Author: David Poulsen
Poulsen, David, 2019 Challenges and opportunities of vertical flows in long-screened and open borehole wells for quantitative groundwater investigation, Flinders University, College of Science and Engineering
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This thesis brings into clear focus the opportunity offered by working with the vertical flow regime within a well to resolve the depth and chemistry of specific inflows, in addition to characterising the intersected aquifer heterogeneity and distribution of hydraulic heads. The specific contributions are 1) an improved understanding of the role of ambient intraborehole flow in relation to purging and sampling; 2) a novel approach to determine the pumped flow profile in a well using a salt tracer; and 3) an innovative new way to determine depth-resolved aquifer chemistry by combining knowledge of ambient flows with depth-specific sampling within a well.
Depth-resolved estimates of aquifer chemistry are critical to a wide range of groundwater investigations. If a well intersects zones of variable concentrations, the pumped sample is a composite of the inflows, which mix in the well. Where discrete concentrations are required, excessive mixing makes samples less useful and potentially misleading. An additional risk where such wells have been left un-pumped is that a pumped sample can be biased due to ambient flows through the well having invaded part of the aquifer, displacing the native groundwater. Three dimensional numerical modelling is used to quantify this sample bias, and show that it is usually infeasible to purge the ambient flow plume from around a long-screened well. It is therefore critical to know the in-well flow regime in ambient conditions and at the pumping rate used for sampling to inform a strategic sampling approach.
The ambient flow regime is shown to be particularly useful to sample groundwater native to defined inflow zones (head in the zone > head in the well), and avoid zones impacted by the invasion of intraborehole flow (head in the zone < head in the well). Depth-specific samples are interpreted as either native groundwater from a discrete source, subject only to analytical error, or a mixture from multiple sources that can be deconvolved, incorporating error in both flow and concentration measurement. Depth-resolved age tracers (Chlorofluorocarbons, Carbon-14 and Helium) in groundwater from three supply wells in the Pilbara region of Western Australia are verified with samples from a multi-depth nest of piezometers. Results show old groundwater at all depths and the simultaneous occurrence of young water at shallower depths in undisturbed dual-porosity fractured aquifers.
This approach improves the utility of long-screened and open borehole wells in groundwater investigations. However, careful consideration needs to be given to how in-well flow rates measured as a function of depth relate to flowing features in the surrounding aquifer. In particular, with screened wells the gravel pack (or void) in the annulus between the screen and the borehole wall is a largely unmeasurable and uncontrollable pathway for vertical flow. This feature makes it possible for flow to enter the well at a point downstream of where it was released from the aquifer. So the practitioner must be mindful that changes of in-well flow rate might not correspond exactly to the intersected permeable features.
Keywords: groundwater, sampling, borehole flow, ambient flow, intraborehole flow, long-screened well, open borehole
Subject: Hydrology thesis
Thesis type: Doctor of Philosophy
Completed: 2019
School: College of Science and Engineering
Supervisor: Peter Cook