Estimating the Potential Toxicity of Chemicals Associated with Hydraulic Fracturing Operations Using Quantitative Structure–Activity Relationship Modeling

Abstract

The United States Environmental Protection Agency (EPA) identified 1173 chemicals associated with hydraulic fracturing fluids, flowback, or produced water, of which 1026 (87%) lack chronic oral toxicity values for human health assessments. To facilitate the ranking and prioritization of chemicals that lack toxicity values, it may be useful to employ toxicity estimates from quantitative structure-activity relationship (QSAR) models. Here we describe an approach for applying the results of a QSAR model from the TOPKAT program suite, which provides estimates of the rat chronic oral lowest-observed-adverse-effect level (LOAEL). Of the 1173 chemicals, TOPKAT was able to generate LOAEL estimates for 515 (44%). To address the uncertainty associated with these estimates, we assigned qualitative confidence scores (high, medium, or low) to each TOPKAT LOAEL estimate, and found 481 to be high-confidence. For 48 chemicals that had both a high-confidence TOPKAT LOAEL estimate and a chronic oral reference dose from EPA's Integrated Risk Information System (IRIS) database, Spearman rank correlation identified 68% agreement between the two values (permutation p-value =1 × 10(-11)). These results provide support for the use of TOPKAT LOAEL estimates in identifying and prioritizing potentially hazardous chemicals. High-confidence TOPKAT LOAEL estimates were available for 389 of 1026 hydraulic fracturing-related chemicals that lack chronic oral RfVs and OSFs from EPA-identified sources, including a subset of chemicals that are frequently used in hydraulic fracturing fluids.

Quantity of flowback and produced waters from unconventional oil and gas exploration

“…Several studies have identified and defined different contaminants commonly found in injected hydraulic fracturing fluids based on the chemicals toxicity (Hurley et al, 2016;Kahrilas et al, 2015;McLaughlin et al, 2016;Mohan et al, 2013;Yost et al, 2016a;Yost et al, 2016b).…”

A geospatially resolved database of hydraulic fracturing wells for chemical transformation assessment

“…(v) Toxicity measurements [79][80][81] or estimates, exposure models, 82 and population data could be appended, akin to the EPA's Risk Screening Environmental Indicators (RSEI) model, 83 using FracFocus ingredient disclosures and input in lieu of the Toxic Release Inventory (TRI).…”

Environmental risks associated with flowback and produced waters in shale gas projects

“…However, there are specific instances in which hydraulic fracturing activities have been implicated in the contamination of groundwater and surface water. Events that may lead to contamination include surface spills, leaks from flowback/produced water storage pits, and well blowouts (Yost et al, 2016b).…”

“…Together with characterization studies carried out for typifying the constituents that are found in flowback water (Abualfaraj et al, 2014;Yost et al, 2016b), a model was proposed in which the temporal increase of the salt concentration in flowback is explained by salt diffusion from capillary-bound or free brine in the shale matrix to the injected water that fills fractures after hydrofracturing (Balashov et al 2015). For these studies the Marcellus Shale Coalition GTI report was used as data source.…”

“…Several studies have been conducted in an attempt to characterize the constituents that are found in flowback water (Abualfaraj et al, 2014;Yost et al, 2016b). The main data sources are four, which can be consulted on the (organic-rich) shale, which has thousands of unconventional well drillings ( Figure 3-1);…”