Natural attenuation is the reduction in concentration of contaminants in groundwater by natural processes inherent in the geologic units containing the aquifer. These processes can be geochemical and biological, and are dependent on factors such as the cation exchange and sorption capacity of the aquifer material, the microbial population, microbial carbon sources and nutrients, and the type and quantity of the contaminant to be attenuated. Reliance on natural attenuation has become a popular option in recent years, and has been proposed for several sites throughout the United States.

To evaluate an aquifer’s suitability and capacity for natural attenuation of groundwater contamination from landfill leachate, the chemical characteristics of the leachate need to be determined for not only the present time but also the future. Prediction of future chemical characteristics usually requires the use of a geochemical model, and such models must be calibrated to actual data before predictions can be made. Thus, the monitoring and evaluation of the changes in leachate chemistry over the life of a landfill is needed to calibrate a model and will facilitate predictions of the long-term nature of the leachate.

In this study, the leachate-chemistry data from a lined municipal solid waste landfill located in southeast Florida collected over a 12-year period are examined and temporal changes in leachate chemistry are evaluated. The computer code PHREEQC is used to analyze these trends and to identify the important chemical processes occurring in the landfill.

There are several trends that have been identified. They include increasing concentrations of total iron and total manganese and decreasing concentrations of total dissolved solids and chlorides over the life of the landfill. A decreasing trend is also noted for pH over the life of the landfill, from initial values of 8.0 to later values of 6.5.

This research represents a logical first step toward the calibration of geochemical models for the prediction of future leachate chemistry from a landfill, and could be an important factor in the decision on whether to use natural attenuation as a treatment method for groundwater contamination from a landfill.