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Description
Arid environments have long been regarded as superior locations for waste disposal due to their remoteness, high evapotranspiration and low annual precipitation. Research indicates that wetting front instabilities can exist in a wide range of unstructured sandy soils under unsaturated conditions thereby allowing contaminant transport to travel more quickly to groundwater than predicted under Darcian flow. An in situ infiltration experiment was performed at a study site located in the western Mojave Desert. The objective of this study was to identify the presence and effect of preferential flow in the soils at Edwards Air Force Base, California. A secondary objective was to assess the validity of the chloride mass balance technique for quantifying advective flow properties of the wetting front specific to this environmental setting. The study monitored the transport behavior of the conservative tracers chloride and fluorescein under unsaturated conditions in the vadose zone of a 64 rn2 undisturbed field site. The study area was instrumented with vacuum suction lysirneters and soil moisture sensors. Soil moisture and solute transport characteristics were observed via instrumentation during several high intensity, winter storm events followed by a single irrigation event. A total of 15.71 cm of rainfall fell on the study area and an additional 12.47 cm of water was applied by irrigation. The western Mojave Desert periodically receives high intensity, frequently spaced precipitation that can yield between 22 and 39 cm/yr with an average annual periodicity of 9 years. The precipitation-induced wetting front was observed to a depth of 335 cm and was displaced to 427 cm following irrigation. Results of this study indicate that bypass or preferential flow occurred when the study site was subjected to several closely spaced, high intensity precipitation events. Chloride and fluorescein, marking the arrival of the irrigation-induced wetting front, were observed at 152 cm below ground surface within one day after irrigation was initiated. Numerical modeling, using the one-dimensional variably saturated flow and transport model HYDRUS, predicted that fluorescein would be confined to the upper 250 cm of the soil. However, fluorescein was observed at a depth of 672 cm below ground surface contrary to Darcian flow predictions suggesting the presence of bypass flow. The chloride mass balance technique was utilized to compare the residence time of pore fluids prior to and post-irrigation. Post-irrigation chloride residence times were greater than the irrigation water applied to the site by a factor of between 1.4 to 12.9 in the shallow interval from ground surface to 400 cm. The over-estimation of chloride ages indicates antecedent water in the immobile soil pore fraction, possessing a relatively higher chloride concentration than irrigation water, contributed to the overall chloride age. Therefore, one must consider the potential influence of antecedent chloride in the immobile fraction when attempting to apply the chloride mass balance technique.
