**4.5 Waste management and disposal risks**

Wastewater is the largest waste stream in oil and gas production. It consists primarily of produced water, which is for the most part brine mixed with hydrocarbons and suspended solids. Produced water is distinct from flowback water, which consists primarily of HFF and is generated for the first few days after hydraulic fracturing operations. Produced water may contain chemicals and additives used in drilling mud, methane, petroleum condensate, heavy metals, naturally occurring radioactive materials (NORM). Typically, flowback and produced water are temporarily stored in on-site pits (also called evaporation sumps) or tanks prior to disposal or reuse/recycling. These pits pose air and water pollution risks, from the release of volatile compounds into the air (evaporation is the purpose of some pits and may be supported by aerators) and the use of unlined pits. Flooding can also lead to pits overflowing and dispersing their hazardous contents across the landscape and potentially contaminating groundwater and nearby surface streams. On-site spills due to broken pipes or deterioration of the exterior walls of pits can also lead to localized soil and water contamination. California's oil-rich Central Valley has a legacy of unlined pits and at least one of the sites is known to have a sub-surface pollution plume that is threatening the Kern River [43].

 The majority of produced water is disposed of through deep-injection wells (class II wells according to the UIC program by EPA). In Pennsylvania, produced

 water was initially send to publicly owned treatment works, but the treatment processes were not adequate to handle the high TDS and chemical-contaminated water and the state prohibited the practice. If the injection wells reach aquifers that may potentially be used as a source of water for drinking or other purposes, the practice may threaten the water supply in water-stressed regions. This is the case in California, where hundreds of injection wells were found to be in potential violation of the Safe Drinking Water Act [44].
