ISOTOPE HYDROLOGY INVESTIGATIONS IN LARGE REFUSE LYSIMETERS

With the aim of studying the hydraulic behaviour and isotopic effects in refuse dumps, measurements of the O-18 and H-2 content of rainwater infiltrating into large cylindrical refuse lysimeters (1.5-4 m height with diameter of about 5 m), installed by the Technical University of Brunswick, Germany, were performed. The 11 lysimeters were filled with household refuse and sewage sludge under different conditions (aerobic/anaerobic, compaction, layering). The longterm hydraulic parameters of flow through the lysimeters (transit time, apparent dispersivity, volume of total water) were estimated with a specially adapted black-box model, which used isotopic content curves of rainwater and infiltrated water sampled at different depths in the lysimeters. The transit times of rainwater through the lysimeters (3-6 years) and the mean flow velocities deduced from them (0.4-1 m year(-1)), correspond to values in the unsaturated zone of sandy aquifers. The dispersivities are in the range of 0.1-1 cm. A special attempt was made to follow individual heavy rain events and estimate the travel times and portions of rainwater percolating swiftly through the lysimeter. It was found that up to 40% of those precipitation events drains off directly, i.e. within a few weeks. However, as it remains only a short time in the lysimeter, this water should not be greatly affected by the lysimeter filling. In addition, by evaluating the delta(2)H-delta(18)O relationship of the water samples from the different lysimeter sampling points, it was possible to differentiate, on a preliminary basis, isotopic effects caused by evaporation and biochemical reactions.