Detailed Pumping Test to Characterize a Fractured Bedrock Aquifer

Abstract

AbstractBedrock fractures and results of a 24‐well, 21‐day pumping test were analyzed to characterize a low‐yielding fractured gneiss aquifer. Several analytical techniques for evaluating aquifer tests in fractured bedrock are reviewed and applied. The time‐drawdown curve for the pumping well and the early time‐drawdown responses at several nearby observation wells indicate linear flow through a single fracture. In contrast, analysis by radial flow techniques of the late time‐drawdown responses at all observation wells suggests that flow occurs through interconnected fractures. Taken together, these time‐drawdown responses are consistent with a model incorporating linear (single fracture) flow at the pumping well and radial (interconnected fracture) flow in an outer region.The elliptical cone of depression reveals a direction of enhanced drawdown which closely parallels one of five observed fracture sets. However, this fracture set comprises fewer than 15 percent of all fractures present. The drawdown response should not be extrapolated to characterize the entire aquifer, because it does not result from an overriding structural feature which would influence the aquifer‐wide hydraulic response.The results indicate the advisability of performing structural analysis along with aquifer analysis to characterize a bedrock aquifer. Further, the results indicate that in an aquifer where the hydraulic response at a pumping well suggests linear single fracture flow, if more observation wells or later time data are available it may be found that the aquifer responds with radial flow. The results also suggest that in certain bedrock aquifers, the larger the area of investigation, the more likely the aquifer will be shown to behave consistently with radial flow analytical techniques. In order to obtain the appropriate data, the goals of a bedrock pumping test and the structural data should be carefully considered before the test duration and observation well network are planned.