Building upon previous success, the U.S. Army Corps of Engineers is taking an innovative approach to treating explosives-contaminated groundwater – enlisting the help of microorganisms.
Beginning in 1994, a pump-and-treat system was used for contaminant removal. However, pump-and-treat removal efficiency had declined over time, prompting Navy officials to seek alternate means to optimize treatment efforts. Seattle District’s successful bioremediation results at the Umatilla Chemical Depot in Oregon, a comparable groundwater-treatment project, pointed a way forward.
“Back in the 80s and 90s, pump-and-treat was the standard remedy for groundwater remediation, but it can only take you so far,” said Mandy Michalsen, senior scientist and technical lead for the project. “It provides great contaminant removal results initially, but over time you’re just not extracting as much.
“Today we have many positive results, like those at Umatilla, showing bioremediation to be more cost effective, more efficient and yielding better results than pump-and-treat,” Michalsen said. “In fact, we received so much attention from our Umatilla work, Navy officials noticed and then reached out to us for assistance.”
Although the groundwater plume at NB Kitsap includes other explosive contaminants such as trinitrotoluene, commonly known as TNT, scientists are focused on RDX – an explosive organic compound often used in military applications.
“Since TNT is less soluble and sticks to the soil, making it easier to treat, we are focusing on RDX which forms the boundary of the groundwater plume,” said Aaron King, project environmental engineer.
Having Umatilla as a bioremediation blueprint of sorts, the Seattle District team first set out to determine what type of additive would yield the best results.
“From our work at Umatilla, we knew certain carbon substrates would work well in encouraging microorganisms to breakdown RDX,” King said. “Of those, we picked corn syrup and ethanol to figure out which one worked best and also to get our RDX degradation rates. We wanted to find the one that was the most energetically favorable so we could reduce the cleanup time.”
Field tests conducted in fall 2013 to spring 2014 presented a clear winner: Corn syrup had a degradation rate roughly three times greater than that for ethanol.
“The addition of corn syrup results in the formation of anaerobic conditions which degrade contaminants in the groundwater,” King explained. “In anaerobic RDX biodegradation, microbes facilitate breakdown of the RDX molecule to form nitrite, nitrous oxide, and formaldehyde, which are ultimately transformed to nitrogen gas and carbon dioxide.”
Using field test results, model simulations projected out over a 20-year timeframe indicate that using pump-and-treat systems without bioremediation would leave RDX concentrations at a 50 microgram-per-liter level. However, when bioremediation is included to optimize pump-and-treat systems RDX concentrations are expected to be about 10 micrograms-per-liter, a significant reduction which subsequently should expedite natural recovery.
Armed with positive results, environmental engineers are preparing for another round of field tests at Bangor this fall. These tests will include increasing the number of injection sites, measuring formation response to injections and estimating local groundwater velocities.
As the Seattle District team continues to make innovative strides forward in bioremediation treatment solutions, its accomplishments are also opening pathways to new initiatives and partnerships.
“We are forging new collaborations all the time, potentially teaming with University of Connecticut on a new groundwater project at Naval Base Kitsap next fall, using stable isotope tracers to track contaminants as they are degraded during our tests,” Michalsen said. “We also obtained a grant from the Navy Environmental Sustainability Development to Integration program to evaluate aerobic bioaugmentation at our Navy site as well, results of which have been promising.
“While most people don’t think of the Corps as leaders in groundwater remediation, our recent cutting edge successes at the Umatilla Chemical Depot and Naval Base Kitsap are starting to change that.”