The environmental impact of the main landfill of the city of Zagreb (Croatia), which contains about 5 million tons of waste, on the adjacent groundwater was studied. The waste is disposed of directly onto the highly permeable alluvial sediments, only few kilometres upstream of a large protected groundwater zone. Systematic monitoring was performed in the framework of a major project aimed to assess strategies for remediation of the landfill. Results obtained in 6 sampling campaigns during 1995-96 were used to determine the redox zones in the leachate plume and to describe horizontal and vertical distributions of selected contaminants. A relatively narrow non-continuous iron-reducing zone was found along the edge of the landfill in the prevailing directions of the groundwater flow. Even after a distance of 1200 m the redox conditions in the aquifer still remained anaerobic (nitrate-reducing), while a permanently aerobic zone was present only upstream from the landfill. The horizontal distribution of the contaminants was highly dependant on the hydrological regime, but the preferential direction of spreading was toward the protected groundwater zone. Moreover, it was shown that this aquifer section is polluted not only in the surface layer but across its whole vertical profile (as deep as 60 m).

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