A saltwater disposal well is where the water from oil and gas well production is discarded.
Called "saltwater" euphemistically by industry, this fluid is considered hazardous waste because of its high salt content, hydrocarbons, and industrial compounds. This fluid is injected into wells so that it doesn't contaminate land or water resources.
Hydraulic fracturing of shale gas well sites produces millions of gallons of this "saltwater" (also known as "produced water"). Placement of high pressure saltwater disposal well sites is often controversial because of the potential for groundwater contamination and small earthquakes.