In Reply Refer To:
Mail Stop 411
November 4, 2011
OFFICE OF GROUNDWATER TECHNICAL MEMORANDUM 2012.01
SUBJECT: Policy for evaluation of well integrity for water-level measurements—initial application to CBR network wells and test phase for all wells
Water-level measurements play an important role in tracking the status of a groundwater resource. In order to ensure that a water-level measurement adequately represents the hydraulic head in the groundwater system penetrated by the open interval of the well, it is necessary to assure that the well is in good hydraulic connection with the groundwater system and that this connection has not changed over time. Well integrity should be evaluated routinely for USGS groundwater-level observation wells that are part of a recurring measurement program. Well integrity also should be evaluated for wells measured regularly, but less frequently, such as wells in a synoptic or mass measurement program.
The process of evaluating well integrity differs depending on well construction, well access, frequency of measurement, and other factors. Depending on these factors, well integrity may be evaluated by analyzing water-level measurements, by physical tests, or by a combination of these techniques. The process of evaluating well integrity for water-level measurements described in this memo relies heavily on the use of basic hydrologic principles and proper documentation.
Purpose of this MemorandumThis memorandum outlines the requirement that all wells funded under the federal Collection of Basic Records (CBR) Program must undergo a routine evaluation of well integrity. In addition, the Office of Groundwater is considering policy options for evaluating well integrity for all USGS groundwater-level observation wells. USGS observation wells include wells measured by USGS personnel and those wells measured by observers. Other organizations that furnish data to the USGS should have similar methods and procedures as the USGS so as to ensure the data that are being published are from wells that are connected to the aquifer in which they are completed. Centers that publish data (including data served on the Internet) from wells measured by other organizations should encourage the agencies furnishing record to adopt the USGS well-integrity procedures or develop their own procedures. Implementation of the policy for CBR wells will allow Water Science Centers, the Water Science Field Team Groundwater Specialists, and the Office of Groundwater to evaluate the requirement for wider implementation for wells not in the CBR program.
Policy for wells in the CBR network
This policy is presented in terms of office- and field-based procedures. The office-based procedures must be followed for all CBR wells and are recommended for all wells. Implementation of the field-based procedures is more flexible, based on the judgment of the Center Groundwater Specialist.
Office-Based Evaluation of Well Integrity
Annual Hydrograph Check
Field-Based Evaluation of Well Integrity
Each Site Visit
Annual Total Depth Measurement
Periodic Hydraulic Testing
If access to the well is suitable for a hydraulic test, some type of repeatable hydraulic test must be conducted upon site establishment, and as necessary throughout the life of the well. As described above, the field evaluation, annual measurement of total depth, and annual evaluation of the hydrograph will provide a good initial check on well integrity. But a quantitative measure of well performance/integrity is valuable and necessary periodically, whenever the above evaluation methods indicate a possible degradation in well performance. The policy for observation well hydraulic testing is summarized in the steps listed below.
Upon establishment of a new well in a measurement program, conduct a baseline hydraulic test (additional detail provided in attachment “Types of Hydraulic Tests for Well Integrity”). Document the results in the site folder or digital documentation for the well. Future tests will be compared to these baseline test results. If a baseline test was not done when the site was established, it should be done as soon as possible.
Conduct the annual hydrograph check, the site visit check for well physical condition, and the annual measurement of total depth as described above.
If the evaluation in step number 2 suggests a degradation of well integrity, repeat the hydraulic test. If the water-level response from the repeat hydraulic test differs significantly from previous tests, the well should be examined more thoroughly (perhaps by using geophysical techniques) and a decision made whether to rehabilitate the well (such as by redeveloping) or to properly abandon the well.
The evaluations described in step number 2 above may not always detect degraded well integrity. Thus a hydraulic test should be repeated as time and resources permit, and re-testing is recommended every 3-5 years for all wells in critical networks or used in critical management decisions. The term “critical” is subjective but should include monitoring wells that a cooperator or the public uses to evaluate the effects of nearby groundwater depletions, climate change, or groundwater/surface-water interaction.
Each step of the well integrity evaluation must be documented in the paper site folder, station analysis, or other digital documentation for the well
Evaluation for wells not in the CBR networkThe above policy will immediately apply to CBR network wells and its application will be evaluated for wells not in the CBR network based on input from Water Science Center and WSFT Groundwater Specialists. The Centers are encouraged to apply the policy to all wells, regardless of whether they are in the CBR network, in order to appropriately evaluate the policy. There are some special cases where implementation of the policy is not feasible or burdensome and the Office of Groundwater would like to identify these
Prospective Evaluation Approach for Wells Measured Less Frequently Than Once per Year
Some network wells are measured infrequently, but are in a regular measurement program. This is common for wells that are part of a synoptic or mass measurement program. Potentiometric surface maps commonly are produced from these measurements. Even though these wells are measured infrequently, it is important that their integrity be evaluated.
Each measurement should be evaluated by comparing it to past measurements. If a measurement is outside the range of expected measurements, as suggested from past measurements, it should be re-evaluated. If potentiometric surface maps are made from the water-level measurements, any measurement that seems to deviate from the conceptual model of the groundwater-flow system should be re-evaluated. Best hydrologic judgment should be used to determine the accuracy of the measurement and the extent to which the measured value is representative of the aquifer hydraulic head. If measurements are judged to be anomalous, field-related well integrity checks may be needed to determine the integrity of the well; methods such as total well depth measurement (and comparison to previous depth measurements), well-integrity hydraulic tests, borehole geophysical logs, and borehole camera surveys can be used for this purpose.
Water Science Center Performance Evaluation
The Water Science Field Team Groundwater Specialists and the Office of Groundwater representatives will review the well-integrity documentation during the triennial Water Science Center Technical Reviews. In addition, the Water Science Field Team Groundwater Specialists and the Office of Groundwater encourages Water Science Center personnel to develop effective screening tools to evaluate well integrity for wells with continuous measurements and to share these with the groundwater community.
Cunningham, W.L., and Schalk, C.W., comps., 2011, GWPD 11-Measuring well depth by use of a graduated steel tape (Version: 2010.1): in Groundwater technical procedures of the U.S. Geological Survey (Cunningham, W. L., and Schalk, C.W., comps.): U.S. Geological Survey Techniques and Methods 1–A1, 151 p. (available only online at http://pubs.usgs.gov/tm/1a1/).
Cunningham, W.L., and Schalk, C.W., comps., 2011, GWPD 17-Conducting an instantaneous change in head (Slug) test with a mechanical slug and a submersible pressure transducer (Version: 2010.1): in Groundwater technical procedures of the U.S. Geological Survey (Cunningham, W. L., and Schalk, C.W., comps.): U.S. Geological Survey Techniques and Methods 1–A1, 151 p. (available only online at http://pubs.usgs.gov/tm/1a1/).
William M. Alley /s/
Chief, Office of Groundwater
Distribution (bcc): GS-W WSC Directors, GS-W CD, GS-W A, GS-W GW All, OGW
ATTACHMENT: Types of Hydraulic Tests for Well Integrity
Types of Hydraulic Tests for Well Integrity
A change in hydraulic test results over time is an indicator of a change in well integrity. The approach used for a hydraulic test is to induce a change in water level in the well and measure the water level as it returns to equilibrium. This test can be done in many ways, and some flexibility is necessary based on well construction, well accessibility, and instrument configuration, for example.
A single-well “slug test” using a mechanical slug or air (pneumatic test) is recommended for wells in a regular measurement program such as the Collection of Basic Records (CBR) Program. If a single-well test using a mechanical slug or air is not possible, another repeatable approach to induce water-level change in the well must be used. This can be done using a pump, bailer, or by pouring clean water into the well. Method repeatability over time is a key aspect to the choice of method. All future tests should be run identically to the baseline test in order to simplify the comparison of results. The text below provides basic guidance on several types of tests. Details for each type are available in the literature (for example, Butler, 1998).
USGS field procedures for conducting a mechanical slug test are provided in Groundwater Technical Procedure 17 (Cunningham and Schalk, 2011).
A pneumatic slug test differs from a mechanical slug test in that the initiation of the change in water level is accomplished with air pressure. This test requires the ability to seal the top of the well and pressurize the well, which lowers the water level; then, the sudden release of the pressure results in the rise of water level. Such tests are always conducted using a pressure transducer, data logger, and field computer. If pressurization is instantaneous, both a falling-head and rising-head test can be conducted. Most of the time, however, well pressurization is not instantaneous and only the rising-head part of the test can be evaluated.
A poured slug test initiates the water-level change by pouring a known volume of water into the well. Use of deionized water is preferable. However, if an observation well is being tested, and no water-quality samples are collected, use of tap water may be acceptable. Because this test creates a sudden rise in the water level, only a falling head test can be conducted. An electric tape, steel tape, or pressure transducer and data logger can be used to record the water level as the water level returns to the pre-test (static) conditions.
Bailing the well
A bailer test initiates the water-level change by suddenly removing a known volume of water from the well using a bailer. The bailer is lowered into the well and allowed to fill with water; rapid removal of the filled bailer creates a sudden drop in water level. The subsequent water-level rise, and return to the static water level, is measured over time. Thus, only a rising head test can be conducted. An electric tape, steel tape, or pressure transducer and data logger can be used to record the water level as the water level returns to the pre-test (static) water level.
Pumping the well
Well tests that involve the pumping of a well (for example, a specific capacity test) are usually not required because of the additional time and equipment required. The USGS measures many irrigation, production, and public water supply wells. Some of these wells have dedicated pumps and some do not. Wells that are pumped regularly, probably are in good connection with the aquifer and do not have to be tested unless there is some indication of decreasing connection. The Water Science Centers, however, may want to take advantage of wells that have a pump installed but are not regularly pumped. These might be back up wells in a public water supply well field, or back up irrigation wells that are used only when surface water is not available. Slug tests may not be effective for wells like this, so using the dedicated pump to determine specific capacity might be the Water Science Center's most feasible option to check well integrity. A baseline specific capacity test (run by the USGS or reported from the original well installation) would be helpful in cases of suspected anomalous water-level measurements.
Key points for all methods
The purpose of the hydraulic test for well integrity is to determine if the response of the well has changed over time. The goal is not to calculate aquifer properties, although it might be a useful additional step. Regardless of the method used to induce water-level change, the following points are relevant for a well-integrity test:
In summary, the main objective of a well-integrity test is to evaluate the hydraulic connection between the well and the aquifer. The Water Science Center staff has flexibility to determine the best method to accomplish this objective. An important aspect is reproducibility, so that, future tests can be compared to tests already conducted and documented.
Butler, J.J., Jr., 1998, The design, performance, and analysis of slug tests: Lewis Publishers, Boca Raton, FL, 252 p.
Cunningham, W.L., and Schalk, C.W., comps., 2011, GWPD 17-Conducting an instantaneous change in head (Slug) test with a mechanical slug and a submersible pressure transducer (Version: 2010.1): in Groundwater technical procedures of the U.S. Geological Survey (Cunningham, W. L., and Schalk, C.W., comps.): U.S. Geological Survey Techniques and Methods 1–A1, 151 p. (available only online at http://pubs.usgs.gov/tm/1a1/)