Performance and Analysis of Squifer Slug Testing and Pumping Tests Policy 20070531

Performance and Analysis of
Aquifer Slug Tests and Pumping Tests Policy
May 31, 2007
Table of Contents
page
Introduction .....................................................................................................................................2
(1) Purpose of policy...................................................................................................................2
(2) Basis for technical approach .................................................................................................2
(3) The purpose of aquifer testing...............................................................................................2
(4) The method chosen by the investigator .................................................................................2
Section I: Slug Tests ......................................................................................................................4
(1) Basic Testing Requirements:.................................................................................................4
(a) Before beginning the test ............................................................................................4
(b) Frequency of Water Level Measurements ..................................................................4
(c) Accuracy of Water Level Measurements....................................................................5
(d) Duration of the Slug Test ............................................................................................5
(2) Method of Analysis:..............................................................................................................5
Section II: Pumping Tests ..............................................................................................................6
(1) Basic Testing Requirements:.................................................................................................6
(a) Before beginning the test ............................................................................................6
(b) Frequency of Water Level Measurements and Pumping Rate Measurements ...........7
(c) Accuracy of Water Level Measurements....................................................................7
(d) Maintaining a Constant Pumping Rate .......................................................................7
(e) Verification of the Pumping Rate ...............................................................................8
(f) Duration of the Pumping Test .....................................................................................8
(g) Other information........................................................................................................8
(2) Method of Analysis:..............................................................................................................8
Section III: Reporting Aquifer Test Results ................................................................................10
References .....................................................................................................................................11

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Introduction
A determination of hydraulic conductivity, transmissivity, and/or specific yield is
required under several different types of permit applications and subsurface investigations
associated with groundwater contamination incidents. This policy provides guidance regarding
the methods used to determine these parameters for projects under the Division of Water
Quality’s (Division) regulatory purview. .
(1) Purpose of policy
The purpose of this policy is to: (a) provide guidance to investigators in selecting and applying
an appropriate method (slug test or pumping test) for assessing an aquifer’s hydraulic
characteristics; (b) alert investigators to the most common oversights associated with performing
and analyzing these test methods; and (c) provide a guidance to regulators to use in evaluating
the adequacy of test results submitted by investigators.
(2) Basis for technical approach
This policy is based on methods and techniques that are generally accepted by educators and
consultants. These include the following sources:
The Bouwer and Rice Slug Test – An Update, by Herman Bouwer, Ground Water Journal, May
– June 1989.
Groundwater and Wells, 2nd Edition, by Fletcher G. Driscoll, Johnson Division, St. Paul, MN,
1986.
Analysis and Evaluation of Pumping Test Data, 2nd Edition, by G. P. Kruseman and N. A. de
Ridder, International Institute for Land Reclamation and Improvement, Wageningen, The
Netherlands, 1990.
(3) The purpose of aquifer testing
Aquifer testing is performed to: a) estimate the hydraulic conductivity, transmissivity, and
specific yield of an unconfined aquifer or storativity of a confined aquifer, to b) quantify the
vertical leakage from one aquifer into another and/or c) to assess the effects of boundaries such
as surface water boundaries or low permeability features such as intrusive dikes. Aquifer testing
results help provide the basis for: hydrogeologic conceptual models; estimation of sustainable
groundwater withdrawal rates; capture zone analyses; and estimation of groundwater
contaminant migration rates. Vertical leakage is assessed by pumping one aquifer and observing
the concurrent hydraulic response of the other aquifer. The Division may require the applicant to
quantify the items mentioned above in situations where there is potential to (a) harm
groundwater users, (b) degrade the existing quality of the groundwater in question, based on its
current classification (see 15A NCAC 02L.0200), or (c) degrade surface water quality.
(4) The method chosen by the investigator
The method used to ascertain representative values of hydraulic conductivity, transmissivity,
specific yield, and other aquifer parameters should be chosen based on site-specific conditions
and the intended application of the information.
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Slug tests actually test the formation immediately adjacent to the well which, in the vast majority
of cases, will not be representative of the entire aquifer, and should only be used in those
situations where a high degree of numerical and volumetrically representative accuracy is not
needed. The analyses of slug tests usually produce approximate values of hydraulic conductivity,
and do not produce values of storage coefficients. Since slug tests only provide hydraulic
conductivity estimates for a very small portion of the formation of interest, where practical,
multiple slug tests should be carried out in different representative wells in order to determine a
range of estimated hydraulic conductivities. Slug tests should never be exclusively relied upon
when designing a groundwater recovery system, or in any other situation where a more
reliable estimate of the effects of long-term pumping (including the establishment of a
hydraulic zone of capture) is required.
Pumping tests performed on aquifers produce values of hydraulic conductivity, transmissivity,
storativity or specific yield that are far more accurate and volumetrically representative of the
aquifer. These tests typically stress the aquifer and draw water from a distance, furnishing
information that defines and demonstrates the aquifer’s ability to store and transmit water.
Pumping tests should have two or more observation wells to allow the collection of data that is
not affected by hydraulic losses near the pumping well, and to allow for a more accurate
determination of aquifer characteristics.
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Section I: Slug Tests
In general, the Division considers “Slug out” tests to be more representative of actual aquifer
conditions. However, “Slug in” tests are acceptable in cases where the well screen and sand pack
interval are fully submerged. “Slug out” tests create a dewatering of the saturated zone and are
acceptable to the Division for obtaining rough estimates of hydraulic conductivity. The rapidly
changing level of the water in the well, and the time at which each level occurred during the
performance of the test, must be measured accurately. This is best performed with a
transducer/datalogger arrangement, which can be programmed to record the time and water level
data at appropriate intervals. Formations that have high hydraulic conductivities may exceed the
limitations of the method and, in the case of most traditional methods of analysis, will produce
analytical results that can only be said to be above a certain value. However, there are several
more recently developed methods of analysis that have been developed to better handle high-
conductivity scenarios. In high conductivity situations, more accurate values of aquifer
parameters can be obtained by performing a pumping test.
(1) Basic Testing Requirements:
(a) Before beginning the test
Select a well-constructed, well-maintained monitor well for the test. Although 2-inch
diameter wells are most often used, wells having larger diameters may also be used. The
age of the well, the depth of the well, borehole diameter, the length of the well screen and
sand/gravel filter pack, the size of the screen slots, the size of the sand/gravel filter
material surrounding the well screen, the method(s) used to develop the well and its
results, and the drilling method used to install the well should all be known to the
investigator. Verify that a good hydraulic connection exists between the well’s interior
and the aquifer formation by removing a few bailers full of water from the well and
measuring the amount of time required for the water level to return to static. If the water
level does not return to static within a reasonable amount of time with respect to the
expected hydraulic conductivity of the material screened, a good hydraulic connection
may not exist, and the well should be redeveloped prior to conducting the slug test. All
new wells used to perform slug test must be properly developed. The integrity of older
wells should also be evaluated to ensure it has not been compromised. Well screens can
become clogged by silt or bacteria over time, and this could affect the accuracy of the
test.
(b) Frequency of Water Level Measurements
A pre-test static water level should be determined initially. The water level should be
measured (along with the elapsed time) as often as possible during the first two minutes
of the test. If a transducer/datalogger arrangement is used, the apparatus may be
programmed to measure the water level at very frequent intervals for the first two
minutes, and at less frequent intervals thereafter (logarithmic time sampling). If the
measurements are taken by hand using an electric tape, the measurements should be taken
as frequently as possible during the first two minutes of the test; then at intervals of ten
seconds for the third and fourth minutes; and at intervals of thirty seconds from the fifth
minute until the conclusion of the test (10 minutes of elapsed time or 95% water level
recovery, whichever occurs first).
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(c) Accuracy of Water Level Measurements
Water level measurements must be accurate to within one eighth of an inch or one
hundredth of a foot.
(d) Duration of the Slug Test
The slug test should be performed for no less than ten minutes, or until such time as the
water level in the test well recovers 95% of its original pre-test level, whichever occurs
first.
(2) Method of Analysis:
The data from the slug test should be analyzed by applying an appropriate and recognized
method of slug test analysis such as the Bouwer and Rice method. It should be
understood that the value of aquifer hydraulic conductivity produced by the analysis of
the test represents an estimate that may vary by as much as a full order of magnitude
from the true value. Knowledge of the saturated aquifer thickness needs to be
incorporated into the analytical methodology, because uncertainty about the aquifer
thickness will lead to additional error. A slug test should never be used as the sole
determinant of an aquifer’s hydraulic conductivity or transmissivity in situations where
an accurate estimate of these parameters is required, such as in the design of a “closed-
loop” system. Contact the Division to determine if the use of slug tests only is appropriate
for a particular investigation or evaluation. Slug tests on multiple monitor wells may be
used to estimate the variability of these parameters across a site.
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Section II: Pumping Tests
Aquifer pumping tests involve the pumping of a well at a constant, sustained rate, or by
maintaining a constant pumping discharge rate in order to hydraulically stress an aquifer and
study its behavior under such conditions. Vertical leakage between aquifers can be assessed by
pumping from one aquifer and observing the concurrent hydraulic response of the other aquifer.
Pumping tests are best performed on a fully penetrating, fully developed well with water-level
measurements frequently taken at the pumping well and at two or more observation wells. The
observation wells should be situated on different sides of the pumping well and at different
distances from it. Further information regarding the appropriate number and spacing of
observation wells for different aquifer settings in references such as Kruseman and deRitter
(1990) and Driscoll (1989).
(1) Basic Testing Requirements:
(a) Before beginning the test
The aquifer test(s) should be well planned from the start. The locations and depths of the
pumping well and observation wells should be based on a conceptual model of the site
hydrogeology and the hydrostratagraphic units to be tested. For example, if the aquifer
formation has a potential to exhibit horizontal anisotropy, the observation wells should be
placed such that they measure water-level response at positions parallel and
perpendicular to the direction of suspected highest horizontal hydraulic conductivity with
respect to the pumping well. If the underlying leakage to a semi-confined aquifer is to be
measured, then observation wells should be placed and sealed into the deeper aquifer to
test the response of the semi-confined aquifer. Additional information with regard to
placement of pumping and observation wells for various hydrogeologic conditions can be
found in Kruseman and deRitter (1990) and Driscoll (1989).
Properly disposing of the pumped water from the aquifer test is an important
consideration of aquifer test planning. If the ground water to be pumped is contaminated
above the appropriate groundwater standards, then in most cases the pumped water must
be treated and disposed of properly after securing the appropriate permit. If the
groundwater to be pumped is not contaminated above the groundwater standards, then the
pumped water should be piped at least 250 feet away and hydraulically downgradient
from the pumping well, or greater if there is a likelihood of the recharging water affecting
the aquifer test results. Alternatively, and particularly when high discharge rates are
planned, an evaluation of the expected cone of influence at the anticipated conclusion of
the test should be made, and the discharge distance adjusted to be beyond the radius of
the maximum cone of influence.
The age of the pumping well, the depth of the well, borehole diameter, the length of the
well screen and sand/gravel filter pack, subsurface soils, the size of the screen slots, the
size of the sand/gravel filter material surrounding the well screen, the method(s) used to
develop the well and its results, and the drilling method used to install the well should all
be known to the investigator. Verify that a good hydraulic connection exists between the
well’s interior and the aquifer formation by removing a few bailers full of water from the
well and measuring the amount of time required for the water level to return to static. If
the water level does not return to static within a reasonable amount of time with respect
to the expected hydraulic conductivity of the material screened, a good hydraulic
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connection may not exist, and the well should be redeveloped prior to conducting the
pumping test. Well screens can become clogged by silt or bacteria over time, and this
could affect the accuracy of the test. It is essential that the pumping well and all
observation wells be properly developed before beginning the pumping test.
More observation wells are recommended for those aquifers that have heterogeneous
geologic and hydraulic characteristics. One of the wells (a “background” well) should be
situated beyond the hydraulic influence of the pumping well and should serve to measure
natural changes in the water level of the aquifer, which may take place during the
pumping test.
Periodic water level measurements should be taken in the pumping well and in the
observation wells over a 24 hour period before the test begins in order to document
ongoing recharge, discharge, or atmospheric or tidal fluctuations affecting the aquifer.
(b) Frequency of Water Level Measurements and Pumping Rate Measurements
Static water levels should be collected from all wells initially, and immediately prior to
the test. Water level measurements within the pumping well should be taken initially and
then at least every ten seconds for the first minute; every twenty seconds from two
minutes to five minutes; every thirty seconds from five minutes to ten minutes; every
minute from ten minutes to twenty minutes; every five minutes from twenty minutes to
an hour; and every fifteen minutes thereafter. Water level measurements in observation
wells should be taken initially and then at intervals determined by factors such as the type
of aquifer (confined or unconfined) being tested, the distance from the observation well
to the pumping well, the pumping rate, and the results of the “pre-test” (see below).
Further information regarding water level measurements can be found in references such
as Kruseman and deRitter (1990) and Driscoll (1989). The flow rate should be monitored
continuously at the beginning of the test and the pump speed or orifice opening adjusted
so as to keep the flowrate constant for a constant discharge rate pumping test. The pump
will tend to slow down as it is forced to work harder, lifting the water from increasingly
deeper levels as the test progresses and the water level in the pumping well falls.
(c) Accuracy of Water Level Measurements
Water level measurements must be accurate to within one eighth of an inch or one
hundredth of a foot. Water level measurements taken inside the pumping well should be
protected from false readings caused by cascading water and other forms of turbulence.
(d) Maintaining a Constant Pumping Rate
The test well should be “pre-tested” prior to the actual test (this can be a formal step-
drawdown test or a more informal method) in order to verify the ability of the well to
furnish water, the ability of the pump to operate and to deliver the water, and the
appropriateness of the intended pumping rate. A pump should never be set to deliver its
maximum pumping rate at the beginning of a test, as the pumping rate will usually
decrease as the water level within the well falls and the pump is forced to raise the water
a greater vertical distance. A pump or its discharge line should be set to deliver up to 2/3
of the system capacity at the beginning of a test, and this can be done by either initially
decreasing the running speed (RPM), or by partially closing off the discharge line with an
in-line valve. In order to maintain a constant discharge rate, the pump’s running speed
will have to be increased, or the discharge control valve opened, as the water level in the
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well falls. The rate of discharge, as indicated by an orifice weir or in-line flow meter, will
have to be watched carefully during the first several minutes of the test, and the flow
control valve or pump speed may have to be adjusted almost continuously during this
time in order to maintain a constant discharge rate.
(e) Verification of the Pumping Rate
Regardless of the type of apparatus used to measure the rate of pumping, the pumping
rate should be periodically verified by another method of measurement. The three most
common methods of flow measurement are an in-line flow meter, an orifice weir, and the
use of a calibrated bucket and stopwatch.
(f) Duration of the Pumping Test
As a general rule, as a test proceeds, the hydraulic stress on the aquifer increases as a
progressively greater volume of the aquifer contributes to the well. The longer a test is
performed, the more representative the data will be. The “industry standard” for the
duration of a pumping test is 24 hours, and longer tests may be advisable for certain
situations, such as the testing of low-conductivity formations or gathering information for
the design of pump-and-treat groundwater remediation systems. Shorter duration tests
may be allowed for more transmissive aquifers; however, the Aquifer Protection Section
Central Office at [phone] should be contacted first for prior approval. Recovery
measurements which begin after the pump is turned off may be taken at the pumping well
and the observation wells to provide additional data for analysis and comparison. These
recovery measurements should be collected using the same type of scheme for frequency
of measurements described above, using the pump shutoff time as the initial time.
(g) Other information
If there is concern about the existing groundwater quality and/or contamination, then
groundwater samples should be collected during the pump test. Any questions regarding
this should be directed to the Aquifer Protection Section Central Office at (919) 733-
3221.
Other information that has been found to be useful in the analysis and interpretation of
pumping tests includes barometric pressure readings (essential if non-vented transducer
cables are used), the temperature of the discharged water, the air temperature and general
weather conditions during the test, and any natural or man-made events that could impact
the accuracy of the pumping test. These events include changes in the pumping rate of a
nearby pumping well; rainstorms; the passage of storm fronts; snowmelt events; abrupt
changes in air temperature; wind conditions; earthquakes; tidal events; and changes in
aquifer loadings (vehicle or railcar displacement). Careful notes should be taken during
the test that record the exact time at which any unusual phenomenon occurs.
(2) Method of Analysis:
Before the water level data from the pumping well and observation wells is analyzed, the data
should be corrected for the influence of factors such as: barometric pressure fluctuations when
testing confined or semi-confined aquifers; local aquifer loadings from railroad cars, etc.; tidal
influences in aquifers hydraulically connected to or otherwise influenced by ocean tides; partial
aquifer penetration of observation wells; turbulent hydraulic losses at or near the pumping well
(“well efficiency” losses); the hydraulic effects of recharge boundaries or barrier boundaries; and
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aquifer recharge from recent precipitation or surface application events. Pre-test water level
measurements taken in the pumping well and observation wells can be examined to assess the
potential influence of these factors.
In many cases, it is usually helpful to plot the drawdown contours in map-view at various times
throughout the test duration. These types of plots may help correctly interpret boundary effects
and anisotropic effects.
The pumping test analysis should be based on a recognized and appropriate method that is
appropriate and valid for the site conditions. It is not acceptable to find the best fit to a set of
field data and conclude, based on this fit alone, that the aquifer system actually is the model used
in the analysis. Other field data from boring logs, geological mapping, and location of
boundaries needs to be consistent with the analysis model chosen. Further information regarding
the applicability of methods of pumping test analysis can be found in references such as
Kruseman and deRitter (1990).
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Section III: Reporting Aquifer Test Results
Scaled maps showing the locations of all wells involved in the test should be provided. The maps
should be clear and legible, with each of the pumping well and each of the observation wells
being clearly labeled and identified. Any other locations where data was gathered during the test
(water supply wells, streams, etc.) should be included and labeled on the maps. Cross-sections
are considered to be very helpful. All aquifer test data and analyses should be clearly
documented, including appropriate aquifer response graphs. It may be appropriate to provide
map-view groundwater drawdown contours at various times of an aquifer pumping test. All
analysis methods should be clearly documented. If commercially available computer software is
used in the analyses, then complete printout sheets of the data, the analyses, and a list of all
analytical assumptions (aquifer type, extent, etc.) should be provided. The Aquifer Protection
Section may request that large data sets (spreadsheets) be provided in electronic form.
The construction details for the pumping well and all observation wells used during the test
should also be submitted. The aquifer test data should be included in the report. The data should
be comprehensive in that it include the identity of the well being monitored, the static water level
in the well, the water level measurements along with the (elapsed) time of each measurement,
and the unit of measurement used. Concurrent confirmatory pumping rate discharge readings,
taken at appropriate intervals, should also be included, as should be time(s) of any adjustments
made to the rate of discharge. Any corrections made to the water level data should be noted,
along with the reason(s) for those corrections. Post-test water level recovery data, when taken,
should also be included.
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References
Bouwer, Herman, The Bouwer and Rice Slug Test – An Update, Ground Water Magazine, May-
June 1989.
Driscoll, Fletcher G., Groundwater and Wells, 2ndEdition, Johnson Division, St. Paul, MN, 1986.
Kruseman, G. P. and de Ridder, N. A., Analysis and Evaluation of Pumping Test Data, 2nd
Edition, International Institute for Land Reclamation and Improvement, Wageningen, The
Netherlands, 1990.
Weight, Willis D. and Sonderegger, John L., Manual of Applied Field Hydrogeology, McGraw-
Hill, 2001.

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