Sorption and movement of ionic and non-ionic pesticides in selected soils of New Zealand

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Massey University
There is a growing concern about the persistence of pesticide residues in soils and their subsequent movement to groundwater and surface water. Sorption of pesticide by soil particles is one of the key processes affecting the fate of pesticides in soil. The overall objective of this research was to examine the sorption and movement of ionic (2,4-D, atrazine and metsulfuron methyl) and non-ionic (phorate and terbufos) pesticides in a range of allophanic and non-allophanic soil materials of New Zealand. Firstly, the methods of measuring pesticide in the soil solution were evaluated. The results suggested that radiotracer techniques can be used to measure low concentrations of pesticide in the soil solution during sorption studies. Based on this, the sorption and movement of pesticides was examined using 14C-labelled compounds. Complete recovery of 14C pesticide residues in soils was achieved by direct extraction of soils with a scintillation cocktail, which contained an organic solvent. The effect of drying soils on the sorption and leaching of an inorganic anion (phosphate; P) and organic pesticides (2,4-D and phorate) was examined using field-moist, freeze-dried, air-dried and oven-dried soil samples. Compared to field-moist and freeze-dried, both air-drying and oven-drying of soil increased the sorption of P, but decreased the sorption of pesticides. Solubilization of organic carbon during air-drying and oven-drying may have increased the accessibility of P to sorption sites on the mineral surfaces and thereby increased the sorption. In contrast, the addition of water soluble organic carbon bound onto pesticides in solution and decreased the apparent sorption onto the soil; thereby increasing leaching. Pesticide sorption was measured in a range of allophanic and non-allophanic topsoil and subsoil samples using a batch equilibrium technique. In general, pesticide sorption, as measured by the distribution coefficient (Kd), increased with an increase in octanol-water partition coefficient (Kow) of the pesticide and followed the order: terbufos > phorate > 2,4-D > atrazine > metsulfuron methyl. The Kd values increased with increasing organic carbon content of the soils and when the sorption was normalised to organic carbon (Koc) there was less variation in Koc values between the soils. Removal of organic carbon decreased the sorption of pesticides and the effect was more pronounced with non-ionic than with the ionic pesticides. The results suggest that organic carbon is the principal sorbent for non-ionic pesticides, whereas clay is also contributing to the sorption of ionic pesticides. Multiple regression models were developed to predict pesticide sorption based on soil properties. The existing empirical equations based on Kow values gave unsatisfactory predictions of pesticide sorption in the soils examined. The contribution of different particle size fractions of soils to sorption and desorption of pesticides was examined using two soils with contrasting characteristics. The Kd values decreased in the order: clay > silt > sand. Organic carbon accounted for most of the variation in Kd values between the particle size fractions, and the removal of organic carbon decreased the Kd values of the pesticides. Following four successive extractions with 0.01 M CaCl2,65-90% and 22-75% of the initially sorbed ionic and non-ionic pesticides, respectively were released from the particle size fractions. Greater amounts of pesticides were released from the soil fractions from which organic carbon had been removed than from natural soil fractions and the difference was greater for the non-ionic than the ionic pesticides. Experiments were conducted to examine the effects of different sources of added carbon (peat, sludge, mushroom compost, pig manure and poultry manure), and dissolved organic carbon (DOC) on the sorption and movement of pesticides in soils. Added carbon sources increased the sorption of pesticides and followed the order: peat > sludge > pig manure > mushroom compost > poultry manure. The differences in the effect of carbon addition on the sorption of pesticides may be related to the differences in their effect on DOC and pH of the soil. Premixing DOC with the pesticide solutions decreased the pesticide sorption whereas premixing DOC with soil increased the pesticide sorption. Column studies showed that pesticide mobility was enhanced by the presence of DOC. Column leaching experiments were conducted to examine the movement of pesticides through repacked soil cores (step-function and pulse inputs) and intact cores (step-function input) using two soils with different pesticide sorption capacities. In repacked soil columns, the step-function experiments showed a symmetrical breakthrough curve (BTC) for a non-sorbed solute (3H20) with a sigmoidal shape, whereas there was an asymmetrical BTC with extensive tailing for a sorbed solute (2,4-D). In the pulse experiments the leaching of pesticides decreased with an increase in the Kd values and leaching decreased in the order: terbufos > phoratc > 2,4-D > atrazine. The results from the intact core experiments suggested that both the sorbed (2,4-D) and non-sorbed (3H20) solutes move preferentially through macropores such as worm holes and root channels. The convection-dispersion equation (CDE) either with an equilibrium or a bicontinuum non-equilibrium sorption process was used to simulate the measured effluent BTCs obtained by simultaneous displacement of 3H20 and 2,4-D. The CDE with an equilibrium sorption process failed to simulate the BTC for 2,4-D in repacked and intact soil columns; whereas the CDE with a bicontinuum non-equilibrium sorption process provided a good description of the experimental data. Both chemical (intraorganic matter diffusion) and physical (preferential flow) processes are involved in the non-equilibrium sorption during the movement of pesticides in soil.
Irregular pagination. Missing p.89 in Vault copy too.
Pesticides, Environmental aspects, Soils, Pesticide content, Soil pollution