Measurement of Flow through the Unsaturated Zone Using a Prototype Funnel Lysimeter

Abstract

Prototype funnel lysimeters were constructed using 2-m dia. tree transplanting equipment. This equipment offers a possible means for quantitative, practical, and cost-effective measurement of flow in the unsaturated zone at the meter scale. The objective of this investigation was to develop and test the funnel lysimeter as a practical field methodology to conduct controlled quantitative tracer tests on large undisturbed soil samples. The steps to achieve this objective were to develop a working prototype, conduct tracer tests at sites with different field soils and a repacked soil in the laboratory, determine the characteristics of tracer breakthrough curves for the test soils, and assess the degree to sample disturbance caused by the lysimeter installation process. Lysimeters were installed by temporarily excavating a 2-m(3) soil sample using the tree transplanter and then replacing the sample after the installation of a sump and funnel-shaped impermeable liner. Tracer experiments were then carried out by applying solutes at the surface and sampling from the sump using an automatic sampler. Seven funnel field lysimeters were installed in Minnesota over two field seasons in three different soil types. Experiments identified two distinct flow types: preferential and matrix flow. In a typical field experiment with the preferential-flow soil, discharge at the lysimeter sump occurred within 15 minutes of the start of a six minute tracer application; discharge of the tracer stated after 23 minutes. For the matrix-flow soil, discharge to the sump typically took several hours to respond to tracer application. Drainage to the sump in the same cycle was composed of 100% antecedent water. The installation method does not lead to major disturbance of the soil but is more effective in sandy soils. The experimental results indicate that the funnel lysimeter methodology is suited for wide application in subsurface hydrology because of its large sample size, a time scale resolution of minutes, and mass balance control. Essential technological improvements to the lysimeter and further tracer experiments are required to fully define the lysimeter flow characteristics.