An estimated 1 million gallons of saltwater generated by oil drilling has leaked from a North Dakota pipeline, company and tribal officials say, and federal authorities are investigating whether any of the liquid reached Lake Sakakawea, one of the nation's largest man-made lakes. Here are some questions and answers about the spill and saltwater:
Q: How does oil drilling produce saltwater?
A: Water at depths below the freshwater table becomes progressively saltier — in some cases, more so than ocean water. This naturally occurring saltwater often accumulates in the porous rock formations that contain oil and gas deposits, so it rises to the surface during production and must be separated. Additionally, water injected deep underground during hydraulic fracturing, or "fracking," can acquire similar levels of salinity before returning to the surface.
Q: What disposal options are there?
A: Fracking fluids are often re-used but eventually must be discarded, while naturally occurring briny water that rises to the surface during oil and gas production must be disposed of continuously. One method is to transport it to an approved disposal well for underground injection. Another is to store the water where it can evaporate, leaving behind solid residues that can be placed in a landfill.