within the well's screened interval to isolate the desired test region of the aquifer. A slug test

is then performed for the test region of the aquifer by inducing water flow through the

isolated section of the well screen. The slug test data collected can be analyzed by a number

of methods. A multilevel slug test method is described by Molz et al. (1990).

Multiple well tests involve withdrawing water from, or injecting water into, one well,

and obtaining water level measurements over time in observation wells. Multiple well tests

are often performed as pumping tests in which water is pumped from one well and drawdown

is observed in nearby wells. A step drawdown test should precede most pumping tests to

determine an appropriate discharge rate. Aquifer tests conducted with wells screened in the

same water bearing zone can be used to provide hydraulic conductivity data for that zone.

Multiple well tests for hydraulic conductivity characterize a greater proportion of the

subsurface than single well tests and thus provide average values of hydraulic conductivity.

Multiple well tests require measurement of parameters similar to those required for single

well tests (e.g., time, drawdown). When using aquifer test data to determine aquifer

parameters, it is important that the solution assumptions can be applied to site conditions.

Aquifer test solutions are available for a wide variety of hydrogeologic settings, but are often

applied incorrectly by inexperienced persons. Incorrect assumptions regarding hydrogeology

(e.g., aquifer boundaries, aquifer lithology, and aquifer thickness) may translate into incorrect

estimations of hydraulic conductivity. A qualified ground water scientist with experience in

designing and interpreting aquifer tests should be consulted to ensure that aquifer test solution

methods fit the hydrogeologic setting. Kruseman and deRidder (1989) provide a

comprehensive discussion of aquifer tests.

Multiple well tests conducted with wells screened in different water bearing zones

furnish information concerning hydraulic communication between the zones. For these

aquifer tests, piezometers should be located and screened in permeable, semi permeable, and

"impermeable" zones. Water levels in these zones should be monitored during the aquifer test

to determine the type of aquifer system (e.g., confined, unconfined, semi confined, or

semi unconfined) beneath the site, and their leakance (coefficient of leakage) and drainage

factors (Kruseman and deRidder, 1989). A multiple well aquifer test should be considered at

every site as a method to establish the vertical extent of the uppermost aquifer and to evaluate

hydraulic connection between aquifers.

Certain aquifer tests are inappropriate for use in karst terranes characterized by a

well developed conduit flow system, and they also may be inappropriate in fractured bedrock.

When a well that is located in a karst conduit or a large fracture is pumped, the water level in

the conduits is lowered. This lowering produces a drawdown that is not radial (as in a

granular aquifer), but is instead a trough like depression that is parallel to the pumped conduit

or fracture. Radial flow equations do not apply to drawdown data collected during such a

pump test. This means that a conventional semi log plot of drawdown versus time is

inappropriate for the purpose of determining the aquifer's transmissivity and storativity.

Aquifer tests in karst aquifers can be useful, but valid determinations of hydraulic

November 1992

4 49