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Trivalent versus hexavalent chromium: why a total-chromium result cannot answer the question

Total chromium measurements mix chromium(III) and chromium(VI)

The public water system record for Montgomery County includes total chromium data, which reflects the combined levels of trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)). These two forms differ chemically and in potential health effects. Because total chromium does not distinguish between these forms, it cannot specify how much chromium-6 is present.

Hexavalent chromium is monitored separately in federal occurrence surveys

The EPA’s Unregulated Contaminant Monitoring Rule (UCMR3) data for Aqua PA’s Main System documents chromium-6 concentrations independently. This provides a more focused look at chromium(VI) levels in the system’s surface water sources, but still does not represent individual home water quality.

Geogenic hexavalent chromium in arid and alkaline aquifers versus industrial plating sources

Chromium-6 can originate from natural sources or industrial activities

Chromium-6 presence in water arises from different sources. Geogenic sources, such as specific rock formations, release chromium(VI) under certain conditions, typically in arid and alkaline aquifers. In contrast, industrial plating and manufacturing processes may contribute chromium-6 contamination in areas of heavy industry. Montgomery County’s surface water supply may have varying inputs, but the exact source distribution for chromium-6 in this system is not detailed in the data.

Analyzing source helps prioritize treatment and testing approaches

Knowing whether chromium-6 originates geogenically or from industrial sources can influence how a household chooses to approach water treatment and testing. Understanding these distinctions aids in evaluating the most appropriate water filtration technology.

The order in which to rule things out, from cheapest check to laboratory test

Start with basic water quality parameters available from the water supplier

Review available water quality reports and consumer confidence reports from your utility, which may provide context for chromium and other metals. These documents do not cover chromium-6 specifically but help establish baseline water quality.

Conduct a certified laboratory test for chromium-6 at your tap

The only way to confirm your home's chromium-6 level is through a laboratory analysis of a water sample from your point of use. This test measures chromium(VI) in micrograms per litre (µg/L or parts per billion), the unit used in the EPA’s monitoring results.

How oxidation state changes between the well, the distribution main and the tap

Chemical changes can alter chromium speciation during water treatment and distribution

Chromium can change from one form to another through oxidation-reduction reactions. For example, chromium-6 may be reduced to chromium-3 in some parts of the treatment process or pipes, altering the measured forms at different points.

Surface water supplies are treated and can affect chromium forms

Because Montgomery County’s Aqua PA system uses surface water, treatment steps such as coagulation, filtration, and disinfection may impact chromium speciation before the water reaches homes. This complexity means that local area monitoring data may not fully represent the chromium-6 level at an individual household tap.

What the UCMR3 nationwide survey was designed to learn, and its limits for one household

UCMR3 provides occurrence data, not compliance or specific home results

The EPA’s UCMR3 survey collected water samples from public water systems nationwide between 2013 and 2015 to monitor emerging contaminants, including chromium-6. Montgomery County’s Aqua PA system showed detections of chromium-6 in 100 out of 104 samples, with a highest detected value of 2.6 µg/L and a median detection of 0.205 µg/L.

Survey data cannot determine if a single household’s water meets any specific level

Because the survey samples system-wide points, the data cannot be used to infer the chromium-6 concentration at a particular home tap. Variations within the distribution system and household plumbing may differ significantly.

What distinguishes this problem from the two problems most often confused with it

Chromium-6 is distinct from total chromium and from other metals in water

Unlike total chromium measurements, hexavalent chromium refers to a specific chemical form with unique chemistry and treatment needs. It is also separate from problems such as iron or manganese staining or bacterial contamination, which require different approaches.

Taste, staining, or corrosion issues are unrelated to chromium-6 concentrations in this context

Chromium-6 is monitored primarily for potential health considerations under research and regulatory review, rather than aesthetic water quality issues. Homeowners should not confuse these separate water quality concerns.

Next step: order a laboratory test for chromium-6 at your tap

The definitive way to understand your household’s chromium-6 concentration is to have your tap water analyzed by a certified laboratory. This test should specify chromium-6 or hexavalent chromium results reported in micrograms per litre (µg/L).

If your results indicate chromium-6 at concentrations of concern, the Pentek Chromium-6 Filter ships ready to configure and can be a practical option for reducing chromium(VI) levels. This technology is designed specifically to address hexavalent chromium in residential water supplies.

Where this information comes from

Source: EPA SDWIS public water system record. Sampled 2013-10-31. These figures describe the public water system's record, not the plumbing inside any individual building.

On record: EPA UCMR3 monitoring (2013-2015) for Aqua PA - Main System (PA1460073), PA: 208 results across 2 listed contaminants, 149 detections above the reporting level; no federal MCL for hexavalent chromium; total chromium MCL 100 ug/L