Graduate Thesis Or Dissertation
 

Nitrate and pesticide transport under pear production in clay and sandy soil

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  • Groundwater contamination on irrigated land is of concern in this nation and around the world. In order to reduce the potential of groundwater contamination by agricultural practices such as irrigation, fertilizer and pesticide application, vadose-zone monitoring and sampling are needed. The main objective of this study was to evaluate impacts of current irrigation treatments and soil structures on the migration of pollutants to groundwater. Passive CAPillary wick pan Samplers (PCAPS) and suction cups were installed in two cracking clays and one sandy soil under the pear tree root zone. PCAPS and suction cups were used to collect nitrate-nitrogen and tracer samples. Tracers were applied to track the spatial and temporal patterns of compounds that mimic nitrate-nitrogen and pesticide movement. The observed magnitude of water leaching over 3 months differed between irrigation methods and soil structures and decreased in this order: flooding over 3 months in clay soil (22.8 cm) > micro-sprinkler in clay soil (16.1 cm) > over-head sprinkler in sandy soil (4.1 cm). Leaching patterns were varied spatially; soil structures, irrigation methods, preferential flow, and high water table may have been responsible for the spatial variation of leaching. Mass recovery of all three tracers, including bromide, blue dye, and rhodamine had the same decreasing order: flooding in clay soil > micro-sprinkler in clay soil > over-head sprinkler in sandy soil. Average blue dye and rhodamine concentrations had the following order: flooding in clay soil > micro-sprinkler in clay > over-head sprinkler in sandy soil. Since blue dye and rhodamine have similar properties to some moderately adsorbed pesticides, we may infer that the risk of pesticide movement in three sites should also decrease in this order. Presumably pesticide movement in clay soil would have been more pronounced for flooding than sprinkler irrigation. On the annual/seasonal basis, the total mass of nitrate-nitrogen leaching differed between irrigation methods and soil structures and decreased in the following order: over-head sprinkler in sandy soil > flooding in clay soil > micro-sprinkler in clay soil. The annual average nitrate-nitrogen concentration observed under over-head sprinkler in sandy soil was 15 mg/l over the maximum allowed concentration level (10 mg/l) by the EPA while seasonal nitrate-nitrogen concentration was low in clay soil under current irrigation practices. Strong evidence suggested the occurrence of preferential flow in this study. Preferential flow may contribute to high water leachate, nitrate and pesticide migration. High correlation coefficients between paired PCAPS indicated that PCAPS have similar responses to water and solute leaching. Several improvements in PCAPS are needed to obtain representative samples under severe flooding conditions. Limited data suggested that ultra-low rate irrigation devices could reduce the water leaching and the potential of pollutant migration to the groundwater because ultra-low rate application devices minimize the soil macropore flow.
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