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 wit h household refuse and sewage sludge under different conditions (aerob ic/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-b ox model, which used isotopic content curves of rainwater and infiltra ted water sampled at different depths in the lysimeters. The transit t imes 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 rainw ater percolating swiftly through the lysimeter. It was found that up t o 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. I n addition, by evaluating the delta(2)H-delta(18)O relationship of the water samples from the different lysimeter sampling points, it was po ssible to differentiate, on a preliminary basis, isotopic effects caus ed by evaporation and biochemical reactions.