Laboratory investigations of effective flow behavior in unsaturated heterogeneous sands

Two-dimensional unsaturated flow and transport through heterogeneous sand w as investigated under controlled laboratory conditions. The unsaturated hyd raulic conductivity of five homogeneous sands and three heterogeneous syste ms composed of these five sands was measured using a steady state flux cont rolled method. The heterogeneous sand systems were established in a laborat ory tank for three realizations of random distributions of the homogeneous sands comprising a system of 207 grid cells. The water flux was controlled at the upper boundary, while a suction was applied at the lower boundary su ch that on the average a uniform pressure profile was established and gravi ty flow applied. Solute breakthrough curves measured at discrete points in the tank using time domain reflectometry, as well as dye tracer paths, show ed that flow and transport took place in a very tortuous pattern where seve ral grid cells were completely bypassed. The degree of tortuosity appeared to be dependent on the degree of saturation, as the tortuosity increased wi th decreasing saturation. Despite the tortuous flow patterns, we found that the effective unsaturated hydraulic conductivity as well as the retention curves for the three realizations of the heterogeneous sand were quite simi lar, thus suggesting that this type of heterogeneous flow system can be tre ated as an equivalent homogeneous medium characterized by effective paramet ers.