Determination of flow and transport properties in a deep unsaturated soil profile
AdvisorWierenga, Peter J.
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PublisherThe University of Arizona.
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AbstractThree goals of this research were: (1) to study the movement of a non-reactive tracer (bromide) and of water through non-aggregated fine sand, with low clay (1.2%) and organic matter content (<1%); 2) to develop an inverse method for estimating the hydraulic properties of unsaturated soil from intermediate- and field-scale infiltration data; and (3) to develop a transient in-situ method for calibrating the neutron probe. All three goals of this research are crucial for understanding and determining water and solute transport at the intermediate to field scale. The field research was conducted in a field-scale research facility--the University of Arizona Superfund lysimeter (400-cm deep and 250-cm in diameter). We found equilibrium conditions, as evidenced by symmetrical bromide breakthrough curves (BTCs), from data collected during an unsaturated infiltration experiment in the lysimeter. Breakthrough of bromide, however, occurred sooner than was expected based on water arrival, and this observation is inconsistent with previous observations of other investigators. About 21% of the pore water (corresponding to approximately 0.03 cm³ cm⁻³) was found to be isolated from the bromide transport. We postulate that this inaccessible water partly existed as very thin films, adsorbed onto soil particle surfaces, and did not participate in anion transport. The combined effect of these films and of anion exclusion caused the bromide tracer to travel faster than the wetting front in this initially dry soil, because the excluded water fraction was larger than the initial water content. The soil hydraulic properties were estimated by an inverse method using in-situ data collected from this deep infiltration experiment. Soil hydraulic properties determined from laboratory experiments often are non-representative of field conditions. The inverse method developed in this study uses transient tension data during wetting of the profile, and the steady state water content found behind the wetting front. The results indicate that the method is fast and yields a unique estimate of the in-situ hydraulic properties at the field scale, without the need to collect excessive amounts of data. The neutron moisture meter used to determine the soil water content was calibrated using a newly developed transient mass balance method. The method was tested in the field scale lysimeter, and at a field site at the Agriculture Center, Maricopa, AZ. Water content errors using this method were less than 0.01 cm³ cm⁻³ for both sites. Application of the method to the lysimeter data showed excellent agreement between the soil water storage obtained using the calibration curve, and the actual volume of soil water added into the system.
Degree ProgramGraduate College
Soil, Water and Environmental Science