By Kelly A. Reynolds, MSPH, PhD
Ahead of the federal regulatory curve, the California Department of Public Health (CDPH) joins other states in the nation in setting recommended notification levels for emerging chemical contaminants in drinking water sources. According to US EPA, 1,4-dioxane is a possible human carcinogen. Due to the chemical’s highly soluble nature and frequent presence in the environment, concern of human exposures from groundwater consumption continues to grow. How dangerous is 1,4-dioxane? What is the risk of cancer in exposed populations and is there a way to control the contaminant in our drinking water supplies?
An emerging contaminant
Dioxane has been found in numerous groundwater supplies in the US, Canada and Japan. In 2008, 73/702 (10 percent) of groundwater sources tested positive for 1,4-dioxane, with 28 (four percent) testing above the CDPH set notification level (then set at 3.0 µg/L) in drinking water. The majority of positive samples were clustered in Los Angeles County, CA.
A US EPA toxicology report on 1,4-dioxane, published in 2010, suggested the harmful level of exposure was even lower than previously thought. In response, several states lowered their guidance related to tolerable concentrations. California and Illinois set new guidance levels at one µg/L. Massachusetts set an even more stringent guidance at 0.3 µg/L.
There are many data gaps related to how often humans are exposed to dioxane and to how much they are exposed. Under US EPA’s Unregulated Contaminants Monitoring Requirements 3 (UCMR3), dioxane will be monitored by 800 US water utilities to a limit of ≤ 0.3 µg/L. With increased monitoring, the number of public and domestic wells testing positive is expected to increase. The question is: how will this data be used to drive increased safety standards?
Dioxane is mostly manufactured for use as a stabilizer in solvents but can also be found in personal care products, such as shampoo, cosmetics and detergents. Food samples have also tested positive. Mostly, electronic, metal-finishing and fabric-cleaning industries utilize dioxane-containing compounds. The properties that protect solvent degradation also lead to the degradation resistance in dioxane.
Humans are exposed to 1,4-dioxane via ingestion, inhalation or dermal absorption pathways. Water can serve as a source for each of the above exposure routes where drinking, showering, bathing and laundering activities contribute. Although recognized as an incomplete database, 182,338 pounds of dioxane are known to have been released into the environment in 2007. Surface water discharges totaled approximately 56,996 pounds around the same time frame. The contaminant binds weakly to soils and is highly water soluble—a combination conducive to increased transport and groundwater access.
The US EPA Integrated Risk Information System (IRIS) provides information relative to specific exposure pathways and cancer risk levels for specific hazards. For non-carcinogens, IRIS provides information on reference dose (RfD) values, which mark the estimated ‘safe’ level of exposure, above which unacceptable adverse effects may occur. RfDs are based on estimated daily intake levels of drinking water. The more water one drinks, the more the intake of contaminants as well. US EPA estimates individual drinking water consumption rates at an average 1.2 L/ day for adults. Exposure below the RfD is thought to be tolerable by humans with no significant risks. Acceptable risk is based on a 10-6 risk level. This means that the drinking water consumed should cause less than one-in-a-million chance of cancer over a lifetime of consumption. Determination of carcinogenic effects is not a simple process. Classification of possible carcinogens is determined relative to the available weight of evidence (see Table 1). Evidence of 1,4-dioxane carcinogenicity is based on animal, not human, evidence. Given tumor production and spread in animals following exposure, the contaminant is considered a probable human carcinogen as well.
Maximum contaminant levels (MCLs) are legally enforceable federal standards indicating the allowable limit for drinking water contaminants. Although state and federal maximum contaminant levels have not been established for 1,4-dioxane, individual states can set their own standards and health advisories. The maximum concentration in drinking water to achieve a 10-6 risk level is estimated at 0.35 µg/L. Therefore, this is the guideline (i.e., Hitting A Moving Target: Regulation of 1,4-Dioxane in Drinking Water Table 1. Cancer classifications based on weight of evidence Classification Evidence H Carcinogenic to humans L Likely to be carcinogenic to humans L/N Likely to be carcinogenic above a specified dose, but not below S Suggestive evidence of carcinogenic potential I Inadequate information to assess carcinogenic potential N Not likely to be carcinogenic to humans Water Conditioning & Purification July 2013 unenforceable limit) now set by US EPA in drinking water. Some states have set even more stringent standards than the federal guidelines. Conventional drinking water treatment systems, both large and small in scale, are generally not designed to remove dioxane. One strategy for making sure regulated contaminants do not exceed MCLs is to blend the contaminated water source with non-contaminated water. The point, therefore, is to dilute the hazard to a practical, if not acceptable, risk level.
A treatment plant in Tucson, AZ has been diluting their drinking water supply to achieve a 1.15 µg/L dioxane concentration. At a cost of millions of dollars, the utility is currently constructing an advanced oxidation process (AOP) water treatment facility in response to the latest US EPA guidelines of 0.35 µg/L limits. Industry groups are questioning the validity of what appears to be a moving target relative to dioxane advisories. As one reporter put it, “The chemophobia of the EPA seems founded on a political agenda and upon pure guess work…”.
Published research evaluating various technologies for efficacy of dioxane removal at the point-of-use/point-of-entry and for municipal water treatment is scarce. Studies funded by the NHDES and the University of New Hampshire (UNH) Environmental Research Group (ERG) evaluated treatment technologies, including air stripping, carbon adsorption, direct UV photolysis and UV-peroxide advanced oxidation to remove dioxane from private groundwater systems. Coconut-based carbon adsorption was found to be the most feasible treatment option based on efficacy (96 percent removal), cost and ease of use.1
Given the contaminants inherent persistence in soil and water environments, resistance to degradation, low volatility and hydrophilic nature, it is difficult to remove using typical industrial remediation (air stripping) or POU technologies as well (i.e., activated carbon adsorption or reverse osmosis). Use of AOP technologies in combination with ultraviolet light or ozone increases AOP efficacy as well. Originally, biodegradation tools were thought to be ineffective but researchers have worked to isolate microbial agents that indeed are able to degrade dioxane. The most effective treatment methods in the future will likely involve multiple barriers. More research is needed to determine the actual risks and control measures needed for public health protection from 1,4-dioxane exposures.
- Curry, M.A. 1,4 Dioxane removal from groundwater using point-of-entry water treatment techniques, University of New Hampshire, Thesis, 2012.
- ASTDR, “Toxicological profile for 1,4-dioxane,” Agency for Toxic Substances & Disease Registry. CAS#123-91-1, Atlanta, GA, 2012.
- US EPA, Exposure Factors Handbook. EPA/600/R-090/052F, Office of Research and Development, National Center for Environmental Assessment, Washington, DC, 2011.
- US EPA, Drinking water standards and health advisories table, June 2007. [Online]. Available: www.epa.gov/region9/water/drinking/files/ dwsha_0607.pdf. [Accessed 14 June 2013].
- DuHamel J, “EPA may change dioxane standards in Tucson water,” Tucson Citizen, pp. http://tucsoncitizen.com/wryheat/2010/09/20/epa-maychange-dioxane-standards-in-tucson-water/,20 September 2010.
- US EPA, Treatment Technologies for 1,4-Dioxane: Fundamentals and Field Applications. EPA 542-R-06-009, 2006. [Online]. Available: www.epa.gov/ tio/download/remed/542r06009.pdf.
About the author
Dr. Kelly A. Reynolds is an Associate Professor at the University of Arizona College of Public Health. She holds a Master of Science Degree in public health (MSPH) from the University of South Florida and a doctorate in microbiology from the University of Arizona. Reynolds is WC&P’s Public Health Editor and a former member of the Technical Review Committee. She can be reached via email at [email protected]