CHANGES IN LEACHATE CHEMISTRY WITH INCREASING AGE AT A MUNICIPAL SOLID WASTE LANDFILL IN FLORIDA

The changes that occur in the chemical composition of leachate from municipal solid waste landfills as they age have generally been based on differences between young and old landfills that can be very different in terms of composition, volumes, location and geometry, and climate. In this study these variables were eliminated by examining over 12 years of quarterly leachate data (1988-2001) from a single cell at a landfill in Florida. Furthermore, the samples were collected directly from the landfill leachate collection system, which eliminates the effects of groundwater interaction. The landfill was operated form 1984-1990, and was capped in 1992 with a plastic liner, which allows for the evaluation of the affects of closure on the chemical concentration of the leachate.

There are several observations that have been identified from the data. They include a lack of correlation between rainfall and leachate strength (expressed as conductivity), high variability in concentration between sampling events (up to 30%) with a decrease in amplitude noted for some parameters after closure, and pH readings which are close to neutral. Parameters that reflect biological activity, such as ammonia, total kjeldhal nitrogen, biochemical oxygen demand, displayed fluctuations but no significant trends, indicating steady microbial activity. Decreasing trends were noted for total dissolved solids (6000 to 2000 mg/l) and conductivity, which incorporate several conservative parameters, including chloride, sodium, and fluoride. These decreasing trends were unaffected by closure.

Chromium, cobalt, nickel and vanadium concentrations exhibit covariance with overall decreasing trends. Although the covariance is not perfect, it is clearly demonstrated and of significant interest. Iron and manganese concentrations demonstrated significant covariance and in contrast to the other metals, display increasing trends after closure (3 to 9 mg/l Iron, 0.15 to 0.25 mg/l Manganese). As these metals are particularly redox sensitive, their increasing concentrations after closure suggest that capping the landfill enhances the redox conditions that promote mobility of iron and manganese.