Ecological risk assessment of accidental release of flowback water: A conceptual framework

ABSTRACT

The aim of the study is to conduct an ecological risk assessment of accidental release of flowback water into freshwater body. Flowback water produced from the hydraulic fracturing process has a complex combination of high concentration of salts, organic compounds and metals. The toxicity of flowback water is assessed and an exposure assessment method for the inorganic constituents of the flowback water is developed. An equation for risk is derived to characterize the risk of the flowback water to the aquatic ecology. A case study is conducted for accidental release of hydraulic fracturing flowback water in Montney unconventional play trend in northern British Columbia. The flowback water quality data for 212 wells, including the concentrations of various salt ions, metal ions, and hydrogen sulfide, is collected for the assessment. The risk quotient is found to be 0.16 (<1), proving no significant risk to the aquatic ecosystem with 90% confidence. However, the overall results of the uncertainty and scenario analysis concludes that the risk to the ecology cannot be completely overlooked. Scenario analysis was done for monthly creek discharge and a relationship between risk quotient and the ratio of spill volume to the creek discharge was derived.

KEYWORDS:

• flowback water
• exposure assessment
• ecological risk assessment
• Montney
• unconventional gas

List of symbols

	=	log octanol water partition coefficient
	=	distance between AWSS and water stream
	=	adsorption coefficient
	=	amount of adsorbate
	=	concentration remaining in solution after adsorption
	=	volume of flowback water spilled
	=	concentration of contaminant in spilled water
	=	mass of soil contaminated
	=	final contaminant concentration in receiving water body
	=	background contaminant concentration in receiving water body
	=	exposure concentration of the nth contaminant
	=	Effect concentration of the nth contaminant
	=	creek discharge
	=	flowback water discharge
	=	probability of exposure
	=	bioavailable fraction
	=	Area of contaminated soil
	=	effect concentration 50
	=	lethal concentration 50
	=	soil bulk density
	=	risk quotient

Acknowledgments

This research was carried out with the financial support of MITACS, BCOGC and SECURE Energy Services. I would like to extend my gratitude to BCOGC for providing data and SECURE for providing their helpful insights for this paper.

Notes

¹ The definition of the salt in this paper has been adopted from (Bright & Addison, 2002) as “salt should include the individually quantified anions chloride, sulfate, and bicarbonate, as well as the individually quantified cations sodium, potassium, calcium, and magnesium.”