{"id":1756,"date":"2026-02-14T04:26:07","date_gmt":"2026-02-14T04:26:07","guid":{"rendered":"https:\/\/books.gw-project.org\/an-introduction-to-hydraulic-testing-in-hydrogeology\/chapter\/ch067-xhtml\/"},"modified":"2026-08-01T09:50:05","modified_gmt":"2026-08-01T09:50:05","slug":"performing-a-slug-test","status":"publish","type":"chapter","link":"https:\/\/books.gw-project.org\/an-introduction-to-hydraulic-testing-in-hydrogeology\/chapter\/performing-a-slug-test\/","title":{"raw":"14.2 Performing a Slug Test","rendered":"14.2 Performing a Slug Test"},"content":{"raw":"
\r\n
\r\n
\r\n\r\nSlug tests are almost always conducted using a single well. The test method requires the water level in the borehole to rapidly rise or fall (Figure 102<\/a>). It is assumed for most slug tests that the water level in the well instantaneously reaches a new position (no time is lost) at time = 0. However, some testing methods allow the change in water level to occur over a period, when a brief pumping interval is followed by a long water\u2011level recovery time. These tests can be analyzed if initiation of the head data collection is started immediately after the maximum head change is obtained. An initial static water\u2011level measurement is needed to establish the pretest condition for all tests. When extensive pumping precedes data collection, time\u2011drawdown data should be collected and a recovery\u2011analysis method applied.\r\n\r\n<\/div>\r\n
\r\n\r\nConducting a slug test requires careful planning including obtaining information on well construction and site conditions. The Kansas Geological Survey, U.S.A., is known for its research on the application of slug\u2011test methods (Butler, 1998). Their recommendations for conducting a successful slug test are incorporated in the following material.\r\n\r\n<\/div>\r\n
\r\n

Assessing the Hydrogeologic Setting and Well Construction<\/h3>\r\n
\r\n\r\nA site conceptual model is needed to assess the nature of the formations being tested. Are the formations aquifers or aquitards (Woessner & Poeter, 2020)? Are they confined or unconfined? Do the wells fully penetrate the formations? Could boundary conditions impact the results? Each of these questions requires resolution prior to conducting tests and analyzing results.\r\n\r\n<\/div>\r\n
\r\n\r\nWater supply wells, observation wells, or open boreholes can be used to conduct slug tests. Designing a slug test requires knowledge of the length and location of the borehole and perforated interval or length of well screen penetrating the formation, the presence of a gravel pack, and the diameter and lengths of the blank well casings (Figure 105<\/a>).\r\n\r\n<\/div>\r\n
\"\"\r\n
\r\n
\r\n
\r\n\r\nFigure 105 \u2011<\/strong> General well configurations used for slug testing in confined and unconfined aquifers. The horizontal lines and vertical dashes represent perforated casing intervals (screens). db<\/sub><\/em> is the diameter of the cased well bore and dp<\/sub><\/em> is the diameter of the perforated casing. When a gravel pack is included, the effective diameter includes the combined perforated interval and the gravel\u2011pack diameters. The length of the perforated interval (L<\/em>) and its position in the aquifer can vary. Wells can be fully or partially penetrating (as shown). The dark gray shading represents grout placed around the blank well casing (e.g., cement or bentonite).<\/span>\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n
\r\n
a) A fully penetrating well in a confined aquifer. The casing diameter and screen diameter are the same.<\/blockquote>\r\n<\/div>\r\n
\r\n
b) A partially penetrating confined aquifer well with a larger well casing diameter than the screen diameter.<\/blockquote>\r\n<\/div>\r\n
\r\n
c) A partially penetrating confined aquifer well with a gravel pack around the screened interval. The effective perforated interval diameter includes the screen and surrounding gravel pack.<\/blockquote>\r\n<\/div>\r\n
\r\n
d) A fully penetrating well in an unconfined aquifer where the dashed yellow line indicates the well installation is completed in a borehole or driven to depth without fill around the screen or casing and the formation is allowed to collapse around the well.<\/blockquote>\r\n<\/div>\r\n
\r\n
e) A partially penetrating well in an unconfined aquifer where the dashed yellow line indicates the well installation is completed in a borehole or driven to depth without fill around the screen or casing and the formation is allowed to collapse around the well.<\/blockquote>\r\n<\/div>\r\n
\r\n
f) A partially penetrating well in an unconfined aquifer where a larger borehole is drilled and then an assembled casing and screen are placed in the borehole. The bore hole outside of the screen is filled with a filter pack (gravel pack) of coarse material (Sterrett, 2007) then the casing interval is grouted. When analyzing the test data, the perforated casing diameter typically includes the gravel pack, effective diameter.<\/blockquote>\r\n<\/div>\r\n
\r\n
g) A partially penetrating well in an unconfined aquifer where the screened interval is a smaller diameter than the well casing. After the borehole was drilled to the screen depth the casing was set and grouted, then a slightly smaller diameter borehole is advanced, and a screen installed.<\/blockquote>\r\n<\/div>\r\n
\r\n
h) A partially penetrating well in an unconfined aquifer with a smaller casing diameter than the well screen. The large borehole is drilled to depth and then the casing and screen were set; then the borehole is grouted.<\/blockquote>\r\n<\/div>\r\n<\/div>\r\n
\r\n

Special Considerations for Water Table Systems<\/h3>\r\n
\r\n\r\nSlug tests can easily be conducted in water\u2011bearing units that are confined. They can also be conducted when a water table is present, and the perforated interval is below the water table (Figure 105<\/a>). However, if the perforated interval extends above the water table, only slug\u2011out or rising\u2011head slug tests should be performed (Figure 106<\/a>). In this setting, falling\u2011head tests are avoided because the water level at the start of the test after the introduction of the slug would be higher than the static water level extending into the vadose zone. When this occurs the water level falls more rapidly than it would in the saturated portion of the aquifer because it flows into the partially saturated zone above the water table. Analysis of the resulting data produces a hydraulic conductivity value that is higher than the value representative of the saturated unit (Todd & Mays, 2005).\r\n\r\n<\/div>\r\n
\r\n\r\n\"\"\r\n\r\n<\/div>\r\n
\r\n
\r\n\r\nFigure 106 \u2011<\/strong> Water\u2011level in a well in an unconfined aquifer after introduction of a solid slug at the start of a slug\u2011in test in a well with the screen completely below the water table and a well with a screen that extends about the water table. a) When the perforated interval is completed below the water table, slug\u2011in or slug\u2011out tests can be successfully completed. b) If the screened interval extends above the water table only slug\u2011out tests should be conducted because a slug\u2011in test will overestimate the saturated horizontal hydraulic conductivity due to some of the water flowing into the unsaturated zone. c) Enlargement of (b) showing the factors affecting the rate of the head decline when a slug\u2011in test is performed in a well with a screen extending above the water table. During the water level decline a portion of the water enters the unsaturated zone and the resulting rate of water level decline is greater than it would be if only the saturated portion of the aquifer was controlling the rate of decline.<\/span>\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n
\r\n

Free Exchange of Water with the Formation<\/h3>\r\n
\r\n\r\nAnother criterion for conducting a successful slug test is that the perforated or screened interval is hydrologically well connected to the formation being tested. This means the number, size, and configuration of the perforations should not inhibit the exchange of water between the formation and the well. If the configuration of perforations (too small or too few) inhibits the movement of water to and from the formation, the test may reflect the \"hydraulic conductivity\" of the well hardware instead of the geologic material (Figure 107<\/a>).\r\n\r\n<\/div>\r\n
\"\"\r\n
\r\n
\r\n
\r\n\r\nFigure 107 \u2011<\/strong> Schematic of a falling\u2011head slug test in an unpumped well penetrating an unconfined aquifer.<\/span>\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n
\r\n
a) Well design with a perforated interval (black portion of the casing with white perforations) that is not well connected to the formation. Head recovery will be controlled by the poorly designed perforated interval not the formational properties.<\/blockquote>\r\n<\/div>\r\n
\r\n
b) Perforated interval that is fully connected to the formation. Water in the well bore freely communicates with the formation.<\/blockquote>\r\n<\/div>\r\n
\r\n
c) Perforated interval adjacent to a portion of the borehole damaged during drilling such that finer material has been left behind. The damaged area will control the rate of change of the borehole water level, this is called a skin effect.<\/blockquote>\r\n<\/div>\r\n
\r\n
d) Insert showing drilling damage to the formation producing a low permeability zone. The smaller arrows indicate the connection to the surrounding aquifer is poor.<\/blockquote>\r\n<\/div>\r\n
\r\n
e) An example of a zone around the perforated interval of a well where the permeability has been enhanced. This can occur by the addition of a gravel pack (coarse material placed outside of the perforated casing) or the removal of natural fines in the formation by well development. When a gravel pack is present the effective radius, de<\/sub><\/em>, of the perforated interval includes the gravel pack.<\/blockquote>\r\n<\/div>\r\n
\r\n\r\nOften the formational material opposite the perforated interval becomes less permeable during the drilling process (Figure 107<\/a>), especially in fine\u2011grained material or when drilling fluids are used in well construction. In some cases, the process of drilling and installing the well results in finer material entering the formation or the clays present are smeared to the borehole wall. This is referred to as forming a skin of lower\u2011permeability material. This skin effect or borehole skin (Fetter, 2001) restricts the exchange of water between the well bore and formation. Therefore, hydraulic\u2011conductivity values computed from slug\u2011test results will underestimate the properties of the formation. Well\u2011development procedures like surging, bailing, and over\u2011pumping can increase the permeability of the skin and should be implemented during well construction (e.g., Sterrett, 2007). In some settings, too much development may not only remove the fines that entered a formational material during drilling but also remove additional natural fines in the formation. This creates a zone of higher\u2011than\u2011normal hydraulic conductivity adjacent to the screen and if it extends a significant distance into the formation may impact test data. In this case the well may recover more rapidly because of the presence of this high hydraulic conductivity zone. As a result, hydraulic\u2011conductivity values will be higher than the formational value. Finally, in many observation or water\u2011supply wells, gravel pack is added to enhance the permeability around the perforated interval to assure perforations are not plugged with formational fines. When this design is used, the effective diameter of the perforated interval needs to include the gravel pack (Figure 107<\/a>e).\r\n\r\n<\/div>\r\n<\/div>\r\n
\r\n

Raising and Lowering the Water Level<\/h3>\r\n
\r\n\r\nTypically, instantaneously inducing a change in water level in the well being tested is accomplished by submerging or removing a solid slug from the water\u2011filled well bore. These actions result in a rise or fall of the borehole water level. The degree of change is a function of the volume of the slug and volume of water per length of the well bore (Figure 108<\/a>).\r\n\r\n<\/div>\r\n
\"\"\r\n
\r\n
\r\n
\r\n\r\nFigure 108\u00a0\u2011\u00a0<\/strong>Slug\u2011test methods used to raise and lower the water level in an unpumped well. This example shows an unconfined aquifer, however, the methods applied to a confined aquifer would be the same. The black rectangles represent solid slugs of a known volume. The red rectangles are transducers suspended in the well. H0<\/sub><\/em> is the maximum change in water level measured from the static at the initiation of the test.<\/span>\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n
\r\n
a) A slug\u2011in or falling\u2011head slug test. The solid cylinder is dropped into the well water so that it is totally submerged (second image). The water level instantly rises.<\/blockquote>\r\n<\/div>\r\n
\r\n
b) A slug out or rising\u2011head slug test. The solid slug is placed in the well and the water level comes back to equilibrium (static water level). The slug is then rapidly removed from the well and water level data collected.<\/blockquote>\r\n<\/div>\r\n
\r\n
c) A pneumatic rising\u2011head slug test where the water level is initially depressed by an increase in air pressure in the closed well bore. Air is pumped into the well until the water level is depressed to the desired level. At that time a valve to the air\u2011tight cap is closed. The air pressure in the casing is read by a pressure gauge or a second transducer installed at the location of the pressure gauge. The test is initiated when the airline is detached, and the valve is opened allowing the air to escape.<\/blockquote>\r\n<\/div>\r\n
\r\n\r\nIf a falling\u2011head slug test is desired, the slug is suspended on a cord and dropped into the well until it is totally submerged. This displaces a known volume of water resulting in a water\u2011level rise. Based on the volume of water displaced, the maximum height to which the water will rise can be computed and checked against the observed maximum level. The decline in water level over time is then recorded. Using a solid slug, a rising\u2011head test can be accomplished by lowering a solid slug on a cord into the well, allowing it to totally submerge, and waiting for the water level to stabilize. Once the water level in the well has returned to equilibrium (static conditions) the slug can be yanked either totally out of the well or high enough that the entire slug is above the static water level. This results in a rapid lowering of the water level, a slug\u2011out test. The rising water level is recorded over time.\r\n\r\n<\/div>\r\n
\r\n\r\nIf the water level in the well bore is close to the surface, a continuous solid rod can be used as a slug when inserted into the well a known distance. The volume of solid rod that is submerged then removed, or the volume the solid rod inserted below the water level is required to compute H0<\/sub><\/em> values as the test is initiated.\r\n\r\n<\/div>\r\n
\r\n\r\nA bailer can be used to lower the water level in a test well. One method uses the bailer to remove a single volume of water. The bailer is lowered into the well until it is totally submerged. Water levels in the well are allowed to return to equilibrium. Then at the start of the test, the bailer full of water is yanked from the well. This removes a volume of water causing a rapid fall in the water level. The rising water levels over time are monitored. It is best to totally remove the bailer from the well to prevent water from leaking back into the well. A second method is referred to as a bail\u2011down method, where the bailer is used to extract water multiple times to lower the water level. Once the water level has reached the desired depth, bailing ceases and water level recovery is recorded. The bail\u2011down method is only useful in lower hydraulic conductivity material. This method is appropriate when storage properties are not being estimated as it assumes the non\u2011instantaneous water level decline allows estimates for K assuming a semi steady\u2011state condition. Neither of the bailer methods are likely to be used when the water in the well bore is contaminated, as special precautions are often required to dispose of the contaminated bailed water. To prevent water\u2011quality impacts, when a solid slug is used to conduct a test in a contaminated well the slug needs to be decontaminated before it is used to conduct tests in other wells.\r\n\r\n<\/div>\r\n
\r\n\r\nA third method involves the use of compressed air to create a rising\u2011head slug test (Figure 108<\/a>c). An increase in the well\u2011bore air pressure is used to lower the water level in the well and hold it at a given depth below the static water level. The well is fitted with an airtight cap, a pressure gauge, and an air line with a valve. The pressure gauge can also be replaced by a second transducer that senses air pressure in the well bore. A small compressor or hand pump is used to increase the air pressure between the water surface and the well cap. The pressure gauge records the pressure in the well bore relative to atmospheric pressure. The pressure readings can be converted to feet (or meters) of water level change. Once the increased pressure level is established in the well bore the valve is closed and the compressed air line removed. When the test starts the airline valve is opened allowing the over\u2011pressurized well\u2011bore air to escape. The depressed water level begins to recover because the well bore is at atmospheric pressure. The submerged transducer measures the change in the overlying water level during recovery.\r\n\r\n<\/div>\r\n
\r\n\r\nFinally, a volume of water can be dumped into the well bore as quickly as possible to create a rise in the water level. This is challenging if large volumes of water are needed to make a measurable change in the water level (large casing diameters or high hydraulic conductivities). In some settings the addition of water for the test may negatively influence other site conditions, including changing the local water chemistry. This method may be appropriate in a river study where small\u2011diameter piezometers (a centimeter in diameter) are driven into the riverbed and slug tests are performed by adding a volume of river water to the wells (e.g., Woessner, 2020). The physical addition of water to the well may not be acceptable if wells are in a contaminated groundwater system as the addition of a different quality water may impact future water\u2011quality\u2011analysis results.\r\n\r\n<\/div>\r\n<\/div>\r\n
\r\n

Recording Water\u2011Level Change<\/h3>\r\n
\r\n\r\nMethods for measuring water levels in wells are presented in Section 4 of this book. As with hydraulic testing using pumping wells, it is important to record early\u2011time water\u2011level changes. When tests are conducted by putting in or extracting slugs or bailers, measuring devices like float recorders are not appropriate because cables often get tangled. In addition, when wells recover quickly (seconds or a few minutes), steel tapes and even electric tapes often cannot be deployed rapidly enough to obtain enough early\u2011time data points for a robust analysis. The best method for obtaining water\u2011level data is a pressure transducer\/data recorder installed at a sufficient depth that the use of a solid slug will not disturb the instrument. If test water levels recover slowly, electric or steel tapes can be used to record levels.\r\n\r\n<\/div>\r\n
\r\n\r\nWhen beginning a test, the static water level must be recorded with a steel or electric tape to establish a reference point for the test measurements. Transducer records are linked with the static water level to convert pressure readings to water\u2011level changes. A micro barometer should be used during the test if closed transducers (i.e., absolute rather than relative transducers) are used in order to determine if atmospheric\u2011pressure adjustments to the test data are necessary. However, the test is typically completed within a short time interval (e.g., 10s of minutes) thus correction for barometric changes may not be necessary.\r\n\r\n<\/div>\r\n
\r\n\r\nWhen solid slugs are used, the initial maximum rise or fall of the head can also be computed based on the volume of the well casing per unit of length and the known volume of the slug. For example, if the water in a 10 cm diameter PVC well casing is perturbed by submerging a 5 cm diameter solid slug that is 30 cm long, the volume of water displaced by the slug would be given by the volume of a cylinder with the dimensions of the slug: volume = \u03c0(square of the radius of the slug)(height of the slug) = 3.14 (2.5 cm)2<\/sup> (30 cm) = 589 cm3<\/sup>. A well bore that is 10 cm in diameter contains 78.5 cm3<\/sup> of water in each centimeter of its length, i.e., (3.14(5 cm)2<\/sup>(1cm)=78.5 cm3<\/sup>). Thus, the water level will rise 1 cm for every 78.5 cm3<\/sup> added. So, the addition of the 589 cm3<\/sup> slug would result in a maximum rise of 589 cm3<\/sup>\/78.5 cm3<\/sup>\/1 cm = 7.5 cm. Thus, the initial measurement (H0<\/sub><\/em>) should be about 7.5\u00a0cm.\r\n\r\n<\/div>\r\n<\/div>\r\n
\r\n

Test Repeatability<\/h3>\r\n
\r\n\r\nSlug tests are relatively easy to perform. Butler (1998) suggests that multiple slug tests should be conducted at a well. He recommends a well should be slug tested at least three times and then the values averaged to produce a representative horizontal hydraulic conductivity value. In some cases, the process of performing the slug test may also result in some well development (surging the fines from the perforated interval) or formation plugging (moving more fines to the perforated interval). If test values during multiple testing are changing consistently (sequentially increasing or decreasing), additional well development may be needed before values are considered reliable and reproduceable. This issue can be checked by computing the maximum measured head change for each test; the normalized head change should be \u2265 0.9 for each test, indicating that formational changes are not occurring during the test (Butler, 1998).\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>","rendered":"
\n
\n
\n

Slug tests are almost always conducted using a single well. The test method requires the water level in the borehole to rapidly rise or fall (Figure 102<\/a>). It is assumed for most slug tests that the water level in the well instantaneously reaches a new position (no time is lost) at time = 0. However, some testing methods allow the change in water level to occur over a period, when a brief pumping interval is followed by a long water\u2011level recovery time. These tests can be analyzed if initiation of the head data collection is started immediately after the maximum head change is obtained. An initial static water\u2011level measurement is needed to establish the pretest condition for all tests. When extensive pumping precedes data collection, time\u2011drawdown data should be collected and a recovery\u2011analysis method applied.<\/p>\n<\/div>\n

\n

Conducting a slug test requires careful planning including obtaining information on well construction and site conditions. The Kansas Geological Survey, U.S.A., is known for its research on the application of slug\u2011test methods (Butler, 1998). Their recommendations for conducting a successful slug test are incorporated in the following material.<\/p>\n<\/div>\n

\n

Assessing the Hydrogeologic Setting and Well Construction<\/h3>\n
\n

A site conceptual model is needed to assess the nature of the formations being tested. Are the formations aquifers or aquitards (Woessner & Poeter, 2020)? Are they confined or unconfined? Do the wells fully penetrate the formations? Could boundary conditions impact the results? Each of these questions requires resolution prior to conducting tests and analyzing results.<\/p>\n<\/div>\n

\n

Water supply wells, observation wells, or open boreholes can be used to conduct slug tests. Designing a slug test requires knowledge of the length and location of the borehole and perforated interval or length of well screen penetrating the formation, the presence of a gravel pack, and the diameter and lengths of the blank well casings (Figure 105<\/a>).<\/p>\n<\/div>\n

\"\"<\/p>\n
\n
\n
\n

Figure 105 \u2011<\/strong> General well configurations used for slug testing in confined and unconfined aquifers. The horizontal lines and vertical dashes represent perforated casing intervals (screens). db<\/sub><\/em> is the diameter of the cased well bore and dp<\/sub><\/em> is the diameter of the perforated casing. When a gravel pack is included, the effective diameter includes the combined perforated interval and the gravel\u2011pack diameters. The length of the perforated interval (L<\/em>) and its position in the aquifer can vary. Wells can be fully or partially penetrating (as shown). The dark gray shading represents grout placed around the blank well casing (e.g., cement or bentonite).<\/span><\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n

\n

a) A fully penetrating well in a confined aquifer. The casing diameter and screen diameter are the same.<\/p><\/blockquote>\n<\/div>\n

\n

b) A partially penetrating confined aquifer well with a larger well casing diameter than the screen diameter.<\/p><\/blockquote>\n<\/div>\n

\n

c) A partially penetrating confined aquifer well with a gravel pack around the screened interval. The effective perforated interval diameter includes the screen and surrounding gravel pack.<\/p><\/blockquote>\n<\/div>\n

\n

d) A fully penetrating well in an unconfined aquifer where the dashed yellow line indicates the well installation is completed in a borehole or driven to depth without fill around the screen or casing and the formation is allowed to collapse around the well.<\/p><\/blockquote>\n<\/div>\n

\n

e) A partially penetrating well in an unconfined aquifer where the dashed yellow line indicates the well installation is completed in a borehole or driven to depth without fill around the screen or casing and the formation is allowed to collapse around the well.<\/p><\/blockquote>\n<\/div>\n

\n

f) A partially penetrating well in an unconfined aquifer where a larger borehole is drilled and then an assembled casing and screen are placed in the borehole. The bore hole outside of the screen is filled with a filter pack (gravel pack) of coarse material (Sterrett, 2007) then the casing interval is grouted. When analyzing the test data, the perforated casing diameter typically includes the gravel pack, effective diameter.<\/p><\/blockquote>\n<\/div>\n

\n

g) A partially penetrating well in an unconfined aquifer where the screened interval is a smaller diameter than the well casing. After the borehole was drilled to the screen depth the casing was set and grouted, then a slightly smaller diameter borehole is advanced, and a screen installed.<\/p><\/blockquote>\n<\/div>\n

\n

h) A partially penetrating well in an unconfined aquifer with a smaller casing diameter than the well screen. The large borehole is drilled to depth and then the casing and screen were set; then the borehole is grouted.<\/p><\/blockquote>\n<\/div>\n<\/div>\n

\n

Special Considerations for Water Table Systems<\/h3>\n
\n

Slug tests can easily be conducted in water\u2011bearing units that are confined. They can also be conducted when a water table is present, and the perforated interval is below the water table (Figure 105<\/a>). However, if the perforated interval extends above the water table, only slug\u2011out or rising\u2011head slug tests should be performed (Figure 106<\/a>). In this setting, falling\u2011head tests are avoided because the water level at the start of the test after the introduction of the slug would be higher than the static water level extending into the vadose zone. When this occurs the water level falls more rapidly than it would in the saturated portion of the aquifer because it flows into the partially saturated zone above the water table. Analysis of the resulting data produces a hydraulic conductivity value that is higher than the value representative of the saturated unit (Todd & Mays, 2005).<\/p>\n<\/div>\n

\n

\"\"<\/p>\n<\/div>\n

\n
\n

Figure 106 \u2011<\/strong> Water\u2011level in a well in an unconfined aquifer after introduction of a solid slug at the start of a slug\u2011in test in a well with the screen completely below the water table and a well with a screen that extends about the water table. a) When the perforated interval is completed below the water table, slug\u2011in or slug\u2011out tests can be successfully completed. b) If the screened interval extends above the water table only slug\u2011out tests should be conducted because a slug\u2011in test will overestimate the saturated horizontal hydraulic conductivity due to some of the water flowing into the unsaturated zone. c) Enlargement of (b) showing the factors affecting the rate of the head decline when a slug\u2011in test is performed in a well with a screen extending above the water table. During the water level decline a portion of the water enters the unsaturated zone and the resulting rate of water level decline is greater than it would be if only the saturated portion of the aquifer was controlling the rate of decline.<\/span><\/p>\n<\/div>\n<\/div>\n<\/div>\n

\n

Free Exchange of Water with the Formation<\/h3>\n
\n

Another criterion for conducting a successful slug test is that the perforated or screened interval is hydrologically well connected to the formation being tested. This means the number, size, and configuration of the perforations should not inhibit the exchange of water between the formation and the well. If the configuration of perforations (too small or too few) inhibits the movement of water to and from the formation, the test may reflect the “hydraulic conductivity” of the well hardware instead of the geologic material (Figure 107<\/a>).<\/p>\n<\/div>\n

\"\"<\/p>\n
\n
\n
\n

Figure 107 \u2011<\/strong> Schematic of a falling\u2011head slug test in an unpumped well penetrating an unconfined aquifer.<\/span><\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n

\n

a) Well design with a perforated interval (black portion of the casing with white perforations) that is not well connected to the formation. Head recovery will be controlled by the poorly designed perforated interval not the formational properties.<\/p><\/blockquote>\n<\/div>\n

\n

b) Perforated interval that is fully connected to the formation. Water in the well bore freely communicates with the formation.<\/p><\/blockquote>\n<\/div>\n

\n

c) Perforated interval adjacent to a portion of the borehole damaged during drilling such that finer material has been left behind. The damaged area will control the rate of change of the borehole water level, this is called a skin effect.<\/p><\/blockquote>\n<\/div>\n

\n

d) Insert showing drilling damage to the formation producing a low permeability zone. The smaller arrows indicate the connection to the surrounding aquifer is poor.<\/p><\/blockquote>\n<\/div>\n

\n

e) An example of a zone around the perforated interval of a well where the permeability has been enhanced. This can occur by the addition of a gravel pack (coarse material placed outside of the perforated casing) or the removal of natural fines in the formation by well development. When a gravel pack is present the effective radius, de<\/sub><\/em>, of the perforated interval includes the gravel pack.<\/p><\/blockquote>\n<\/div>\n

\n

Often the formational material opposite the perforated interval becomes less permeable during the drilling process (Figure 107<\/a>), especially in fine\u2011grained material or when drilling fluids are used in well construction. In some cases, the process of drilling and installing the well results in finer material entering the formation or the clays present are smeared to the borehole wall. This is referred to as forming a skin of lower\u2011permeability material. This skin effect or borehole skin (Fetter, 2001) restricts the exchange of water between the well bore and formation. Therefore, hydraulic\u2011conductivity values computed from slug\u2011test results will underestimate the properties of the formation. Well\u2011development procedures like surging, bailing, and over\u2011pumping can increase the permeability of the skin and should be implemented during well construction (e.g., Sterrett, 2007). In some settings, too much development may not only remove the fines that entered a formational material during drilling but also remove additional natural fines in the formation. This creates a zone of higher\u2011than\u2011normal hydraulic conductivity adjacent to the screen and if it extends a significant distance into the formation may impact test data. In this case the well may recover more rapidly because of the presence of this high hydraulic conductivity zone. As a result, hydraulic\u2011conductivity values will be higher than the formational value. Finally, in many observation or water\u2011supply wells, gravel pack is added to enhance the permeability around the perforated interval to assure perforations are not plugged with formational fines. When this design is used, the effective diameter of the perforated interval needs to include the gravel pack (Figure 107<\/a>e).<\/p>\n<\/div>\n<\/div>\n

\n

Raising and Lowering the Water Level<\/h3>\n
\n

Typically, instantaneously inducing a change in water level in the well being tested is accomplished by submerging or removing a solid slug from the water\u2011filled well bore. These actions result in a rise or fall of the borehole water level. The degree of change is a function of the volume of the slug and volume of water per length of the well bore (Figure 108<\/a>).<\/p>\n<\/div>\n

\"\"<\/p>\n
\n
\n
\n

Figure 108\u00a0\u2011\u00a0<\/strong>Slug\u2011test methods used to raise and lower the water level in an unpumped well. This example shows an unconfined aquifer, however, the methods applied to a confined aquifer would be the same. The black rectangles represent solid slugs of a known volume. The red rectangles are transducers suspended in the well. H0<\/sub><\/em> is the maximum change in water level measured from the static at the initiation of the test.<\/span><\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n

\n

a) A slug\u2011in or falling\u2011head slug test. The solid cylinder is dropped into the well water so that it is totally submerged (second image). The water level instantly rises.<\/p><\/blockquote>\n<\/div>\n

\n

b) A slug out or rising\u2011head slug test. The solid slug is placed in the well and the water level comes back to equilibrium (static water level). The slug is then rapidly removed from the well and water level data collected.<\/p><\/blockquote>\n<\/div>\n

\n

c) A pneumatic rising\u2011head slug test where the water level is initially depressed by an increase in air pressure in the closed well bore. Air is pumped into the well until the water level is depressed to the desired level. At that time a valve to the air\u2011tight cap is closed. The air pressure in the casing is read by a pressure gauge or a second transducer installed at the location of the pressure gauge. The test is initiated when the airline is detached, and the valve is opened allowing the air to escape.<\/p><\/blockquote>\n<\/div>\n

\n

If a falling\u2011head slug test is desired, the slug is suspended on a cord and dropped into the well until it is totally submerged. This displaces a known volume of water resulting in a water\u2011level rise. Based on the volume of water displaced, the maximum height to which the water will rise can be computed and checked against the observed maximum level. The decline in water level over time is then recorded. Using a solid slug, a rising\u2011head test can be accomplished by lowering a solid slug on a cord into the well, allowing it to totally submerge, and waiting for the water level to stabilize. Once the water level in the well has returned to equilibrium (static conditions) the slug can be yanked either totally out of the well or high enough that the entire slug is above the static water level. This results in a rapid lowering of the water level, a slug\u2011out test. The rising water level is recorded over time.<\/p>\n<\/div>\n

\n

If the water level in the well bore is close to the surface, a continuous solid rod can be used as a slug when inserted into the well a known distance. The volume of solid rod that is submerged then removed, or the volume the solid rod inserted below the water level is required to compute H0<\/sub><\/em> values as the test is initiated.<\/p>\n<\/div>\n

\n

A bailer can be used to lower the water level in a test well. One method uses the bailer to remove a single volume of water. The bailer is lowered into the well until it is totally submerged. Water levels in the well are allowed to return to equilibrium. Then at the start of the test, the bailer full of water is yanked from the well. This removes a volume of water causing a rapid fall in the water level. The rising water levels over time are monitored. It is best to totally remove the bailer from the well to prevent water from leaking back into the well. A second method is referred to as a bail\u2011down method, where the bailer is used to extract water multiple times to lower the water level. Once the water level has reached the desired depth, bailing ceases and water level recovery is recorded. The bail\u2011down method is only useful in lower hydraulic conductivity material. This method is appropriate when storage properties are not being estimated as it assumes the non\u2011instantaneous water level decline allows estimates for K assuming a semi steady\u2011state condition. Neither of the bailer methods are likely to be used when the water in the well bore is contaminated, as special precautions are often required to dispose of the contaminated bailed water. To prevent water\u2011quality impacts, when a solid slug is used to conduct a test in a contaminated well the slug needs to be decontaminated before it is used to conduct tests in other wells.<\/p>\n<\/div>\n

\n

A third method involves the use of compressed air to create a rising\u2011head slug test (Figure 108<\/a>c). An increase in the well\u2011bore air pressure is used to lower the water level in the well and hold it at a given depth below the static water level. The well is fitted with an airtight cap, a pressure gauge, and an air line with a valve. The pressure gauge can also be replaced by a second transducer that senses air pressure in the well bore. A small compressor or hand pump is used to increase the air pressure between the water surface and the well cap. The pressure gauge records the pressure in the well bore relative to atmospheric pressure. The pressure readings can be converted to feet (or meters) of water level change. Once the increased pressure level is established in the well bore the valve is closed and the compressed air line removed. When the test starts the airline valve is opened allowing the over\u2011pressurized well\u2011bore air to escape. The depressed water level begins to recover because the well bore is at atmospheric pressure. The submerged transducer measures the change in the overlying water level during recovery.<\/p>\n<\/div>\n

\n

Finally, a volume of water can be dumped into the well bore as quickly as possible to create a rise in the water level. This is challenging if large volumes of water are needed to make a measurable change in the water level (large casing diameters or high hydraulic conductivities). In some settings the addition of water for the test may negatively influence other site conditions, including changing the local water chemistry. This method may be appropriate in a river study where small\u2011diameter piezometers (a centimeter in diameter) are driven into the riverbed and slug tests are performed by adding a volume of river water to the wells (e.g., Woessner, 2020). The physical addition of water to the well may not be acceptable if wells are in a contaminated groundwater system as the addition of a different quality water may impact future water\u2011quality\u2011analysis results.<\/p>\n<\/div>\n<\/div>\n

\n

Recording Water\u2011Level Change<\/h3>\n
\n

Methods for measuring water levels in wells are presented in Section 4 of this book. As with hydraulic testing using pumping wells, it is important to record early\u2011time water\u2011level changes. When tests are conducted by putting in or extracting slugs or bailers, measuring devices like float recorders are not appropriate because cables often get tangled. In addition, when wells recover quickly (seconds or a few minutes), steel tapes and even electric tapes often cannot be deployed rapidly enough to obtain enough early\u2011time data points for a robust analysis. The best method for obtaining water\u2011level data is a pressure transducer\/data recorder installed at a sufficient depth that the use of a solid slug will not disturb the instrument. If test water levels recover slowly, electric or steel tapes can be used to record levels.<\/p>\n<\/div>\n

\n

When beginning a test, the static water level must be recorded with a steel or electric tape to establish a reference point for the test measurements. Transducer records are linked with the static water level to convert pressure readings to water\u2011level changes. A micro barometer should be used during the test if closed transducers (i.e., absolute rather than relative transducers) are used in order to determine if atmospheric\u2011pressure adjustments to the test data are necessary. However, the test is typically completed within a short time interval (e.g., 10s of minutes) thus correction for barometric changes may not be necessary.<\/p>\n<\/div>\n

\n

When solid slugs are used, the initial maximum rise or fall of the head can also be computed based on the volume of the well casing per unit of length and the known volume of the slug. For example, if the water in a 10 cm diameter PVC well casing is perturbed by submerging a 5 cm diameter solid slug that is 30 cm long, the volume of water displaced by the slug would be given by the volume of a cylinder with the dimensions of the slug: volume = \u03c0(square of the radius of the slug)(height of the slug) = 3.14 (2.5 cm)2<\/sup> (30 cm) = 589 cm3<\/sup>. A well bore that is 10 cm in diameter contains 78.5 cm3<\/sup> of water in each centimeter of its length, i.e., (3.14(5 cm)2<\/sup>(1cm)=78.5 cm3<\/sup>). Thus, the water level will rise 1 cm for every 78.5 cm3<\/sup> added. So, the addition of the 589 cm3<\/sup> slug would result in a maximum rise of 589 cm3<\/sup>\/78.5 cm3<\/sup>\/1 cm = 7.5 cm. Thus, the initial measurement (H0<\/sub><\/em>) should be about 7.5\u00a0cm.<\/p>\n<\/div>\n<\/div>\n

\n

Test Repeatability<\/h3>\n
\n

Slug tests are relatively easy to perform. Butler (1998) suggests that multiple slug tests should be conducted at a well. He recommends a well should be slug tested at least three times and then the values averaged to produce a representative horizontal hydraulic conductivity value. In some cases, the process of performing the slug test may also result in some well development (surging the fines from the perforated interval) or formation plugging (moving more fines to the perforated interval). If test values during multiple testing are changing consistently (sequentially increasing or decreasing), additional well development may be needed before values are considered reliable and reproduceable. This issue can be checked by computing the maximum measured head change for each test; the normalized head change should be \u2265 0.9 for each test, indicating that formational changes are not occurring during the test (Butler, 1998).<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n","protected":false},"author":8,"menu_order":60,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-1756","chapter","type-chapter","status-publish","hentry"],"part":589,"_links":{"self":[{"href":"https:\/\/books.gw-project.org\/an-introduction-to-hydraulic-testing-in-hydrogeology\/wp-json\/pressbooks\/v2\/chapters\/1756","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/books.gw-project.org\/an-introduction-to-hydraulic-testing-in-hydrogeology\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/books.gw-project.org\/an-introduction-to-hydraulic-testing-in-hydrogeology\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/books.gw-project.org\/an-introduction-to-hydraulic-testing-in-hydrogeology\/wp-json\/wp\/v2\/users\/8"}],"version-history":[{"count":4,"href":"https:\/\/books.gw-project.org\/an-introduction-to-hydraulic-testing-in-hydrogeology\/wp-json\/pressbooks\/v2\/chapters\/1756\/revisions"}],"predecessor-version":[{"id":3546,"href":"https:\/\/books.gw-project.org\/an-introduction-to-hydraulic-testing-in-hydrogeology\/wp-json\/pressbooks\/v2\/chapters\/1756\/revisions\/3546"}],"part":[{"href":"https:\/\/books.gw-project.org\/an-introduction-to-hydraulic-testing-in-hydrogeology\/wp-json\/pressbooks\/v2\/parts\/589"}],"metadata":[{"href":"https:\/\/books.gw-project.org\/an-introduction-to-hydraulic-testing-in-hydrogeology\/wp-json\/pressbooks\/v2\/chapters\/1756\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/books.gw-project.org\/an-introduction-to-hydraulic-testing-in-hydrogeology\/wp-json\/wp\/v2\/media?parent=1756"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/books.gw-project.org\/an-introduction-to-hydraulic-testing-in-hydrogeology\/wp-json\/pressbooks\/v2\/chapter-type?post=1756"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/books.gw-project.org\/an-introduction-to-hydraulic-testing-in-hydrogeology\/wp-json\/wp\/v2\/contributor?post=1756"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/books.gw-project.org\/an-introduction-to-hydraulic-testing-in-hydrogeology\/wp-json\/wp\/v2\/license?post=1756"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}