Contaminated groundwater sampling and quality control of water analyses

The objective of groundwater sampling for site characterisation is the collection of samples that represent the underlying conditions at a site and ensuring that sample integrity is maintained from field to laboratory. The authors describe the tools available to the field sampler for the collection of groundwater samples, methods of on-site water quality analysis and the appropriate preservation and handling of samples. There are a variety of portable sampling devices available for the collection of groundwater, however, each application has different requirements and is dependant on the contaminant(s) of interest and most importantly, the specification of the borehole to be sampled. A number of different sampling devices and their applicability are presented. Traditionally, to ensure sample representivity the removal of stagnant water from a monitoring well was accomplished by purging a fixed number of well volumes, generally between three to five volumes, prior to sample collection. In recent years research has shown that low-flow purging (pumping at a rate that doesn’t disturb the stagnant water in a well) produces samples that are representative of the formation water. In addition ‘no purge’ sampling is becoming an increasingly accepted method of collecting representative groundwater samples for some determinands, in particular VOCs and some metals using diffusion methods. The merits of different purge methodologies are discussed. On-site water quality measurements are carried out predominantly to monitor effective purging of water at the sampling point before sample collection, and to measure unstable parameters that cannot be subsequently reliably determined in the laboratory. On-site measurements such as pH, Specific Electrical Conductance (SEC), Oxidation Reduction Potential (ORP), Dissolved Oxygen (DO), Temperature and alkalinity can be used to provide a check on a subsequent laboratory analysis. Techniques for the preservation and analysis of samples and quality assurance and quality control are also presented.