Summary The disposal of liquid wastes containing heavy metals and organic solvents into lagoons excavated to beneath the water table in a shallow, unconsolidated sand aquifer has resulted in local groundwater pollution. The development of the pollution plume appears to be controlled by the morphology of the aquifer, the distribution of permeability within it and the head distribution in the vicinity of the lagoons. On the basis of redox reactions three geochemical zones have been identified down hydraulic gradient in the transition from strongly reducing conditions near the lagoon and at the base of the aquifer to oxidizing conditions in natural groundwater. Heavy metals are attenuated within a short distance of the lagoons, probably as a result of precipitation as sulphides and carbonates, but organic wastes have been found in excess of 300 m from the site. With increasing distance from the lagoons changes in bacterial populations and the character of organic compounds present at the base of the aquifer suggest that organic transformations are occurring despite little change in the concentration of total organic carbon, although ultimately biodegradation to methane and carbon dioxide takes place. Studies are continuing in order to develop a three dimensional mathematical model integrating chemical reactions with groundwater flow.

Abstract. A sampling manifold has been developed that allows the rapid collection of ground water from multilevel samplers during tracer tests. Its reliability was proven in an experiment lasting 51 days.

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A 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.