The Effect of Unsaturated/Saturated Zone Property upon the Hydrogeochemical and Microbiological Processes Involved in the Migration and Attenuation of Landfill Leachate Components

Abstract

Landfilling of domestic wastes at ‘dilute and disperse' sites on the two major aquifers in the U.K., the Triassic Sherwood Sandstone and the Cretaceous Chalk, continues to highlight the vulnerability of groundwater resources. This paper describes the findings of hydrogeochemical and microbiological investigations designed to elucidate the processes of migration and attenuation of landfill leachate components which operate in the unsaturated/saturated zone. Microbial enumeration has shown that bacteria colonise the unsaturated and saturated zones of the Chalk, and the investigated unsaturated zone profile of the Sandstone to a depth of at least 40 m. Domestic waste landfilling on the Chalk appears to result in an increased bacterial population. The high buffering capacity of this rock type has a significant impact on pollution plume development by counteracting the potentially low pH conditions, attributable to volatile acid components of landfill leachate, which inhibit biodegradation. In the Sandstone, increased bacterial populations only correlate with horizons where anaerobic biodegradation has been identified by pore water and gas profile analysis. The limited buffering capacity of this formation indicates that a substantial depth of unsaturated zone is required to provide dilution of landfill leachate so as to maintain biodegradation and significant attenuation of pollution plume development. Simple laboratory activity tests, using recovered core material incubated under aerobic conditions at 10 °C and dosed with a range of concentrations of volatile acid solutions, have highlighted the constraints on biodegradation associated with low buffering capacity. The long term objective of these studies has been to provide data for landfill design assessments capable of achieving the high standards which present day environmental pressures demand.