The Impact of Excavation and Landfilling on the Hydrogeology of a Clay Pit

摘要

Extensive quarrying of the Jurassic Oxford Clay in the area south and west of Bedford, UK, has resulted in the presence of a series of disused brickpits. Current demand for waste disposal sites has led to the development of several of these pits as landfill sites. The current study addresses the impact which the removal of large volumes of clay, and the subsequent use of several of the pits as landfill sites, has had on the local hydraulic regimes of the pits. Water bearing strata beneath the Oxford Clay have low permeabilities of 10"07 to 10'09 m/s, which, coupled with relatively flat lying piezometric surfaces, means that any contaminants which may escape from the landfills will spread extremely slowly. Groundwater quality is naturally poor, with salinities of the order of 500 to 1000 mg-Cl/1. To date no conclusive evidence of widespread leachate migration exists. However, study of piezometric data and groundwater quality analyses suggests that prolonged periods of clay extraction have had the greater impact on the local hydraulic regimes of the pits. Potential flow directions between the two water bearing units beneath the pits appear to reverse on the down gradient side of several of the pits. This is considered a consequence of the old quarrying practice of digging drainage trenches into the Kellaways Sand to collect surface run off. Pumping of the collected water also served to lower groundwater levels in the Kellaways Sand.