Hexavalent Chromium (Cr-6)

Nationwide interest in levels of hexavalent chromium (Cr-6) in drinking water followed a report released in December 2010 covering major cities, including the Washington, D.C. region.  There is much uncertainty about the significance of very low levels of Cr-6 in drinking water so we are providing the following facts to keep our customers informed about the issues regarding Cr-6 and WSSC’s actions.


What is chromium and where do you find it?

Chromium is a naturally occurring metal found in rocks, soil, plants, and animals.  Chromium is also used industrially in steel making, metal plating, leather tanning, paints, dyes and wood preservatives. It is also discharged by industrial facilities such as coal burning power plants. It is most commonly present in the environment in three forms:  trivalent (Cr-3), hexavalent (Cr-6) and Cr-0, the metal form.  Cr-3 is an essential micro-nutrient in the human diet and generally not considered toxic. Cr-6 is known to pose human health risks from inhalation, and a new Environmental Protection Agency (EPA) risk assessment, not finalized yet, has raised concerns about the risk to human health from exposure to very low levels of Cr-6 by ingestion.


Is Cr-6 allowed in drinking water?

Cr-6 is currently regulated under the EPA's Safe Drinking Water Act only as a component of “total chromium” (along with Cr-3).  The allowable level (or Maximum Contaminant Level, MCL) for total chromium is 100 parts per billion (ppb).  However, the current MCL was set more than 20 years ago based on the best available science at that time when Cr-6 was not believed to be a significant health risk from ingestion of drinking water.  So the current MCL does not necessarily address the recent concerns that have been raised about Cr-6 at levels much lower than the MCL.


Testing for Cr-6

Currently, the EPA does not require that Cr-6 be separately tested in drinking water. However, it does require that we measure total chromium and that measurement includes Cr-6 along with Cr-3. Total chromium levels in WSSC water are routinely less than 2 ppb – 50 times less than the EPA limit. This is chronicled annually, along with other required water quality test results, in our Water Quality Report.


In light of the recent concerns in January 2011 the EPA issued guidance for water utilities who wish to voluntarily monitor for Cr-6. Following this guidance, WSSC began testing separately for Cr-6 at a variety of points within our system.  


Sampling points included the source water we take into our Water Treatment Plants (WTP) from the Potomac and Patuxent Rivers, and treated water at those plants and a variety of public locations, including a fast-food restaurant, a local college and a coffee shop. WSSC uses careful sampling procedures to collect representative samples and avoid sample contamination.


We performed tests on a monthly basis for a year, going beyond the EPA recommendations that we test quarterly. We then tested on a quarterly basis for an additional year, and we are in the process of testing a more limited set of points quarterly for a final year. All data from those tests have been made available to regulators, our customers and the public through our website, and we will keep those results online.


Here is a table detailing the results of our Cr-6 samples testing.  It will be updated as additional tests are conducted. 

WSSC Voluntary Cr-6 Monitoring Results (in ppb) (Quarterly since April 2012)






Potomac WTP raw 0.090 0.066 0.067   0.072 0.048
Potomac WTP tap 0.077 0.200 0.130  0.093 0.048
Bethesda (restaurant) 0.087 0.180 0.150  0.084 0.076
Piscataway WWTP (operator's lab) 0.100 0.240 0.170  0.170  0.11
Patuxent WTP raw 0.024 0.025  *ND  0.024  0.027
Patuxent WTP tap 0.033 0.034  *ND 0.024 0.035
Colesville (restaurant) 0.047 0.070 0.029  0.051 0.052
Bowie (college campus) 0.072 0.100  0.067  0.084 0.095
WSSC Voluntary Cr-6 Monitoring Results (in ppb) (Feb 2011 - Jan 2012 monthly)














Potomac WTP raw *ND  0.070 0.054 0.110 0.190 0.065  0.140 0.067 0.072 0.100 0.100 0.086
Potomac WTP tap *ND   0.070 *ND**  0.093 0.093  0.200 0.098 0.080 0.120 0.100 0.110 0.084
0.068 *ND


0.100 0.220  0.220 0.210 0.140 0.130 0.100 0.110 0.083
Piscataway WWTP 
(operator's lab)
0.073 *ND  0.100 0.150 0.280  0.270 0.270 0.190 0.180 0.160 0.130 0.120
Patuxent WTP raw *ND *ND *ND   0.053 0.038  0.022 0.028 0.039 *ND 0.027 0.039 0.022
Patuxent WTP tap *ND *ND 0.062   0.057 0.052  0.036 0.034 0.042 0.029 0.033 0.043 0.030
*ND *ND 0.074** 0.130  0.056  0.049 0.038 0.051 0.058 0.042 0.057 0.036
(college campus)
*ND *ND 0.095**  0.120 0.180  0.160 0.160 0.110 0.10 0.090 0.087 0.080


*ND - Non-detection - results are below the minimum level (0.05 ppb or 0.02 ppb) the lab method can reliably detect.   It does NOT necessarily mean that the concentration is zero, just that it’s too low for the lab to confidently assign a number if it is present.

** Sample results may be invalid due to improper sample preservation, but are shown for informational purposes.


Results from sampling to date:

Results to date indicate a very low level presence of Cr-6 when it is detected above the minimum reporting limit at the eight sample locations.  See the Cr-6 Frequently Asked Questions for information about the potential significance of Cr-6 at these very low levels.


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