Exhibit A6 - Groundwater Management Plan_FINAL_MEC 12-9-24.pdf

This document is revised periodically. The current version resides on the ESH Web page.

GROUNDWATER MANAGEMENT PLAN

December 2024

Author: Date:

Timothy Zei Hydrogeologist, Environmental Protection, Environmental Program Department

Approved: Date:

Lori Huntoon Manager, Environmental Program Department

Approved: Date:

Marc Clay Director (Interim), Environment Safety & Health Division

Distribution electronically ESH Website

Dec. 2, 2024

Dec. 6, 2024

GROUNDWATER MANAGEMENT PLAN

December 2024

Groundwater Management Plan

CONTENTS

Contents

1.0 Introduction

2.0 Groundwater Zones (Aquifers) and Well Locations

3.0 Subsurface Groundwater Migration

3.1 Glacial Deposits

3.2 Upper Bedrock

4.0 Groundwater Monitoring

4.1 Purpose Statement

4.2 Groundwater Monitoring Network

4.3 Influence of Surface Water

4.4 Areas of Contamination in the Upper Aquifer

5.0 Groundwater Well Policies

5.1 Wellhead Protection

5.2 Setback Zones

5.3 Monitoring Well Construction

5.4 Well Sealing and Abandonment

6.0 Sources

Appendix: Borehole and Well Specifications

1.0 Introduction

The Groundwater Management Plan (GMP or Plan) is a Department of Energy (DOE) mandate for effective integration of requirements among site programs and activities to ensure that federal, state, county, local, and other requirements pertaining to groundwater protection and management are incorporated in a consistent, effective manner.

This Plan documents the Fermi National Accelerator Laboratory (Fermilab) effort to prevent, eliminate, and/or minimize adverse impacts of operations to groundwater beneath the site. Its purpose is to determine the extent and understand the impact of past activities, remediate adversely affected areas, and monitor current operations. In addition, the data collected by the groundwater monitoring program provides a basis for projecting the potential impacts of planned activities on future groundwater conditions through groundwater modeling. The Plan is driven by Fermilab’s commitment to protect groundwater resources and it is an integral element of the Environmental Management System. Groundwater surveillance and compliance monitoring at Fermilab is carried out as required under the Resource Conservation and Recovery Act (RCRA) and the Fermilab Environmental Monitoring Strategy subject to DOE Order 458.1.

The Environmental Program Department is responsible for implementing and maintaining the GMP and its network of monitoring wells and piezometers. Groundwater sampling activities will be coordinated between the Environmental Monitoring group and other groups (e.g., Tritium Management) as needed.

2.0 Groundwater Zones (Aquifers) and Well Locations

Multiple groundwater zones (aquifers), regularly monitored at Fermilab, range from several feet deep to more than 400 feet below the ground surface (bgs) as depicted on Figure 1. Up to three groundwater zones have been observed in the unconsolidated glacial sediments. Similarly, three groundwater zones are monitored in the bedrock beneath the site.

The uppermost aquifer (as deep as 15 feet bgs) in the unconsolidated glacial sediments (comprising the Prairie Aquigroup) frequently exhibits hydraulic connection with nearby surface water bodies including lakes, ponds, and ditches. An intermediate aquifer in the Prairie Aquigroup (approximately 20-40 feet bgs) exists at some locations at Fermilab1. The depths of these relatively shallow glacial aquifers overlap and have a potential interaction with the majority of Fermilab’s subsurface accelerator structures. An additional aquifer (approximately 40-60 feet bgs) exists at the base of the glacial sediments in either the Batestown Member till or Henry Formation sand at several locations and could be in direct hydraulic connection with the underlying aquifer at the top of the bedrock. In previous communications with the Illinois Environmental Protection Agency (IEPA)2, groundwater in the glacial deposits overlying the bedrock at Fermilab has been determined to be Class II General Resource Groundwater. The Illinois Pollution Control Board (IPCB) publishes groundwater quality standards for Class II groundwater in 35 Illinois Administrative Code (IAC) Part 620.

Groundwater residing in or below the Silurian dolostone bedrock aquifer, the upper surface of which is 50 to 80 feet below the ground surface in the Joliet Formation at Fermilab, is classified as Class I potable resource groundwater. The IPCB has established groundwater quality standards found in 35 IAC Part 620 and defines Class I groundwater as a non-degradable resource, which is to be highly protected. The classification of an aquifer as a Class I groundwater resource involves numerous criteria, including a hydraulic conductivity of 0.28 feet/day (typical of sands or fractured bedrock) or greater, a sustained yield of 150 gallons/day or more, and a depth of at least 10 feet bgs. Deeper experiment structures such as the Neutrinos at the Main Injector (NuMI) tunnels directly intersect and interact with the bedrock aquifers down to approximately 360 feet bgs.

The locations and general aquifer designations of groundwater wells at Fermilab are shown in Figure 2. Wells sampled for environmental surveillance monitoring or as part of ongoing RCRA Facility Investigation (RFI) corrective actions are colored as red. An additional 124 wells are used as piezometers (pore-water pressure measuring apparatus) to gather information on groundwater flow directions site-wide. These data are used in conceptualizing the horizontal and vertical flow of groundwater and the potential transport of contaminants from past, present and future operational areas.

1 In the Central Utility Building (CUB) Pipe and Tile Field (SWMU 12), the intermediate groundwater zone is referred to as the “deep till zone.”

2 IEPA acceptance of the Compliance Commitment Agreement, dated July 26, 2006, and IEPA letter dated June 10, 1998 in response to a Class II demonstration in a 1995 RCRA RFI Work Plan.

Figure 1. General Fermilab geologic section (based on NuMI Tunnel), with typical monitoring depths and groundwater-bearing intervals. Sample/piezometer status is for 2024.

Figure 2. Groundwater well locations at Fermilab. Locations shown are wells within the EPD monitoring and sampling network, the LBNF temporary well network, and the ISD operational supply network as of December 2024.

A total of five semi-private water supply wells are presently in use across the Fermilab site. The majority of these supply wells are installed in the Joliet Formation and deeper Silurian limestone formations (approximately 100 to 240 feet deep) and are of minor use. An exception to this, Well W-4, is installed to a depth of 1,432 feet into the Ironton-Galesville Formation regional Cambrian- Ordovician sandstone aquifer system and serves as a back-up supply for Industrial Cooling Water (ICW) needs. Fermilab’s primary source for domestic water is provided by the City of Warrenville municipal supply system.

3.0 Subsurface Groundwater Migration

3.1 Glacial Deposits

Groundwater flow direction in the glacial deposits is either downward through the till units to the underlying aquifers or laterally to nearby discharge sinks such as field tiles, ditches, creeks, or ponds. Due to the high vertical gradient, downward flow is a major component of the flow direction within the upper glacial deposits. Horizontal flow is only a factor locally in the vicinity of discharge sinks. Based on Shelby Tube samples collected in limited locations, vertical seepage velocities range between 0.036 to 0.12 feet/year and horizontal seepage velocities range between

0.0006 to 0.14 feet/year.

3.2 Upper Bedrock

The groundwater flow direction in the upper bedrock aquifer (Joliet Formation) is toward the east (generally following the regional dip of the formation) and toward the south (following the bedrock surface topography) but is locally dependent on groundwater withdrawals (Figure 3). The NuMI tunnel captures groundwater in its vicinity, acting as a drain and creating an oblong cone of depression in the groundwater elevation contour map. This cone of depression also includes a recovering cone of depression centered on former water supply Well W-1, which ceased operation in the 2005 and was sealed in 2013. Future projections of the response of groundwater flow direction to withdrawal stresses will continue to be evaluated.

Figure 3. Groundwater elevation contours as measured December 5-12, 2022.

4.0 Groundwater Monitoring

4.1 Purpose Statement

Monitoring groundwater beneath Fermilab is important in order to determine impacts of current or ongoing Laboratory activities as well as to characterize contamination from past practices thereby enabling implementation of remediation as necessary. Groundwater monitoring is essential to quickly identify radioactive contamination of groundwater resources should radionuclides migrate downward from activated soil in the vicinity of beamlines or other experimental areas. It is also necessary to monitor for the existence and subsequent migration of chemical contamination.

The Illinois Groundwater Protection Act (IGPA) authorizes the groundwater monitoring requirements that affect Fermilab. Illinois requirements for groundwater quality and monitoring are presented in 35 IAC Part 620. Additional requirements are included in IEPA and USEPA directives (letters) attached to Fermilab’s RCRA Part B Permit which was issued by the IEPA in 2016.

4.2 Groundwater Monitoring Network

Previously, groundwater monitoring at Fermilab consisted of samples taken from water supply wells associated with previous farms located on site when Fermilab acquired the land in 1970. The construction of these wells consisted of casings drilled through the glacial deposits with open boreholes into the bedrock which received water from the upper dolomite aquifer system.

Consequently, the location of the wells was not related to the operational areas of the laboratory.

This construction and these locations meant that potential contamination in the glacial tills from sources at a distance from these wells could be diluted to the extent that it would not be detected from water samples collected at these locations. This well network was also not able to address the question of whether or not Fermilab was influencing the groundwater quality beneath the Site as wells are not situated at locations that could provide control data on the quality of the groundwater entering the site.

The current GMP is significantly more proactive, employing upgradient and source-specific monitoring wells and piezometers to allow for more effective monitoring of groundwater conditions beneath Fermilab. It has reduced the use of pre-existing water supply wells for anything other than gathering data on potentiometric surfaces to determine groundwater flow direction and risk monitoring as deemed appropriate. Groundwater samples from upgradient monitoring wells are used to evaluate radionuclides and/or chemical contaminants that may be entering the groundwater for comparison with samples taken from source-specific monitoring wells. Source-specific monitoring wells are located and constructed solely for the purpose of the early detection of contaminants from a potential source so that timely and effective preventive or remedial action may be taken. The characterization of geologic materials from borehole drilling and the installation of piezometers are used primarily to identify deposits for groundwater class designation and to evaluate the differences in water levels over distances to evaluate flow directions and gradients.

These are used together in determining the appropriate location and depth for source-specific monitoring wells. The continual monitoring of water levels in piezometers ensures that potential changes in groundwater flow direction will be detected. Permanent shifts in groundwater flow direction would necessitate the installation of new source-specific monitoring wells.

Two general types of potential contamination exist at Fermilab: radionuclide and chemical. The groundwater sampling program concentrates on analysis for the accelerator-produced radionuclides 3H, 7Be, 22Na, 54Mn, and 60Co. Low levels of 3H (less than 100 pCi/ml in non-regulated Class II groundwater) have been detected in source-specific wells screened in the glacial tills beneath the Neutrino shielding berm and at the Central Utilities Building (CUB) tile field.

Radiochemical contamination due to soil activation from site operations has been addressed for historic operational areas through a vulnerability investigation (Fermilab-TM-2097) and from current operations through shielding studies. Chemical contamination has also occurred in the past and is currently being monitored at the CUB Pipe and Clay Tile Field as discussed in Section 4.4.

4.3 Influence of Surface Water

Surface water monitoring for radionuclide and chemical contamination is performed at a number of locations within the site and at the site boundary to ensure that site operations have no detrimental influence on surface waters discharged from Fermilab. Recharge to the saturated zone in the glacial deposits occurs through infiltration of precipitation and percolation from surface waters. The various confining layers effectively insulate the bedrock aquifers from potential surficial radionuclide and chemical contamination due to dilution and radioactive decay during the significant amount of time required for water to percolate downward. Vertical migration rates measured at the Site range between 0.12 and 0.036 feet per year. Estimated recharge rates to the bedrock aquifer from the glacial deposits range from approximately 0.3 to 4.8 inches/year with a median rate of 2.9 inches/year.

Monitoring of the surface water is also conducted to evaluate the potential for groundwater contamination from direct introduction of surface water into wells on-site. Fermilab reduces this possibility through compliance with Wellhead Protection regulations located in 35 IAC Part 611, Subpart B, Section 611.212 in addition to proper well abandonment procedures.

4.4 Areas of Contamination in the Upper Aquifer

4.4.1 RADIOCHEMICAL CONTAMINATION

Meson/Neutrino Fixed Target Areas Five monitoring wells screened in the upper dolomite monitor for the presence of tritium and accelerator-produced isotopes in the Class I groundwater. These wells include one upgradient (MS-1) and four downgradient (NS1-4, NS2-4, NS2-5 and S-1290) of the historic activation areas in the Meson/Neutrino Fixed Target area. Five piezometers in this area supplement information on groundwater levels to ensure that these monitoring wells continue to be screened in locations downgradient of the areas of concern. No radionuclides have been detected in these wells. This area was removed as a SWMU from the RCRA Part B Permit and is voluntarily being monitored for accelerator produced isotopes on an annual basis.

4.4.2 CHEMICAL CONTAMINATION

CUB Pipe and Clay Tile Field

Currently there are 14 monitoring wells located at the abandoned CUB tile field near the center of the Main Ring to monitor for chloride and lead. Five of the wells are screened in the upper glacial till unit, five in the intermediate till units (referred to at SWMU 12 as “deep till”), one to the surface of the dolomite, and three are screened within the dolomite. Heavy metals including chromates and large quantities of sodium chloride were discharged into the tile field from 1974 to 1976.

Chromates were historically discharged from the use of zinc chromate as a rust inhibitor.

Wastewater from regeneration processes released chloride and accelerator-produced isotope discharges in this area.

IEPA requested semiannual groundwater monitoring of this Solid Waste Management Unit (SWMU) under the laboratory’s RCRA Part B Permit to monitor dissipation of these constituents.

The sampling list was reduced in 2001 to two parameters (total chloride and total lead) due to non-detection and detections below groundwater quality standards for most of the parameters. The historical and recent data for these two parameters are routinely evaluated for potential reduction in the monitoring program. Specific sampling procedures for the CUB tile field wells are described in ESH Standard Operating Procedure Sampling Ground Water for Chemical Analysis at the CUB Tile Field (Procedure No. ESHS – 102).

4.5 GROUNDWATER QUALITY STANDARDS FOR FERMILAB

Current groundwater quality standards (as of December 2024) for the Fermilab site are summarized in Table 1 below.

Table 1 Groundwater Quality Standards

Constituent Unit

Groundwater Quality Standard

Class I (Potable Resource)

Class II (General Resource)

Tritium pCi/L 20,000.0 Not Established

Chloride mg/L 200.0 200.0

Lead mg/L 0.0075 0.1

Notes: pCi/L = picocuries per liter mg/L = milligrams per liter

The current groundwater quality standards are provided for the user’s convenience but should be verified using the IPCB’s website3.

3 https://ilga.gov/commission/jcar/admincode/035/03500620sections.html

5.0 Groundwater Well Policies

5.1 Wellhead Protection

Fermilab no longer supplies community-wide potable water. If the Laboratory decides to provide such in the future, Section 14.2 of the Illinois Environmental Protection Act will require minimum setback zones of 200 feet to be maintained around all community water supply wells.

The primary purpose of wellhead regulations is to protect the local groundwater resource and water supply wells installed within the area from contamination. This is achieved through the proper identification and management of hazardous materials used, stored, and/or disposed of within this area; by the proper location, construction, control, and abandonment of wells (including monitoring and water supply wells); and the identification and prevention of other potential routes of groundwater contamination.

The IGPA amended the Illinois Environmental Protection Act in order to enhance protection of community wells. The amendments provide for well setbacks, land use regulation, groundwater quality standards, and detailed assessment of threatened community wells and their aquifers, as necessary. The Illinois Environmental Protection Act regulates community wells and potentially contaminating land uses.

The amended Illinois Environmental Protection Act specifically defines three types of potentially contaminating land uses: potential primary sources, potential secondary sources, and potential routes. The primary sources include disposal areas for hazardous municipal wastes and storage sites with large amounts of substances. Secondary sources include storage facilities for smaller amounts of hazardous substances and domestic wastewater treatment facilities. Potential routes refer to possible pathways for contamination of groundwater and include abandoned wells, drainage wells, and sand and gravel mining operations.

5.2 Setback Zones

Well setback zones regulate the spatial relationship between water supplies and potential contamination routes and sources, and protect existing water supply wells from encroachment by new potential routes or contamination sources. The minimum setback zones are established for the location of new potential route, potential primary or secondary source of contamination. Siting of new potential primary or secondary sources or potential routes is prohibited within the setback zone. Conversely, new water wells must be at least 200 feet from an existing source or route.

The second level of protection provided under the IEPA involves the use of a voluntary maximum setback zone of up to 1,000 feet for community water supply wells. Maximum setback zones allow the well owner, state, county, or municipal government to regulate land use beyond the required setback zone. No maximum setback zones have been established around the three main production wells at Fermilab. These wells are classified as non-transient, non-community supply wells under the regulation of the Department of Public Health and as such are not covered under the community water supply regulations governing establishment of maximum setback zones (Illinois Environmental Protection Act, Section 14.3).

5.3 Monitoring Well Construction

The Appendix to this Plan documents different aspects of drilling boreholes, selecting materials, and installing monitoring devices. It is presented so that standard practices and procedures will be employed by all Division/Departments and contractors who are associated with the design, drilling, and installation of monitoring wells at Fermilab. The Fermilab ESH Manual Chapter 8011 defines the specific Division/Department responsibilities and administrative program to manage current and future monitoring wells. Standards that apply to monitoring wells, with the exception of those installed to monitor chemicals leaking from underground storage tanks which are installed within the excavation made for the installation of the underground storage tank, are found in 77 IAC Section 920.170.

5.4 Well Sealing and Abandonment

Monitoring wells that are no longer needed for their intended purpose shall be sealed within 30 days unless an extension is obtained from the appropriate county health department or it is determined by the ESH Department that these wells should continue to be included in the site-wide groundwater monitoring network. An extension shall be obtained by application to the ESH Director. The sealing shall be done in such a manner that the well cannot act as a conduit for migration of contaminants from the ground surface to the water table or between aquifers.

Abandonment procedures shall comply with regulations set forth in 77 IAC Section 920.120.

Abandonment Procedures

Well sealing will be done by a licensed water well driller, pursuant to the Illinois Water Well and Pump Installation Contractor’s License Act. Well sealing procedures shall be done in compliance with IAC Title 77 requirements.

Notification Responsibilities/Procedures

The Fermilab ESH Division and the DOE Fermi Site Office will provide notification of the abandonment to appropriate agencies unless specifically outlined in the project approval by the ESH Division Director.

A permit for sealing wells must be obtained through the appropriate county health department (either DuPage or Kane Counties). An Application for Permit to Construct, Modify or Abandon a Water Well is available from the Illinois Department of Public Health (IDPH) website.4 Once the form is completed, it must be submitted to the health department for the appropriate county.

Alternatively, the DuPage County Health Department (DCHD) provides a Water Well Sealing Application form on its website.5 Preparation of the application forms will be coordinated through the Fermilab ESH Division and the DOE Fermi Site Office.

The licensed contractor must provide advance notice to the Fermilab ESH Division Director 72 hours prior to the commencement of work to seal a well. The sealing form provided by the health

4 https://dph.illinois.gov/content/dam/soi/en/web/idph/files/forms/water-well-permit-042716.pdf 5 https://www.dupagehealth.org/DocumentCenter/View/7864/Water-Well-Sealing-Application-PDF department shall be completed by the contractor and submitted to the Fermilab ESH Division within 10 days of completion of the well sealing. For wells located in DuPage County, a Water Well Sealing Form/Well Location on Site Form must be completed. These forms are available at the DuPage County Health Department-Environmental Health Services website.6 For wells located in Kane County, an IDPH Water Well Sealing Form should be completed. These forms are available at the IDPH website.7 Reporting forms will be reviewed, approved, and then final reporting sent through the DOE Fermi Site Office to the appropriate county health department.

Whenever possible, a licensed drilling contractor familiar with environmental drilling methods and procedures will be used to seal and abandon monitoring wells. Environmental Protection personnel must verify that the materials used to seal and abandon the monitoring wells are appropriate and will not degrade groundwater quality.

6 https://www.dupagehealth.org/DocumentCenter/View/171/Water-Well-Sealing-Well-Location-PDF 7 https://dph.illinois.gov/content/dam/soi/en/web/idph/files/forms/ww-well-sealing-frm-042716.pdf

6.0 Sources

ASTM D5092/D5092M-16, Standard Practice for Design and Installation of Ground Water Monitoring Wells in Aquifers, October 2017.

Bauer, R. Bedrock Geologic Setting and Geotechnical Properties in Northeastern Illinois.

Geologic and Hydrogeologic Setting of the Chicago Area Shortcourse. Association of Engineering Geologists North Central Section, September 1995.

Cossairt, J. Donald, Rod Walton and Paul Kesich. The Investigation of Environmental

Radiological Vulnerabilities at Fermilab. Fermilab TM-2097. January 2000.

DuPage County Code, Chapter 34, Ordinance Number OH-0002-90, DuPage County Health

Department Private Water Supply Ordinance, December 1990.

DuPage County Health Department, 2003, Private Water Supply Ordinance Article 18-4.

Forrest, C. W., et al. Groundwater Protection by Local Government, Department of Urban and Regional Planning University of Illinois at Urbana-Champaign, September 1993.

Groundwater & Wells - Third Edition, edited by Robert Sterrett, Johnson Screens, 2007.

Ground Water, Volume II: Methodology, EPA/625/6-90/016b, July 1991.

Handbook of Suggested Practices for the Design and Installation of Ground-Water

Monitoring Wells, EPA/600/4-89/034, March 1991.

Illinois Water Well Construction Code, Title 77 IAC, Part 920, Illinois Department of Public

Health, 1992.

Illinois Water Well Construction Code, 2000, Title 77: Public Health, Chapter I:

Department of Public Health, Subchapter r: Water and Sewage, Part 920.

Kane County, Illinois, 2004, Water Supplies/Wells Ordinance Number Ord. 04-199, 5-11-

2004.

Kesich, P.M. Application of the Stratigraphic Approach to Hydrogeologic and Geotechnical

Investigations. in Proceedings for Midwest GeoSciences Group Workshop. Advances in Site Characterization for Environmental and Engineering Projects at Glaciated Sites, 2004.

Landon, R. A., and J. P. Kempton. Stratigraphy of the Glacial Deposits at the National

Accelerator Laboratory Site, Batavia, Illinois, Circular 456, Illinois State Geological Survey, Urbana, Illinois, 1971.

Manual of Ground-Water Quality Sampling Procedures, Scalf, M. R., McNabb, J. F., Dunlap, W. G., Cosby, R. L., and Gryberger, F. S., Robert S. Derr Environmental Research Laboratory, Office of Research and Development, USEPA, 1981.

Meyer, S. A Review of Northeastern Illinois Hydrogeology and Possible Directions for

Future Studies. ISGS. Illinois Groundwater Association, Fall Meeting 1999. Octave Chanute Aerospace Museum, Rantoul, Illinois.

Nielsen, D. M., Correct Well Design Improves Monitoring, Environmental Protection, Volume 4, Number 7, July 1993.

Nielsen, D. Fundamentals of Ground Water Monitoring Well Design and Installation, NGWA Seminar "Theory and Practice of Ground Water Monitoring and Sampling," October 1-3, 1990.

Practical Handbook of Ground-Water Monitoring, Edited by David Nielsen, Lewis

Publishers, 1991.

RCRA Ground-Water Monitoring: Draft Technical Guidance, EPA/530-R-93-001, November 1992.

Rust Environment & Infrastructure, Aquifer Pumping Test Report, September 1996.

Rust Environment & Infrastructure, NS1 and NS2 Hydrogeologic Site Characterization and

Well Installations, September 1996.

STS Consultants, Ltd., Subsurface Exploration and Analytical Testing Results, Neutrino and

Meson Experimental Areas, Project #25635-XF, November 1988.

STS Consultants, Ltd., Report of Subsurface Exploration and Monitoring Well Installation, Neutrino Experimental Area, Project #25635-XH, December 1989.

STS Consultants, Ltd., Hydrogeological Evaluation Report Neutrino Main Injector, Project

#12050-DL, April 1997.

US-EPA, Environmental Services Division, Environmental Compliance Branch Standard

Operating Procedures and Quality Assurance Manual, February 1, 1991 (ECBSOPQAM).

Vaiden, R. C., et al. Geological-Geotechnical Studies for Siting the Superconducting Super

Collider in Illinois: Results of Drilling Large-Diameter Test Holes in 1986, Illinois State Geological Survey, Environmental Geology Notes 124, Champaign, Illinois, 1988.

Appendix: Borehole and Well Specifications

Drilling Methods

Once the available data have been assembled and the well design proposed, a drilling method has to be selected. There are many different methods of drilling that are suited for different conditions.

The selection of the drilling method best suited for a particular job should be based on the following factors in order of importance:

1. Hydrogeologic environment;

a. type(s) of formation(s)

b. depth of drilling

c. depth of desired screen setting below the water table

2. Types of pollutants expected, if any;

3. Location of drilling site;

4. Design of monitoring well desired; and

5. Availability of drilling equipment.

In all cases, minimizing the risk of potential cross contamination between hydraulically separate geologic layers or acceleration of natural travel time through geologic formations of low conductivity shall be the most important factor to consider when selecting the drilling method(s) for a project. The chosen method should introduce the least possible amount of foreign material into the borehole and produce the least possible disturbance to the formation. Any required drilling fluid mixtures shall use a water source free of bacterial, chemical, and radiological contamination.

Borehole Requirements

The borehole shall be bored, drilled, or augured as close to vertical as possible, and checked with a plumb bob or level. Slanted boreholes will not be acceptable unless specified in the design. The borehole shall be of sufficient diameter so that well construction can proceed without major difficulties. To assure adequate size, a minimum 2-inch annular space is required between the casing and the borehole wall (or the hollow-stem auger wall). For example, an 8-inch borehole is required for installation of a 4-inch outside diameter (OD) casing. However, if the inside diameter (ID) of the casing is 4 inches, the borehole will have to be larger than 8 inches to include the 2-inch annular space and the outside diameter (OD) of the casing (4-inch ID plus the casing wall thickness). The 2-inch annular space around the casing will allow the filter pack, bentonite pellet seal, and the annular grout to be placed at an acceptable thickness. Also, the 2-inch annular space will allow up to a 1.5-inch diameter tremie tube for placing the filter pack, pellet seal, and grout at the specified intervals. An annular space less than the 2-inch minimum will not be acceptable.

When installing a well inside of hollow-stem augers, the inside diameter (ID) of the augers is the area to be considered when determining the 2-inch annular space.

Construction Techniques

Construction techniques shall follow acceptable standard guidance practices. Particular attention should be made to compliance requirements as applicable. In addition, the objectives of data, analyses, and information that are required from a monitoring point should be determined and should dictate the techniques used.

Well Casing and Screen Materials The casing of a monitoring well provides access from the ground surface to a point in the subsurface. Along with the associated seals and grout, the casing prevents the borehole from collapsing, prevents hydraulic communication between geologic zones, and permits piezometric head measurements and groundwater quality sampling. A common design flaw when constructing monitoring wells is the use of inappropriate well casing or screen materials to meet site-specific requirements. This can result in the chemical alteration of the sample, degradation of well casing and screen materials, or physical failure of the well. Unique site-specific and logistical factors should be the controlling criteria in selecting casing and screening materials.

It is important to evaluate the following concerns regarding what materials to use in constructing the monitoring well early in the design phase:

1. Will the material impact on the chemical integrity, or "representativeness" of any samples through sorption or leaching processes, residual materials, or storage and handling practices?

2. What is the durability of the material to long term exposures to potentially hostile human-induced, or natural chemical environments?

3. What is the material's structural (tensile, compressive, and collapse) strength?

4. How easy is the material to handle?

5. What are the cost constraints or advantages?

The two main categories of materials commonly used for casing and screens in monitoring well construction are:

1. Thermoplastics (PVC) This material is used in a wide range of natural geochemical and contaminant conditions.

It has a high strength and a low weight per unit length. It is readily available and has a very low purchase price. However, it may degrade in high concentration of solvents and cannot be driven.

2. Metals (Carbon and Stainless Steel) These materials are used in a wide range of natural geochemical and contaminant conditions. They have a high strength in a wide range of temperature. They are readily available, moderately priced, and can be driven. They are, however, susceptible to corroding under some geochemical conditions (pH <3.5, TDS> 2000 ppm) and have a high weight per unit length.

Design specifications may require the use of other materials in which case the justification must be documented in the project plan. In most cases, the material chosen will come in various lengths that must be joined together. Only mechanical joining will be used for monitoring well construction. For thermoplastics, no solvent primers or cements will be allowed. Flush-joint threaded casing shall be used so the inner and outer casing will maintain uniform diameters at the joints. Effective seals will also be required at the casing joints to prevent bentonite grout or contaminated water from entering the well. O-rings free of per- and polyfluoroalkyl substances (PFAS) shall be used to seal the couplings. For metal casings, only threaded joints shall be allowed.

In all cases, the materials used for casing and screening the monitoring well shall be thought out carefully to meet the program objectives and the site-specific conditions. The material type used shall be documented fully on the well completion report.

It is important to make sure that all material is cleaned prior to installation, especially for monitoring wells from which samples will be collected for chemical analyses. This requires washing with an acceptable detergent and rinsing thoroughly with clean water for PVC and steam-cleaning and high pressure, hot water washing for metals. This ensures removal of cutting oils, lubricants, protective coatings of synthetic waxes, or additives that may be used to enhance the durability of the materials which may be left on the casing and screen after their manufacture.

Factory cleaning of casing or screen in a controlled environment is superior to any cleaning efforts attempted in the field. After cleaning, all materials should be wrapped in plastic and kept off of the ground. The type and extent of pre-installation decontamination of materials is dependent on the type of monitoring to be done at the well site. These factors should be addressed during the initial stages of program development.

Casing Diameter The diameter of casing used for constructing a monitoring well depends on the intended purpose of the well. The important factors that will affect the required casing diameter are:

1. The diameter of any downhole equipment that may be utilized in the monitoring well;

2. The type of drilling method preferred;

3. The total depth of the well and strength of casing to be used;

4. The ease of well development;

5. The volume of water required to purge the well;

6. The rate of recovery of the well after purging; and

7. The unit cost per foot of casing material and drilling.

The outside diameter of casings is standardized. However, due to the range in wall thickness of casing types, the inside diameter can vary.

Well Intake The proper selection of the monitoring well screen and filter pack is essential in the design of a hydraulically efficient monitoring well. This is especially true for unconsolidated and certain types of poorly consolidated geologic materials. A properly designed well intake allows formation water to flow freely into the well while minimizing or eliminating the entrance of fine-grained materials.

For water bearing zones that are naturally turbid and will not yield clear formation water (free of visible sediment) even after extensive development, the borehole can be overdrilled to allow for placement of a sump in the well below the well screen. A sump usually consists of a small section of well casing located below the well screen. Sumps serve as catch basins or storage areas for sediment that flows into the well and drops out of suspension. This is common in formations containing large proportions of fine-grained materials, >50% silts or clays. The sediment can then be periodically pumped out of the sump preventing the well screen from clogging or "silting up".

Sediment laden water can greatly lengthen filtering time and create chemical interference in certain sample analyses.

1. Filter Packs In the construction of a monitoring well it is imperative that the natural stratigraphic setting be distorted as little as possible. This requires that the development of void space be minimized in unconsolidated formations. As a consequence, boreholes that are over-sized with regard to the casing and well intake diameter generally should be filter-packed. This prevents the collapse of the borehole around the screen with the subsequent creation of void space and the loss of stratification of the formation. Collapse also frequently results in the failure of well seals placed on top of the collapsed zone, although well development prior to seal installation may help to minimize this potential problem. The artificial introduction of coarse-graded material into the annular space between a centrally-positioned well intake also serves to filter out fine materials from the formation adjacent to the well. This results in an increase in the effective diameter of the well and an accompanying increase in the amount of water that flows toward and into the well.

The materials used as filter pack in a monitoring well should be chemically inert to alleviate the potential for alteration of groundwater sample chemical quality. It should be composed primarily of clean quartz sand. The individual grains of the filter pack materials should be well rounded and consist of less than 5 percent non-siliceous material. A filter pack comprised of other types of crushed stone should not be used because of potential chemical alteration of groundwater and problems from non-rounded material.

The selection of filter pack grain size is a function of the formation. The first step in its design is to obtain samples of the formation intended to be monitored and perform sieve analyses on the samples. The filter pack material size is then selected on the basis of the finest formation materials present. The filter pack grain size should be selected by multiplying the 70 percent retained grain size (D30) of the formation materials by a factor between 4 (for fine and uniform formations) and 6 (for coarse and non-uniform formations). The uniformity coefficient (D60/D10) of the filter pack materials should not exceed 2.5 and the gradation of the filter material should form a smooth and gradual size distribution when plotted.

When placing the filter pack into the borehole, a minimum of 6 inches of the filter pack material shall be placed under the bottom of the well screen to provide a firm footing and an unrestricted flow under the screened area. The filter pack shall be placed by the tremie or positive displacement method to 3 to 5 feet above the well screen. For water table observation wells constructed in areas where the depth to the water table is less than 5 feet, the required filter pack height above the top of the well screen may be reduced to 6 inches to allow for the required amount of annular space sealant to be placed. Placing the filter pack by "pouring" will be acceptable for shallow wells less than 25 feet in depth. A secondary filter pack may be used in cases where it is difficult to keep annular pack material (bentonite grout) from migrating into the primary filter pack or infiltrating down to the screened interval of the well, decreasing contact to the surrounding formation or altering the quality of groundwater samples taken from the well. The necessity for a secondary filter depends on the particle-size distribution of the formation. The upper half of the secondary filter should be in contact with a lithologic layer of equal or lower permeability and thickness to prevent the slurry or sealant from migrating around the secondary filter and into the coarser filter pack. It should extend at least 1 foot above the top of the primary filter pack.

2. Screen Slot Size The screen intake opening (slot) size shall be designed in coordination with the filter pack grain size. Both of these are designed as a function of the formation determined through sieve analysis.

The chosen slot size should retain greater than 90 percent of the filter pack material. The length of the monitoring well intake will depend on the purpose of the well. Well intakes typically range in length from 2 to 20 feet. Shorter screen lengths provide more specific information about vertically distributed water quality, hydraulic head, and flow in the formation adjacent to the intake. Along with slot size and length, other intake factors include intake type and corrosion and chemical degradation resistance. The intake type along with the slot size control the amount of open area in the well intake, which establishes the hydraulic efficiency of the intake, the ease at which formation water can flow into the well, and allows for effective well development. Commercially manufactured well intakes shall be used in all monitoring well designs. No hand-slotted or drilled casings should be used. Commercially available screen types include:

1. Louvered (shutter-type);

2. Bridge-slot;

3. Machine-slotted; and

4. Continuous-slot wire-wound.

The preferred type of screen is the continuous-slot type. The configuration used in this screen type provides significantly greater open area per given length and diameter than is available with any other intake type, provides a wider range of available slot sizes than any other type of intake, has slot sizes that are accurate to within ± 0.003 inch, and is more effective in preventing formation materials from becoming clogged in the openings.

The well intake is more susceptible to corrosion and/or chemical degradation than any other part of the monitoring well and also provides the highest potential for sorption or leaching phenomena to occur. This is because the intake has a large surface area of exposed material and is placed in a position designed to be in contact with potential contaminants or adverse conditions. The same guidelines should be used for selection of the screen material as is used in casing material selection.

Annular Seals The purpose of the annular seal is to fill the annular space produced as a result of the installation of the well casing into the borehole and prevent the vertical movement of water and/or contaminants.

1. Filter Pack Seal-Bentonite Pellet Seal (Plug) A seal shall be placed on top of the filter pack consisting of a pure bentonite pellet material with a solids content of at least 20 percent. The preferred method of placing bentonite pellets is by the positive displacement or the tremie method. Use of the tremie method minimizes the risk of pellets bridging in the borehole and assures the placement of pellets (also sand and grout) at the proper intervals. The tremie tube shall have a side discharge port to minimize damage to the filter pack during grout placement. Pouring of the pellets is acceptable in shallow boreholes (less than 25 feet) where the annular space is large enough to prevent bridging and to allow measuring (with a tape measure) to insure that the pellets have been placed at the proper intervals. In order to insure that the pellets have been placed at the proper intervals, the pellets shall be tamped with an appropriate tamping tool while the measuring is being conducted. The tamping process minimizes the potential for pellet bridging by forcing any pellets that have lodged against the borehole wall and/or the well casing, down to the proper interval. Improper placement of the bentonite seal can result in persistent sample contamination (high pH, trace metals, and TDS). This may result in the loss of the ability of the well to meet its intended use and require abandonment of the monitoring well.

The bentonite seal shall be placed above the filter pack, 3 - 5 feet above the top of the well screen, with a minimum of two feet vertical thickness. The hydration time for the bentonite pellets shall be a minimum of eight hours or the manufacturer's recommended hydration time, whichever is greater. In all cases, the proper depths shall be documented by measuring and not by estimating.

Other forms of bentonite, such as granular bentonite and bentonite chips, have limited applications and are not recommended for the bentonite seal unless special conditions warrant their use. In any case, deviation from bentonite pellets for the seal, such as a 30 percent solids bentonite grout, shall be justified in the project plan. If for some reason, the water table is temporarily below the pellet seal interval, potable water (or higher quality water) shall be used to hydrate the pellets.

2. Grouting The Annular Space The annular space between the casing and the borehole wall shall be filled with either a high solids, pure (no additives), bentonite grout, or a cement/bentonite grout. Each type of grout to be used shall be evaluated as to its intended use and integrity. The grout shall be placed into the borehole, by the tremie method, from the top of the bentonite seal to within 2 feet of the ground surface or below the frostline, whichever is greater. The tremie tube shall have a side discharge port to minimize damage to the bentonite pellet seal during grout placement. The grout shall be allowed to "set" or cure for a minimum of 24 hours before the concrete surface pad is installed. All grouts shall be prepared in accordance with the manufacturer's specifications. Bentonite grouts shall have a minimum density of 9.4 lbs/gal to ensure proper setup. Cement grouts shall be mixed using 6.5 to 7 gallons of water per 94-lb bag of portland cement (Type I). The addition of bentonite (5-to-10 percent) to the cement grout is for elasticity and the reason for its use shall be documented. All materials used in the grouting process shall be fully documented.

Surface Completion The primary purpose of any surface completion shall be to prevent surface runoff from entering and infiltrating down the annulus of the well and to protect the well from accidental damage or vandalism.

1. Above Ground Riser Pipe and Outer Protective Casing The well casing, when installed and grouted, shall extend above the ground surface a minimum of

2.5 feet. A vent hole shall be drilled or cut into the top of the well casing cap to permit pressure equalization. An outer protective casing shall be installed into the borehole after the annular grout has "set" for at least 24 hours. The outer protective casing shall be of anodized aluminum construction with a locking cap. Generally, an outer protective casing used over a 2-inch well casing is 4 inches square by 5 feet long. Similarly, a protective casing used over 4-inch well casings is 6 inches square and 5 feet long. Round protective casings are also acceptable. A protective casing shall have sufficient clearance around the inner well casing, so that the outer protective casing will not come into contact with the inner well casing after installation. The protective casing shall have a minimum of two weep holes for drainage. These weep holes shall be a minimum 1/4 inch in diameter and drilled into the protective casing just above the top of the level of concrete inside the protective casing to prevent water from standing inside of the protective casing. The protective casing is installed by pouring concrete into the borehole on top of the grout. The protective casing is then pushed into the wet concrete and borehole a minimum of 2 feet. Extra concrete may be needed to fill the inside of the protective casing so that the level of the concrete inside of the protective casing is at or above the level of the surface pad. The protective casings shall extend a minimum of 3 feet above the ground surface or to a height so that the cap of the inner well casing is easily accessible when the protective casing is opened.

2. Flush-surface Completion If the monitoring well is installed in a high traffic area it might be desirable to complete the well flush with the ground surface and install a watertight traffic cover. Traffic covers are designed to extend from the ground surface down into the concrete plug around the well casing. The cover shall have a seal that makes the unit watertight when closed and secured. The traffic cover shall be installed as far above grade as practical to minimize standing water and promote runoff.

However, this completion method is not recommended and prior approval must be obtained from the ES&H Division before it is incorporated into the monitoring program design.

3. Concrete Surface Pad A concrete surface pad shall be installed around each well casing. Concrete shall be placed into the formed pad and into the borehole (on top of the grout) in one operation making a contiguous…