Nitrate and Arsenic in Indiana and Michigan Private Wells: What USGS and EPA Data Shows

USGS National Water Quality Assessment Program data for the Midwest agricultural region shows approximately 20 to 25 percent of shallow private wells exceed 5 mg/L nitrate, the Environmental Working Group health threshold, even when most stay below the EPA maximum contaminant level of 10 mg/L (verify with USGS Circular 1225 and current NAWQA publications). Arsenic presents a separate but equally invisible problem: the USGS Indiana Water Science Center has documented arsenic in bedrock aquifer wells in northern and central Indiana, and Michigan EGLE has documented trace arsenic in glacial-drift aquifer zones across multiple counties. The EPA MCL for arsenic is 10 micrograms per liter. Private wells in Indiana and Michigan are exempt from federal Safe Drinking Water Act oversight. No agency monitors them. No utility tests them. Private well owners in both states are entirely responsible for knowing what is in their water, and neither nitrate nor arsenic produces a taste, color, or odor at the concentrations that make them dangerous.

Why private wells fall outside the Safe Drinking Water Act

The federal Safe Drinking Water Act, administered by the EPA, applies to public water systems that serve at least 25 people or have 15 or more service connections. Private wells serving individual households fall entirely outside that framework. The EPA does not regulate private wells. State environmental agencies in Indiana and Michigan set guidance but not enforceable ongoing testing requirements for private well owners after initial construction.

The EPA SDWIS (Safe Drinking Water Information System) database, which tracks compliance violations and monitoring results, contains no records for private wells precisely because they are not covered systems. When you see EPA SDWIS cited for water quality data, it reflects public system performance. Private well water quality in Indiana and Michigan is unknown to any agency unless the homeowner tests it and reports a problem to their county health department.

Indiana's roughly 800,000 private wells and Michigan's even larger private well population represent a significant share of total drinking water sources in both states. For a deeper overview of the testing frequency and contaminant priorities that apply specifically to Indiana private wells, see our Indiana private well testing guide. For general well water testing principles that apply across both states, see our annual well water testing guide.

USGS NAWQA data: what the Midwest agricultural well record shows

The USGS National Water Quality Assessment Program is the most comprehensive long-term groundwater quality dataset for the United States. NAWQA studies systematically sample wells across major aquifer systems and land-use settings, and the Midwest agricultural region is one of the most thoroughly documented. The core finding relevant to Indiana and Michigan private well owners is that nitrate concentrations in shallow wells under agricultural land are meaningfully elevated compared to wells in forested or undeveloped settings.

USGS Circular 1225 and related NAWQA publications (available through the USGS NAWQA program page) document that approximately 20 to 25 percent of Midwest shallow agricultural wells exceed 5 mg/L nitrate as nitrogen. The 5 mg/L figure is not the EPA MCL; it is the level above which the Environmental Working Group considers chronic exposure a potential cancer risk based on epidemiological research. Most of the wells exceeding 5 mg/L remain below the EPA MCL of 10 mg/L, meaning they would not trigger a regulatory action even in a public water system. For private well owners drinking that water daily for years, the EWG threshold is worth knowing about.

Wells below 100 feet in glacial-drift aquifers under row-crop land have the highest documented nitrate concentrations in the NAWQA dataset. Deeper wells penetrating consolidated bedrock aquifers show substantially lower nitrate, because the confining layers above the bedrock reduce infiltration of surface-applied fertilizers and animal waste.

Source: USGS NAWQA Circular 1225 and NAWQA agricultural studies. Percentages are approximate ranges from published data; verify current figures at usgs.gov/nawqa.

Arsenic in Indiana bedrock aquifers: what USGS and ISDH data documents

Arsenic contamination in Indiana groundwater follows a geologic pattern distinct from nitrate. Rather than being driven by surface land use, arsenic in Indiana wells is largely geogenic, meaning it originates from the minerals in the aquifer rock itself. Indiana's Silurian and Devonian age bedrock formations in the southern and central portions of the state contain arsenic-bearing minerals that can release arsenic into groundwater under certain chemical conditions.

The USGS Indiana Water Science Center has documented arsenic detections in bedrock aquifer wells in Kosciusko, Fulton, and Cass counties in northern Indiana, as well as in portions of the Silurian bedrock belt running through central Indiana. These detections are not uniformly distributed. A well in one part of a county may test clean while a neighboring well in a different stratigraphic zone detects arsenic above the EPA MCL of 10 micrograms per liter. This spatial variability is precisely why county-level guidance is insufficient and individual well testing is the only reliable approach. See the USGS Indiana Water Science Center for published data.

The Indiana State Department of Health guidance on private wells (available at in.gov/isdh) includes arsenic as a recommended test parameter for Indiana well owners, particularly for those in areas with documented bedrock arsenic occurrence. ISDH recommends testing arsenic at least once and repeating after any well work or if neighboring wells detect elevated levels.

Arsenic's health significance is substantial. Long-term exposure above the EPA MCL is associated with increased risk of bladder, lung, and skin cancer, cardiovascular disease, and diabetes according to EPA and WHO assessments (see EPA arsenic in drinking water). Unlike bacterial contamination, arsenic does not cause acute illness at typical well water concentrations. The risk is cumulative over years of exposure, which is why many Indiana homeowners have unknowingly consumed arsenic above health thresholds for extended periods. If you have a bedrock well in central or northern Indiana and have never tested for arsenic, that test should be scheduled immediately. It costs roughly $20 to $40 at a certified Indiana lab.

For Indiana homeowners on deep bedrock wells, the water quality picture often includes high hardness from limestone geology alongside potential arsenic. Indiana wells in the 200 to 400 foot range in Silurian limestone typically yield water in the 15 to 25 GPG hardness range based on USGS data for the region. Hardness at that level has no health implication but is relevant to equipment sizing. See our Indiana hard water guide for the full context on hardness across the state.

Arsenic in Michigan glacial-drift wells: what Michigan EGLE documents

Michigan's groundwater arsenic story differs from Indiana's in its source geology. Where Indiana arsenic comes largely from bedrock formations, Michigan arsenic is associated with glacial sediments deposited during the Pleistocene ice ages. The glacial till and outwash deposits that form Michigan's primary aquifer zones contain iron and manganese oxides that can carry arsenic. When groundwater chemistry conditions shift, particularly in low-oxygen (reducing) zones, that arsenic can mobilize from the sediment into solution.

The Michigan Department of Environment, Great Lakes, and Energy (EGLE) has documented arsenic detections in glacial-drift aquifer wells across multiple Michigan counties. Portions of the western Lower Peninsula and certain Upper Peninsula areas have been more frequently documented. Michigan EGLE arsenic data for well water is available through the Michigan EGLE Private Water Supply Program. County-level data varies, and statewide summaries from EGLE and USGS Michigan Water Science Center reports are the most reliable current sources.

Michigan well owners face a parallel challenge with hardness. Grand Rapids area wells in the glacial outwash aquifer average around 10 to 12 GPG, while Kalamazoo area wells can reach 18 to 19 GPG according to USGS hardness data. Neither hardness level has a health significance, but it affects treatment design. A softener-first approach without checking for arsenic risks missing arsenic entirely, since softeners do not reduce arsenic. See our Michigan hard water by city guide for hardness context by region.

Michigan's private well population is substantial, and the state's Private Water Supply Program provides some resources for well owners. Michigan law requires testing at the time of well construction, during certain real estate transactions, and when wells are rehabilitated or deepened. Ongoing monitoring after move-in is the homeowner's responsibility. For Michigan well owners who have never had a comprehensive panel that included arsenic, that test is overdue regardless of where in the state the well is located.

EPA limits, EWG thresholds, and what each number means for your household

Two separate standards govern how nitrate and arsenic are evaluated in drinking water, and understanding the gap between them matters for private well owners who are making their own decisions without regulatory oversight.

For nitrate, the EPA primary maximum contaminant level is 10 mg/L measured as nitrogen. This limit was established primarily to protect infants under six months of age from methemoglobinemia, a condition in which nitrate interferes with oxygen transport in the blood. The acute risk to infants is the clearest documented health outcome at concentrations above 10 mg/L. Separately, epidemiological research reviewed by the Environmental Working Group suggests associations between chronic exposure at concentrations as low as 5 mg/L and increased colorectal cancer risk in adults. The EWG health guideline of 0.14 mg/L is far below both the EPA MCL and the 5 mg/L threshold, reflecting a long-term cancer risk framing rather than acute infant safety. Private well owners should know where their water falls relative to both the 5 mg/L EWG level and the 10 mg/L EPA MCL, not just one or the other.

For arsenic, the EPA MCL is 10 micrograms per liter (10 ppb). This MCL was set in 2001 and became effective in 2006, lowered from the previous 50 ppb limit. The EPA's own maximum contaminant level goal (MCLG) for arsenic is zero, reflecting that no safe level of exposure to a carcinogen has been established. The practical regulatory floor is 10 ppb as a balance between public health and treatment feasibility and cost for public systems. For private well owners choosing their own treatment standard, the MCLG of zero provides a more conservative target.

Both nitrate and arsenic are undetectable by taste, color, or odor at the concentrations that cause harm. Neither produces immediate symptoms in healthy adults. The harm from nitrate above infant safety thresholds and the cancer risk from arsenic both develop over time. This makes routine testing the only protective action available to a private well household. Annual testing for nitrate is the minimum recommended frequency in Indiana and Michigan agricultural areas. Arsenic testing should be done at least once for every well in Indiana or Michigan, and repeated if there are changes to the well or if neighboring wells detect elevated levels.

For context on PFAS contamination in Indiana public water systems, which is a separate public data issue, see our Indiana PFAS UCMR5 data guide. PFAS is a documented concern in some Indiana communities and is worth understanding alongside the well water picture even for homeowners not on municipal supply.

Treatment that actually removes nitrate and arsenic

Once a lab result confirms nitrate or arsenic above the relevant threshold, the question is which treatment technology reliably removes it. Several options exist, and the right choice depends on the concentration, the presence of other contaminants in the well, household size, and water use patterns. The most important thing to understand upfront is that standard water softeners do not remove either contaminant effectively. Softeners exchange calcium and magnesium ions for sodium. Nitrate and arsenic require different treatment chemistry.

For nitrate above 10 mg/L as N, the primary residential treatment options are:

• Point-of-use reverse osmosis: A five-stage reverse osmosis system at the kitchen sink removes 85 to 95 percent of nitrate through a semi-permeable membrane. This is the most practical and cost-effective solution for most households where the primary concern is drinking and cooking water. An under-sink RO produces filtered water at the kitchen tap without treating water used for bathing and laundry.
• Whole-house anion exchange for nitrate: Specialized nitrate-selective anion exchange resin can be installed as a whole-house unit for households where nitrate exposure from bathing is a concern, or where infants need nitrate-free water at all taps. This is a more expensive and operationally complex solution than point-of-use RO and is typically used when nitrate levels are very high or household needs are specific.

For arsenic above 10 micrograms per liter, the options include:

• Point-of-use reverse osmosis: The same five-stage RO system that reduces nitrate also provides substantial arsenic reduction, typically 90 to 95 percent for pentavalent arsenic (As V), which is the form present in most oxygenated well water. For trivalent arsenic (As III), pre-oxidation with an aeration or oxidation step may be needed to convert it to the pentavalent form before RO treatment. A water test specifying arsenic speciation is helpful for treatment design.
• Whole-house arsenic-selective media: Activated alumina and iron-based adsorbent media (such as GFO, granular ferric oxide) can be used in a whole-house tank to adsorb arsenic from all household water. These systems require periodic media replacement or regeneration and professional sizing based on arsenic concentration and household flow rate. They are appropriate when arsenic is elevated enough to warrant dermal exposure reduction, which is uncommon at typical residential well concentrations just above the MCL.

For Indiana well owners who also have iron or hydrogen sulfide, the treatment sequence matters. Iron and sulfide can foul arsenic adsorption media and reduce RO membrane life significantly. In that case, the correct sequence is: iron and sulfide treatment first, then softening if hardness is high, then arsenic or nitrate treatment at the point of use. See our guide to iron and sulfur treatment in Indiana for the full sequence logic, and our Noblesville well water diagnostic for a practical case study of sequencing multiple contaminants.

For softening in combination with arsenic or nitrate treatment, see our Indiana water softener guide and our no-salt conditioner vs softener comparison for how salt-free options interact with downstream treatment systems.

How to get tested: Indiana and Michigan certified laboratories

For Indiana well owners, only laboratories certified by the Indiana State Department of Health should be used for regulatory-quality analysis. The ISDH certified laboratory list is available at in.gov/isdh. The Indiana DNR well records database at wellrecords.in.gov is the starting point for understanding your well depth and aquifer type before interpreting lab results.

For Michigan well owners, certified laboratory lists are maintained by the Michigan EGLE. The EGLE Private Water Supply Program page provides guidance on testing requirements, certified labs, and what to do when results come back above MCLs. County health departments in both states often coordinate testing programs that can reduce cost for routine panels.

A complete well water panel for nitrate and arsenic concern should include: nitrate (as N), arsenic, total coliform and E. coli, iron, manganese, hardness, pH, and TDS at minimum. If your well is shallow and under agricultural land, annual nitrate testing is advisable. If you have a deep bedrock well in a documented arsenic zone in Indiana or a glacial-drift well in a Michigan county with documented arsenic occurrence, arsenic should be on every panel until you have at least three consecutive non-detect results.

Aqua Otter's free in-home well water consultation covers the full panel for our service area customers across Indiana and Michigan. We bring calibrated meters, pull a sample for certified lab analysis, and return to walk through results and treatment options in person. There is no obligation. Call (248) 621-8411 or use our online scheduler. For other water quality questions specific to municipal supply in our service areas, see our guides to Indianapolis tap water quality, Fort Wayne water quality, and Grand Rapids, MI water quality.

Frequently asked questions about nitrate and arsenic in Indiana and Michigan wells

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