Capture Zone Geometry in a Fractured Carbonate Aquifer

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This study examined a fractured carbonate aquifer that has a transition from porous media type (continuum) flow near the bedrock surface to discrete fracture (non continuum) flow at depth. Three depth zones were delineated using a borehole flowmeter, borehole video logs, and pumping tests. The upper zone is fractured to the degree where it behaves hydraulically as a continuum, the middle zone is less fractured and behaves as a discretely fractured aquifer, and the lower zone is least fractured, has no measurable fracture interconnection, and behaves as an aquitard. These zones were not related to lithologic boundaries, showing that monitoring well design based solely on lithology may be inappropriate in some fractured systems.

The geometries of capture zones in this aquifer were determined by combining the field observations with numerical modeling. The capture zone geometries are very complex, containing thin a really extensive features around fractures in the middle zone which extend over an area 17.5 times greater than the capture area in the upper continuum zone. A capture zone computed with lumped aquifer parameters leads to inaccurate conceptualization of capture zone geometry at this site.

The presence of open core hole monitoring wells affected the flow regime under both ambient and pumping conditions. The wells act as short circuits between otherwise isolated fractures and fracture zones. By connecting the continuum to the non continuum flow regime with the wells, ambient flow in the non continuum regime was increased by a factor of 20. Under pumping conditions, the presence of the monitoring wells alters the capture zone of the pumping well. Discrete fractures provide a connection between the pumping and observation wells at depth that causes separate cones of depression to be formed around observation wells in the upper aquifer, and thus, the capture zone in the near-surface aquifer may include multiple, isolated areas around monitoring wells.