From The newly released EPA Study On fracking "Properties affecting the likelihood of a spilled chemical reaching and impacting a drinking water resource include mobility, solubility, and volatility. Of the 453 chemicals for which physicochemical properties were available, 18 of the top 20 most mobile ones were reported in the EPA’s FracFocus database for 2% or less of wells. Choline chloride and tetrakis (hydroxymethyl) phosphonium were exceptions and were reported in 14% and 11% of wells, respectively. These two chemicals appear to be relatively more common, and, if spilled, would move quickly through the environment with the flow of water. The majority of the 453 chemicals associate strongly with soils and organic materials, suggesting the potential for these chemicals to persist in the environment as long-term contaminants. Many of the 453 chemicals fully dissolve in water, but their aqueous solubility varies greatly. Few of the chemicals volatilize, and thus a large proportion of most hydraulic fracturing chemicals tend to remain in water. Oral reference values and oral slope factors meeting the criteria used in this assessment were not available for the majority of chemicals used in hydraulic fracturing fluids, representing a significant data gap for hazard identification.1,2 Reference values and oral slope factors are important for understanding the potential human health effects resulting from exposure to a chemical. Chronic oral reference values and/or oral slope factors from selected federal, state, and international sources were available for 90 (8%) of the 1,076 chemicals used in hydraulic fracturing fluids. From U.S. federal sources alone, chronic oral reference values were available for 73 chemicals (7%) of the 1,076 chemicals, and oral slope factors were available for 15 chemicals (1%). Of the 32 chemicals reported as used in at least 10% of wells in the EPA’s FracFocus database (excluding water, quartz, and sodium chloride), seven (21%) have a federal chronic oral reference value. Oral reference values and oral slope factors are a key component of the risk assessment process, although comprehensive risk assessments that characterize the health risk associated with exposure to these chemicals are not available. Of the chemicals that had values available, the health endpoints associated with those values include the potential for carcinogenesis, immune system effects, changes in body weight, changes in blood chemistry, cardiotoxicity, neurotoxicity, liver and kidney toxicity, and reproductive and developmental toxicity. However, it is important to note that evaluating any potential risk to human populations would require knowledge of the specific chemicals that are present at a particular site, whether or not humans are exposed to those chemicals and, if so, at what levels and for what duration, and the toxicity of the chemicals. Since most chemicals are used infrequently on a nationwide basis, potential exposure is likely to be a local or regional issue, rather than a national issue. Accordingly, consideration of hazards and risks associated with these chemical additives would be most useful on a site-specific basis and is beyond the scope of this assessment."

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