Leaching of Lead (Pb) by preferential flow through agricultural soils

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June, 2016
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Until recently, scientific studies on lead (Pb) leaching was uncommon owing to the assumption that Pb is strongly bound by soil matrix. Nevertheless, Pb transport in soils has recently received a lot of attention when concentrations beyond permissible limits were detected in groundwater and surface water bodies. A study was conducted to determine Pb leaching by preferential flow through soils under close canopy cocoa, open canopy cocoa, and maize fields. It included three experiments: effect of different Pb concentrations on transport, Pb transport as affected by macropore and matrix flow, and Pb sorption at different soil depth classes. The experimental design was randomized complete block with four replications. Intact soil columns were collected up to soil depths of 0 - 30 cm from the three study sites using polyvinyl chloride (PVC) pipes. Some of the intact soil columns were thoroughly mixed and repacked into other PVC pipes under similar bulk densities such that macropores were virtually disrupted, leaving a maximum proportion of matrix pores. Different concentraions of Pb were then applied at 0, 35, 70 and 105 mg/L to saturated intact soil columns to determine the effect of Pb concentrations on transport. In order to ascertain the effect of macropore flow on Pb transport, 105 mg/L Pb was spiked on other saturated intact and the repacked soil columns. Results from the two soil columns were then compared to determine macropore flow effect on Pb transport. The Pb sorption experiment was conducted by sectioning these intact and repacked soil colunns according to the three depth classes (0-10, 10-20, and 20-30 cm) after they had drained for 72 hours. Effluent concentrations of Pb from the two leaching experiments were used to develop breakthrough curves (BTCs) each. Significant differences (p< 0.05) were observed among Pb effluent concentraions of intact soil columns under each test site as the concentrations of Pb applied were increased. Significantly higher (p< 0.05) leaching of Pb in terms of relative concentrations were recorded in the order: close canopy cocoa > open canopy cocoa > maize, as the concentrations of Pb applied were increased on the BTCs. In addtion, intact soil cloumns recorded significantly higher (p< 0.05) leaching of Pb from the BTCs, than repacked soil columns under each study site. The influence of macropore flow accounted for significantly higher (p< 0.05) saturated hydraulic conductivities, Pb fluxes, breakthrough concentrations, and significantly shorter (p< 0.05) transit time for Pb transport in intact soil columns than repacked soil columns under each study site. The hydraulic property and tranport parameters above were also significantly highest (p< 0.05) under close canopy cocoa among the test sites. There were significant positive correlations (r= 0.67 to 0.99) between the selected physical and hydraulic properties (except bulk density), and transport parameters (except transit time) of both intact and repacked soil columns. Pb sorption generally decreased with depth and increasing macroporosity in the soils under each test site. Significantly higher (p< 0.05) Pb sorption were recorded in the corresponding depths of repacked soil columns than intact soil columns under each study site. Soils under maize sorbed the highest amount of Pb at the respective soil depths. From the study, organic matter improved hydraulic conductivity and eventually enhanced water and Pb fluxes in the soils under close canopy cocoa than the two study sites. Generally, soils under close canopy cocoa were more laible to Pb leaching as compared to soils under open canopy cocoa and maize. Macropore flow substancially contributed to Pb leaching in the soils under each study site. The effect of macropore flow on Pb leaching was higher in soils with higher macroporosities. Thus the amount of Pb that leached out from intact soil columns was in the order: close canopy cocoa > open canopy cocoa > maize. Contrastingly, effect of matrix flow on Pb transport was relatively lower. Hence, applying heavy doses of Pb directly or indirectly by agricultural and industrial acivities to soils under close canopy cocoa could leach Pb into the vadoze zone and groundwater. On the other hand, soils under maize are apt to surface tranport of Pb into water bodies. The study also showed that percolating Pb solutions in the soils may enhance Pb sorption at the upper layers of the soil profile than the lower layers especially in soils with lower macroporosities and also in the soil matrix.
A dissertation submitted to The Department of Crop and Soil Sciences, Faculty of Agriculture in partial fufilment of the reqirements for the award of Master of Philosophy in Soil Science,