Porosity and herbicide leaching in soils amended with olive-mill wastewater

Abstract

Organic amendments used to enrich soils of low organic matter contents can affect sorption and movement of pesticides in soils. In this work, we studied the effect of olive oil mill wastewater (OOW) soil amendment on soil porosity and on leaching of the herbicides clopyralid (3,6-dichloropicolinic acid) and metamitron (4-amino-3-methyl-6-phenyl-1,2,4-triazin-5(4H)-one) in clay soil columns. OOW amendment resulted in an increase in the organic carbon content of the soils and a reduction in soil porosity, the latter confirmed by mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) studies. MIP and SEM data showed the reduction in porosity is basically due to a reduction in larger pores (radius > 0.1 μm) and an important increase in finer pores (radius < 0.1 μm). These changes in porosity are produced by the combined effect of the suspended and soluble organic matter and salts in the OOW and the solubilization-insolubilization of the soil carbonate minerals promoted by OOW. Soil columns were handpacked with the unamended clay soil and with the same soil which had been treated for 3 yrs with two different doses of OOW (low dose: 300 ml/m 2 a year and high dose: 600 ml/m 2 a year). Clopyralid moved more rapidly than metamitron in the unamended soil due to greater sorption and degradation of metamitron. Total amounts of clopyralid leached from the OOW amended soils were significantly reduced (75 and 25% for the lower and higher dose, respectively) when compared with the unamended soil (100%), whereas metamitron did not leach at all from the amended soils. Sorption and degradation studies with soil slurry suggested this reduction may be mainly due to an increase in sorption and degradation processes in amended soils, as a consequence of the increase in the organic carbon content. However, the decrease in mobility produced by OOW amendment is greater than that suggested from the sorption and degradation increases. The reduction in large size conducting pores and the increase in the small non-conducting pores, induced by OOW amendment, produce an increase in the residence time of the herbicides in the immobile water phase, enhancing diffusion, sorption and degradation processes, thereby retarding mobility. The retarding effect was more pronounced for metamitron than for clopyralid due to the higher sorptivity and degradability of the former herbicide. These results suggest the possible use of the OOW or similar wastewater amendment in reducing the contamination of groundwater by pesticide drainage.