Determining Aquifer Characteristics

Hydraulic conductivity K, transmissivity T, and storativ-ity S are the hydraulic properties which characterize an aquifer. Before the quantities required to solve ground-water engineering problems, such as drawdown and rate of flow, can be calculated, the hydraulic properties of the aquifer K, S, and T must be determined.

Determining the hydraulic properties of an aquifer generally involves applying field data obtained from a pumping test. Other techniques such as auger-hole and piezometer methods can be used to determine K where the groundwater table or aquifers are shallow.

Pumping test technology is prominent in the evaluation of hydraulic properties. It involves observing the drawdown of the piezometric surface or water table in observation wells which are located some distance from the pumping well and have water pumped through them at a constant rate. Pumping test analysis applies the field data to some form of the Theis equation in general, such as s = ^W(u, a, 3, ...) 9.8(1)

where u = Sr2/4Tt and a,3 = dimensionless factors defining particular aquifer system conditions. In general, matching the field data curve (usually a plot of s versus r2/t) with the standard curve (known as the type curve) drawn between W and u for various control values of a, 3, . . ., calculates the values of S and T. This process is explained in the next section. Techniques requiring no matching have since been developed.

Various site conditions are associated with a pumping test in a well-aquifer system. The following list summarizes different site conditions (Gupta 1989):

I. Type of pumping

A. Drawdown

B. Recovery

C. Interference

II. State of flow

A. Steady-state

B. Nonsteady (transient) state

III. Area extent of aquifer

A. Aquifer of infinite extent

B. Aquifer bound by an impermeable boundary

C. Aquifer bound by a recharge boundary

IV. Depth of well

A. Fully penetrating well

B. Partially penetrating well

V. Confined aquifer

A. Nonleaky aquifer

B. Leaky confining bed releasing water from storage

C. Leaky confining bed not yielding water from storage but transmitting water from overlying layer

D. Leaky aquifer in which the head in the overlying aquifer changes

VI. Unconfined aquifer

A. Aquifer in which significant dewatering occurs

B. Aquifer in which vertical flow occurs near the well

C. Aquifer with delayed yield

Selecting a proper type curve is essential for the data analysis. During the last decades, several contributors have developed type curves for various site conditions or combinations of categories. Starting with Theis, who made the original type curve concept, other contributors to this field include Cooper and Jacob (1946) and Chow (1952) for confined aquifers, and Hantush and Jacob (1955), Neu-man and Witherspoon (1969), Walton (1962), Boulton (1963) and Neuman (1972) for unconfined aquifers.

Confined Aquifers

This section discusses the methods used in determining aquifer characteristics for confined aquifers.

105 106 107 108

r2A IN m2/n(lv

FIG. 9.8.2 Relations s versus r2/t and W(u) versus u.

105 106 107 108

r2A IN m2/n(lv

FIG. 9.8.2 Relations s versus r2/t and W(u) versus u.

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