TABLE 1.

Summary of major compounds present in FPW along with their biodegradability and toxicity concerns.

Chemical compounds	Biodegradabilitya	Toxicity and other concerns	References
Geogenic compounds Hydrocarbons and organic compounds
VOCs (BTEX)	++	Reproductive toxicity; Neurotoxic; Carcinogen; soluble in water	Lueders, 2017; Varjani, 2017; Folkerts et al., 2020
PAHs (Naphthalene, Phenanthrene)	++	Genotoxic; Reproductive toxicity	Ghosal et al., 2016; Butkovskyi et al., 2017; Folkerts et al., 2020
Salts, radionuclides and other ions
Major cations and anions (Na+, Ca2+, Mg2+, K+, Cl–, HCO${}_3^{-}$, SO${}_4^{2-}$, Br–)	−	Inhibit microbial degradation; Toxic to freshwater ecosystems; Disruption of soil structure and damage irrigational crops; Formation of disinfection byproducts in downstream drinking water plants	Oetjen et al., 2018a; Tasker et al., 2018; Sedlacko et al., 2019; Folkerts et al., 2020
Trace metal (Li, Ba, Fe, Mn, Sr, As, B, Pb)	−	Display toxicity based on concentration and bioavailability	Folkerts et al., 2020
NORM (Ra, U)	−	Carcinogen	Tasker et al., 2018
Fracturing fluid additives/Anthropogenic compounds
Biocides (GA, DBNPA, QACs)	– to ++	Inhibit microbial degradation of co-contaminants in FPW	Ertekin et al., 2016; Rogers et al., 2017; Mumford et al., 2018; Akyon et al., 2019
Surfactants (PEG, PPG, AEOs, NPEOs)	+ to ++	No adverse effects known	Heyob et al., 2017; Rogers et al., 2018; Hanson et al., 2019
Transformation products
Halogenated organic compounds	NA	Potential recalcitrant and toxic compounds	Hoelzer et al., 2016; Luek et al., 2018

^a “−” not amenable to biological treatment, “+” potentially biodegradable, “++” highly biodegradable, NA, not available.