Report 4: Trihalomethanes (THMs) at Flint monitoring sites test below EPA’s Standards
Trihalomethanes (THMs) are a type of disinfection byproducts (or DBPs) formed in drinking water when disinfectants (chemicals used to kill microorganisms like chlorine) react with organic or inorganic matter in the water. During our sampling trip in Flint, we sampled at eight normal distribution system monitoring sites on August 18 for THMs. All sites had LOW THMs, due to improved water quality post-installation of a new $1.6 million Granular Activated Carbon (GAC) treatment filter. The reduction of THMs by GAC is optimal when it is brand new, and the GOOD NEWS is that the THMs we measured at the monitoring sites ranged from 24-39 ppb (average = 30 ppb) which is now well below EPA standards.
There are legitimate concerns that the monitoring sites do not represent worst case scenarios in the distribution system. So, we tested tap water of residents living farther away from the water treatment plant; we took four samples from consumers homes where the chlorine was all gone (the scientific term is “decayed”), and which should represent pretty “bad cases” for testing. Not surprisingly, the THMs were higher at 52-87 ppb at these sites, with 2 out of 4 over the 80 ppb EPA standard. We even tried to see what the “worst possible” THMs could be on the day we sampled, by taking water samples near the treatment plant, and letting chlorine react in a glass container after collection. This test procedure can be expected to make water with the highest possible THMs. All of those samples were over the EPA 80 ppb standard, although at 83-92 ppb they were just barely over.
THMs Main Conclusion: The good news is that the GAC filters seem to be working, thanks to the efforts of the Flint’s water treatment plant personnel. The GAC should also help address taste and odor problems.
Caveats: Although this is unequivocal good news compared to recent history, we also note the following:
1) THMs are now back to levels typical of when Detroit water was being used.
2) The ability of GAC to remove THMs will slowly be lost over the next half year, and replacing it to get back to where things are now will cost more money.
3) When we sampled, the chlorine in the distribution system was too low to meet Federal standards, which tends to make THMs measurements lower. Unfortunately, no matter how much chlorine Flint adds, it is probably not possible to get chlorine to more than 95% of consumers’ homes because of rampant corrosion of iron pipes making up the distribution system.
Take away message: We agree with Dr. Joan Rose, that in the grand scheme of things, worse things can happen than having THMs higher than EPA standards. In fact, we believe that worse things are happening as we speak, including 1) permanent damage to the water pipe system from corrosion, 2) chlorine residuals that are too low in many homes, and 3) initial data showing very high lead levels.
Primary Author: Dr. Marc Edwards
Acknowledgments: Dr. Peter Vikesland and Jody Smiley at Virginia Tech for analyzing the THMs and helping with the sampling plan.