When Drinking Water Contaminates Ground Water

Although it may seem counter-intuitive and even ludicrous, there are situations where ground water has been “contaminated” by municipal drinking water in West Virginia, with potentially serious complications and cost implications for cleanups of contaminated sites under West Virginia’s environmental regulatory programs.  It should be noted that this scenario is not unique to West Virginia. 

The problem results from the presence of trace amounts of certain volatile organic compounds (VOCs) in public water supply systems where the raw water supply has been treated (disinfected) with chlorine and chloramine.  In particular, the problem is related to the group of VOCs known as the trihalomethanes, which are formed when chlorine and chloramines react with trace amounts of naturally-occurring organic materials present in water.  The total trihalomethanes (TTHMs) consist of four VOCs: bromodichloromethane, bromoform, chloroform, and dibromochloromethane.  Under the Safe Drinking Water Act (SDWA), USEPA regulates TTHMs in public water supply systems under the Stage 1 and 2 Disinfectants and Disinfection Byproducts Rule and the USEPA has established an MCL in drinking water of 80 parts per billion (ppb).  This standard is for the sum of all four TTHMs.  This is where the trouble begins.  For example, under the WV Voluntary Remediation Program, the WVDEP has established risk based concentration (RBC) de minimis values for each of the four individual TTHMs in groundwater, which range from 120 parts per trillion to 8.5 ppb.  Obviously, these values are significantly less than the USEPA MCL for the TTHMs under the SDWA.    

Why is this important?  Discharges of drinking water to the subsurface can result in situations where the concentrations of the individual TTHMs in groundwater, typically chloroform, are greater than the RBC de minimis values for the four individual VOCs under the WV Voluntary Remediation Program, but the sum of the four TTHMs are less than the USEPA MCL standard.  In other words, groundwater at a site could be in compliance with the USEPA drinking water MCL for the TTHMs, but still exceed a RBC de minimis value for one of the individual TTHMs (e.g., chloroform).  Under such a scenario, a property owner could be looking at potentially performing expensive groundwater investigations and remediation to address the chloroform “contamination.” 

Although chloroform contamination can be associated with some chemical manufacturing activities, the common sources of TTHMs observed in groundwater in West Virginia include leaks from municipal water distribution and sanitary sewer systems.  Also, monitoring well drillers often use a small amount of municipal water during environmental drilling and well construction because it is “clean,” which also results in “releases” of municipal drinking water to groundwater.  Even if TTHMs are not present in the drinking water, the chlorine and chloramines that are present can react with the naturally-occurring organic materials in the groundwater aquifer and generate detectable concentrations of TTHMs.  Although such “releases” of “clean” drinking water might not seem important, because of the extremely low RBC de minimis values for the individual TTHMs and laboratory detection limits, they are often detected in the parts per billion or even parts per trillion range during monitoring well sampling events, in particular, if the wells have not been fully developed prior to sampling.  Over the last 15 years, we have observed several sites in West Virginia where such “releases” to groundwater have occurred.  At one site in particular, 11 groundwater monitoring wells had been installed and sampled at considerable expense to “chase” and delineate a chloroform plume in groundwater that was discharging from a water treatment plant.  Since no chemical manufacturing had been associated with this property, it was ultimately determined that the USEPA MCLs were the appropriate applicable regulatory standard, and not the RBC de minimis values.   

Additional information about TTHMs, disinfection byproducts, and applicable MCLs is available at http://water.epa.gov/drink/contaminants/basicinformation/disinfectionbyproducts.cfm. 

This article was authored by Greg Tieman, Acacia Environmental Group LLC. For more information on the author see here.