Abstract

This thesis concerns aqueous batch–type adsorption studies, laboratory degradation studies and laboratory packed soil column and field leaching studies which were conducted to determine the influence of soil properties on retention, degradation and mobility of atrazine and metolachlor in Ngabu clay, Thyolo clay, Makoka sandy clay loam, Bvumbwe loamy sand and Chancellor College sandy loam. A snapshot survey was also conducted to assess ground and surface water contamination by atrazine and metolachlor in the Zomba/Bvumbwe region. No herbicide residue was detected in the groundwater samples. In surface water samples atrazine was detected in 38% and metolachlor was detected in 15% of the samples. The concentrations of the herbicides were at their highest soon after the first run off event after herbicide application. The concentrations, however were generally below the World Health Organization's (WHO's) recommended maximum guideline values. Following the first run off event concentrations of herbicides steadily decreased with time, decreasing to zero within eight weeks of herbicide application at 37% of the water sampling points that had herbicide contamination. Light soaking rains, higher clay content, flat land, longer distance between agricultural land and surface water body (filtering area), lower herbicide application rates and herbicide incorporation seemed to reduce herbicide export to surface water. The L-type and C-type sorption isotherms were observed for atrazine and metolachlor on all soils, such that the adsorption of atrazine and metolachlor are described well by Freundlich (r2 = 0.96 to 0.99), Linear (r2 = 0.90 to 0.990, Langmuir (r2 = 0.80 to 0.96) and Temkin (r2 = 0.94 to 0.99) isotherms. The atrazine and metolachlor adsorption conformed to the isotherms in the following order: Freundlich > Temkin > Linear > Langmuir. The adsorption coefficients (kd), at 250C, of both herbicides were related to soil organic carbon content (r = 0.88** for atrazine and r = 0.99*** for metolachlor) and cation exchange capacity (r = 0.98*** for atrazine and r = 0.89** for metolachlor). The kd for atrazine was also related to clay content (r = 0.81*). After the first 24-hour desorption period, the amounts of herbicide desorbed from the Bvumbwe, Chancellor College, Makoka, Ngabu and Thyolo soils ranged from 10-40, 4-22, 12-27, 0.75 -10 and 3-25 %, respectively, of the herbicide that had adsorbed. The degree of desorption depended on type of soil and the initial concentration of herbicide. Desorption was hysteric in all cases, being more irreversible at the lowest herbicide concentrations adsorbed. vi Desorption was inversely related with organic carbon (r2 = -0.85 for metolachlor and -0.75 for atrazine), clay (r2 = -0.78 for metolachlor and -0.64 for atrazine) and cation exchange capacity (r2 = -0.91 for metolachlor and -0.74 for atrazine). Thus the Ngabu soil with highest organic matter and high clay content had least desorption whereas the Bvumbwe soil with lowest organic matter and clay contents had the most desorption. Degradation of the two herbicides was initially fast and then followed by a slow degradation process. The degradation of atrazine and metolachlor was described well by simple first order (SFO), bi exponential (DFOP) and hockey stick (HS) kinetic models (r2 = 0.95 - 0.99, 0.97 - 0.99 and 0.91 – 0.99, respectively). Atrazine and metolachlor degradation conformed to the kinetic models in the following order: DFOP>SFO>HS. The half –lives for SFO model varied from 25 - 45 days for atrazine and 28 - 58 days for metolachlor, and were significantly correlated with adsorption coefficients (r2= 0.99 for atrazine and r2 = 0.87 for metolachlor), and clay (r2= 0.88 for atrazine and r2 = 0.92 for metolachlor) and organic matter contents (r2= 0.83 for atrazine and r2 = 0.77 for metolachlor) of the soils. The mobility of herbicides was affected by the intensity of herbicide adsorption by soil constituents (kd), solubility of the herbicide in water, initial soil water content at the time of herbicide application, the level of water input after herbicide application and herbicide longevity (half life). The leaching of herbicides was inversely related to soil kd (r2 = - 0.99*** for atrazine and r2 = - 0.91*** for metolachlor). Horizontal movement of atrazine was affected by soil texture and amount and timing of rainfall. Mobility index (MI) values showed that leaching of the two herbicides followed the order Bvumbwe>Chancellor College>Makoka>Thyolo>Ngabu. This order was confirmed by the groundwater contamination potential (GWCP) ratings derived using the simple decision aid model. These results indicate that export of herbicides to water bodies can be reduced by maintaining high organic matter in soils, not applying herbicides when soil is too wet or too dry, following recommended land husbandry practices that reduce soil erosion and maintaining a percentage of the agricultural land as a filtering zone. The results also show that recommendations on application rates for herbicides should consider the clay mineralogy of the soil in addition to type of weeds, crop and clay content of the soil.

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School	: School of Natural and Applied Sciences
Issued Date	: 2009

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