Slug Tests

by Glenn M. Duffield, President, HydroSOLVE, Inc.

What Is A Slug Test?

Typical well configuation for slug test in unconfined aquifer

Figure 1. Typical control well configuration for slug test in unconfined aquifer with fully submerged well screen.

In hydrogeology, a slug test is a controlled field experiment in which the water level in a control well is caused to change (rise or fall) instantaneously and the subsequent water-level response (displacement from static) is measured in the control well and one or more surrounding observation wells through time.

The goal of a slug test, as in any aquifer test, is to estimate hydraulic properties of an aquifer system such as hydraulic conductivity.

Analysis of overdamped slug test in unconfined aquifer with KGS Model

Figure 2. Estimation of aquifer properties from time-displacement data collected during an overdamped slug test in an unconfined aquifer using the KGS Model (Hyder et al. 1994) type-curve solution (data from Butler 1998).

Typically, aquifer properties are estimated from a slug test by fitting mathematical models (type curves) to displacement data through a procedure known as curve matching (Figure 2).

Common types of slug tests include the following:

Rising-head slug tests
Falling-head slug tests

Slug tests conducted in wells screened across the water table may exhibit a phenomenon known as the double straight-line effect and require special interpretation.

Attempting to match straight-line methods (e.g., Bouwer and Rice 1976) to slug test data with curvature leads to ambiguity. Reduce uncertainty by applying recommended normalized head ranges.

Besides slug tests, other types of aquifer tests used to determine aquifer properties include pumping tests and constant-head tests.

Slug Test Initiation

Figure 3. Falling-head test data showing (1) noninstantaneous test initiation and (2) noise during water-level recovery.

Slug test initiation involves raising or lowering the water level in the control well as rapidly as possible. For a given slug test, select a method of initiation that introduces minimal noise into water-level readings.

Slug tests may be initiated in a number of ways including the following:

solid slug
bailer
pneumatic pressurization

Of these initiation methods, the pneumatic technique is often preferred for its ability to reduce noise immediately after the start of a test especially in high-hydraulic conductivity (high-K) aquifers (Butler 1998). For wells screened across the water table, however, the pneumatic method is not viable and an alternate method of test initiation is required.

When using a solid slug or bailer to initiate a slug test, ensure that its diameter allows sufficient clearance inside the well to prevent blockage and avoid interference with sensors and cables.

Water-Level Measurements

During a slug test, water levels in wells may be measured by manual techniques or through the use of sensors connected to automatic data loggers. For most slug tests, especially in high-K aquifers, the use of sensors and data loggers is essential to obtain sufficiently rapid readings.

Manual measurement techniques include chalked steel tape and electric water-level sounders.
Pressure transducers combined with data loggers provide rapid and accurate measurements.

Even when working with reliable transducers and data loggers, it's good practice to obtain periodic manual measurements at each well to confirm transducer readings.

A linear schedule is appropriate for recording water levels a during a slug test. For high-K aquifers, several readings per second are recommended. After lowering the transducer to within 0.5 m of the water surface (Butler et al. 2003), start the data logger and wait for the water level to equilibrate prior to initiating the slug test.

Pressure transducers are available in vented and nonvented models. Vented transducers measure pressure relative to the ambient barometric pressure. Nonvented transducers measure absolute pressure including the pressure of the air column above the sensor. A barometric sensor is required to correct readings from nonvented transducers for changes in barometric pressure.

Well Construction Details and Related Field Data

You'll need the following field measurements to determine K from a slug test:

casing (inside) radius
well radius
borehole radius
screen length
depth to top of screen from water table (unconfined aquifer) or overlying confining unit (confined aquifer)
slug radius and length (solid slug)
saturated thickness of aquifer
static depth to water in well

Normalized Head

Initial displacement, designated as H₀, represents the change in water level (rise or fall) at the start of a slug test from the static (pre-test) position. Subsequent displacement readings, H, are recorded as the water level in the well returns to static.

Displacement readings are often transformed to normalized head, H/H₀, for graphical analysis. Normalized heads range from 0 (static condition) to 1 (the initial displacement).

AQTESOLV allows you to toggle the display of normalized heads for slug test analysis.

Test Duration

The decision to terminate a slug test is best made by monitoring the progress of water-level recovery during the test. Ideally, the test should continue until recovery reaches static or H/H₀ ≤ 0.05 (Butler 1998).