Salt contamination of land and water occurs because the oil production process brings salt water to the surface. The water is hot. Often the produced water is supersaturated, many times saltier than the ocean.
Historically in North Dakota during the period of the 1950s to the late 1970s, salt contamination happened because the oil industry chose to dump its salt waters into unlined evaporation pits.That decision is peculiar because the American Petroleum Association promulgated the standard of care of the industry in the 1930s. The API standard of care was "never dump salt in unlined pits in zones that are hydrologically sensitive." The concern was that salts leach both laterally and vertically. The API's solution to pollution was dilution. However, the API knew that many situations do not have an adequate supply of fresh water to absorb the salt the idustry brought to the surface. The practical remedy was to create pits. However, they were not created with sufficient capacity to contain all the fluids generated, especially when rainfall or snow melt flowed into the pits.
In North Dakota and Northeast Montana, when pits were located out of the flowing zone of watersheds and in relatively impervious clay zones, the problem was not significant. The reality was that few pits were prepared with impervious liners. As a result, in the late 70's, North Dakota required pit liners.
Even when drilling operation fluids were contained and produced water was impounded along with produced water, on retirement of the well, many of those evaporation pits which were lined, were cut to drain and dry out the contents of the pit. The goal was to enable farmers to drive across abandoned oilfield site without bogging down. Scientists now know that the accumulated pit salts, on average the 250 tons in each, imperil adjoining lands by leaching vertically and latterly. The oil industry, and some soil scientists, knew that the salt plumes would migrate. The distance liquid salts move is astounding. Some salt plumes have traveled under producing land without damage and yet reappeared as much as a mile away and there kill soil and vegetation. Salt plumes corrupt watersheds and wells. The studies completed suggest that in Bottineau County's Renville and Hastings Townships pit salts have leached into the Fox Hill Aquifer.
Salt kills aquatic plants, animals and soil microbes. Loss of microbes destroys soil structure. Concentration of sodium (Na+) changes soil permeability. Interruption of vertical water flows makes the soil wet, preventing drying out. Salt initially destroys desirable vegetation and finally kills even salt tolerant species such as foxtail barley and the familiar cattail plants. Salt changes the “tilth of the soil” by removing air spaces. Salted soil is said to be dead. Depending on the background origin of the soil, slope of the land and precipitation, some dead soils erode dramatically. Think of the eroded zones in shallow soils of land surrounding the North Dakota Badlands. (See photos.)
When spills are new, the remedy is to quickly capture the salt water. The best remedy is to vacuum the salt water then pump it down using deep disposal wells, deep enough not to imperil aquifers. Another effective remedy, when the capture has not occurred promptly, is more expensive but nearly always appropriate. The site should be criss-crossed with drain tiles and these must be connected to collection basins. Not only the original produced water collected in these basins but also subsequent precipitation must be captured and pumped down hole to gradually dilute the concentration of salt. In some instances, the produced water drainage tiles and collection wells must be pumped for years and of course always downhole into deep disposal wells. Another method is to “dig and haul" the contaminated soil to a hazzardous waste site.
When abandoned evaporation pits' leachate is the problem, the solutions are more expensive. The method endorsed by North Dakota State Geologist Ed Murphy is to establish barriers to arrest further movement. That is, curtain the site with drain tiles and pump the liquid salt flowing therefrom for years. Covering the site with a mound of clay is important because it limits flow of snow melt and rain water into the liquid salt and thereby greatly diminishes leaching. Some contaminated leachate from lands known as "Brownfields" and the contaminated aquifers below them have been pumped for more than 17 years.
North Dakota has hundreds of thousands of acres of salted lands in need of these measures. The costs of remediation may well exceed billions of dollars.