| Popis: |
Modeling the field scale movement of chemicals in unsaturated soil is of intense interest to both the public and private sectors and has become an area of active theoretical research in a number of environmentally based disciplines. However, the experimental data needed to validate existing solute transport models and to inspire the development of more refined approaches is very limited. In this research study, the movement of a mobile tracer (Br−) was monitored as it moved through the unsaturated zone beneath the soil surface of a 0.64 ha loamy sand field. Under flux-controlled, steady state water flow achieved by bidaily sprinkler irrigation, a narrow pulse of 58.9 mol/m−3 NaBr(aq) was applied uniformly to the field and subsequently leached downward while monitored by vacuum solution samplers replicated 16 times at each of 6 depths between 0.3 and 3.0 m and 6 times at the 4.5-m depth. Six deep soil cores to a maximum of 25 m were taken to characterize the final field average bromide depth profile after the pulse had passed the 4.5-m depth. Although the mass recovery of the area-averaged pulse was near 100% at all depths, the coefficient of variation (CV) of mass recovery between samplers at a given depth was near 50%. Lateral variations in apparent vertical solute velocity or in solute transport volume were considerable, with CVs near 50% in the shallow monitoring depths. However, variations in transport volume with depth at a given site were also large, even though the solution samplers for different depths were displaced laterally by only 0.3–0.6 m at different sites. The mean vertical velocity of the area-averaged solute pulse was significantly less than the ratio of the average net water flux to the average volumetric water content, until approximately 1.8 m. The difference between the two average velocities near the surface was large enough (nearly a factor of 2) to suggest that transient effects from the bidaily irrigations were influencing solute transport. |
Plný text