Unconventional oil and gas (O&G) resources require hydraulic fracturing to be extracted from relatively impermeable formations, where large volumes of fracturing fluid, including water, proppants and chemical additives, are injected into the formation at high pressures to increase hydrocarbon recovery. Upon release, wastewater flows to the surface together with the targeted hydrocarbon resource.

The main research question of this thesis is to determine what chemical risks are associated with UO&G activities in relation to water quality. The two main topics of interest are related to how chemical risk assessment should be performed for UO&G activities and how these risks can be mitigated.

Evidence for chemical contamination of groundwater exists and considering the high number of chemicals, the high concentrations and the positive toxicity responses of UO&G related samples, measures must be put in place to mitigate or prevent contamination. Chemical risk assessment needs to be performed on a site- and activity-specific basis to ensure environmental and human health. Additionally, the large number of chemicals, the whole UO&G water cycle and the downhole conditions that may affect chemical fate need to be considered. Proper wastewater management including treatment needs to be implemented. Moreover, the use of green chemicals in fracturing fluid may reduce negative effects on environmental and human health in case a contamination occurs.

Many uncertainties and unknowns related to publicly available chemical and toxicological information remain, pointing towards the importance of the precautionary principle in UO&G regulation.

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