1-1. Groundwater Monitoring Guidance (Nov. 2025).pdf

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Appendix A

Groundwater Monitoring Guidance

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TABLE OF CONTENTS

APPENDIX A: GROUNDWATER MONITORING GUIDANCE ................................................ A-1 A. Overview ..................................................................................................................................... A-1

1. Introduction ..................................................................................................................... A-1

2. References ....................................................................................................................... A-2

B. Monitoring Well Types and Construction .................................................................................. A-3

1. Objectives of Monitoring Wells...................................................................................... A-3

2. Types of Groundwater Monitoring Systems ................................................................... A-3

3. Choice of Monitoring System ......................................................................................... A-7

4. Minimum Construction Standards .................................................................................. A-7

a) Materials ............................................................................................................. A-9

b) Assembly and Installation ................................................................................... A-9

c) Well Development ............................................................................................ A-10

d) Recordkeeping and Reporting........................................................................... A-11

5. Direct Push Technology ................................................................................................ A-12

a) Advantages of DPT ........................................................................................... A-12

b) Disadvantages of DPT ...................................................................................... A-13

6. References ..................................................................................................................... A-13

C. Locations and Depths of Monitoring Wells .............................................................................. A-15

1. Importance .................................................................................................................... A-15

2. Approach to Determining Monitoring Locations and Depths ...................................... A-15

a) Background Monitoring .................................................................................... A-16

b) Site Characterization Monitoring ...................................................................... A-16

c) Attainment and Postremedial Monitoring ......................................................... A-16

3. Factors in Determining Target Zones for Monitoring .................................................. A-16

a) Groundwater Movement ................................................................................... A-17

i) Geologic Factors ................................................................................... A-17

ii) Groundwater Barriers............................................................................ A-18

iii) Karst Terrane ........................................................................................ A-20

iv) Deep-Mined Areas ................................................................................ A-23

b) Contaminant Distribution.................................................................................. A-23

4. Areal Placement of Wells ............................................................................................. A-23

5. Well Depths, Screen Lengths, and Open Intervals ....................................................... A-24

6. Number of Wells ........................................................................................................... A-26

7. Well Yield ..................................................................................................................... A-26

a) Fractured Rock .................................................................................................. A-27

b) Heterogeneous Unconsolidated Formations ..................................................... A-27

c) Areas of Uniformly Low Yield ......................................................................... A-27

8. References ..................................................................................................................... A-28

D. Groundwater Sampling Techniques .......................................................................................... A-30

1. Importance of Sampling Technique .............................................................................. A-30

2. Sample Collection Devices ........................................................................................... A-32

3. Sample Collection Procedures ...................................................................................... A-32

a) Protective Clothing ........................................................................................... A-32

b) Water Levels ..................................................................................................... A-32

c) Field Measurements .......................................................................................... A-32

d) Purging .............................................................................................................. A-33

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i) Criteria Based on the Number of Bore Volumes .................................. A-35

ii) Criteria Based on Stabilization of Indicator Parameters ....................... A-36

iii) Low Flow Purging ................................................................................ A-36

iv) Special Problems of Low-Yielding Wells ............................................ A-37

v) No-Purge Methods ................................................................................ A-38

vi) Summary on Purging ............................................................................ A-39

e) Management of Purge Water ............................................................................ A-40

f) Private Wells ..................................................................................................... A-41

g) Filtering ............................................................................................................. A-41

h) Sample Preservation.......................................................................................... A-42

i) Decontamination of Sampling Devices ............................................................ A-43

j) Field Sampling Logbook................................................................................... A-44

k) Chain-of-Custody .............................................................................................. A-45

4. References ..................................................................................................................... A-45

E. Well Decommission Procedures ............................................................................................... A-47

1. Introduction ................................................................................................................... A-47

2. Well Characterization ................................................................................................... A-47

3. Well Preparation ........................................................................................................... A-48

4. Materials and Methods .................................................................................................. A-48

a) Aggregate .......................................................................................................... A-48

b) Sealants ............................................................................................................. A-49

c) Bridge Seals ...................................................................................................... A-50

5. Recommendations ......................................................................................................... A-50

a) Casing Seal........................................................................................................ A-50

b) Wells in Unconfined or Semi-Confined Conditions ......................................... A-51

c) Wells at Contaminated Sites ............................................................................. A-51

d) Flowing Wells ................................................................................................... A-51

e) Wells with Complicating Factors at Contaminated Sites ................................. A-52

f) Monitoring Wells .............................................................................................. A-52

6. Existing Regulations and Standards.............................................................................. A-54

7. Reporting....................................................................................................................... A-54

8. References ..................................................................................................................... A-54

F. Quality Assurance/Quality Control Requirements ................................................................... A-55

1. Purpose .......................................................................................................................... A-55

2. Design ........................................................................................................................... A-55

3. Elements ........................................................................................................................ A-55

4. References ..................................................................................................................... A-58

Figure A-1: Recommended Construction of an Open Borehole Well ................................................... A-4 Figure A-2: Recommended Construction of a Single-Screened Well ................................................... A-5 Figure A-3: Example of a Well Cluster ................................................................................................. A-6 Figure A-4: Examples of Target Zones ................................................................................................. A-8 Figure A-5: Monitoring Well Screens Placed Too Deeply Below the Target Zone to

Detect Contamination ......................................................................................................... A-8 Figure A-6: Effect of Fractures on the Spread of Contamination ........................................................ A-19 Figure A-7: Ineffective Monitoring Wells in a Carbonate Aquifer ..................................................... A-21 Figure A-8: Summary of Procedures for Well Decommissioning....................................................... A-53

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Table A-1: Advantages and Disadvantages of Different Sampling Devices ....................................... A-34

Table A-2: Procedure for the Management of Well Purge Water from Groundwater

Sampling ............................................................................................................................ A-42

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APPENDIX A: GROUNDWATER MONITORING GUIDANCE

When groundwater is an affected medium, monitoring it is an extremely important part of site characterization, fate and transport assessment, and ultimately, demonstrating attainment of a cleanup standard at Act 2 sites. Taking this under consideration, the Groundwater Monitoring Guidance identifies technical considerations for performing detailed yet concise hydrogeologic investigations and groundwater monitoring programs at Act 2 sites. The purpose of this guidance is to ensure consistency within the Department and to inform the regulated community of DEP’s technical recommendations and the basis for them.

The methods and practices described in this guidance are not intended to be the only methods and practices available to a remediator for attaining compliance with Act 2 regulations. The procedures used to meet requirements should be tailored to the specific needs of the individual site and Act 2 project and based on the history, logistics, and unique circumstances of those sites. The guidance is not intended to be a rigid step-by-step approach that is utilized in all situations. The Department recommends that site remediators consult with DEP Regional Office staff for assistance in evaluating and understanding site characterization information for a more efficient Act 2 cleanup.

A. Overview

1. Introduction

Monitoring of groundwater quality is an important component in the application of and compliance with Act 2 of 1995, the Land Recycling and Environmental Remediation

Standards Act (Act 2, 35 P.S. §§ 6026.101-2026.908). The goal for monitoring groundwater quality is to obtain reliable data and information that is representative of aquifer characteristics, groundwater flow direction, and physical and chemical characteristics of the groundwater.

Before beginning a hydrogeologic investigation at an Act 2 site, a conceptual site model

(CSM) should be developed based on site geology and hydrogeology and the characteristics of the release. The CSM should estimate distribution of predominant geologic units, flow conditions, location of aquifers and aquitards (if known), water table surface and other pertinent hydrogeologic factors present at the site. Coupled with hydrogeologic properties at the Act 2 site, the CSM should consider the type of contaminant which has been released and its physical properties (e.g., petroleum-based or solvent-based, weathered vs. fresh, etc.), the manner of release to the environment, and the volume of the release as can best be determined.

Typical groundwater quality monitoring at Act 2 sites may include:

• Background monitoring: relating to determination of background conditions in accordance with the Act 2 background cleanup standard (e.g. establishing if a groundwater contaminant is naturally occurring, an areawide problem typically resulting from historic, areawide releases, or from an upgradient source). The results of background groundwater monitoring will form a basis against which future monitoring results will be compared to established background values for specific regulated substances of concern, develop groundwater quality trend

261-0300-101 / March 27, 2021 / Page A-2 analyses, or remediation effectiveness under Act 2 when the background cleanup standard is selected.

• Site Characterization: During site characterization, groundwater monitoring wells may be installed and sampled at an Act 2 site throughout the area(s) of contamination, as well as in areas not affected by the release of any regulated substance. Some of the data collected at the monitoring well locations may include groundwater elevations, which are then used to calculate groundwater flow direction and hydraulic gradient, permeability of aquifer materials, porosity of the aquifer, the types of regulated substances present and their concentrations, and the spatial variation in concentration, both horizontally and vertically. A fate and transport assessment most likely should be implemented during this phase of the Act 2 investigation.

• Attainment monitoring: Attainment monitoring of groundwater is performed to demonstrate that the selected Act 2 cleanup standard has been attained at the Point of Compliance (POC). Refer to Section II.B of this guidance for additional information on this concept. Attainment monitoring is also utilized to determine the effectiveness of groundwater cleanup activities.

• Postremedial monitoring: Postclosure monitoring is conducted to determine any changes in groundwater quality after the cessation of a regulated activity or activities. This monitoring may also be part of a postremedial care plan, such as periodic monitoring of sentinel wells. Analytes most likely to be included are those which were monitored during site characterization and/or attainment monitoring.

2. References

Alaska Department of Environmental Conservation, September 2013, Division of Spill

Prevention and Response Contamination Sites Program, Monitoring Well Guidance.

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B. Monitoring Well Types and Construction

1. Objectives of Monitoring Wells

Monitoring wells should be located and constructed to provide the controlled access necessary to characterize the groundwater at an Act 2 site. Wells should be constructed by a driller who is licensed by the Commonwealth of Pennsylvania (Act 610 of 1956, 32 P.S. § 645.12, and 17 Pa. Code Chapter 47). Drillers do not need to be licensed to install piezometers, temporary well points, or in-situ sampling probes.

Monitoring wells should effectively achieve one or more of the following objectives:

• Provide access to the groundwater system for collection of water samples.

• Measure the hydraulic head at a specific location in the groundwater flow system.

• Provide access for conducting tests or collecting information necessary to characterize the chemical properties of aquifer materials or their hydrologic properties.

While achieving these objectives, the groundwater monitoring system should also preserve the conditions of the subsurface that is penetrated, but not monitored. For example, a well designed to monitor a bedrock aquifer should be designed and installed with minimal or no impact to the flow system in the unconsolidated material overlying the bedrock.

Although monitoring (or observation) wells may be used to measure water levels and then determine the configuration of the water table, or other potentiometric surface, the focus of this appendix is groundwater quality monitoring. Specifically, this appendix provides guidance for the monitoring of groundwater at Act 2 sites.

2. Types of Groundwater Monitoring Systems

Groundwater monitoring systems range from the simple to the complex. Each system has its own value and use in the monitoring environment. Various types of groundwater monitoring systems are described below. General recommendations for the construction of single-screened wells and open boreholes are shown in Figures A-1 and A-2. Site-specific circumstances may require modifications to the recommended construction details.

Open boreholes - These boreholes are typically drilled into competent bedrock with the casing extending completely through the overburden (unconsolidated material) and into the competent rock below. Note that a vertical conduit is created which may intercept active groundwater flow zones (controlled by primary porosity and secondary porosity;

i.e. fractures, bedding planes, solution cavities) previously not in contact with each other, potentially resulting in cross contamination. Recommended installation details are shown in Figure A-1.

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Figure A-1: Recommended Construction of an Open Borehole Well

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Figure A-2: Recommended Construction of a Single-Screened Well

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Figure A-3: Example of a Well Cluster

Single screened wells - These wells consist of a prefabricated screen of polyvinylchloride plastic, stainless steel, etc., that is inserted into an open borehole. Clean sand or gravel is placed around the annular space of the screen for the entire vertical distance of the screen length and slightly higher past the connecting screen and well casing. Recommended installation details are shown in Figure A-2.

Well clusters - Well clusters, or a well nest, consist of the construction of open boreholes or screened monitoring wells in a specific location, with each well monitoring a different depth or zone of groundwater. An example of a well cluster is shown in Figure A-3.

Well points - Well points are usually short lengths (i.e., 1-3 feet) of screen attached to a hardened metal point so that the entire unit can be driven, pushed, or drilled to the desired depth for monitoring. (This method is usually limited to shallow, unconsolidated formations.)

Piezometers - These are small diameter wells, generally non-pumping, with a very short well screen or section of slotted pipe at the end that is used to measure the hydraulic head at a certain point below the water table or other potentiometric surface.

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3. Choice of Monitoring System

The type of monitoring system chosen depends on the objectives of monitoring at the site. Once the target zones, or areal locations and depths that are the most likely to be impacted by the release are defined, monitoring is often adequately accomplished by using open rock boreholes or single-screened wells that monitor the entire saturated thickness, or a large portion of the target zone.

Where contamination has been detected and definition of vertical contaminant stratification is desired, wells that monitor more discrete intervals of the target zone, or individual aquifers, usually need to be constructed. In this case, well clusters such as shown in Figure A-3 will often be the construction design of choice, although open holes that monitor a short vertical interval or single water-bearing zone also may have application. As the flow beneath the site is better understood, the monitoring system typically will target more specific depths and locations.

Well points, or in-situ sampling probes (direct push technology), can be valuable reconnaissance tools for preliminary site characterizations, or for determining the locations of permanent monitoring wells (see EPA, 1993 and ITRC, 2006). However, in-situ sampling probes can miss a light nonaqueous phase liquid (LNAPL) on the water table and may have problems penetrating coarse sands and gravel (where contamination may be located). Other potential problems include very slow fill times in clayey sediments and significant capture of fines in the sample.

Special well construction will be needed to monitor for certain types of contaminants.

For example, if an LNAPL is a concern, the well screen should be open, bridging the top of the water table and within the zone of fluctuation, so that the LNAPL contaminants will not be cased-off.

4. Minimum Construction Standards

To properly meet the objectives listed in Section B.1, monitoring wells should be designed and constructed using minimum standards in each of the following categories.

1) Materials

2) Assembly and installation

3) Well development

4) Recordkeeping and reporting

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Figure A-4: Examples of Target Zones

Figure A-5: Monitoring Well Screens Placed Too Deeply Below the Target Zone to Detect

Contamination

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Different standards and practices may be necessary depending upon the monitoring objectives of an individual site. Monitoring wells constructed to meet multiple objectives should employ the standards of the most rigorous objective. For instance, a well point may be suitable for monitoring hydraulic head, but may not be optimum for collecting samples. Therefore, a well proposed to monitor head and collect water samples should be designed as a conventional, screened well and not as a well point. In addition, construction methods, materials, and well development of each point in the plan must not compromise the objective of other monitoring wells in the well system.

a) Materials

Materials that are used in construction of a monitoring well should not contaminate the groundwater being monitored. A list of materials should include, but not be limited to, the drilling tools and equipment, casing, riser pipe, well screen, centralizers (if needed), annular sealant, filter pack, and drilling fluids or additives. All materials should be of adequate size and of competent strength to meet the objectives of the monitoring point. All materials introduced into the boring should be free of chemicals or other contaminants that could compromise the monitoring well or other downgradient wells. Practices must be employed to minimize the potential for contamination of the materials during storage, assembly, and installation. Specific cleaning procedures should be employed in situations where the materials might introduce contaminants to the groundwater system. Well screens and risers should be coupled using either water-tight flush-joint threads or thermal welds. Solvent welded couplings are not recommended for monitoring well construction.

b) Assembly and Installation

Equipment and techniques should be used that create a stable, open, vertical borehole of large enough diameter to ensure that the monitoring well can be installed as designed, while minimizing the impact on the zone(s) being monitored. When drill cuttings and groundwater removed during construction will likely be contaminated, procedures commensurate with the type and level of contamination should be followed for the handling, storage, and disposal of the contaminated material. Whenever feasible, drilling procedures that do not introduce water or other liquids into the borehole should be utilized. When the use of drilling fluids is unavoidable, the fluid should have as little impact on the constituents of interest as possible. If air or other gas is used as the drilling fluid, the compressor should be equipped with an oil air filter or an oil trap.

The well screen and riser assembly should be installed using procedures that ensure the integrity of the assembly. If water or other ballast is used, it should be of known and compatible chemistry with the water in the boring. Unless designed otherwise, the assembly should be installed plumb and in the center of the boring.

Centralizers of proper spacing and diameter can be used. Depending upon the physical environment, the well should be finished as a secure stick-up or flushmount at the discretion of the project geologist. Either completed type of well should be securely capped to prevent the entry of foreign material.

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Installation of the filter pack, sealants, or other materials in the annular space should be done using tremie pipes or other accepted practices. Protective casing and locking well caps must be installed, and any other necessary measures must be taken to ensure that the monitoring well is protected from vandalism and accidental damage. To reduce misidentification, all monitoring wells constructed in developed areas, or in any location where they may be mistaken for other structures (such as tank-fill tubes, drains, and breather tubes), should have a locking cap conspicuously labeled “Monitoring Well” (preferably by the well-cap manufacturer). In addition, locks for the monitoring wells should use a key pattern different from locks on other structures at the site. It is also advisable that the well identification number be placed on both the inside and outside of the protective casing.

c) Well Development

After installation, groundwater monitoring wells should be developed to:

• Correct damage to the geological formation caused by the drilling process;

• Restore the natural water quality of the aquifer in and around the well;

• Optimize hydraulic communication between the geologic formation and the well screen; and

• Create an effective filter pack around the well screen.

Well development is necessary to provide groundwater samples that represent natural undisturbed hydrogeological conditions. When properly developed, a monitoring well will produce samples of acceptably low turbidity (less than

10 Nephelometric Turbidity Units (NTUs) as recommended by U.S. EPA, 2013).

Low turbidity is desirable as turbidity may interfere with subsequent analyses, especially for constituents that sorb to fine-grained materials, such as metals

(CEPA, 2014). Well development stresses the formation and filter pack so that fine-grained materials are mobilized, pulled through the well screen into the well, and removed by pumping.

Well development should continue until as much of the fine-grained materials present in the well column have been removed as possible. It is important to record pumping rates utilized during well development. Purging and sampling rates should not exceed the maximum pumping rate used during well development. When it is likely that the water removed during development will be contaminated, procedures commensurate with the type and level of contamination should be utilized and documented for the handling, storage, and disposal of the contaminated material. Development methods should minimize the introduction of materials that might compromise the objective of the monitoring. If air is used, the compressor should have an oil air filter or oil trap.

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Repeated well development may be conducted as necessary at the discretion of the project geologist, especially if clogged screens or biofouling are evident.

d) Recordkeeping and Reporting

Because interpretation of monitoring data from a monitoring well is spatially dependent on both the activity being monitored and other monitoring wells in the system, records and samples of the materials used to construct and drill the monitoring well should be kept. Following construction, accurate horizontal and vertical surveys should be performed. The surveys should be completed by personnel knowledgeable in land surveying techniques. A permanent reference point should be made by notching the riser pipe. Whenever possible, all reference points should be established in relation to an established National Geodetic

Vertical Datum (NGVD). Monitoring well locations should be surveyed to

1 linear foot, and monitoring well elevations should be to the nearest .01 foot.

Elevations of the protective casing (with the cap off or hinged back), the well casing, and the ground surface should be surveyed for each monitoring well (see

Nielsen, 1991). DEP-permitted facilities are generally required to record the latitude and longitude for each monitoring well (this also is recommended for non-permitted facilities).

A groundwater monitoring network report should be prepared. This report should include copies of the well boring logs, test pit and exploratory borehole logs;

details on the construction of each monitoring point; maps, air photos or other information necessary to fully describe the location and spatial relationship of the points in the monitoring system; and a recommended decommissioning procedure consistent with the applicable regulatory program and the well decommissioning procedures recommended in Section E of this appendix.

Monitoring well logs should be prepared and should describe, at a minimum, the date of construction; the thickness and composition of the geologic units

(identification of stratigraphic units should be completed on the well log using the

Unified Soil Classification System); the location and type of samples collected;

the nature of fractures and other discontinuities encountered; the nature and occurrence of groundwater encountered during construction, including the depth and yield of water-bearing zones; headspace of photoionization detector (PID) readings collected; any observations of contamination (e.g. NAPL); and the static water level upon completing construction.

A well completion plan should also be included in the monitoring network report.

Each plan should include information on the length, location, slot size, and nature of filter pack for each screen; type, location and quantity of material used as annular seals and filler; description of the type and effectiveness of well development employed; and notes describing how the well, as constructed, differs from its original design and/or location.

The reports described above do not relieve the driller from the obligation to submit, for each well drilled, a Water Well Completion Report to the Department

261-0300-101 / March 27, 2021 / Page A-12 of Conservation and Natural Resources (DCNR), Bureau of Topographic and

Geologic Survey, as required by Act 610 (the Water Well Drillers License Act).

5. Direct Push Technology

Direct Push Technology (DPT) devices are investigative tools that drive or ‘push’ small-diameter rods into the subsurface via hydraulic or percussive methods without the use of conventional drilling. DPT has been in use in the environmental industry for more than two decades and its utilization as a tool for performing subsurface investigations in

Pennsylvania and many other states has grown concurrently with its evolving technology.

Monitoring wells installed using DPT could either be field-constructed, similar to conventionally drilled and installed wells, or installed using pre-packed well screens.

The pre-packed well screen assemblies consist of an inner slotted screen surrounded by a wire mesh sleeve which acts as a support for filter media (sand). The sand is packed between the slotted screen and the mesh. It is important to note that only DPT pre-packed wells are considered suitable for Act 2 sites, due to quality assurance concerns regarding field-construction and associated problems placing the filter pack around the screens of small-diameter wells.

a) Advantages of DPT

Depending on site conditions, DPT offers an attractive alternative to conventional auger drilling and split spoon sampling. The smaller size of DPT rigs enables well installation and sampling in areas not accessible to traditional large auger rigs.

As DPT methods utilize a smaller diameter boring than conventional drilling, less solid waste is generated. Similarly, less liquid waste will be generated from smaller diameter monitoring wells. Because less waste is generated, worker exposures are reduced.

Overall, there is minimal disturbance to the natural formation using DPT in comparison with auger drilling.

From an economic standpoint, DPT has several advantages versus conventional drilling. In relation to project schedule and budget, the time-effectiveness of DPT installation may enable the remediator to investigate more areas of a site than traditional hollow stem auger (HSA) drilling would allow and in a shorter time.

Fewer well construction materials may enable a remediator to install additional monitoring points on a limited budget.

Most importantly, short-term and long-term groundwater monitoring studies conducted by others have produced results demonstrating that water samples collected from DPT installed wells are comparable in quality to those obtained from conventionally constructed wells.

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b) Disadvantages of DPT

DPT cannot completely replace the use of conventional drilling/monitoring well installation as limitations of the technology are evident in certain situations. DPT is only useful at generally shallow depths (less than 100 feet below surface grade) and in unconsolidated formations. DPT is not suitable for formations containing excessive gravel, cobbles, boulders, etc., or for bedrock drilling due to the obvious lack of augering capabilities.

DPT may be utilized for monitoring well installation below confining layers or as

‘nested’ wells with extreme caution. DPT utilizing only a macrocore barrel and drive rods may not provide for the advancement of casing to keep the borehole open and seal off each separate zone of saturation, which therefore can potentially allow for the mixing of separate zones of saturation when the push rods are withdrawn from the borehole. Therefore, DPT may be utilized for this purpose only if the project geologist can ensure that the threat of cross-contamination from separate zones of saturation above clean zones of saturation will not occur.

If large volumes of aqueous sample are required, DPT installed monitoring wells may not be suitable due to the small diameter of the well screen.

Since DPT causes smearing and compaction of the borehole sides, proper well development techniques are vital to ensure that natural hydraulic permeabilities are maintained. Several studies have demonstrated that hydraulic conductivities can vary by an order of magnitude lower for wells installed by DPT versus wells installed by conventional HSA. For this reason, DPT-installed wells may not be suitable for aquifer characteristics testing, nor for efficient groundwater recovery.

Great care needs to be taken to ensure adequate well development when using

DPT for well installations.

6. References

Aller, L., and others, 1989, Handbook of Suggested Practices for the Design and

Installation of Ground-Water Monitoring Wells, National Water Well Association (This publication covers nearly all aspects of design and construction of monitoring wells.

Each of the eight chapters has an extensive reference list.).

American Society for Testing and Materials, 2010, Standard Practice for Direct Push

Installation of Prepacked Screen Monitoring Wells in Unconsolidated Aquifers, ASTM-D6725-04.

Anderson, K.E., 1993, Ground Water Handbook, National Ground Water Association (A quick reference containing tables, formulas, techniques and short discussions covering, among other things, drilling, well design, pipe and casing, and groundwater flow).

California Environmental Protection Agency, 2014, Well Design and Construction for

Monitoring Groundwater at Contaminated Sites.

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Driscoll, F.G., 1986, Groundwater and Wells, Second Edition, Johnson Filtration

Systems, Inc., St. Paul, Minnesota 55112, 1089 pp.

Gaber, M.S., and Fisher, B.O., 1988, Michigan Water Well Grouting Manual, Michigan

Department of Public Health (Very thorough coverage of grout and sealant formulation, characteristics, handling, and placement.).

Interstate Technology Regulatory Council, 2006, The Use of Direct Push Well

Technology for Long-Term Environmental Monitoring in Groundwater Investigations.

Nielsen, D.M., (Editor), 1991, Practical Handbook of Ground-Water Monitoring, NWWA, Lewis Publishers, Inc., Chelsea, Michigan 48118, 717 pp.

Ohio Environmental Protection Agency, 2016, Technical Guidance for Ground Water

Investigations, Chapter 15, Use of Direct Push Technologies for Soil and Ground Water

Sampling, Revision 1.

U.S. Environmental Protection Agency, 1993, Subsurface Characterization and

Monitoring Techniques-A Desk Reference Guide, Volume 1: Solids and Ground Water

(Largely 1 to 2-page thumbnail descriptions of methods and equipment.).

U.S. Environmental Protection Agency, 1997, Expedited Site Assessment Tools for

Underground Storage Tank Sites.

U.S. Environmental Protection Agency, 2005, Groundwater Sampling and Monitoring with Direct Push Technologies.

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C. Locations and Depths of Monitoring Wells

1. Importance

The locations and depths of monitoring wells are the most important aspects of a groundwater monitoring network. A monitoring point that is misplaced, or not constructed properly to monitor constituents with unique physical characteristics, is of little use and may misrepresent the quality of the groundwater migrating to or from a site.

On the other hand, a properly positioned and constructed monitoring well that detects the earliest occurrence of contamination could save both time and money spent on cleanup of a site. It is important to note that the placement and construction of a groundwater monitoring network at an Act 2 site shall be conducted by a professional geologist licensed in Pennsylvania (25 Pa. Code §§ 250.204(a), 250.312(a), and 250.408(a)).

2. Approach to Determining Monitoring Locations and Depths

Different approaches and efforts for determining the location and depth of wells may be necessary based on the type of monitoring to be done. However, before well locations are chosen for any type of monitoring, the existing data should be evaluated. This can reduce the costs of implementing the monitoring program and can help to make appropriate choices for three-dimensional monitoring locations.

The most efficient way to accomplish the location and depth of monitoring wells for an

Act 2 study is to formulate a CSM, or conceptual groundwater flow model. A conceptual groundwater flow model is the illustrative delineation and formulation of the important controlling components of groundwater flow and thus contaminant transport from recharge areas to discharge zones or withdrawal points. Without a proper conceptualization of groundwater flow, a groundwater model can give spurious results.

On the other hand, a well-developed conceptual model may allow groundwater flow to be accurately approximated without using computer modeling or complex analytical procedures. The groundwater conceptual model is an important tool in the study of groundwater flow on both a local and even larger scale. The goal of the conceptual model is to represent the controlling aspects of groundwater flow at the site being investigated. Important controlling components of groundwater flow can include geological characteristics, geologic structural and stratigraphic relationships, anisotropy, calculated groundwater flow directions and recharge and discharge relationships.

Information may be obtained through site visits, site records and previous studies, interviews with present and past workers, aerial photographs, scientific publications on the local and regional hydrogeology, geophysical surveys, borings, wells, aquifer tests, etc. If enough information is available, the designer can determine the groundwater flow paths and design a complete monitoring network. However, actual testing of aquifer parameters and borehole geophysics provides the best information to evaluate placement and construction of monitoring wells, especially in newly established sites or facilities where little site information is available.

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a) Background Monitoring

The determination of background water quality is paramount in understanding the effect of an activity or site on groundwater quality. Often, insufficient site information is available so that initial well locations may depend on casual observations and assumptions regarding groundwater flow. If subsequent information shows that monitoring wells are misplaced, new wells should be installed.

b) Site Characterization Monitoring

Appropriately placed monitoring wells are necessary to detect groundwater quality at an Act 2 site. The more that is known about the history of operations at the site, (potential) contaminant flow paths, and the constituents that may have been discharged to the environment, the more likely that monitoring wells installed during the site characterization phase of the investigation will be optimally placed and constructed to monitor the impact on groundwater quality.

Monitoring well locations should be concentrated in those areas that will most likely first be impacted by the known discharges on the site, which typically will be located within or comprise the uppermost aquifer. As groundwater data is collected, additional monitoring wells may need to be installed to fully characterize the groundwater contaminant plume(s) present. The greater the complexity of the hydrogeology and the spread of contamination, the more monitoring wells that may be necessary to characterize the contamination.

c) Attainment and Postremedial Monitoring

Any number of wells, including all installed during the site characterization phase, may be used for attainment monitoring. These wells will demonstrate attainment of the chosen cleanup standard at the POC. The impact of any remediation conducted at the Act 2 site on the groundwater flow paths (e.g.

pumping the aquifer) should be considered for placement of attainment monitoring wells. Postremedial monitoring would likely be conducted in the same wells as attainment monitoring to monitor for any residual rebound occurring in the aquifer after remediation activities have been completed.

3. Factors in Determining Target Zones for Monitoring

The prime requirement for a successful monitoring system is to determine the “target” zones - the spatial locations and depths that are the most likely areas to be impacted by the site being investigated. The dimensions of target zones depend on the vertical and horizontal components of flow in the aquifers being monitored, the size of the Act 2 site, the potential contaminants released, and the distance that contamination may have traveled from the facility since being released. Figure A-4 shows how different target zones could be formed based on these factors.

Horizontal and vertical components of groundwater flow are best determined by constructing planar and cross-sectional flow nets based on the measurement of water levels in piezometers. Where the vertical components of flow are negligible, wells, rather

261-0300-101 / March 27, 2021 / Page A-17 than piezometers, drilled into the aquifer to about the same depth, will allow preparation of a contour map of water levels representing horizontal flow. This should be adequate to prepare a planar flow net and determine the target zone.

With regard to upgradient wells, target zones (as defined above) do not exist. Upgradient wells should be drilled to depths that are screened or open to intervals similar to that of the downgradient wells, or to depths that yield water that is otherwise most representative of the background quality of the water being monitored by the downgradient wells. In other words, upgradient wells should be installed within the same hydrogeologic aquifer to the respective downgradient wells.

The numerous site details to consider when establishing target zones may be grouped into either groundwater movement or the spatial distribution of contamination.

a) Groundwater Movement

In what direction is groundwater flowing? If flow paths are not easily determined, what will influence the direction of groundwater flow? The answers to these questions are critical to selecting target zones and the optimal locations of monitoring wells.

Using the groundwater levels from piezometers or wells at the site, the groundwater flow direction and hydraulic gradient can be determined. At least three monitoring points are needed to determine the horizontal flow direction and hydraulic gradient; however, at some sites, knowledge of the vertical component of flow may be important. This is best accomplished by using well pairs of

“shallow” and “deep” piezometers or short-screened wells.

It may appear to be a simple task to place monitoring wells in downgradient positions using a map of the groundwater elevation contours, or by anticipating the flow direction based on topography or discharge points. However, at many sites, three-dimensional flow zones must be understood to install appropriate monitoring points (see Section C.5 of this appendix). Figure A-5 shows how a well can miss the vertical location of contamination at a site. Water level measurements, piezometer and well construction logs, geologic well logs, and groundwater flow direction maps should be reviewed carefully when assessing the dimensions of target zones.

i) Geologic Factors

The geology of a site can complicate the selection of the target zones for monitoring. Geologic factors can produce aquifers that are anisotropic. In an anisotropic aquifer, the hydraulic conductivity is not uniform in all directions so that groundwater moves faster in one direction than another and oblique to the hydraulic gradient. Anisotropy can result from various sedimentary or structural features such as buried channels, bedding planes, folds, faults, voids, and fractures.

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In Pennsylvania, most of groundwater flow in bedrock is through fractured rocks. Fracture flow in bedrock (or hardened sediments) requires additional considerations compared to flow in unconsolidated materials.

Consolidated materials may exhibit small effective porosities and low hydraulic conductivities that impede groundwater flow. However, the development of secondary porosity may allow substantial flow of groundwater through fractures, joints, voids, cleavage planes and foliations. These features tend to be highly directional, exhibit varying degrees of interconnection, and may produce local groundwater flow regimes that are much different from the regional trends.

Geologic factors influence the direction of groundwater flow by controlling the transmissivity. For example, Figure A-6 shows the effect of fractures on the spread of contamination. Although the gradient indicates flow to the north, groundwater also follows the major fractures and spreads to the northeast. Monitoring wells “1” and “2” located to the north of the site may detect contamination, but the lack of a monitoring well to the northeast will miss an important direction of migration.

Common sedimentary bedding planes also could have a similar effect on groundwater flow.

It is important to identify hydrostratigraphic intervals which may or may not be interconnected at the site when conducting a groundwater investigation. Monitoring wells should not be screened across two intervals as groundwater flow and concentrations of contaminants may differ significantly in each interval.

ii) Groundwater Barriers

The presence of hydrogeologic barriers should also be considered when locating wells in a groundwater monitoring system. A groundwater barrier is a natural geologic or artificial obstacle to the lateral movement of groundwater. Groundwater barriers can be characterized by a noticeable difference in groundwater levels on opposite sides of the barrier. Geologic faults and dikes along with tight lithologic formations such as shale and clay layers are common examples. Important types of barriers include the following:

Geologic faults - Fault planes…