RSL, MCL, MCLG, UPUS – What’s it all mean?


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RSL, MCL, MCLG, UPUS – What’s it all mean?

By: Nate Wanner

While many people understand that there is a limit to how much contamination can be present in groundwater, they are often surprised to learn that there is not just one limit. These numbers vary depending on the chemical, regulatory agency or program, best available analytical and treatment technologies and costs, and other factors. Confusion over this alphabet soup is common, and EPA recently revised Section 2.5 of its User’s Guide to describe how MCLs are used in data screening and how they are different than RSLs.

Groundwater standards for drinking water begin by identifying contaminants that may adversely affect public health. For instance, benzene (found in gasoline and other materials) can cause cancer and other health issues. Conversely, iron and potassium are essential dietary minerals. EPA also focuses their efforts and research on contaminants that occur with some degree of frequency in groundwater.

Next, EPA determines a maximum contaminant level goal (MCLG) for contaminants it decides to regulate in public drinking water supplies. MCLGs are developed by EPA’s Office of Water, and are somewhat similar to Regional Screening Levels (RSL) for tapwater developed by EPA’s Superfund program. Both MCLGs and RSLs are developed based on the level of a contaminant in drinking water below which there is no known or expected risk to health. In general, these are developed with a one in a million target risk – if one million people are exposed to a particular level of contaminant, one of the million people can be expected to have an adverse health affect. These calculations are based upon a number of assumptions regarding the age, mass and gender of people, how much contaminant they ingest, how long they are exposed to the contaminant, and a host of other factors. As new research becomes available and models are refined, MCLGs and RSLs are adjusted to match the newest and most commonly accepted data.

While many people would expect drinking water standards to exactly match the MCLGs or RSLs, this is not necessarily the case. Sometimes the values are so low that analytical laboratories are unable to detect compounds at that level, or cannot do so without using exorbitantly expensive technology. Other times, treatment technologies and cost are taken into consideration. At the end of the day, maximum contaminant levels (MCLs) are set as close to risk-based goals (MCLGs) as feasible. MCLs are enforceable standards that must be met for drinking water provided by public water supplies. Unlike MCLGs and RSLs, MCLs are rarely updated.

Not all contaminants have MCLs, although other contaminants are commonly regulated by other EPA or state programs. These programs may have different risk tolerances; such as, “1 in 100,000” or “1 in 10,000.” For instance, in Ohio’s Voluntary Action Program (VAP), contaminants without an MCL are evaluated at a “one in 100,000” risk level represented by an Unrestricted Potable Use Standard (UPUS). The Ohio VAP and other regulatory programs also commonly account for cumulative risk across all contaminants, meaning that a little bit of a lot of things is equally bad to a lot of one thing. There can also be different groundwater standards if the water won’t be used as drinking water.

To summarize, here is a “quick and dirty” summary of common acronyms used with drinking water standards:

  • MCL – maximum contaminant level. The highest concentration of a contaminant allowed in drinking water provided by a public water supply.
  • MCLG – maximum contaminant level goal. The concentration that an MCL will be set at, if it is feasible.
  • RSL(for tapwater) – regional screening level. The concentration below which risk from a contaminant is generally considered negligible and not warranting additional response or investigation. Whether or not a concentration above an RSL actually poses a risk is the subject of additional investigation.
  • UPUS – unrestricted potable use standard. Ohio EPA Voluntary Action Program’s highest concentration of a contaminant permissible in drinking water when an MCL has not been promulgated. This standard may be adjusted downward (more stringent) if more than one contaminant is present.

Published in Cox-Colvin’s January 2019 Focus on the Environment newsletter.


Nate Wanner is a Cox-Colvin Senior Scientist with over 15 years of experience leading and completing environmental projects, in addition to six years as an educator and IT director. He holds a BS in Geology: Water Resources from Ohio University and a Masters in Geographic Information Systems from Penn State University. His areas of expertise include brownfields, underground storage tanks, due diligence and database services. Nate is an Ohio EPA VAP Certified Professional (CP), a Certified Professional Geologist (CPG) with the American Institute of Professional Geologists (AIPG) and is a registered Professional Geologist (PG) in Kentucky.