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.

A slug test is a controlled field experiment, performed by groundwater hydrologists to estimate the hydraulic properties of aquifers and aquitards, in which the water level in a control well is caused to change suddenly (rise or fall) and the subsequent water-level response (displacement or change from static) is measured through time in the control well and one or more surrounding observation wells. Slug tests are frequently designated as rising-head or falling-head tests to describe water-level recovery in the control well following test initiation. Other terms sometimes used instead of slug test include baildown test, slug-in test and slug-out test.

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).

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.

Learn more about the following slug test topics:

Types Of
Slug Tests

Rising-Head And Falling-Head Tests

Slug tests are often classified as either rising-head or falling-head tests depending on the direction of water-level recovery in the control well.

  • A rising-head test is initiated by rapidly lowering the water level in the control well and then taking measurements of the rising water level in the well. Baildown test and slug-out test are alternate terms for rising-head test.
  • A falling-head test is conducted by rapidly raising the water level in the control well and subsequently measuring the falling water level. Slug-in test is another term for falling-head test.

Overdamped And Underdamped Tests

Slug test response is often described as either overdamped or underdamped. Overdamped response occurs in aquifers of low to moderate hydraulic conductivity (K) while underdamped response may be observed in high-K aquifers.

Slug test data with concave upward curvature
Figure 3. Data from a typical overdamped falling-head slug test in an unconfined aquifer with fully submerged well screen (data from Batu 1998).

Overdamped response (Figure 3) is encountered most often in slug tests and is analyzed using mathematical models by Bouwer and Rice (1976), Hvorslev (1951) and Cooper et al. (1967) among others.

Underdamped slug test in a high-K aquifer
Figure 4. Data from a typical underdamped slug test in a high-K confined aquifer with fully submerged well screen.

Underdamped response may occur in aquifers of high hydraulic conductivity. In an underdamped test, the response is oscillatory (Figure 4) and requires specialized methods for analysis.

Wells Screened Across Water Table

Double straight-line effect in slug test data
Figure 5. Double straight-line effect from filter pack drainage observed in slug test data from well screened across the water table (data courtesy G. Zemansky).

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

Field Data
Checklist

Gather the following field measurements to determine K from a slug test (see Figure 1 for well construction details):

  • casing (inside) radius
  • well (screen) radius
  • borehole radius
  • screen length
  • filter pack material (if present)
  • depth to top of screen from water table (unconfined aquifer) or overlying confining unit (confined aquifer)
  • slug radius and length (solid slug)
  • static (pre-test) depth to water in well
  • saturated thickness of aquifer

The saturated thickness of the aquifer under investigation may not be known from direct measurement at the control well in certain groundwater studies (e.g., LNAPL monitoring). In such cases, the thickness must be be estimated from other data sources such as nearby wells, published reports, geologic maps, geophysics, etc. Another strategy is to perform a sensitivity analysis for a range of aquifer thickness values and observe how varying the thickness affects your estimate of hydraulic conductivity.

Conducting
A Slug Test

Test Initiation

Example of noninstantaneous slug test initiation
Figure 6. 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 methods:

  • 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 slug tests in 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 the instrument's diameter allows sufficient clearance inside the well to prevent blockage and avoid interference with sensors and cables.

Slug tests are sometimes initiated by pouring water into the control well; however, this technique is not recommended because water running along the inside well casing wall can result in noninstantaneous test initiation.

Water-Level Measurements

During a slug test, water levels in wells may be measured by manual techniques or through the use of pressure tranducers 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 observation well to (1) confirm transducer readings and (2) provide backup readings in the event of accidental data loss. At a minimum, one should take manual readings before and after a slug test.

A linear schedule is appropriate for recording water levels 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, wait for the water level to equilibrate and obtain a manual reading 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 (air) pressure.

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).

Normalized Head

Initial displacement, designated as H0, represents the change in water level (rise or fall) from the static (pre-test) position that occurs at the start of a slug test. 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).

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

Additional
Resources

Links To Guidance Documents

Find additional guidelines and procedures in the collection of guidance documents.

Recommended Reading

  • The Design, Performance, and Analysis of Slug Tests (Butler 1998)

Check out the aquifer testing reference list for additional literature pertaining to slug tests.