Indiana Background Levels - Establishing and Using Background Concentrations in Cleanups

Overview

Background concentrations are the concentrations of chemicals that would be present at a site in the absence of the release being investigated. In Indiana, background is one of the remediation objective options available under the Risk-based Closure Guide (WASTE-0046-R2) - if a chemical’s concentration at a site does not exceed its background concentration, no remediation is required for that chemical regardless of whether it exceeds Published Levels.

This matters most for naturally occurring chemicals, particularly metals. Arsenic, chromium, iron, manganese, and other metals are naturally present in Indiana’s soils and groundwater at concentrations that frequently exceed health-based screening levels. Without a background evaluation, a site could be required to remediate naturally occurring conditions to levels below what nature provides - an impossible and unnecessary task.

The R2 recognizes background as a remediation objective under R2 Task 6, and IC 13-25-5-8.5(b)(1) provides the statutory basis by allowing remediation objectives based on “background levels of hazardous substances and petroleum that occur naturally on the site.”

Types of Background

Naturally Occurring Background

Naturally occurring background refers to concentrations of chemicals in soil, groundwater, or sediment that result from natural geologic, hydrologic, or biological processes - not from any human activity. Common examples in Indiana include:

• Arsenic in soils derived from glacial till (Indiana’s glacial geology contributes naturally elevated arsenic in many areas)
• Iron and manganese in groundwater from dissolution of aquifer minerals
• Barium in soils derived from carbonate bedrock

IDEM has published area-specific background studies that provide reference data for certain regions. A 2013 study in Terre Haute (conducted in partnership with Indiana University-Purdue University Indianapolis and the Midwestern States Environmental Consultants Association) examined background lead, arsenic, and PAH concentrations in surface soils. A 2017 study in Indianapolis examined background lead and arsenic in surface soils. Both are available as PDFs on the IDEM Technical Guidance for Cleanups page.

Area-Wide Anthropogenic Background

Area-wide anthropogenic background refers to concentrations resulting from diffuse, non-point-source human activities that are not attributable to a specific release. Examples include:

• Lead in urban surface soils from historical leaded gasoline and paint
• PAHs in surface soils in industrial areas from decades of atmospheric deposition
• Pesticides in agricultural soils from historical application

The statutory language in IC 13-25-5-8.5(b)(1) refers to background levels that “occur naturally,” which may limit the formal applicability of anthropogenic background as a remediation objective. However, IDEM’s published background studies for Terre Haute and Indianapolis include lead and PAHs - chemicals with significant anthropogenic contributions in urban areas. In practice, the distinction between naturally occurring and area-wide anthropogenic background should be discussed with the IDEM project manager early in the project, as the acceptability of an anthropogenic background approach may depend on the specific chemicals, site context, and program.

When to Use Background

Background is most useful when:

• Metals exceed Published Levels but are likely naturally occurring. This is the most common application in Indiana. Many sites have arsenic, iron, or manganese at concentrations above the Published Levels but within the range of natural variability for the area.
• Site-wide concentrations are uniform rather than showing a release pattern. If a chemical’s concentration is roughly the same across the site including areas unaffected by the release, the chemical may be present at background levels.
• The chemical is not associated with the release. If the release involved petroleum or chlorinated solvents, naturally occurring metals at the site are not release-related and should be evaluated against background rather than Published Levels.

Background is not appropriate when:

• The chemical is clearly associated with the release (e.g., metals at a plating facility)
• Concentrations show a spatial pattern consistent with the release source (elevated near the source, decreasing with distance)
• The chemical is not naturally occurring and has no plausible area-wide anthropogenic source

Sampling Design

Selecting Background Sampling Locations

Background samples must be collected from locations that are representative of the same geologic setting as the site (same soil type, same geologic formation, same depth interval), unaffected by the release being investigated (upgradient, outside the contamination footprint, or at a separate reference location), and comparable in land use history (an undeveloped wooded area is not appropriate background for an urban industrial property when evaluating anthropogenic chemicals).

For groundwater, background wells should be installed in the same aquifer, at the same depth interval, and upgradient of the release. For soil, background samples should be collected from the same soil horizon and geologic unit at locations outside the area of influence of the release. Use USGS and Indiana Geological and Water Survey maps to confirm the geologic context.

Number of Background Samples

The number of background samples must be sufficient to characterize the range and variability of background concentrations. The EPA ProUCL Technical Guide recommends at least 8-10 background samples for statistical comparisons, though more may be needed if variability is high. In practice, 15-20 samples produce more robust statistics, especially in Indiana’s geologically variable glacial deposits. Background samples should span the spatial extent of the site’s geologic setting, not be clustered in one location, and should be collected from the same media and depth intervals as site samples.

Analytical Methods

Background samples must be analyzed using the same analytical methods and reporting limits as site samples. If site samples were analyzed by EPA Method 6010/6020 for metals, background samples must use the same methods. Inconsistent analytical methods undermine the statistical comparison.

Statistical Methods

Comparing Site Data to Background

The purpose of the statistical evaluation is to determine whether site concentrations are significantly different from background concentrations. Common approaches include:

Upper tolerance limit (UTL) or upper prediction limit (UPL) comparison. Calculate a UTL (e.g., 95% UTL at 95% confidence) for the background dataset. Site concentrations below the UTL are considered consistent with background.

Two-sample hypothesis testing. Statistical tests such as the Wilcoxon rank-sum test (Mann-Whitney U test) compare the distributions of site and background data. If the test indicates no statistically significant difference, the site data is consistent with background.

EPA ProUCL. EPA’s ProUCL software is the standard tool for environmental statistical analyses and is widely accepted by state regulatory agencies. It calculates UCLs, UTLs, performs distribution testing, and supports background comparison analyses. ProUCL includes specific modules for background versus site comparisons and handles censored datasets.

Handling Non-Detects

Background datasets frequently include non-detect results, especially for metals near the reporting limit. Statistical methods must properly account for censored data (values reported as less than the detection limit). ProUCL includes methods for datasets with non-detects, including Kaplan-Meier and regression on order statistics (ROS) approaches.

Do not substitute zero or the detection limit for non-detects without understanding the statistical implications. The substitution method affects the calculated background concentration and can bias the comparison in either direction.

Representative Concentration vs. Maximum

The R2 Task 5 requires representative concentrations, not maximum concentrations, for risk evaluation. Similarly, background should be characterized by a statistically valid upper bound (UTL or UCL), not by the single highest background sample. A single elevated background result does not establish a background level - the statistical evaluation provides the defensible basis.

Using Background as Remediation Objectives

Under R2 Task 6, background concentrations are one of the options for specifying remediation objectives. When a chemical’s representative concentration at the site does not exceed the established background concentration, no remediation is required for that chemical even if it exceeds Published Levels.

The R2’s framework for applying background:

1. Establish background using defensible sampling and statistical methods
2. Compare site representative concentrations (Task 5) to background
3. If site concentrations are at or below background: the chemical does not require remediation. Document this in the Task 7 remedy necessity determination.
4. If site concentrations exceed background: the chemical requires further evaluation against Published Levels or site-specific risk-based values

Documentation Requirements

A background evaluation submitted to IDEM should include:

• Rationale for the background approach (why background is appropriate for the chemicals and media in question)
• Description of background sampling locations and their relationship to the site geology
• Map showing background sample locations relative to the site
• Background analytical results with full laboratory reports
• Statistical analysis with methodology described (software used, parameters selected, distribution assumptions)
• Comparison of site data to background with clear conclusions
• Discussion of how background fits into the overall conceptual site model

Comparison to Ohio

Feature	Indiana	Ohio
Background as remediation objective	Allowed under R2 Task 6 and IC 13-25-5-8.5(b)(1)	Allowed under OAC 3745-300-08(B)(4)
Who establishes background	Consultant proposes, IDEM reviews	CP establishes, subject to Ohio EPA audit
Statistical software	ProUCL widely used and accepted	ProUCL commonly used; no specific mandate
Area-wide anthropogenic background	Statutory language says “naturally occurring”; discuss with IDEM PM	Addressed through Urban Setting Designation program
State-specific background data	IDEM published Terre Haute (2013) and Indianapolis (2017) surface soil studies	Ohio EPA has published limited background data for select contaminants

Ohio’s Urban Setting Designation (USD) program provides a formalized mechanism for addressing area-wide background in urban settings. The USD allows properties in designated urban areas to use modified groundwater standards based on the assumption that municipal water supply is available. Indiana does not have an equivalent program - area-wide background issues are handled case by case through the R2 framework and in coordination with the IDEM project manager.

Practical Tips

Start the background study early. Background sampling takes time - you need to identify locations, install wells (for groundwater), collect samples, and wait for results. If background is likely to be important (metals sites, glacially-derived soils), plan the background study as part of the initial investigation, not as an afterthought.

Check the IDEM background studies first. Before designing your own background sampling program, review the IDEM-published background studies for Terre Haute and Indianapolis. If your site is in a similar geologic setting, these studies provide useful reference data (though they may not substitute for site-specific background sampling).

Match the geology precisely. IDEM will evaluate whether your background locations represent the same geologic setting as the site. If your site is in glacial till, your background locations must be in glacial till. If the site is in alluvial deposits along a river, background must come from the same alluvial setting. Use USGS and Indiana Geological and Water Survey maps to confirm the geologic context.

Collect more samples than you think you need. Background variability is often higher than expected, especially for metals in glacially-derived soils. A dataset with 8 samples and high variability produces a wide confidence interval and a high UTL - which may not provide the tight background characterization you need.

Document non-release conditions at background locations. IDEM may question whether your background locations are truly unaffected. Document the land use history, inspect for potential contamination sources, and photograph the sampling locations. If a background location later turns out to be contaminated, your entire background study is compromised.

Use ProUCL correctly. ProUCL has many options, and selecting the wrong distribution assumption or the wrong test can produce misleading results. Follow EPA’s ProUCL Technical Guide and document your parameter selections in the report.

Source

IDEM Nonrule Policy Document WASTE-0046-R2: Risk-based Closure Guide, July 8, 2022 (background provisions in Tasks 5-6). IC 13-25-5-8.5(b)(1) (remediation objectives based on background). IDEM Background Lead, Arsenic and PAH Surface Soil Levels - Terre Haute (2013). IDEM Background Lead and Arsenic Surface Soil Levels - Indianapolis (2017). EPA ProUCL Technical Guide for Statistical Analysis of Environmental Background and Site Data. Indiana Geological and Water Survey publications.