Borehole sampling techniques and field analysis of groundwater in landfill pollution studies

Abstract

AbstractA borehole drilled at a landfill for site investigation and/or monitoring purposes will generally intercept a vertical interval within an aquifer which may have a wide range of permeabilities and so the entry of a pollutant into a borehole column will not be uniform. Drilling methods and the need to avoid the introduction of contaminants to groundwater mean that hydraulic testing may be restricted, particularly in unconsolidated formations. Techniques that do not add water should be used in preference to injection or tracer tests. Sampling and testing during borehole drilling are necessary to define the groundwater flow system within the borehole, to enable an effective completion to be made. In unconsolidated formations, silt and clay horizons may restrict vertical groundwater movement with the result that pollutants become stratified and move in discrete zones within the aquifer. Therefore taking a bulk or average water sample from a borehole completed to monitor full aquifer thickness will usually result in dilution and mixing of the contaminant with cleaner groundwater. Formation disturbance caused by percussion drilling can also cause vertical groundwater flow and mixing outside the borehole casing making the isolation of discrete zones difficult.Groundwater samples obtained during site investigation drilling or from routine sampling visits will be subject to temperature and pressure change. As a sample is depressurised, variables such as pH, Eh, dissolved gas content and the speciation of certain constituents can be altered. Quantification of these changes and their minimisation means that strict procedures should be adhered to in sample handling and analysis on site.This paper, covering landfill research work funded mainly by the Department of the Environment and carried out by the Fluid Processes Research Group of the British Geological Survey describes techniques and equipment used to overcome some of the problems cited, to obtain representative groundwater samples.