by Glenn M. Duffield, President, HydroSOLVE, Inc.
What Is A
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.
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).
Learn more about the following slug test topics:
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.
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
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.
A Slug Test
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
- 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.
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.
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).
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.
Links To Guidance Documents
- Ohio Environmental Protection Agency
- North Carolina Division of Environmental Quality
- U.S. Environmental Protection Agency [pdf]
Find additional guidelines and procedures in the collection of guidance documents.
- The Design, Performance, and Analysis of Slug Tests (Butler 1998)
Check out the aquifer testing reference list for additional literature pertaining to slug tests.