Submitted to: Transactions of the ASAE
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/15/2000
Publication Date: N/A
Citation: Interpretive Summary: Soil cores are often used to assess herbicide movement through soil or to assess contaminant transport models. When macropores (root channels, worm holes, cracks, etc.) are present, the suitability of soil cores for these purposes is questionable because vertical water movement is through only a small portion of the total porosity. Our objectives were to determine if soil samples are adequate to 1) assess vertical herbicide movement and 2) assess a contaminant transport model in a soil subject to macropore flow. The results indicate that soil samples can be a poor indicator of subsurface herbicide movement and a poor method to assess contaminant transport models. These findings will help researchers design experiments whose objective it is to: 1) assess vertical pesticide movement or 2) assess the predictive ability of contaminant transport models. This research indicates that, in soils subject to macropore flow, it may be unwise to rely solely on soil cores to indicate vertical pesticide movemen or to assess the vertical predictive ability of contaminant transport models.
Technical Abstract: When macropores are present, the suitability of soil cores to assess herbicide movement through soil or as the sole means to evaluate contaminant transport models is questionable. Therefore, our objectives were to determine if soil samples are adequate for these purposes. To accomplish this, atrazine and alachlor were surface-applied to 30 by 30 by 30 cm blocks of undisturbed, no-till silt loam soil at three moisture levels (dry, intermediate, wet) then subjected to a 0.5-h, 30 mm simulated rain. Percolate was collected in about 10 mL increments from the base of the blocks using a 64-cell grid lysimeter. After percolation ceased, the soil was sampled by slicing the blocks into eight, 3.75 cm-thick, horizontal slabs. The contaminant transport model GLEAMS was used to predict herbicide concentration in the percolate after calibration of herbicide concentration in the bottommost soil section and percolate volume. Herbicide concentrations in percolate were significantly greater than background and higher in the dry soil blocks than in the wet soil. In contrast, a negative correlation was observed between herbicide concentration in the percolate and that in the bottommost soil layer. Moreover, herbicide concentration in most subsurface soil samples were not significantly greater than background. Although calibration of GLEAMS resulted in accurate prediction of percolate volumes and bottom layer soil herbicide concentrations, herbicide concentrations in percolate were under-predicted in most instances. Our results indicate that soil samples can be a poor indicator of subsurface herbicide movement and inadequate as the sole means to assess contaminant transport models in soils subject to macropore flow.