Design Considerations for the Acquisition of Hydrochemical Data from Deep Boreholes

Deep (>300 m) subsurface hydrochemical data are important components of geological investigations of potential radioactive waste disposal sites. These data can be interpreted to yield information that supports conceptual hydrogeological models, constrains palaeohydrogeological evolution and characterises subsurface hydrochemical conditions of a site. The most important sources of these data are the analyses of groundwater samples collected during extraction tests performed in boreholes either during drilling or after drilling has been completed. To maximise the information obtained from their interpretation, the quality and quantity of hydrochemical data obtained should be such that they are representative of the compositions of the in-situ groundwaters, the in-situ location of all groundwaters should be known, and the key trends in groundwater compositions should be identifiable from the data set. The design of deep borehole-based hydrochemical data acquisition programmes to meet such objectives is a complex task. It is useful to simplify the design process by focusing attention on those activities and parameters that are most likely to affect the quality and quantity of hydrochemical data obtained from the final implemented design. In this paper we examine those design parameters associated with drilling (cutting method, drilling fluids, flushing method, drill string and bottomhole assembly), fluid extraction testing (timing, test tools, fluid extraction method), sampling (sampling locations, timing of sampling, numbers of samples, sample collection methods), and sample analysis (analytical precision and accuracy, location and timing of analyses) activities. For each parameter we discuss the design options available, and identify those with features that are most likely to be beneficial or detrimental to the quality and quantity of hydrochemical data. Preferred design options have been identified where possible. However, each investigation will have a number of external constraints associated with it arising from either the investigation itself, the site or available technology. As a result, depending on the circumstances of each investigation, it may not be possible to include all the preferred design options into a final design.