Permeability Testing in Unconsolidated Materials
by: Sebastien Fortin, E.I.T., M.Sc.
Field Methods for the Saturated Zone
This section briefly presents some of the most widely used field-testing methods for determining hydraulic conductivity in the saturated zone. The discussed methods are:
Slug Tests;
Pump Tests;
Tracer Dilution Tests.
Slug Tests
The slug test essentially consists of measuring the recovery of head in a well after a near-instantaneous change in head at that well (a nearby observation well can also be used in certain situations). In the standard configuration, a slug test begins with a sudden change in water level in a well. This can be done, for example, by rapidly introducing a solid object ("slug") or equivalent volume of water into the well (or removing the same), causing an abrupt increase (or decrease) in water level. Following this sudden change, the water level in the well returns to static conditions as waster moves out of the well or into it (when change was a decrease in water level) in response to the gradient imposed by the sudden change in head.
These head changes through time, which are termed the response data, can be used to estimate the hydraulic conductivity of the aquifer formation through comparison with theoretical models of test responses. The design, performance, and analysis of slug tests is described in details by Butler (1998).
Pump Tests
Pumping tests may be conducted to determine (i) the performance characteristics of a well and (ii) the hydraulic parameters of the aquifer. This latter purpose is to provide data from which the principal factors of aquifer performance (transmissivity, hydraulic conductivity and storage coefficient) can be calculated. A pumping test consists of pumping a well at a certain rate and recording the drawdown in the pumping well and in nearby observation wells at specific times. There are two primary types of aquifer tests: constant-rate tests and step-drawdown tests. In the constant-rate test, the well is pumped for a significant length of time at one rate, whereas in a step-drawdown test the well is pumped at successively greater discharges for relatively short periods. Data from both types of aquifer pumping tests can be analyzed to determine important hydraulic characteristics of an aquifer and/or of a well.
Measurements required for pump tests include the static water level just before the test is started, time since the pump started, pumping rate, pumping levels or dynamic water levels at various intervals during the pumping period, time of any change in discharge rate, and time the pump stopped. It is important to also monitor water level recovery following cessation of the pump for a period approximately as long as for the pumping phase. The recovery measurements are extremely valuable in verifying the aquifer coefficients calculated during the pumping phase of the test.
A number of analytical techniques have been developed over the years to interpret pump test data to extract the desired hydraulic parameters. These methods are presented in details in various textbooks such as Freeze and Cherry 1979), Driscoll (1986), Domenico and Schwartz (1990) and other manuals.
Tracer Dilution Tests
In the tracer dilution test, changes in concentration with time of an initially uniform column of tracer (e.g. a salt, bromide or other solutions) in a borehole are used to determine seepage velocities and hence indicate the ambient groundwater flow velocity about the borehole. To determine the direction of the flow and hence the groundwater velocity the method needs to be extended to measuring tracer breakthrough curves from observation boreholes of differing orientations. Conducting a tracer test can be expensive and time consuming since observation boreholes are required and less than ideal sets of orientations have to be used.
In the tracer-dilution test, a solution of known concentration is circulated/mixed within the well-screen section. The decline of tracer concentration (i.e., "dilution") with time within the well screen can be monitored directly using a vertical array of ion-specific electrode probes located at known depth intervals. Based on the dilution characteristics observed, the vertical distribution (i.e., heterogeneity) of hydraulic properties and/or flow velocity can be estimated for the formation within the well screen section. The presence of vertical flow within the well screen can also be identified from the probe/depth dilution response pattern. A description of the performance and analysis of tracer-dilution test characterization investigations is provided in Halevy et al. (1966), Hall et al. (1991), and Hall (1993).
Alternatively, the tracer pumpback is a constant-rate pumping test that is initiated after the average tracer concentration has decreased (i.e., diluted) to a sufficient level within the well screen (usually a 1 to 2 order of magnitude reduction from the original tracer concentration). The objective of the pumpback test is to "capture" the tracer that has moved from the well to the surrounding aquifer. Tracer recovery is monitored by measuring the tracer concentration in water pumped from the well. The time required to recover the centroid of tracer mass/concentration provides information of the aquifer effective porosity. Effective porosity is a primary hydrologic parameter controlling contaminant transport.
Forward to Field Methods for the Vadose Zone.
Return to Methods of Permeability Testing.
Consult list of References on Permeability Testing.
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