U.S. Geological Survey
U.S. Geological Survey Open-File Report 95-398
Factors considered for well installation include the nature of materials that make up and overlie the aquifer (for example, unconsolidated or consolidated materials; if consolidated materials are fractured or have openings caused by dissolution); the depth to water, to the top of the aquifer of interest, and to the zone in the aquifer to be monitored; the type of drilling equipment available; access to the site; well casing and screen materials, length, and diameter, and cost. In unconsolidated deposits, a common monitoring-well design (fig. 4) consists of a well screen and casing installed in a well bore with an annular space backfilled with filter packs and annular and surface seals. Specific aspects of NAWQA-design monitoring wells, however, can vary depending on requirements to meet specific data-collection objectives, site conditions encountered, and the drilling method used. For example, some Flowpath Studies will include investigation of ground-water quality as ground-water discharges to surface water, which might require installation of streambed piezometers that are steel, wire-wound well screens attached to the bottom of steel casing that are hand driven to the desired depth into the aquifer beneath the stream. In another situation, an aquifer might contain fine-grained sediment, which in order to prevent silting in of the screen, would require attachment of a riser pipe to the bottom of the well screen to serve as a collection area.
Study Units that select or install wells in semiconsolidated deposits and rock must ensure that the wells meet the design criteria for the study component (table 3). Three possible designs consist of (1) an open borehole at the interval of interest, with well casing installed in the borehole above this interval (the annular spacing between the casing and borehole wall is sealed with grout) (fig. 5a); packers installed in the open borehole above and below the interval of interest to isolate part of the borehole for water-quality sampling (fig. 5b); or a well screen with filter pack installed at the interval of interest, with an annular seal installed above this interval (fig. 5c). Generally, all three of these designs meet the design criteria for Land-Use and Flowpath Studies (table 3) but have advantages and disadvantages that must be evaluated when selecting one of these three designs, a modification of one of these designs, or an alternative design (W. Lapham, U.S. Geological Survey, written commun., 1995--see footnote 1).
For Flowpath Studies, clusters of monitoring wells usually will be installed. Clusters of monitoring wells are used when one well is considered inadequate in terms of characterizing the vertical distribution of hydraulic head or water quality. Several designs of well clusters suitable for water-level measurements and water-quality sampling are illustrated in figure 6 and include (1) monitoring wells with short screens, each installed in its own borehole (fig. 6a); (2) multiple monitoring wells, each with a short screen, installed in a single borehole, with an annular seal between each screened interval (fig. 6b); and (3) a single well, which contains a series of multiport samplers, installed in a single borehole, with each port separated by an annular seal, or by a packer (fig. 6c). The decision to use one design over another depends on a number of factors related to the objective of the Flowpath Study; the advantages of given well-cluster designs are discussed in another document (W. Lapham, U.S. Geological Survey, written commun., 1995--see footnote 1), and their use is described in greater detail in Jelinski (1990); LeBlanc and others (1991); Stites and Chambers (1991); Pickens and others (1978). Short-screened wells installed in separate boreholes (fig. 6a) is the design recommended for NAWQA studies. Spacing wells about 5 to 10 ft apart in a cluster generally maintains well integrity without comprising the intent of collecting data at a well cluster.
It is important to select the appropriate materials, and type, diameter, and length of casing and screen, as these can affect the quality of a ground-water sample. Biased water-quality data can arise from chemical and physical interaction between ground water and materials used to construct monitoring wells (tables 5 and 6). These biases can result from leaching, sorption desorption, or volatilization. Leaching and sorption/desorption studies that examined casing materials are described by Hewitt (1994a and b, 1992); Parker and Ranney (1994); Ranney and Parker (1994); Parker, Hewitt, and Jenkins (1990); Parker and Jenkins (1986); Gillham and O'Hannesin (1990); Reynolds and others (1990); Reynolds and Gillham (1986); Cowgill (1988); Barcelona and others (1983); Sosebee and others (1983); and Curran and Tomson (1983). Parker (1992) provides a recent summary of the findings of several of these and other studies.
The well screen potentially can alter water quality because of the large surface area exposed to ground water. The screen is the part of the monitoring well most susceptible to corrosion and (or) chemical degradation, and provides the highest potential for sorption or leaching of contaminants (Aller and others, 1989, p. 192). Thus, when selecting the screen materials, resistance to leaching or sorption/desorption for the broad suite of NAWQA constituents is a major consideration (table 5). Therefore, PVC is the material of choice for well casing screens installed for NAWQA ground-water studies. In cases where the well will be used only for sampling one class of chemical constituents, casing and screen materials can be selected to minimize bias caused by that material (table 5).
Table 5. Relative leaching or sorption/desorption ranking of well-casing and screen materials for indicated water-quality constituent classes
[Applies in general to classes of compounds indicated. Actual amounts and rates of leaching or sorption/desorption of individual constituents can differ within each major constituent class. The tendency of a material to leach compounds can differ from the ability of the materials to sorb constituents or compounds. 1, least leaching or sorptive/desorptive; 5, most leaching or sorptive/desorptive; PTFE, polytetrafluoroethylene; PVC, polyvinylchloride]
----------------------------------------------------------------------------------- | | Water-quality constituent class(a) | =================================================================================== | | | | | | | | | | | | | Inorganic | Organic | | | | constituents | compounds | | | Material | | | ----------------------------------------------------------------------------------- | | PTFE | 1 | 2-4(b) | ----------------------------------------------------------------------------------- | | PVC(c) | | | ----------------------------------------------------------------------------------- | | - Flush-threaded joints | 1-2 | 2 | ----------------------------------------------------------------------------------- | | - Glued joints(d) | 3 | 5 | ----------------------------------------------------------------------------------- | | Stainless steel(e) | 4 | 1-2 | ----------------------------------------------------------------------------------- | | Galvanized steel | 5 | 4 | ----------------------------------------------------------------------------------- | | Carbon steel | 5 | 4 | ----------------------------------------------------------------------------------- a) Includes constituents to be analyzed according to the laboratory schedules shown on table 2. b) PTFE can be highly sorptive of some organic compounds, although these losses might diminish as equilibrium of casing with ground water is approached (Parker and Ranney, 1994; Ranney and Parker, 1994). c) PVC is the best compromise choice if measuring all constituent classes in table 2. d) Volatile organic compounds leached from glue can include THF (tetrahydrofuran), MEK (methylethylketone), MIBK (methylisobutylketone) and cyclohexanone (Sosebee and others, 1983). e) Generally, stainless steel 316 is more resistant to corrosion than stainless steel 304.
The PVC casing selected should be National Sanitary Foundation-approved schedule 40 (or 80) and flush jointed and threaded. In low-yielding materials, such as till and loess, leakage of water through improperly sealed PVC joints can contribute a significant amount of water to a well compared to the amount of water contributed through the well screen (van der Kamp and Keller, 1993). Under such circumstances, O-rings or Teflon tape on threaded joints below the water table helps prevent this leakage, as does a properly installed annular seal.
Joints of PVC or other plastic casing used for NAWQA-installed wells must be threaded and not glued. Organic compounds that leach from PVC primer or adhesive can compromise sample integrity (Sosebee and others, 1983). Compounds listed as ingredients in one or more of six PVC adhesives and one primer included THF (tetrahydrofuran), MEK (2-butanone or meth ylethylketone), MIBK (methylisobutylketone), cyclohexanone, and DMF (N, N-dimethylformamide). In addition to sample contamination, such compounds can mask or made the identification of other VOCs difficult by co-eluting with other VOCs during sample analysis.
The length of a well screen is determined on the basis of the scale and objectives of the investigation. The length of a well screen is important in relation to the vertical interval of in vestigation. In terms of water-level and water-quality measurements, a short screen generally provides measurements of hydraulic head and ground-water quality that more closely represent point measurements in the aquifer than measurements provided by a long screen. Ground-water-quality samples also reflect an integrated measurement of water quality vertically throughout the screened (or open) interval. Pumping a well with a long screen is more likely to induce mixing of waters of different chemistry than pumping the same well with a short screen. Thus, concentrations of constituents in samples obtained from wells with long screens are less likely to reflect the maximum concentrations of those constituents at any point within the screened interval than samples obtained from wells with short screens.
Table 6. Some general characteristics of materials used for well casing and screens (modified from T.E. Imbrigiotta, U.S. Geological Survey, written commun., 1989)
[PTFE, polytetrafluoroethylene; PVC, polyvinylchloride; SS, stainless steel]
PTFEa,b
- Virgin PTFE readily sorbs some organic solutes (Parker and Ranney, 1994).
- Ideal material in corrosive environments where inorganic compounds
are of interest.
- Useful where pure product (organic compound) or high concentrations
of PVC solvents exist.
- Potential structural problems because of its low tensile and
compressive strengths, low wear resistance, and the extreme
flexibility of the casing string as compared to other engineering
plastics (Driscoll, 1986, table 21.6; Dablow and others, 1988;
Aller and others, 1989, table 25).
- Potential problems with obtaining a seal between the casing and the
annular sealant because of PTFEs low coefficient of friction and
antistick properties as compared to other engineering plastics
(Aller and others, 1989, p, 151).
- Maximum string length of 2 inch schedule 40 PTFE casing should not
exceed about 375 feet (Nielsen and Schalla, 1991, p. 262).
- Expensive.
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PVCa,b
- Leaching of compounds of tin or antimony, which are contained in
original heat stabilizers during formulation could occur after long
exposure.
- When used in conjunction with glued joints, leaching of volatile
organic compounds from PVC primer and glues, such as THF
(tetrahydrofuran), MEK (methylethylketone), MIBK
(methylisobutylketone) and cyclohexanone could leach into ground
water. Therefore, threaded joints below the water table, sealed
with o-rings or Teflon tape, are preferred.
- Cannot be used where pure product or high concentrations of a PVC
solvent exist.
- Maximum string length of 2 inch threaded PVC casing should not
exceed 1,200 to 2,000 feet (Nielsen and Schalla, 1991, p. 250).
- Easy to cut, assemble, and place in the borehole.
- Inexpensive
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STAINLESS STEELa
- Generally has high corrosion resistance, but varies with type.
- Corrosion can occur under acidic and oxidizing conditions.
- Corrosion products are mostly iron compounds, with some trace
elements (see below).
- Primarily two common types:
(1) Stainless steel 304 (SS304): Iron alloyed with the following
elements (percentages approximate): chromium (18-20 percent),
nickel (8-11 percent), manganese (2 percent), silicon (0.75
percent), carbon (0.08 percent), phosphorus (0.04 percent), sulfur
(0.03 percent).
(2) Stainless steel 316 (SS316): Iron alloyed with the following
elements (percentages approximate): chromium (16-18 percent),
nickel (10-14 percent), manganese (2 percent), molybdenum (2-3
percent), silicon (1 percent), carbon (0.08 percent), phosphorus
(0.04 percent), sulfur (0.03 percent).
- Corrosion resistance is good for SS304 and excellent for SS316.
- Expensive.
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GALVANIZED STEELa
- Less corrosion resistance than SS304 or SS316 and more resistance
to corrosion than carbon steel (below).
- Oxide coating could dissolve under chemically reduced conditions
releasing zinc and cadmium.
- Weathered or corroded surfaces present active adsorption sites for
organic and inorganic constituents.
- Inexpensive.
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CARBON STEELa
- Corrosion products (for example, iron and manganese oxides, metal
sulfides, and dissolved metal species) can occur.
- Sorption of organic compounds onto metal corrosion products is possible.
- Weathered surfaces present active adsorption sites for organic and
inorganic constituents.
- Inexpensive.
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a) Residues, such as threading lubricants used in production or
contamination during shipping, require that all materials be
cleaned inside and out prior to installation.
b) Possible construction alternative is to use a PTFE screen with
threaded PVC casing.
Screen lengths for monitoring wells installed by NAWQA for Land-Use and Flowpath Studies typically should range from 2 to 10 ft. The actual length used should reflect study objectives and aquifer conditions. For example, a screen length of 5 ft might be too long for a well used in a NAWQA Flowpath Study, if information suggests that marked vertical differences in the distribution of hydraulic head or water quality occur on the order of a few feet or less. A 5-ft screen placed immediately below the water table, however, probably is appropriate for most NAWQA Land-Use Studies. As a general rule, screen lengths of 10 ft or less generally are appropriate for most NAWQA Land-Use Studies and screen lengths of 1 to 5 ft in length generally are appropriate for most NAWQA Flowpath Studies (table 3).
The diameter of monitoring wells for water quality typically range from one-half to 6 in, with the 2-in diameter well being the USGS and industry norm. Ideally, the preferred well-casing diameter would be suitable for running aquifer tests as well as for collecting water-quality data. Typically, wells for an aquifer test consist of one large-diameter pumping well (4-in diameter or greater) that is associated with wells that can be of smaller diameter in which drawdown is measured as pumping proceeds. The larger diameter normally is required for the pumping well to ensure that the well can be pumped at a rate sufficient to cause measurable drawdown in the observation wells. Therefore, a well with small diameter (2 in or less) generally is not suitable as the pumping well for aquifer testing. The hydraulic data acquired from aquifer tests are particularly important to meet objectives of the Flowpath Studies. Therefore, plans for Flowpath Studies should include installation of at least one larger diameter well per well cluster along the flowpath that will be suitable for aquifer testing, if (1) a larger diameter well is needed to obtain hydraulic data, (2) if a suitable well does not already exist, or (3) if the hydraulic data required are not available.
It is not always possible to select the optimum construction material and screen design. For example, for a Flowpath Study of trace-element concentrations in ground water in an area inaccessible to a drill rig, a hand-driven monitoring well constructed of steel casing and drive point might be the only alternative. The quality of data obtained from such a monitoring well may be difficult to interpret. Where a less-than-optimum well design is used, the increased risk of data bias needs to be considered and any potential bias must be explicitly identified, defined, and reported.
Decontamination of well-installation equipment prior to use reduces contamination of drill holes, aquifers, pore water at the screened interval, and cross-contamination between wells. Procedures for decontamination of equipment, casing and screens, and other materials used for well installation are provided in table 7 and in greater detail in Aller and others (1989), U.S. Environmental Protection Agency (1987), Moberly (1985), and Richter and Collentine (1983), and in W. Lapham, U.S. Geological Survey, written commun., 1995--see footnote 1.
Decontamination of equipment and materials and documenting decontamination procedures and quality-control data is to be a standard operating procedure for NAWQA Study Units. The frequency of decontamination depends on subsurface conditions at the drill site and objectives of the sample collection. Decontamination of equipment between drill sites also is an important precaution to prevent possible cross-contamination between sites. At sites where well clusters are being installed, decontamination of equipment between drill holes will help prevent cross-contamination between boreholes. The degree to which each step of the six-step protocol that follows must be completed, however, depends on the Study-Unit data-collection requirements, including target contaminants and concentrations in ambient samples; the confidence level needed for the data; and the contaminants expected to be contributed by the equipment and methods used for installation and completion of the well.
1. Select a location for decontamination procedures:
o Avoid spilling decontamination fluids at drilling site;
o Prepare clean area for cleaned equipment.
2. Select equipment requiring decontamination.
3. Determine the frequency of decontamination of the equipment.
4. Select the cleaning technique and type of cleaning solutions
to be used for decontamination. The routine procedure for NAWQA
for cleaning well-installation equipment and materials is:
o Wash outside (and inside where applicable--for example, well
casing and screen) of equipment and materials used during well
installation using a low-sudsing, non-phosphate detergent;
o Complete decontamination procedure with high-pressure steam
cleaning using potable tap water.
5. Contain residual contaminants and cleaning solutions, if
necessary, and dispose according to regulations.
6. Plan to collect some quality-control samples to evaluate the
effectiveness of decontamination procedure (for example, a sample
of the rinse water that was used to steam clean or remove all
residues and additional samples of rinse water taken from the
equipment after it has been decontaminated).
Selection of a drilling method from among the methods available requires consideration of the study objectives, as well as site conditions and economics (tables 8 and 9). Because a primary purpose of installing the wells for NAWQA Land-Use and Flowpath Studies is for water-quality sampling, a drilling method that has minimal effect on ground-water chemistry should be a primary consideration during selection of a method.
For installation of wells for sampling ground-water quality, preferred drilling methods are those that minimize (1) the possibility of contamination of aquifers and aquifer pore water by foreign drilling fluids, and (2) cross-contamination between aquifers by drilling fluid, pore water, and drill cuttings. Given that the primary objective of installing wells for NAWQA studies is for water-quality sampling, the selection of a drilling method that minimizes the potential for subsurface contamination by the drilling process should be of primary concern, and well-installation needs should be budgeted accordingly. Use of other drilling methods must be done with the knowledge that increased time and cost might have to be committed to adequately develop and purge the well prior to sampling.
In some cases, a method of drilling that minimizes the potential for subsurface contamination by the drilling process might severely limit collection of other data at the well that are also important to meeting the study-component objectives. For example, many of the most useful borehole-geophysical logs must be run in uncased, fluid-filled boreholes. This requirement is at odds with the most preferred methods of drilling for installing wells for water-quality sampling. Study Units must weigh the cost benefit of data desired against the practical constraints of the drilling methods being considered and the primary objective of collecting ground-water-quality samples that accurately represent ground-water chemistry.
Well-construction information must be documented at the time of well installation, as discussed in the section "Documentation."
Logistical considerations
Drilling considerations
Economic considerations
[The relations given in this table generally apply; there are exceptions in some cases depending on specific techniques employed during drilling1]
1 Good formation samples obtained only when a sample tube is added to cable tool; otherwise samples are disaggregated.
Well completion ensures that the hydraulic head measured in the well is that of the aquifer(s) of interest, ensures that only the aquifer(s) of interest contribute(s) water to the well, and prevents the annular space from being a vertical conduit for water and contaminants. Such completion steps are critical to the long-term goals of NAWQA, which dictate that many of the wells installed by the program are to be used for water-quality sampling for decades. Well completion in unconsolidated deposits for the NAWQA Program consists of installing the well casing and screen, and filling and sealing the annular space between the well casing and borehole wall, and completing the documentation required, as discussed on page 43 in the section "Documentation." Compliance with State regulations for well completion, as for well drilling, is mandatory.
Specific details of well completion require consideration of several hydrogeologic factors, including (1) the depth to water, to the top of the aquifer of interest, and to the zone in the aquifer to be monitored; (2) the nature of materials that make up the aquifer to be monitored and that overlie the aquifer--for example, whether the materials are consolidated or unconsolidated; (3) expected water-level fluctuations; (4) expected direction of the vertical head gradient--down ward, upward, or fairly uniform with depth; (5) whether the aquifer is confined or unconfined; and (6) the design of the monitoring well(s) (figs. 4-6). Completion requirements and practices can differ considerably among wells (W. Lapham, U.S. Geological Survey, written commun., 1995--see footnote 1).
In most cases, wells installed for NAWQA Land-Use or Flowpath Studies will consist of flush-threaded PVC pipe with short (2- to 10-ft) well screens (table 3). Installation of a filter pack around the well screen and sealing of the annular space between the well casing and the borehole wall also will be necessary for those installations in which the annular space could remain open after well installation. Each major element of well completion has specific design objectives, which are discussed briefly here. A more detailed discussion of the major elements of well completion has been written (W. Lapham, U.S. Geological Survey, written commun., 1995--see footnote 1), and additional information related to completion procedures is given in ASTM (1992) and Driscoll (1986).
The well casing and/or screen is installed in the borehole as the first step in well completion. After installation of the well casing and, if needed, the well screen, the major elements of well completion consist of the following:
1. If a well screen is used and a filter pack is required, the primary filter pack is installed around the well screen.
2. A secondary filter pack is installed above the primary filter pack.
3. Annular seals are installed to about frost level.
4. A surface seal is installed.
5. A protective casing is installed around the well at land surface.
An example of these major elements of well completion, in this case for a shallow well with a filter-packed well screen installed in unconsolidated materials, is provided in figure 4. This is a typical design for wells for Land-Use and Flowpath Studies.
The primary filter pack (also commonly called a sand or gravel pack) is material that fills the annulus around and just above the well screen to retain and stabilize material from the adjacent screened unit (fig. 4). A filter pack has a greater grain size than that of the aquifer material in the vicinity of the screen. Filter-pack grain size and gradation are designed to stabilize the hydrologic unit adjacent to the screen and permit only the finest grains to enter the screen during development, resulting in relatively sediment-free water for sampling after development. The primary filter pack should consist of relatively inert material such as quartz, contain no limestone or other calcareous materials such as shell fragments, and contain no organic material such as wood fragments or lignite. Alternatively, filter-pack material of known chemistry (ASTM, 1992) can be used, such as glass beads.
The primary filter pack commonly is extended up the annulus to a minimum of 5 ft above the top of the screen (Hardy and others, 1989), if a secondary filter pack is impractical. The primary filter pack must not intersect multiple water-bearing units, nor cross confining units that otherwise would not be screened (table 3). Intersection of such units can result in an artificial, vertical, hydraulic connection along the annulus between these units, and can affect the chemistry of the ground water being sampled.
The secondary filter pack (fig. 4) is a finer grained material than the primary filter pack, placed in the annulus between the primary filter pack and the overlying annular seal, or be tween different types of annular seals (ASTM, 1992, p. 124). The purpose of the secondary filter pack is to prevent material used for the annular seal from infiltrating and clogging the filter pack and from affecting ambient water chemistry. The secondary filter pack should consist of inert material, consistent with that of the primary filter pack. A length of secondary filter pack of about 1 to 2 ft is recommended (Hardy and others, 1989, p. 16; ASTM, 1992, p. 129, figs. 2 and 3).
Annular seal(s) are installed from above the secondary filter pack or the extended primary filter pack to near land surface, in order to seal the annular space between the casing and borehole wall (fig. 4). These seals prohibit vertical flow of water between aquifers and prevent cross-contamination of aquifers by contaminants. They also protect against infiltration of water and contaminants from the surface.
A 3- to 5-ft plug should be placed above the extended primary or secondary filter pack (ASTM, 1992). The plug is formed from a hydrated material such as bentonite or cement that acts as a sealant. The choice of a sealant material must minimize possible effects on the constituents to be analyzed from the well (table 10). Penetration of the sealant into the underlying filter pack should be limited to less than a few inches (Hardy and others, 1989).
[Information from ASTM (1992), Aller and others (1989), Hardy and others (1989), Driscoll (1986), Gillham and others (1983), and Claassen (1982)]
BENTONITE
(A hydrous aluminum silicate composed primarily of montmorillonite)
Advantages:
Disadvantages:
CEMENT
(Composed of calcium carbonate, alumina, silica, magnesia, ferric oxide, and sulfur trioxide with pH ranges from 10 to 12)
Advantages:
Disadvantages:
Drill cuttings removed from the borehole sometimes are used as grout instead of bentonite or cement, but the effectiveness of these materials as a sealant needs careful evaluation and is not to be used for NAWQA wells. For NAWQA, bentonite, cement, or mixtures of bentonite and cement probably are the most common grout materials that will be used. Generally bentonite is recommended for grout if the well is used for water-quality sampling. However, as in the case of the underlying seal, the choice of a material depends on the purpose of the well. Detailed discussions of characteristics of annular seals and methods of placement can be found in ASTM (1992) and Driscoll (1986).
The surface seal prevents surface runoff down the annulus of the well and, in situations in which a protective casing around the well is needed, holds the protective casing in place. The depth of installation of a surface seal can range from several feet to several tens of feet below land surface. Local regulatory agencies might specify a minimum depth of installation. Because of likely desiccation of bentonite, a cement surface seal is recommended.
A protective casing should be installed around the well to prevent unauthorized access to the well and to protect the well from damage. The protective casing is installed at the same time as the surface seal and should extend to below the frost line (ASTM, 1992). One design for protective casing is a steel casing with vented locking protective cover and weep hole, which permits condensation to drain out of the annular space between the protective casing and well casings (fig. 4). ASTM (1992, p. 132) also calls for coarse sand or pea gravel or both to be placed in the annular space between the protective casing and the well to prevent entry of insects. A second design is a steel casing with bolted or locked manhole cover enclosing a well that is flush with the land surface.
Wells drilled for the NAWQA Program should be developed to enhance flow of water to the well, to remove sediments that are artifacts of well installation, and to provide water representative of the unit being sampled. Developing a well mitigates artifacts associated with drilling, such as changes in aquifer permeability, sediment distribution, and ground-water chemistry. Redevelopment of a well can be necessary because of sedimentation in the well casing, or clogging of the aquifer or well screen.
Development of a well is to be documented as discussed in the section "Documentation." Documentation includes: (1) the method of development; (2) equipment being used; (3) the volume of water removed; (4) a measure of the clarity of water removed from the well over time, such as turbidity, or at a minimum, the visual appearance of the discharge water; and (5) information about well characteristics, such as the total depth of the well, the well diameter, the depth(s) to the screened or open intervals, and the water level in the well.
Information collected during development can be used to evaluate requirements for sam pling. Estimates of the recovery rate and recovery time of the water level in the well after pump ing can be used to estimate the time required for purging the well prior to sampling. If the recovery rate of the water level in the well is slow even after development, it might be necessary to plan purging of the well on one day and sampling the well on the following day. The recovery time can be used to determine the pumping rates to purge and sample the well and to determine if an alternative method of pumping the well is required.
Factors that affect the well development and the effort required depend on aquifer characteristics, the drilling method used to install the well, and well characteristics. Traditionally, methods of well development are selected to optimize on the capacity of a well. Because a primary objective for wells used in the NAWQA Program is water-quality sampling, methods for developing wells must be evaluated and selected on the basis of the probable effect on ground-water chemistry. The best development techniques to restore the chemical quality of aquifer pore water to its predrilled condition are those that avoid the introduction of air, foreign water, and other foreign fluids into the aquifer during the development process. This reduces well-development options, but is critical to ensuring that the effect of development on ground-water chemistry is minimized. The following methods for well development in the NAWQA Program, in the order of recommended use, are: bailing; mechanical surging; pumping or overpumping, and backwashing; indirect eduction jetting; backwashing; and jetting and surging with water or air (W. Lapham, U.S. Geological Survey, written commun., 1995--see footnote 1; U.S. Bureau of Reclamation, 1977; Anderson, 1984; Driscoll, 1986; Aller and others, 1989; and Shuter and Teasdale, 1989).
The criteria for well selection and information about wells selected for sampling are to be documented in permanent files. Careful and complete documentation aids in interpretation of ground-water data and provides historical reference for future use of the well. In addition, doc umentation of network-design information for each Study-Unit Survey, Land-Use Study, and Flowpath Study is required. Types of network-design information include network identifica tion, personnel involved, well selection and installation information, and field activities. An example form is provided in an internal document entitled "gw.network.documentation" (P. Leahy, U.S. Geological Survey, written commun., February 4, 1994).
A paper or electronic well site file should be maintained for each well sampled by NAWQA Study Units. Information in this file begins with a well-information check list (fig. 7). As information about the well site is collected, it is added to the site file.
It is USGS policy that all of the routine ground-water data collected must be stored in the computer files of the National Water Information System. The USGS, Office of Ground Water, interprets "routine" data collection of the USGS to include "all ground-water data collected by WRD basic-data programs and district projects" (Office of Ground Water Technical Memorandum No. 93-03, written commun., 1993).
Documentation of the methods and materials used for well installation is required for each new well installed and for each existing well selected to the extent that the information is available (fig. 8). Documentation of newly installed wells is to be completed at the time of or directly after well completion and development. Documentation includes lithologic, driller's, and well-construction logs (U.S. Geological Survey, 1980, p. 2-80 to 2-81); and a record of well development (fig. 9). A record should be kept of other logs collected during and after drilling, such as drilling-rate and fluid-loss logs (U.S. Geological Survey, 1980, p. 2-80), bore hole-geophysical logs, and field and laboratory analyses of aquifer materials and water samples, results of tests to determine construction integrity of the well, water-level measurements, and aquifer tests.
Also in the well file are location map(s) and site sketches (see for example fig. 10). The location map(s) and site sketch need to be of sufficient detail and scale to enable field location of a well by field personnel unfamiliar with the site. Information on the location map(s) typically includes roads, topography, water bodies, and cultural features. Compass directions or latitude/longitude and a horizontal scale need to be indicated on the location map(s). Distances from milepost markers or other permanent cultural features to the well site also are useful in formation. The site sketch identifies the well location in relation to nearby features such as roads, railroad lines, fences, houses, barns, and out buildings. Compass directions need to be indicated on the sketch, and distances between features and the well typically are included on the sketch. A sketch of the well head identifies features of the well such as the height of the top of the casing in relation to land surface, the locations of measuring and sampling points, and general characteristics of the protective casing. Written descriptions of the site and well characteristics compliment the site and well-head sketches with other useful information, such as site access, whether or not the well is locked, whether or not owner notification is required prior to sampling, tools required, difficulties that might be encountered in locating the well, measuring the water level, or sampling the well, and the possible presence of animals.
Well records must identify the location of the well and describe the point from which water level is measured (the "measuring point"). The locations and altitudes of wells sampled as part of Study-Unit Surveys and most Land-Use Studies will be determined from Geological Survey 7 1/2-minute quadrangle maps. For some Land-Use Studies and for Flowpath Studies, greater accuracy in locations and altitudes of wells will be required than can be determined from 7 1/2-minute quadrangle maps. In these cases, standard second-order ground-surveying techniques (Davis and others, 1966) generally are to be used. An alternative to ground surveying is to use a global-positioning system. In addition, the records should show if a vertical reference point has been established near the well that can be used to check the measuring point and to re-establish a measuring point that has been changed or destroyed.
Data required for creation of a site file in the Ground-Water Site Inventory data base1 (GWSI)
----------------------------------------------------------------------------------- | | Component number for| Example code | | | data entry into GWSI| (Description of | | Data description | | code) | ----------------------------------------------------------------------------------- | Agency Code | C4 | USGS | | Site (Station) Identification | C1 | 394224075340501 | | (Latitude/longitude/sequence no.)| | | | Station Name | C12 | Alpha | | Latitude | C9 | 394224 | | Longitude | C10 | 753405 | | Station locator sequence | | | | number | C815 | 01 | | District/User | C6 | 24 (Maryland) | | State | C7 | 10 (Delaware) | | County Code | C8 | 003 (Sussex) | | Agency Use | C803 | A (Active) | | Station Type | C802 | Y (Well) | | Data Reliability | C3 | C (Field Checked) | | Site Type | C2 | W (Well) | | Use of site | C23 | O (Observation) | -----------------------------------------------------------------------------------
Data required for storage of sample analyses in the Water-Quality data base (QWDATA)
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| | Example code |
| | (Description of code) |
| Data description | |
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| Agency Code | USGS |
| Station Identification | 394224075340501 |
| Sample Medium | 6 (ground water) |
| Sample Type | 2 (blank sample) |
| Hydrologic ("Hydro") Event | 9 (routine sample) |
| Hydrologic ("Hydro") Condition | 9 (stable, normal stage) |
| Begin Date and Time (month/day/year, | 090988, 1530 hrs |
| standard military time) | |
| Analysis Status | H (initial entry) |
| Analysis Source | 9 (USGS laboratory and field) |
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1) From Ground-Water Site Schedule Form No. 9-1904-A, February, 1987.
Figure 9. Example of a form to summarize development of a well.
Figure 10. Examples of well-location and site-sketch maps and information related to site conditions and restrictions.
Photographs of each well sampled and features in the vicinity of those wells are required. The purpose of the chronological series of documentary photographs of each well site is to provide a visual record of land use near the well, which can aid in the explanation and interpretation of analytical results, and can aid in locating the well in the future. When changes occur at or near the well that might affect hydrologic interpretation of data from the well, a new set of photographs is required to document those changes. For example, changes in the reference datum of the well or changes in land use near the well might warrant a new set of photographs. A new set of photographs also would be appropriate if they will aid in locating the well in the future. The set of photographs are (1) one photograph of the well and surrounding area as seen when approaching the well; (2) one close-up photograph of the well and water-level measuring point; (3) four photographs that show the area near the well--one each to the north, east, south, and west; and (4) any additional photographs to document features that might influence the chemistry of water collected from the well. General information and the identification of important features shown on the photographs need to be recorded. The minimum general information includes the date of the photographs, and location and identification of the well (site identification number or station name, latitude and longitude, written description). Features identified on the photographs will include at least the measuring point used for water-level measurements and the sampling point used for water-supply wells.
A field sheet (fig. 11) that documents land use and land cover and other site characteristics in the vicinity of each well is to be completed in its entirety the first time a well is sampled. This information must be rechecked each time a well is sampled and, if any changes have occurred, the changes must be noted on a new sheet. This information is especially important documentation for wells sampled in Land-Use and Flowpath Studies, but also may help explain water-quality results from wells sampled for the Study-Unit Survey, particularly if those wells are resampled at a later time. The land-use and land-cover field sheet filled out by 1991 Study-Unit staff (fig. 11) was modified slightly from the form used in the pilot NAWQA Program (Hardy and others, 1989; Scott, 1989) based on experience with its use during that program. The form in figure 11 currently is being evaluated for use by the 1994 Study Units.
