Land use effects on sorption of pesticides and their metabolites in sandy soils. II. Atrazine and two metabolites, deethylatrazine and deisopropylatrazine, and prometryne

Abstract

There is very limited information about the effect of land use on sorption behaviour of organic chemicals. The effects of land use on the sorption behaviour of prometryne, atrazine, and its 2 main metabolites, deethylatrazine (DEA) and deisopropylatrazine (DIA), in sandy soils were studied. This study would provide fate data for these chemicals specifically for Australian soils. Sorption coefficients (Kd) were determined using batch studies and a single solution concentration (2 mg/L for atrazine, DEA, and DIA, and 3 mg/L for prometryne) after initially determining sorption isotherms for a subset of 6 soils for atrazine and prometryne using a range of concentrations. Generally, the Kd values for prometryne were significantly (P < 0.001) greater than for the other 3 compounds. There was no consistent trend in the Kd values for atrazine and the 2 metabolites. This study demonstrated that prometryne was least likely to pose a risk to groundwater contamination but in some cases both atrazine metabolites (DEA and DIA) were less sorbed in the soils than the parent compound. Consequently the metabolites could pose a greater risk to groundwater quality than the parent compound. In the surface soils, significantly (P < 0.05) greater Kd values were determined under native vegetation for DIA, atrazine and prometryne. In the subsurface soils significantly (P < 0.001) greater Kd values were observed under native vegetation for all 4 chemicals. This may be related to differences in the nature and type of OC content of soils under the 2 land uses. There were significant (P < 0.001) positive relationships for the market garden soils between Kd and OC for atrazine, DIA, and DEA (r2 = 0.85, 0.86, and 0.51, respectively). The relationship for prometryne and OC was not significant for all soils. The relationship between Koc and pH was significant (P < 0.001) for prometryne (r2 = 0.70 all soils, r2 = 0.64 native vegetation only). Prometryne Koc values were also fitted to a model to predict changes in sorption of ionic pesticides with changes in pH. The data fitted the predictive model reasonably well at pHw >5, but at more acidic pH values the data deviated from the model predictions suggesting an increase in the cationic form of the pesticide that readily bind to negatively charged clay particles rather than OC. This study demonstrated that land use significantly (P < 0.05) affected the sorption behaviour of DIA, atrazine, and prometryne. It also highlighted the potential risk that pesticide metabolites may pose in areas where groundwater is used as a drinking water supply. Clearly there is a need for the metabolites to be considered when establishing guidelines for drinking water.