MONITORING WELL DESIGN AND CONSTRUCTION

INTRODUCTION
The guidelines that follow are modified from the 1991 revision of Test Methods for Evaluating
Solid Waste, Volume II, Chapter 11, published by the United States Environmental Protection
Agency (USEPA). This document is commonly referenced by its document number, SW-
846. The California Environmental Protection Agency (Cal EPA) has incorporated
appropriate sections of SW-846 into this document, in an effort to minimize redundant or
contradictory guidance between the Cal EPA and USEPA.
Although developed for monitoring and corrective actions at permitted facilities under the
Resource Conservation and Recovery Act (RCRA), the methods and materials discussed in
Chapter 11 of SW-846 are applicable to all hazardous waste sites. As such, SW-846 is
readily adaptable for investigations pursued under the authority of the Cal EPA.
1.1 Purpose
This document is intended to provide guidelines for the construction of monitoring
wells used for the hydrogeologic characterization of hazardous waste sites. The
purpose of this document is to aid in the selection of materials, provide
recommended quality assurance and quality control (QA/QC) procedures, and give a
standardized approach to the presentation of monitoring well construction records.
The recommendations contained herein represent minimal criteria judged necessary
to obtain quality data and assure reasonable and independently verifiable
interpretations.
The American Society for Testing and Materials (ASTM) has developed guidelines for
well construction and decommissioning (ASTM, 1990 and 1992). The Cal EPA has
incorporated these guidelines, where technically and legally relevant, into the Cal
EPA guidance framework. The Cal EPA is striving to keep up to date with the
development of external guidelines, and every attempt has been made to incorporate
the intent of those documents into the Cal EPA guidelines. As new techniques gain
acceptance and existing techniques are refined, this document will be updated
accordingly to meet the state of the science.
The recommendations presented here are a subset of the larger site
characterization process. The additional investigative tools necessary to adequately
characterize a site are outlined in the Guidelines for Hydrogeologic Characterization
of Hazardous Substance Release Sites (Cal/EPA, 1995).
1.2 Application
Monitoring wells provide a means to assess ground water quality, estimate ground
water flow direction and velocity, and calculate aquifer hydraulic properties. With this
information, hydrogeology can be characterized, contamination can be defined, and
appropriate remedies can be designed to mitigate ground water contamination. The
following guidelines are presented in an effort to promote the proper construction of
monitoring wells and increase the overall quality of site characterizations throughout
the state.
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1.3 Limitations
The recommendations presented here represent minimal criteria that can aid
obtaining quality data and assuring reasonable and independently verifiable
interpretations. Some sites may require investigative efforts above and beyond the
scope of this document, while at other sites a less rigorous application of this
guidance may be appropriate. It is the obligation of the responsible parties and the
qualified professionals performing site investigations to consult with pertinent
regulatory agencies, identify all requirements and meet them appropriately.
This document discusses broad categories of materials and methods that can be
used in monitoring well construction. It does not define specific operating
procedures for material selection or well construction. The qualified professional in
charge of the field investigation should specify the methods, equipment and operating
procedures in an appropriate work plan and document any significant departures
from the work plan that were necessary during the course of the investigation.
The following sections provide a basic summary of monitoring well design and
construction techniques. A comprehensive guide to choosing appropriate drilling
techniques is presented by Aller et al. (1989). Although many of the techniques
presented in this section may be applied to the design and installation of
piezometers, this section is geared to the design and construction of monitoring
wells.
This document does not supersede existing statutes and regulations. State
regulations and ordinances which address monitoring well construction and
performance standards include:
• Department of Water Resources Bulletin 74-90, California Well Standards;
• Title 22, California Code of Regulations, Division 4.5, Chapter 14, Article 6,
Environmental Health Standards for the Management of Hazardous Waste;
• Title 23, California Code of Regulations, Division 3, Chapter 15, Article 5 and
Chapter 16, Article 4, Regulations of the State Water Resources Control
Board and Regional Water Quality Control Boards;
• California Business and Professions Code, Division 3, Chapter 9.
Additionally, county, city and local water agencies may also have ordinances for well
construction. Federal, state and local regulations, statutes, and ordinances should
be identified when required by law, and site characterization activities should be
performed in accordance with the most stringent of these requirements where
applicable, relevant and appropriate.
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2 MONITORING WELL DESIGN AND CONSTRUCTION
2.1 Borehole Construction
When a monitoring well installation is planned, sufficient thought should be given to
the quality of the borehole that will contain the well. The following factors should be
considered in the borehole construction:
• drilling method;
• borehole diameter;
• annular space;
• borehole alignment;
• total depth of the hole;
• selection of backfill material;
• development of the well.
The diameter of a monitoring well borehole should be sufficiently large to contain the
well casing and provide an adequate annular space (as measured from the outside
of the casing to the borehole wall). Additional allowances should be made as needed
for other pipes that may be installed in the annular space, such as sand fill pipes or
sounding tubes.
The annular space is the gap between the outside of the casing and the borehole
wall. The annular space should be large enough to allow clearance of a 1.5-inch I.D.
tremie pipe and for a sufficient width of filter pack and annular seal material.
Recommended annular space widths are as follows:
• between casing and borehole wall - 2.5 inches minimum;
• between well casing and conductor casing - 2 inches minimum;
• between surface conductor casing and borehole wall - 3 inches minimum;
• maximum annular space - 5 inches.
Annular space widths larger than 5 inches may reduce the ability to develop a well, or
may contribute to casing damage from heating during grout curing.
In situations where precise lithologic data are needed (e.g., dipping or folded strata),
or the location of target zones is critical, borehole alignment becomes an important
criterion for monitoring well screen placement. Borehole alignment can be assessed
through a borehole deviation survey, using a borehole dipmeter or similar downhole
tool. Fortunately, misalignment is usually not significant for shallow monitoring well
boreholes (less than 200 feet deep, based on Cal EPA experience); therefore, the
additional cost for borehole deviation surveys is usually not justified. However, where
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precise geologic or hydrogeologic information is needed from deep boreholes
(significantly greater than 200 feet), borehole deviation surveys are recommended.
The depth of each monitoring well is determined by site-specific hydrogeologic
conditions and monitoring objectives. For example, wells may be designed to
monitor the water table, within a water-bearing zone or at the base of an aquifer.
Regardless of monitoring depth, the depth of completion of the monitoring well
borehole should generally be within one foot of the bottom of the screened interval.
Sometimes boreholes are drilled to a depth greater than the final design depth of the
monitoring well, either for exploratory purposes or by error. Boreholes that are not
sealed below the final design depth (whether collapsed or left open) may create a
vertical conduit for preferential flow. Purging and sampling of the completed well
may bring up a non-representative volume of water from below the screen.
Therefore, boreholes should be backfilled with a low-permeability material (e.g., a
cement-bentonite grout mixture) to the design depth. In highly permeable formations,
where vertical preferential flow is less critical, sand may be used in place of the grout
seal to stabilize the hole to the design depth.
2.2 Stratigraphic Control
Adequate stratigraphic control is critical to the geologic investigation. Cal EPA
recommends that every borehole should be continuously sampled. When continuous
sampling of every borehole is not feasible, selected boreholes should be continuously
sampled; their number and locations should be chosen to provide representative
coverage of site geology and areas of interest to the study. For boreholes that are not
continuously sampled, Cal EPA recommends that samples be collected at all suspected
changes in lithology. For boreholes that will be completed as monitoring wells, at least
one sample should be collected from the interval that will contain the monitoring well
intake (i.e., the screened or open (uncased) interval).
Borehole samples should be classified according to their lithology or pedology. Care
should be taken to ensure that samples of every geologic formation, especially all
confining layers, are collected, and that the nature of stratigraphic contacts is
determined.
The RP should prepare stratigraphic cross-sections, both in the direction of groundwater
flow and orthogonal to ground-water flow. The number and locations of the
cross-sections should be sufficient to illustrate the geologic and hydrogeologic features
that may influence contaminant transport. Cross-sections should be based on both the
monitoring well boring logs and the boring logs from the subsurface boring program.
Site stratigraphy represented on the cross-sections should be compared against
known regional stratigraphy to verify the well/boring logs and to prepare an analysis of
site-specific stratigraphy. Cal EPA recommends that in complex geologic settings
borehole geophysical logging, surface geophysical surveys, and/or cone penetrometer
surveys be performed both to verify the logs of cuttings or samples and to assist in
establishing stratigraphic control. When planning such surveys it is important to
remember that drilling methods and well casings/screens will influence the selection
of geophysical methods (e.g., electrical resistivity logging cannot be performed in
cased wells).
2.3 Driven Wells
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Driven wells consist of a steel well screen that is either welded or attached with drive
couplings to a steel casing. The well screen and attached casing are forced into the
ground by hand using a weighted drive sleeve, or with a heavy drive head mounted
on a hoist. As the well is driven, new sections of casing are attached to the well in 4-
or 5-foot sections.
Several problems are commonly associated with the installation of driven wells.
First, it is very difficult or impossible to drive a well through dense silts, clays or
materials containing boulders. If penetration in these materials is accomplished, the
well screen may be destroyed in the process. In addition, silts and/or clays can clog
the well screen to the point where the well cannot be satisfactorily developed. Two
techniques, described in Aller et al. (1989) have been employed to attempt to
alleviate these problems. Driven wells may be helpful as a tool for preliminary field
studies requiring installation of shallow piezometers. However, in most cases, Cal
EPA discourages the sole use of the driven well construction method for the purpose
of installing monitoring wells. This is primarily because of the inability to collect
representative samples of the materials that are penetrated during well installation,
and the inability to seal the well properly unless an outer casing is driven first.
However, if samplers can be driven in advance of the casing to allow subsurface
sample collection, the driven well method may be a viable well installation option.