Using subsurface flow barriers to reduce nitrate leaching
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Leaching of applied fertilizer below the rootzone in the form of nitrate is thought to be a major source of groundwater contamination. Thus, it is important to develop a method to reduce nitrate leaching while maintaining crop yield;Repacked soil columns were used in a laboratory study to examine the relative effectiveness of different types of barriers on leaching and to identify the minimum effective size of a barrier to reduce anion leaching. Of all the subsurface barriers examined, a compacted soil layer above the applied chloride was the most effective barrier which delayed initial breakthrough of chloride by 0.9 relative pore volumes, reduced the peak concentration by more than 50%, and delayed the appearance of the peak by about 1.2 relative pore volumes compared with the no-barrier case;A 5-cm diameter barrier, which had only an extra 2 cm cover to the applied chloride, started delaying the initial breakthrough of chloride. The peak concentration was reduced by 40% and the appearance of the peak was delayed by 0.6 relative pore volumes compared with the no-barrier case;The relative effectiveness of subsurface barriers against anion leaching was investigated in a field study by using lysimeters. Of all the subsurface barriers tested in 1989, a plastic sheet placed above the banded chemicals was the most effective barrier which reduced the leaching of chloride and nitrate by 23 and 21%, respectively, and increased total nitrogen in corn shoots by 100% compared with the no-barrier case. The compacted soil layer reduced the leaching of anions by 12% and increased total plant nitrogen by 20% compared with the no-barrier case;In 1990, the effect of vermiculite on anion leaching was studied. The comparison of solution banding and solution-vermiculite mixture banding revealed that vermiculite had a significant effect on delaying the initial breakthrough of chloride and nitrate. For the plastic sheet above the band, the combined effect of the barrier and vermiculite on anion leaching was observed while the compacted soil layer caused delayed leaching without reducing peak concentrations.