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Abstract

Six pesticides – simazine, glyphosate, alpha-cypermethrin, clopyralid, MCPA and sulfometuron methyl – have been studied to examine dissipation rates in the field, leaching in the field and sorption to various soil types in the laboratory including sorption to some fractions of the soils.
A range of different soils – Ferrosol (2), Vertosol (2), Kurosol, Dermosol – covering a broad spectrum of organic, mineral and chemical properties were selected for the study. The trial sites and soils were chosen to represent a wide range of climatic settings and geographical areas.
Standard field trials were conducted over two seasons with the pesticides applied to cultivated soils in an attempt to examine dissipation under Tasmanian conditions. The resulting data were then used by CSIRO to modify the Pesticide Impact Rating Index software for Tasmanian conditions (PIRI-Tas). Assessment of the sorption of pesticide to the soils was undertaken in the laboratory and data were also then used to modify PIRI.
Sorption coefficients and half-life values for the target Tasmanian soils varied from the previously published range used in PIRI, highlighting the value of local validation of key input data in model development. In addition, insight into the processes controlling the environmental fate of the pesticides studied was gained, and areas where further research is required to allow ‘fine tuning’ of pesticide fate predictions were identified.
The key findings include a wide range of sorption coefficients (Kd) for each pesticide, most varying over an order of magnitude depending on soil type. Organic matter was strongly correlated with sorption of all pesticides. This included glyphosate despite organic matter not being a known sorption site for this pesticide. Soil pH improved the correlations for MCPA, clopyralid, sulfometuron and simazine, but not glyphosate. We recommend that soil pH be considered in future pesticide behaviour models to improve predictive resolution. Even the normalised sorption coefficient Koc was found to vary over one order of magnitude for sulfometuron methyl and by a factor of four with MCPA. Several pesticides were well outside the pre-project PIRI values (e.g. clopyralid, MCPA and simazine) and these pesticides have significantly modified risk profiles in the new PIRI-Tas version.
Fractionation of three of the field soils was undertaken to examine the role of the mineral vs. organic components on sorption for MCPA and sulfometuron methyl. In laboratory studies for MCPA, organic matter removal significantly reduced sorption of the soils tested and the reduced sorption coefficients were strongly negatively correlated to soil pH. For sulfometuron methyl, similar reductions in sorption occurred, though not to the same extent on the Vertosols, and there was no correlation with soil pH for the treated soils. More work on fractionation is required to resolve the controlling factors on sorption but soil pH and organic matter appear to be key factors for most pesticides studied.
The mean half-lives for both sulfometuron methyl and simazine tripled from spring 2006 to autumn 2007 trials, while glyphosate nearly doubled. This reflects the cooler conditions over the autumn–winter trials. Clopyralid and MCPA had mixed results and they appear to be more affected by soil moisture and leaching in combination with temperature and other factors such as organic matter and soil pH levels. Alpha-cypermethrin showed no changes with season which may relate to strong sorption to soils and perhaps the greater role of photodegradation in field dissipation.
Leaching was of a very limited nature in the spring 2006 trial with the exception of simazine at Northdown and Pyengana where approx 1% of applied pesticide leached to the 20–30 cm and 30–40 cm levels respectively. There was also minor leaching of MCPA. Leaching increased significantly in the autumn 2007 trial as might be expected due to the wetter and cooler winter conditions at most sites. Minor leaching of simazine occurred at Pyengana, the Unifarm (Kurosol only) and Northdown. In the autumn 2007 trial there was more significant leaching of clopyralid at both Pyengana and Northdown. The Unifarm Vertosol was resistant to leaching of all pesticides in both seasons.
Simazine’s (triazine) relatively high application rates (6 kg/ha), long autumn–winter half-life, low and variable sorption and demonstrated leaching potential make it the most significant environmental risk for water quality.
Variations in sorption with soil pH are significant as demonstrated by wide variations in the normalised sorption coefficient Koc for certain weakly adsorbed pesticides e.g. sulfometuron and MCPA. This may partly explain regional and industry (forestry vs. agriculture) variations in environmental risk.
The Vertosols represented a special environmental case with pesticides having long half-lives in these soils, relatively weak sorption coefficients and limited leaching behaviour. This means that Vertosols retain high amounts of pesticide at the soil surface where runoff or topsoil erosion could lead to movement toward surface waters.
More research on controlled leaching of simazine, clopyralid and MCPA in a Tasmanian context is required along with data on the bioavailability (desorption) of pesticides like glyphosate, simazine and MCPA from soils and sediments.

Item Type:	Report (Project Report)
Authors/Creators:	Doyle, RB and Oliver, G and Brown, PH and Ratkowsky, DA and Cumming, JP and Hingston, J
Keywords:	Pesticide, half-life, sorption, Tasmania, water, soil, leaching
Publisher:	Tasmanian Institute of Agricultural Research
Additional Information:	Report is published as part of the "The Tasmanian River Catchment Water Quality Initiative"
Related URLs:	Organisation
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