Factors Affecting Ground-Water Samples

A number of factors influence the ability of samplers to collect representative ground-water samples. Table 1 provides a summary of the factors that must be evaluated for each site undergoing ground-water sampling to determine how each might affect the representative nature of samples to be collected and the sampling strategy and methods to be used.

TABLE 1 . Factors Influencing the Representative Nature of Ground-Water Samples
1. Factors related to formation and well hydraulics

• Ground-water flow paths and flow through wells
• Hydraulics within a well between sampling events

2. Factors related to sampling point placement, design, installation, and maintenance

• Placement of the sampling point with respect to the source(s) of contamination being monitored
• Placement of the sampling point intake in the preferential flow pathway (the zone of highest hydraulic conductivity) within the formation(s) of concern
• Installation method used for sampling point construction (i.e., drilling method or direct-push installation method)
• Suitability of sampling point design with regard to material selection, diameter, depth of the sampling point intake, screen length, and screen slot size for the hydrogeologic and geochemical environment being monitored
• Methods used during sampling point construction, including placement of annular seal materials and care in placement of filter pack materials
• Method, timing, and duration of sampling point development
• Long-term maintenance of the sampling point to ensure that the sampling point can continue to provide suitable samples for analysis with regard to representative chemistry that is not impacted by compromises in well integrity (e.g., cracked surface seals)

3. Factors related to geochemical changes associated with sample collection

• Pressure changes in the sample
• Temperature changes in the sample
• Entrainment of artifactual particulate matter in the sample
• Agitation or aeration of the water column in the well or the sample during sample collection