G.H. Groenewold | Geology Reports

USGS2006

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.

Groundwater↗

USGS1982

Natural groundwater recharge in an upland area of central North Dakota, U.S.A.

The magnitude of groundwater recharge to coal aquifers in a 150-km 2 area in west-central North Dakota was determined using three separate approaches: (1) the net water level rise in water-table wells; (2) calculations of the fluid flux between nested piezometers, using the Darcy equation and measured values of hydraulic conductivity and vertical gradients; and (3) evaluation of the inputs to and outputs from the coal aquifer, using a steady-state control volume approach in which the aquifer was divided into semi-rectangular cells bounded by equipotential lines and flow lines. Measurements of potential gradients and hydraulic conductivity permitted indirect determination of all components of flow into and out of the cell except the recharge input, which was determined by difference. All methods yielded consistent results on the order of 0.04-0.01 m yr. −1 These values, which represent 2–9% of the annual precipitation, are consistent with results of other studies on recharge throughout the prairies of North America. Evaluation of site hydrology and stable-isotope data indicates that recharge is restricted in both time and place. Most recharge occurs in late spring and in the fall following heavy rainfall events. During these seasons the ground is not frozen and vegetation is not transpiring large amounts of water. Some recharge may occur during very heavy localized summer storms, but it is not considered volumetrically significant. Major permanent depressions on the site are a source of significant recharge. In addition, the extensive area of ephemeral standing water bodies that result from snowmelt can produce significant amounts of infiltration over the entire site.

central North Dakota↗