There's an old saying that I won't spell out completely, but which most readers will certainly have heard at least once in their lives, to wit: "Don't sh-- where you eat." It is an all-purpose warning about not pursuing incompatible activities in the same place, particularly activities that produce either physical waste or emotional complications.
In this case the waste part is wastewater emitted by oil wells drilled into shale deposits which must undergo extensive hydraulic fracturing (often called fracking) before the oil can be freed. What most people do not know is that for every barrel of shale oil extracted, three to five barrels of water laden with fracking chemicals and salt, toxic minerals and radioactivity (from the deep rock) also comes up, most of it water originally injected under high pressure to fracture the shale and release the oil.
Some 9 millions barrels a days of shale oil is currently produced in the United States each day, according to the U.S. Energy Information Administration. That means between 27 and 45 million barrels of fracking wastewater is produced EACH DAY; in gallons that's between 1.1 and 1.9 billion gallons. And, of course, multiply by 365 and you'll get yearly totals. That's a lot of wastewater and it has to go somewhere and that somewhere is starting to pollute underground water supplies and surface soil and water, and to interfere with oil production itself.