Mechthild Schmitt-Jansen, Janet Riedl, Stefanie Rotter, Rolf Altenburger;
Helmholtz-Centre for Environmental Research – UFZ, Leipzig Halle
During the last few years the technology of hydraulic fraccing became popular for the commercial exploitation of shale gas. During this process, an aqueous mixture of chemicals is injected under hydraulic pressure in the gas-containing shale formations. Recent applications of the technology caused concern regarding the safety of hydraulic fraccing and possible unintended consequences. These concerns include the introduction of chemicals into the underground environment.
To estimate potential ecotoxicological risks of chemical mixtures used during the fraccing process, we developed an assessment strategy based on available knowledge for individual compounds composing selected fraccing fluids.
In a first step, a database was created containing available ecotoxicological knowledge on the fraccing fluid chemicals used in the past. In total 149 substances were considered in this database, derived from 18 fraccing fluids provided by ExxonMobil. After a critical revision of substance identity of the chemical components effect concentrations for three biological models (fish, algae and Daphnia) were summarised in the database for 118 substances. Data were retrieved from the US-EPA Ecotox-database (http://cfpub.epa.gov/ecotox/) and the ESIS-database (http://esis.jrc.ec.europa.eu/). As far as available acute and chronic data were selected deriving from biological testing based on standard OECD-protocols. Acute experimental data (mainly EC50) could be retrieved for 55% of the chemicals for fish toxicity, 44% for Daphnia and 36% for algae. Experimental data for chronic toxicity were available for 16% of the chemicals for fish, 8% for Daphnia and 36% for algae. Obvious data gaps were amended by modelling the minimum toxicity (narcosis) using the ECOSAR software (ECOlogical Structure Activity Relationships) of the US-EPA (http://www.epa.gov/oppt/newchems/tools/21ecosar.htm).
In a second step hazard quotients (HQ) were calculated for individual compounds. The HQ is the ratio of an exposure estimate of a chemical in relation to an effect concentration. Risks of environmental hazards are assumed to be significant for HQs > 1. Initial concentrations of chemicals composing the fraccing fluids and the effect concentrations retrieved in the above mentioned database were used for calculation of compound-specific HQ values. For 38 of the 118 chemical compounds a HQ > 1 was calculated indicating risks deriving from these substances.
In a third step a hazard index (HI) was calculated for each fraccing fluid to consider compound mixtures. It is based on the assumption of concentration additive effects of chemical mixtures. The HI is the summation of all HQs of compounds composing a fraccing fluid. The HI values of the 18 fraccing fluids range from 100 to a maximum of about 1000000 indicating substantial risks deriving from the original fraccing fluids.
In conclusion the compound-based assessment strategy applied in this study seems to be suitable to (I) identify potentially hazardous substances contained in fraccing fluids by considering the intrinsic toxicity of individual compounds as well as the concentration in which they are used and (II) to assess the cumulative risks of compound mixtures of fraccing fluids to the environment. (III) The highly variable range of HI values, calculated for the different fraccing fluids indicate substantial potential risks of the fluids to the environment but also offers the perspective to optimize the fraccing process according to environmental concerns.
Obvious data gaps became evident with regard to an unambiguous identity of the chemicals in use as well as the availability of effect data suitable for safety assessment. Additionally, the extrapolation of indicated risks for sensitive environmental compartments such as surface water bodies or drinking water recourses potentially affected by the fraccing process needs to be considered when applying this approach.
