Simulation of Solute Transport in Heterogeneous Soils. Volume 2. Numerical Experiments

摘要

Groundwater contamination from surface application of chemicals and land disposal of hazardous wastes is one of the most serious hazards to the Earth's hydrologic environment. Lack of information on contaminant transport in the unsaturated portion of the Earth's crust, the zone that separates freshwater aquifers from surface pollutants, has precluded the development of an accurate and efficient method of predicting the extent and timing of groundwater pollution from various forms and amounts of waste inputs. The main aim of the study is to evaluate the effect of such heterogeneity on the dispersion of contaminants in unsaturated soils by the use of stochastic mathematical models. Spatial variability of saturated hydraulic conductivity is examined to assess the effect of uncertainty in leaching of pesticides from heterogeneous soils. Saturated hydraulic conductivity, an important soil parameter that controls the transport of pesticides in soils, is assumed to be composed of a homogeneous mean value and a perturbation caused by the spatial variability of soil properties producing a stochastic process in the mean flow direction. The spatial heterogeneity of porous soils is characterized by the variance and the correlation scale of the saturated hydraulic conductivity in the transport domain. In the first part of the study, numerical experiments are used to investigate the development of scale-dependent macrodispersivity in the unsaturated heterogeneous soils. In the second part of the study, the significance of the variance on the spatial and temporal distribution of tracer spreading is demonstrated for Hawaii Oxic soils. The significance of variance regarding the spatial and temporal distribution of tracer concentrations is demonstrated using solute breakthrough curves at various depths in the soil profile. Macrodispersivity values in heterogeneous soils are proportional to the variance at smaller travel distances and converge to the same value at larger travel distances. For greater correlational distances, a faster breakthrough of solutes at various depths was observed.