GROUNDWATER SAMPLING IN FRACTURED ROCK WELLS

Addressing contaminated groundwater in fractured rock challenges the state-of-the-art, owing to the complex nature of fracture flow and the high cost of investigation. Both passive and active methods are available for sampling wells in fractured rock to characterize contamination. However, how these methods are applied can result in very different concentration determinations. This can lead to misleading interpretations when it comes to addressing bedrock contamination sources, directions of contaminant migration, trends in monitoring data and risk. Concentration determinations are influenced by ambient borehole flow patterns, the number and depths of water contributing fractures, the depth of the sampler inlet, the sampling rate and drawdown achieved during sampling. Low flow sampling used in sampling overburden wells may not be an optimum approach when it comes to bedrock wells. Spreadsheet flow and concentration averaging models are used to examine how different well conditions impact sampling results and to address the following issues: what do concentration data collected using different sampling methods mean and don’t mean; how do different hydrogeologic conditions and sampling approaches influence concentration determinations; and what cost effective approaches are available to improve the representative nature of sampling in fractured rock wells?