Illinois offers a diverse geological setting for groundwater drilling, ranging from thick glacial deposits to deep sedimentary bedrock formations. Water well professionals in the state must navigate not only these varied subsurface conditions but also a comprehensive set of regulations. This post provides an overview of Illinois’ prevailing geological conditions relevant to well drilling, highlights the major aquifers and typical well depths, and details the permitting requirements and procedures. It also identifies the key state agencies involved in groundwater regulation, utility clearance mandates before drilling, and well log reporting expectations. Technical accuracy and compliance are crucial for successful and lawful water well construction in Illinois.
Geological Overview of Illinois
Illinois’ groundwater availability is largely dictated by its complex glacial and bedrock geology. During the Pleistocene ice ages, multiple glaciations covered most of Illinois, leaving behind layers of glacial till (clays and silts) interbedded with sand and gravel deposits. These unconsolidated materials blanket the state in varying thicknesses – from only a few feet in some southern and northwestern areas to hundreds of feet thick in northern and central Illinois. Beneath the glacial drift lies sedimentary bedrock, which includes limestone, dolomite, and sandstone units that serve as important aquifers.
Much of the state’s shallow groundwater is found in sand and gravel aquifers within ancient bedrock valleys or along modern rivers. Glacial meltwaters filled these bedrock-cut valleys with permeable sand and gravel, creating high-yield aquifers (for example, the Mahomet Bedrock Valley in east-central Illinois). In areas between these valley aquifers, the glacial till is predominantly clayey and yields little water, often requiring drillers to go deeper into bedrock for adequate supply. The distribution of groundwater resources is thus uneven – some regions have abundant shallow aquifers, while others rely on deeper sources.
The sedimentary bedrock underlying Illinois dips gently to the southeast and consists of layers of sandstone, shale, coal, and carbonate rock. In northern Illinois, shallow bedrock is often dolomite (Silurian-age) which, where extensively fractured, forms a productive aquifer. Across central and southern Illinois, bedrock aquifers include Mississippian limestones and Pennsylvanian sandstones at moderate depths, and deep Cambrian-Ordovician sandstones at greater depths. Notably, the region around Chicago has both a shallow fractured-dolomite aquifer and a deep sandstone aquifer system that have historically been heavily utilized.
Major Aquifers and Typical Well Depths
Illinois classifies “major aquifers” as geologic units at least 50 square miles in area that can yield at least 70 gallons per minute of potable water to wells. By this definition, the state’s principal aquifer systems fall into three broad categories: (1) Quaternary sand and gravel aquifers, (2) shallow bedrock aquifers (primarily dolomite/limestone in the north and sandstone in some areas), and (3) deep bedrock aquifers (Cambrian-Ordovician sandstones).
Sand and Gravel Aquifers: Unconsolidated sand and gravel deposits associated with ancient river valleys are among the most productive sources of groundwater. These include the extensive Mahomet Aquifer in east-central Illinois, which alone supplies roughly 200 million gallons per day of drinking and irrigation water. Sand and gravel aquifers underlie only about 25% of Illinois’ land area, yet they account for an estimated 65% of the state’s total groundwater potential yield. The highest-yielding sand/gravel aquifers are often those along major rivers (Mississippi, Illinois, Wabash, Ohio) and in deep buried bedrock valleys. In many southern and western counties, usable sand may be found at shallow depths – wells in these areas commonly range from 30 to 100 feet deep into alluvial sands. In contrast, where glacial drift is thicker (such as the Mahomet Valley region), sand and gravel aquifers are deeper; domestic wells tapping these may reach 200 to over 300 feet to find the saturated sand units.
Shallow Bedrock Aquifers: Where permeable bedrock lies near the surface, it can serve as a reliable aquifer. In the northern third of Illinois, shallow Silurian dolomite (carbonate rock) directly underlies the glacial drift and has significant secondary porosity (cracks and solution cavities) that store groundwater. Wells completed in these shallow bedrock aquifers are typically on the order of 100–300 feet deep, and in some northern counties they may extend 300 to over 600 feet into the formation to obtain sufficient yield. Studies indicate that shallow bedrock (dolomite) aquifers provide about 68% of the available yield from bedrock sources in Illinois. In areas of central and south-central Illinois, other bedrock units (Pennsylvanian and Mississippian sandstones, for example) can be tapped at relatively shallow depths (often 100–200 feet) for farm and domestic wells. Where the upper bedrock is less permeable or covered by thick fine-grained materials, yields are lower and wells may need to be drilled deeper or into alternate units.
Deep Bedrock Aquifers: Beneath the shallower strata lie the deep sandstone aquifers of Cambrian and Ordovician age – notably the St. Peter, Ironton-Galesville, and Mt. Simon sandstones. These aquifers, which underlie large portions of Illinois, have historically been vital for municipal and industrial water supply, especially in northeastern Illinois. However, they occur at great depths; wells in the deep sandstone aquifer system around the Chicago area commonly reach 800 to 1,500 feet in depth. In many areas, these deep wells are artesian (pressurized). Over the decades, heavy pumping has significantly lowered pressures in the deep aquifer. In fact, studies by the Illinois State Water Survey have found that the deep-bedrock aquifer is essentially being mined (withdrawn faster than it is recharging) under current usage rates. Accordingly, many communities have supplemented or replaced deep-well supplies with surface water or shallower groundwater to ease the stress on these formations. Still, deep sandstone wells remain an important backup and rural water source. Drillers tackling these aquifers must be prepared for very deep boreholes and often highly mineralized water as depth increases.
In summary, typical water well depths in Illinois vary widely with geology: a drilled well in a shallow sand aquifer might be 50–150 feet deep, whereas a well completed in bedrock could be a few hundred feet deep in one area or over a thousand feet in another. It is not uncommon, for example, for a rural homeowner in western Illinois to have a 80-foot sand-point well in a river sand deposit, while a Chicago suburb draws from a 1200-foot sandstone well. Understanding the local aquifer context is paramount. Fortunately, Illinois has abundant groundwater data – more than 400,000 Illinois households rely on private wells, and millions of well and boring records have been collected over the years – which can guide drillers on expected depths and formations.
Water Well Permitting Requirements
Illinois law requires that anyone constructing or deepening a water well (including private and farm wells) obtain a permit before starting drilling. The Illinois Water Well Construction Code (415 ILCS 30 and associated regulations at 77 Ill. Adm. Code 920) governs well construction standards and the permitting process. Permits for private, semi-private, and non-community wells are administered by the Illinois Department of Public Health (IDPH) through delegated county health departments. There are 90 local health departments in Illinois that review well installation plans, issue permits, and inspect new well construction on behalf of IDPH. In counties without a local health department, IDPH itself handles permits.
Permit Application: To apply for a well construction permit, the drilling contractor (or property owner through a contractor) must submit an application form with required details to the local health authority. The application will ask for information such as the well location, proposed depth, intended use, and a diagram of the site showing distances to potential contamination sources. Illinois regulations specify minimum setback distances (horizontal separation) between a well and sources like septic systems, sewer lines, fuel tanks, or existing wells to protect water quality. The site plan should indicate features like property lines, buildings, septic tanks and leach fields, underground storage tanks, and any abandoned wells within the vicinity. The drilling plan must also be adapted to the local geology (for example, proposing an appropriate casing depth and grouting method for the formations at the site), in compliance with the construction code.
Licensed Contractor Requirement: Only an IDPH-licensed Water Well Contractor may obtain a well construction permit and perform the work (with limited exceptions for an owner drilling on their own property, in some cases). Illinois licenses roughly 700 individuals and companies to construct wells and install pumps, ensuring they have the qualifications to meet state standards. The contractor’s license number and signature must appear on the permit application, certifying that the well will be built in accordance with the Illinois Water Well Construction Code. If a separate contractor will be installing the pumping equipment, that contractor must also be licensed and will typically need to obtain a related pump installation permit or at least submit a completion report for the pump (as discussed later).
Fees and Approval: The state law allows a permit fee up to $100, and many counties charge a fee in that range (for example, Logan County’s permit fee is exactly $100). Once the application is submitted with the fee, the local health department will review it for compliance. This review includes checking that the proposed well location meets all setback requirements and that the construction plan (casing, grout, etc.) is appropriate for the geology. If everything is in order, a permit is issued. The permit is typically valid for a certain period (e.g. 6 months to a year) for the work to be completed.
Pre-Drilling Notifications: After obtaining the permit but before commencing drilling, the well contractor is required to notify the authorities of the start date. State code Section 920.130(g) mandates that the contractor inform the Department (IDPH or the local health department) at least two business days prior to beginning well construction. This notification allows the health inspector the opportunity to schedule an inspection of the drilling or installation if needed. Notification can usually be done by a simple phone call or written notice to the health department. (Note that this is a separate requirement from utility location notification, which is addressed later.)
Construction Standards: All wells must be constructed to meet Illinois’ minimum sanitary standards. Key requirements include proper casing installation, grouting of the annulus to at least 20 feet (or more depending on geologic conditions) to seal out surface contamination, and installing a sanitary well cap and pitless adapter for any buried discharge line. Wells must also maintain specified distances from grade (e.g., casing extending at least 8-12 inches above grade or flood level) and be completed in a way that prevents contamination from entering around the casing. If a well penetrates multiple aquifers, the zones must be isolated to prevent cross-connection. The Illinois Water Well Construction Code provides detailed specifications that drillers must follow, and local inspectors may check aspects like grouting and casing depth during or after construction.
State Agencies and Groundwater Regulation
Several state agencies and programs play roles in regulating and managing groundwater in Illinois:
- Illinois Department of Public Health (IDPH): IDPH is the primary regulatory agency for private water wells. Through its Division of Environmental Health, IDPH licenses well drillers and pump installers and oversees the construction, modification, and sealing of all non-community water wells. IDPH works closely with local county health departments to administer permits and inspections for new wells. IDPH also enforces the Illinois Water Well Construction Code and the Water Well Pump Installation Code, which set the standards for how wells must be built and pumps installed. If a well is abandoned or no longer in use, IDPH regulations require that it be properly sealed by a licensed contractor to prevent it from becoming a conduit for contamination.
- Illinois Environmental Protection Agency (IEPA): The IEPA Bureau of Water regulates public water supplies, including municipal/community wells. While IDPH handles private wells, any well that serves as a source for a municipal or community water system must meet additional requirements under the Illinois Environmental Protection Act and Safe Drinking Water Act rules. IEPA reviews and permits the development of new community supply wells as part of a community water system plan. They ensure that such wells are located and constructed safely (often requiring extensive hydrogeologic investigation and pump testing) and that the water quality is tested and meets drinking standards. IEPA also administers wellhead protection programs under the Illinois Groundwater Protection Act, designating protection zones around community wells to minimize contamination risks. Public water supply wells are subject to routine sampling and monitoring overseen by IEPA. In summary, if you are drilling a high-capacity well for a town or subdivision, IEPA (rather than IDPH) will be heavily involved in the approval and oversight.
- Illinois State Water Survey (ISWS): The ISWS, part of the University of Illinois’ Prairie Research Institute, is not a regulatory body but serves a crucial scientific and data-management role. The ISWS maintains the state’s extensive database of well records and groundwater data. In fact, Illinois has more than 500,000 well and boring logs on file, going back over a century. Whenever a new well is completed, a copy of the Well Completion Report is typically forwarded to the ISWS to be added to the database. The ISWS operates the Illinois Water Inventory Program (IWIP), which tracks withdrawals from high-capacity wells (pumping >100,000 gallons/day). Owners of such wells (e.g. irrigation, industrial, municipal wells) are required to register them and report annual pumpage to IWIP, which helps the state monitor groundwater use trends. The Water Survey also conducts research on aquifer conditions, groundwater levels, and modeling of water resources. Their Illinois Water Supply Planning sections have produced valuable regional studies, and they provide an online well database and interactive map (ILWATER) where drillers can look up logs and aquifer information.
- Illinois State Geological Survey (ISGS): Also a part of the Prairie Research Institute, the ISGS complements the Water Survey by focusing on the geology. ISGS geologists map aquifer extents, bedrock topography, and the thickness of glacial deposits. For example, ISGS has identified the areal extent of major sand and gravel and bedrock aquifers across Illinois. These maps and datasets (often available through the Illinois Geospatial Data Clearinghouse) are valuable for understanding where high-yield aquifers are likely to be found and at what depth. The ISGS is the official repository for drilling records alongside ISWS, and it houses rock cuttings and core samples from wells. When planning a drilling project, consulting ISGS maps or reports can inform you of the subsurface strata you will encounter.
- Local Government and Ordinances: Counties in Illinois may enact local well ordinances that impose additional requirements on well drillers beyond the state code. Typically, these ordinances adopt the state standards by reference (as seen in the Logan County Water Supplies Ordinance, for example). Some local health departments might require extra steps such as submitting a well sketch post-construction, water sampling for bacteria or nitrates upon completion, or stricter well placement rules in certain sensitive areas. It’s important to check with the county for any local amendments. Additionally, if the drilling site is within a city or village limits, local plumbing or building codes might require notification or inspections (for instance, some municipalities require the well location to be approved relative to utility easements or flood zones). Overall, while state law preempts most standards, local authorities can be more restrictive to protect their specific groundwater conditions.
Utility Clearance Before Drilling
One absolutely crucial step before drilling any well (or any excavation) in Illinois is to ensure all underground utilities are located and marked. Illinois law requires all excavators, including well drillers, to contact the state’s one-call notification system (JULIE – Joint Utility Locating Information for Excavators) at least 48 hours (two working days) before digging. This “Call 811” requirement is mandated by the Illinois Underground Utility Facilities Damage Prevention Act (the “Illinois One-Call Law”). By calling 811 or placing an online locate request, the driller triggers a notice to all utility companies with buried lines in the area – including electric, gas, water, sewer, telephone, and fiber optic utilities. Within two days, each utility will send a technician to mark the location of their underground lines on the property with color-coded flags or paint.
Compliance with the one-call law is critical for safety and liability reasons. Hitting a buried gas pipeline or electric line with a drilling rig auger could be catastrophic. Even lesser damages (like severing a telecom cable or water main) can result in hefty fines and repair costs. In 2023 the Illinois Commerce Commission, which enforces the one-call law, issued hundreds of citations and over $4 million in penalties for safe digging violations. Thus, professional well contractors make the JULIE call routine for every job. Remember that the law also specifies the locate request has a 14-calendar-day window in which the excavation must begin, and locates expire after 28 days – so if your drilling is delayed beyond that, you need to call in a fresh ticket. Additionally, within the City of Chicago, a separate system known as DIGGER handles locates (JULIE will automatically route Chicago requests to DIGGER as needed).
Besides utility location, it’s good practice to scout the site for any farm tiles, private lines, or obstructions that might not be covered by a JULIE locate. Always discuss with the landowner where any private irrigation lines, septic laterals, or other hidden utilities might be. Taking these precautions ensures that when you roll the rig onto the site and start drilling, you won’t get any unpleasant surprises from below.
Well Logs and Reporting
After a well is constructed, Illinois requires thorough documentation of the well’s details and the geologic materials encountered. The driller must complete a Water Well Construction Report (well log) for each well and submit it to the appropriate authorities within 30 days of finishing the well. This report typically includes: the well location (often given by street address and coordinates or township-range-section), the depth and diameter of the borehole, the casing and screen specifications, the grout material and depth, static water level, pump information (if known), and a log of geologic strata encountered (with depths of sand, clay, rock, etc.). The log provides a permanent record of the subsurface conditions and construction method, which is useful for future reference by regulators, owners, and scientists.
In practice, the driller will provide one copy of the completed well log to the property owner, submit one copy to the local health department, and in many cases an additional copy is forwarded to the Illinois State Water Survey’s well archive. The local health department (or IDPH) reviews the construction report to ensure compliance with the permit and code. They will check that the well was indeed constructed as permitted – for example, that the casing was installed to the proper depth, that grout was placed to the required level, and that no unapproved variances occurred. If water samples were required (many counties test new private wells for bacteria and nitrates), the results might be attached to the well report. Once approved, the well log is filed in the state’s database. The Illinois State Geological Survey and Water Survey maintain an online tool (ILWATER) where many well logs can be retrieved by professionals for project planning. These records are incredibly useful – by examining logs of nearby older wells, a driller can infer what formations and water levels to expect for a new well in the area.
In addition to new well reports, Illinois regulations also require reporting when wells are sealed (abandoned). A Well Sealing form documenting the method of plugging and materials (typically neat cement or bentonite grout) must be submitted to the health department. This ensures that abandoned wells are properly taken care of, since an unsealed abandoned well can pose a direct pathway for contamination into an aquifer or a physical hazard if left open. Well contractors should advise owners on the importance of sealing any old wells on their property and may often get contracted to perform the sealings.
For wells that are converted or modified (for example, deepening a well or replacing a casing), reports are also required. And if a separate contractor installs the pump after the well is drilled, Illinois expects a Pump Installation Report to be filed, detailing the pump set depth, pump make/model, etc., so that a complete record of the water well system is on file. All of these reporting requirements create transparency and traceability for Illinois’s groundwater infrastructure.
Conclusion
Drilling water wells in Illinois demands both geological savvy and regulatory diligence. The state’s varied geology – from shallow sand and gravel aquifers to deep sandstone reservoirs – means drillers must tailor their methods and designs to local conditions for a successful well. At the same time, a robust regulatory framework is in place to protect groundwater resources and public health. By securing the proper permits, adhering to construction codes, calling in utility locates, and filing accurate well logs, water well professionals can ensure they remain in compliance with Illinois laws. The involvement of agencies like IDPH and IEPA underscores the importance of doing the job right: safe well construction safeguards drinking water quality for homeowners and communities alike. Utilizing the wealth of data from Illinois’ geological and water surveys can give drillers a technical edge, helping them anticipate subsurface challenges. In sum, understanding both the geology beneath your rig and the rules that govern the work is key to thriving as a water well professional in Illinois.
References
- Illinois Department of Public Health (IDPH) – Private Water & Closed Loop Wells Program (IDPH Environmental Health Division) – Describes IDPH’s licensing of well contractors and regulation of well construction (except community water wells).
- Illinois Department of Public Health – Application for Permit to Construct, Modify, or Abandon a Water Well (Form IL482-0620) – Illustrates information required on well permit applications, such as site diagram and contractor certification.
- Illinois State Water Survey – Illinois Water Supply Planning Interactive Maps (2016) – Notes that groundwater resources are unevenly distributed in Illinois due to complex glacial and bedrock geology, and defines major aquifers as those ≥50 square miles yielding ≥70 gpm.
- Illinois State Geological Survey – Major Sand and Gravel Aquifers (ISGS GIS Dataset Metadata, 1996) – States that major sand & gravel aquifers in Illinois are Quaternary deposits in pre-glacial bedrock valleys or along rivers, typically within 300 ft of the surface and capable of ≥70 gpm.
- Illinois EPA – 2014 Integrated Water Quality Report, Groundwater Section (Appendix) – Provides statistics on Illinois aquifers, e.g. sand and gravel aquifers underlie ~25% of the state but provide ~65% of total groundwater yield; shallow dolomite bedrock aquifers provide ~68% of bedrock yield; an estimated 400,000 Illinois homes use private wells.
- Illinois State Water Survey – Wells and Pumping Systems for Domestic Water Supplies (ISWS Circular 117) – Describes typical well depths: sand-and-gravel wells ~30–100 ft in southern/western IL (and 200–300 ft in deep drift areas), bedrock wells ~100–200 ft in parts of central/northern IL but 300–600+ ft in much of northern Illinois.
- Logan County Department of Public Health – Potable Water Program – Example of local enforcement of Illinois’ well code: requires permit before installation, $100 fee, 2-day notice before drilling, well log submission within 30 days, and pump installer report if applicable.
- Illinois.gov Press Release – April is Safe Digging Month in Illinois (2024) – Illinois Commerce Commission reminder that state law requires calling JULIE at 811 at least 48 hours (two working days) before any excavation; one-call locates are valid for 14 days. Notes penalties: in 2023, ICC issued 709 citations totaling $4.1 million for digging violations.
- USGS/Illinois State Water Survey – NGWMN Illinois Final Report (FY2019) – Mentions the Mahomet Aquifer provides approximately 200 million gallons per day of water for east-central Illinois (drinking and irrigation), illustrating the significance of this high-yield aquifer.
- Illinois EPA/Illinois State Water Survey – Northeastern Illinois Water Supply Planning (Water 2050 plan insights) – Warns that heavy use of the deep Cambrian-Ordovician sandstone aquifer in NE Illinois has led to mining of the resource (withdrawals exceed recharge), prompting regional efforts to reduce deep aquifer withdrawals.
- Illinois State Geological Survey – Geological Records Repository – Confirms that ISGS, as the state repository, holds over 500,000 well and boring records for Illinois. The ILWATER online map allows access to water well data (scanned well logs, groundwater levels, etc.) for private and public use.
