GROUNDWATER MONITORING BASICS

What "background" means in landfill groundwater monitoring

30 September 2026 · about a six minute read

Part of a series. If you are new to the subject, start with what RCRA groundwater monitoring is, and why it needs statistics.

Every statistical test a municipal solid waste landfill runs on its groundwater asks the same question: is the water at the edge of the landfill different from the water that the landfill has not touched? The water the landfill has not touched is called background. This article explains what the federal rule says background is, where it comes from, and why it is the part of the work that most often decides whether a result is right.

It covers landfills regulated under RCRA Subtitle D, the part of the Resource Conservation and Recovery Act that governs municipal solid waste landfills. The groundwater rules for those landfills are in 40 CFR Part 258, Subpart E. States with approved programs write their own rules on top of it, and a facility's permit can add more, so check what applies to your site.

What the rule says background is

A landfill's monitoring network has two jobs. Some wells show the groundwater passing the point of compliance at the edge of the landfill. Those are the compliance wells, often called downgradient wells, because groundwater flows toward them from the waste. The other wells must, in the words of the rule:

Represent the quality of background ground water that has not been affected by leakage from a unit.

Those are usually upgradient wells, upstream of the landfill in the direction groundwater flows. The rule allows background to come from other wells too, where the site's hydrogeology makes it impossible to say which wells are upgradient, or where other wells would represent background as well or better than the upgradient ones. The owner or operator must establish background for each constituent the monitoring program requires.

Why one sample is not enough

Groundwater quality varies even where nothing is wrong. Concentrations move with the seasons, with rainfall and recharge, and with small differences in how each sample is taken and analyzed. A single background result says nothing about how much a concentration normally varies, and so it cannot tell a real increase from an ordinary one.

So the rule sets a minimum. During the first semiannual sampling event, at least four independent samples must be collected and analyzed from each well, background and downgradient, for the constituents in Appendix I, the list of constituents Part 258 requires in detection monitoring. After that, at least one sample is taken from each well at every semiannual event, and background grows over time.

Four is a floor, not a target. EPA's Unified Guidance, the agency's guidance on the statistics these rules require, says four observations are rarely enough to describe how a constituent varies. It recommends at least 8 to 10 independent background observations before running most statistical tests, and treats even that as a temporary minimum until more background has been collected.

Why background decides the result

The Unified Guidance is direct about this:

High quality background data is the single most important key to a successful statistical groundwater monitoring program, especially for detection monitoring.

Every test the rule allows, whether an analysis of variance, a prediction limit, a tolerance limit or a control chart, compares compliance well results with background. If background is wrong, the test is wrong, whatever method is used.

The guidance names several ways background goes wrong:

• It no longer describes current conditions. If groundwater has changed over time, for example through cycles of drought and recharge, background measured five or ten years ago may not reflect today's uncontaminated water. Older results may have to be set aside in favour of more recent ones.
• The laboratory methods changed. Results from improved analytical methods may not be comparable with older results.
• The wells differ from each other naturally. At many sites, naturally occurring constituents are at different levels in different wells even where nothing has leaked. Upgradient wells then say little about what is normal at a given compliance well, and each compliance well may need to be compared with its own history instead. The guidance calls these two approaches interwell and intrawell comparisons.

Background also matters beyond the tests. The guidance notes that when upgradient and compliance wells change together, the cause may be a change across the whole aquifer, or a laboratory problem, rather than the landfill.

Background can become the standard

Background has one more role. If monitoring moves into assessment, results are compared with groundwater protection standards, which are usually drinking water limits called maximum contaminant levels. Where the natural background concentration of a constituent is higher than its maximum contaminant level, or where no such limit exists, the rule uses the background concentration as the standard instead.

If something here does not match how background is set at your facilities, we would like to hear it. Write to support@koynasystems.com.