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Optimized Remediation of Groundwater Contamination at the Former Nebraska Ordnance Plant

Published Dec. 18, 2019
The membrane interface and hydraulic profiling tool (MiHPT) rig. Multiple probes are submerged approximately 100 feet below ground while real-time data is monitored within the trailer.

The membrane interface and hydraulic profiling tool (MiHPT) rig. Multiple probes are submerged approximately 100 feet below ground while real-time data is monitored within the trailer.

Three solar arrays, including this one at the Main Treatment Plant, help power groundwater treatment at Mead, resulting in a savings of over 500,000 kilowatt hours of energy to date.

Three solar arrays, including this one at the Main Treatment Plant, help power groundwater treatment at Mead, resulting in a savings of over 500,000 kilowatt hours of energy to date.

Preliminary model output showing 2D and 3D membrane interface and hydraulic profiling tool (MiHPT) locations from the AMA overlaid on the existing plume boundary. Once field work is complete, the groundwater model will be updated to refine the plume boundary and improve the accuracy of overall cleanup times.

Preliminary model output showing 2D and 3D membrane interface and hydraulic profiling tool (MiHPT) locations from the AMA overlaid on the existing plume boundary. Once field work is complete, the groundwater model will be updated to refine the plume boundary and improve the accuracy of overall cleanup times.

The Former Nebraska Ordnance Plant, located in rural Nebraska near the town of Mead, was a 17,250-acre load, assemble and pack facility that produced bombs, boosters and shells in support of World War II and the Korean Conflict. The facility included munition load lines and an Atlas Missile Area, added in 1959.  There are currently four groundwater plumes, each up to four miles long. The primary contaminants within the groundwater are trichloroethene (TCE), a common solvent, and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), an explosive. Despite the existing contamination, the former NOP property is used today for residential, agricultural, and research purposes by the University of Nebraska-Lincoln. The U.S. Army Reserve and Nebraska Air National Guard also own portions of the property.

Groundwater contamination is treated and contained by a 2,500 gallon per minute treatment system that combines the following technologies: granular activated carbon, air stripping, and ultraviolet and ozone oxidation. This system has removed more than 49,000 pounds of TCE and RDX and prevents the plume from expanding downgradient. Plume boundaries are monitored by a network of nearly 400 wells.

Though the existing treatment system removes significant contaminant mass, the groundwater model predicts that contamination will remain above cleanup goals (5 µg/L for TCE and 2 µg/L for RDX) until 2127.  For this reason, the U.S. Army Corps of Engineers and the Formerly Used Defense Site Program are considering optimization options to reduce the overall groundwater cleanup timeframe for this site. The most recent such effort is a new source investigation to locate, and test potential treatment options to treat source material.

The source investigation includes high resolution sampling across the width of the plumes using direct-push profiling with membrane interface and hydraulic profiling tool (MiHPT) from the surface to the top of bedrock (approximately 100 – 130 feet below ground surface). This tool provides down-hole information such as hydrocarbon and solvent contamination as well as soil electrical conductance and permeability. Wherever MiHPT readings indicate the presence of potential contamination, groundwater samples are also collected and analyzed for TCE and/or RDX to verify the MiHPT readings. A review of the field data, once complete, will help determine what options exist to remediate any potential source areas.

Preliminary results have indicated that TCE concentrations adjacent to the likely AMA source areas (e.g. missile silos, flame pits, and the former lagoon) were low or non-detect, suggesting that source material is no longer present near the source. However, all samples, including clean locations, will help refine the groundwater model and improve the accuracy of cleanup timeframe estimates. Source optimization field work at Load Line 1 began in 2019 and will finish at Load Line 2.

Several other optimization efforts have been completed at the former NOP over the years. In 2019, the ozone generator that treats groundwater at the Advanced Oxidation Process plant was upgraded to increase treatment efficiency and decrease long-term operation costs. Ultraviolet photolysis, another AOP, has been utilized at six locations within the plumes to treat RDX in groundwater prior to it reaching the treatment system. This resulted in an annual savings of 120,000 pounds of carbon as well as the ability to shut down of one of the four treatment plants.

In 2015, the U.S. Geological Survey, in collaboration with the Nebraska Water Service, conducted a Heliborne Electromagnetic survey, to collect and interpret wide-spread information about the aquifer from a helicopter. This data allowed for more informed monitoring well placement.

Between 2014 and 2017, three solar arrays have been installed at Mead to improve energy efficiency of the groundwater treatment systems.  From January 1, 2014, through September 30, 2019, the solar arrays have generated over 500,386 kilowatt-hours of energy.

In 2012, a pilot study began within the Load Line 2 groundwater plume in an attempt to add in situ treatment to the overall cleanup process. This study included enhanced bioremediation (sodium acetate injections for RDX and sodium lactate injections for TCE) and a zero valent iron permeable reactive barrier. The results of this pilot study were summarized in 2017 and concluded that cost to implement these systems, as well as technical issues associated with the sodium acetate injections, did not warrant large-scale application.

Regardless of what the outcome of the most recent source optimization effort reveals, the U.S. Army Corps of Engineers and the FUDS Program are committed to finding ways to reduce the overall cleanup timeframe at the former NOP and other complex groundwater contamination sites across the country.