Leachate generated by an oil-and-gas brine pond site in North Dakota
Two unlined ponds were used for holding and evaporation of brines produced with oil and gas at a well site in north-central North Dakota. The brine-evaporation ponds were in use from 1959 up to the late 1970s when they were backfilled and leveled. Continued salt-water migration at this site since closure has decreased crop yields in surrounding fields and has killed trees in a shelterbelt within an area of approximately 10 acres. An apparent resistivity survey delineated a 360,000-ft2 area of extremely low resistivity. Isoconcentration maps indicate that a highly saline leachate plume extends laterally in a 500-foot radius around the ponds and vertically to a depth of 70 feet below the surface. Ground-water recharge at this site is low because of the semiarid climate and the low hydraulic conductivity of the near-surface sediments and, as a result, very little flushing of the brine from the sediment beneath the ponds has occurred. Pore water within the unsaturated zone beneath the reclaimed ponds contains essentially the same ionic concentrations as that: of brine impounded in these pits 10 to 25 years ago. Based upon the results of this research, we estimate that brine leachate will continue to migrate at slow rates from this site for tens and possibly hundreds of years if no action is taken. The construction of a mound over the site and/or an infiltration gallery around the perimeter would minimize the spread of brine and make it possible to return this land to production in the foreseeable future.