January 8, 2007
6:00 - 7:30 pm
Application and Refinement of Treatment Technologies for RDX Biodegradation
Brian Caldwell, PG,
Ronnie Britto, PhD, PECPG
Tetra Tech, Inc.
Oak Ridge, Tennessee
RDX is a cyclic nitramine explosive that is commonly found in soil and groundwater at ammunition plants across the country. One such plant is the Iowa Army Ammunition Plant (IAAP), where contamination has been associated with past manufacturing, training, storage and disposal activities. Following a feasibility study in 2004, biological treatment was selected as the remedy of choice to decontaminate RDX in groundwater, both at offsite and onsite locations.
A treatability study (TS) was performed on offsite groundwater at the IAAP to examine the degradation potential of RDX as a result of the addition of a carbon substrate. High fructose corn syrup (HFCS) was injected as the carbon substrate followed by periodic sampling of the groundwater at selected locations to monitor key chemical and biochemical changes. Injection point spacing was estimated, and substrate was introduced as a single flash injection. Overall, results indicated that reducing conditions could be attained by the addition of HFCS. However, several complications to the specifics of the remedy were indicated as a result of the TS, primarily including the choice of carbon substrate and its introduction and fate within the contamination horizons, which in turn significantly impact the efficacy of the remedial system.
Although the results of the TS indicated the benefits of HFCS, they also showed that this substrate was consumed very rapidly in the aquifer. The quantities of carbon dioxide (as high as 920 mg/L) that were produced point to a very active groundwater biological system. However, much of this activity may not have been targeted directly at RDX biodegradation. At some point following injection, fermentation may have dominated biological proceedings resulting in very high carbon dioxide formation. This phenomenon is sometimes observed in aquifers with more coarse-grained material which have higher conductivities where carbon sugars have been injected to enhance contaminant biodegradation. Therefore, HFCS, while often being the prime choice to enhance biodegradation, may not possess the ideal properties for some aquifers.
During the TS, HFCS was injected via direct push points followed by a collapsing and closure of the borings. However, in injection horizons with significantly high porosity and hydraulic conductivity (such as that present at IAAP), flash injections followed by significant time periods can result in pulsating amendment fronts rather than the preferred constant amendment budget. An alternative approach is to use permanent injection wells, and more frequent but volumetrically smaller injections.
This presentation outlines the progression from TS to pilot-scale implementation in the treatment process using the results of the TS to optimize RDX biodegradation in offsite groundwater. The objectives of the next phase of treatment are as follows:
a. Use of an alternative carbon substrate to HFCS that could result in sustained attainment of reducing conditions and more rapid and robust biodegradation of RDX;
b. Optimization of injection protocol to include the injection mechanism and frequency that could enhance complete biodegradation of RDX; employment of a computational model that can be used to examine and ascertain injection quantities, frequencies, and monitoring locations;
c. Provision of an extended and more detailed suite of biochemical and biological parameters to verify results of this phase; and
d. Development and testing of a system of metrics to verify biodegradation and quantify the consequent mass reduction of RDX.
Page updated May 26, 2018