Chapter 19: Single Vertical Dikes Analysis and Evaluation of Pumping Test Data Second Edition

Background Two types: 1.Igneous 2.Sedimentary Igneous dikes are formed when magma rises through the subsurface and intrudes, or cuts through, pre-existing strata. Igneous dikes are normally coarse-grained due to slow rate of cooling. Weathered material may form sedimentary dikes when sediment fills a pre-existing crack. Sedimentary dikes are not as common as igneous dikes. Both types have high aspect ratios. 2

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Background Dikes can be highly permeable. If a single, permeable, vertical dike bisects an aquifer whose transmissivity is several times less than that of the dike, a specific flow pattern will be created when the dike is pumped. “Trough of Depression” similar to cone of depression 8

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Assumptions and Conditions  The dike is vertical and of infinite extent over the length influenced by the test.  The width of the dike is uniform and does not exceed 10 m  The flow through the fracture system in the dike is laminar (Darcy!)  The uniformly fractured part of the dike can be replaced by a representative continuum to which spatially defined hydraulic characteristics can be assigned  The fractured part of the dike is bounded above and below by impermeable weathered zone and solid rock, respectively.  The well fully penetrates the fractured part of the dike and is represented by a plane sink; flow through the dike towards the well is parallel  The hydraulically connected country rock aquifer is confined, homogeneous, isotropic, and of infinite areal extent.  All water pumped from the well comes from storage within the composite system comprised by the dike and aquifer.  The ratio of hydraulic diffusivity of the dike to the aquifer is not less than 25.  Well-losses and well-bore storage are negligibe. 10

Methods for Observation Wells Partial solutions for the drawdown in observation wells: – Boonstra – Boehmer’s curve fitting method – Boehmer – Boonstra’s curve-fitting method 11

Boonstra – Boehmer’s Curve Fitting Method To analyze the drawdown behavior for early and medium times along the pumped dike: 12

Boonstra – Boehmer’s Curve Fitting Method 13

Example: Observation Well in the Dike, p

Example: Observation Well in the Dike, p. 284 W d T d = 2.6 x 10 4 m 3 /d W d S d = 4.3 x m S T = 3.2 x m 2 /d 15

Boehmer – Boonstra’s Curve-Fitting Method To analyze early and medium time drawdown behavior in observation wells drilled in the aquifer along a line perpendicular to the dike and even with the pumped well: 16

Boehmer – Boonstra’s Curve-Fitting Method 17

Example: Observation Well in the Aquifer, p

Example: Observation Well in the Aquifer, p. 285 T/S = 2.7 x 10 5 m 2 /d Combining results from observation wells in the dike and aquifer, we get separate values for transmissivity and storativity of the aquifer: T = 9.3 m 2 /d S = 3.4 x

Methods for Pumped Wells 1.For early and medium pumping times 2.For late pumping times 20

Early and Medium Pumping Times At early times, all the pumped water is from dike storage (no water from aquifer storage). At medium times, most of the drawdown is governed by parallel flow from the aquifer into the dike (no water from dike storage). 21

Early and Medium Pumping Times Early time: Medium time: 22

Early and Medium Pumping Times 23

Late Pumping Times At late times, the flow in the aquifer is no longer considered parallel, but rather pseudo-radial. Solution for the drawdown in the pumped well during late time: For a pumping test of usual duration, this method can only be applied to fractures or thin dike structures. 24

Late Pumping Times 25

Example: pumped well, p

Example: pumped well, p (W d T d ) √(ST) = 425 m 4 /d 3 / 2 Substituting the values of dike width and transmissivity (W d T d ) and aquifer storativity and transmissivity (ST) obtained with Boonstra – Boehmer’s method into (W d T d ) √(ST), the value is