amount of incipient rainfall or irrigation, are clearly important factors that do affect the probability of pesticide residues reaching groundwater. Similarly, properties of the pesticide itself (especially its inherent mobility and chemical/biological stability) correlate closely with pollution potential, but their evaluation is outside the scope of this review. 

Figure 1. Experimental apparatus for laboratory pesticide leaching studies.

Climatological information for the site will be necessary. Precipitation data, air temperature and pan evaporation data, and any use of irrigation water must be carefully recorded. Irrigation and natural recharge is a critical aspect of pesticide leaching. 

These reports all focus on nonpoint sources of pesticides leaching to ground water. However, the extent of occurrence of pesticides in ground water from agricultural and industrial point sources is also not known. A separate investigation into this topic would be warranted. 

Carsel, R. F., L. A. Mulkey, M. N. Lorber, L. B. Baskin, "The Pesticide Root Zone Model (PRZM) A Procedure for Evaluating Pesticide Leaching Threats to Groundwater," accepted for publication, Ecological Modeling 1985. 

The appropriate means to "test" a model is dependent on the biases of the model tester and the purposes of his exercise. Words such as calibration, validation, and verification have been used to describe a model testing procedure. For this study, PRZM was calibrated to three field sites to determine appropriate parameters for longer term simulations. These long term simulations employed the same parameters as the calibration simulations, and their purpose was to examine trends in pesticide leaching as expressed by PRZM output. 

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