Michal Snehota

Water and entrapped air redistribution in heterogeneous sand sample: Quantitative neutron imaging of the process

Saturated flow in soil with the occurrence of preferential flow often exhibits temporal changes of saturated hydraulic conductivity even during the time scale of a single infiltration event. These effects, observed in a number of experiments done mainly on heterogeneous soils, are often attributed to the changing distribution of water and air in the sample. We have measured the variation of the flow rates during the steady state stage of the constant head ponded infiltration experiment conducted on a packed sample composed of three different grades of sand. The experiment was monitored by quantitative neutron imaging, which provided information about the spatial distribution of water in the sample. Measurements were taken during (i) the initial stages of infiltration by neutron radiography and (ii) during the steady state flow by neutron tomography. A gradual decrease of the hydraulic conductivity has been observed during the first 4 h of the infiltration event. A series of neutron tomography images taken during the quasi-steady state stage showed the trapping of air bubbles in coarser sand. Furthermore, the water content in the coarse sand decreased even more while the water content in the embedded fine sand blocks gradually increased. The experimental results support the hypothesis that the effect of the gradual hydraulic conductivity decrease is caused by entrapped air redistribution and the build up of bubbles in preferential pathways. The trapped air thus restricts the preferential flow pathways and causes lower hydraulic conductivity.

DETERMINATION OF HYDRAULIC PROPERTIES OF A TROPICAL SOIL OF HAWAII USING COLUMN EXPERIMENTS AND INVERSE MODELING

SUMMARY A method for determining soil hydraulic properties of a weathered tropical soil (Oxisol) using a medium-sized column with undisturbed soil is presented. The method was used to determine fitting parameters of the water retention curve and hydraulic conductivity functions of a soil column in support of a pesticide leaching study. The soil column was extracted from a continuously-used research plot in Central Oahu (Hawaii, USA) and its internal structure was examined by computed tomography. The experiment was based on tension infiltration into the soil column with free outflow at the lower end. Water flow through the soil core was mathematically modeled using a computer code that numerically solves the one-dimensional Richards equation. Measured soil hydraulic parameters were used for direct simulation, and the retention and soil hydraulic parameters were estimated by inverse modeling. The inverse modeling produced very good agreement between model outputs and measured flux and pressure head data for the relatively homogeneous column. The moisture content at a given pressure from the retention curve measured directly in small soil samples was lower than that obtained through parameter optimization based on experiments using a medium-sized undisturbed soil column.

Alternative methods to control water infiltration for landfills: a case study in the tropics.

Landfill caps made of synthetic materials are expensive. Compacted clay caps are also subject to desiccation cracks which negate the overall purpose of infiltration reduction. Vegetative cover caps, also called evapotranspiration covers, have been tested in arid climates. In this research, a vegetative cover using a naturally occurring soil of Hawaii was tested. As the annual rainfall at the site is more than the evapotranspiration demand, a portion of the run-off was routed offsite using run-off-enhancing structures such as rain gutters. The site contained six test plots on 4% slopes to simulate different components of water balance: two control plots, two plots with 20% surface area covered by the gutters and two more plots with 40% area covered by the gutters. The gutters were spaced uniformly and enhanced run-off in early stages of vegetation growth. With growth, the vegetation covered the gutters and the gutters did not receive as much rainwater. The difference in run-off between the 20% and 40% was not significant. If the gutters had been placed at a single location in the plots, they would not have been covered with vegetation, thus making them effective to produce runoff in most stages of vegetation growth.

Transport of selected contaminants in recycled water through a Hawaii soil

Leaching behavior of seven organic contaminants, including lindane, atrazine, Nnitrosodimethylamine (NDMA), estrone, 17�-estradiol, octylphenol, and nonylphenol, found in the wastewater of a treatment plant in Honolulu were evaluated for leaching in an Oxisoil (a red soil with high iron oxide content) of Oahu using both batch sorption and packed soil column experiments. Soils from two depths (0.6 m and 3.9 m) were used for the study. Two column leaching experiments were conducted using a solution of the contaminants in an artificially created ground water (deionized water with small concentrations of calcium chloride) and treated wastewater. The breakthrough curves obtained from the column experiments were used in inverse numerical modeling (HYDRUS 1D) to obtain the transport parameters for the column experiments. Sorption characteristics of each compound were obtained through batch sorption studies with different controlling parameters such as pH, ionic strength, and organic carbon content. The results of the experiments indicated that NDMA possessed the maximum potential to leach through the tested soil. Both atrazine and lindane showed moderate leaching properties. The estrogenic and the phenolic compounds were strongly adsorbed to the soil and were retained in the columns. Transport parameters obtained from the column study will be applied to the field setting to determine the arrival time and extent of contamination of ground water where widespread irrigation with wastewater over potable water aquifers to be practiced. Increases in leaching under different controlling parameters (pH, ionic strength, etc.) will also provide additional information about leaching potential.