Organic mulch is a complex carbon material that is typically populated with its own consortium of microorganisms. The organisms in mulch break down complex insoluble organics to soluble carbon, which can then be utilized by these and other microorganisms as an electron donor for treating contaminants via reductive pathways. Mulch has advantages over other electron donors: it is cheaply available, long-lasting, and naturally present in the environment. Over the last decade, organic mulch permeable reactive barriers (PRB) or biowalls have enjoyed increased public interest as a relatively inexpensive technology for addressing contaminated groundwater. The mulch PRB is a passive technology and consequently requires no aboveground injection system, thereby greatly reducing operating and maintenance costs. To date, biowalls have been installed to remediate groundwater contaminated with a variety of electrophilic compounds, including chlorinated solvents and inorganics such as nitrate and perchlorate. This field demonstration represented the first application of mulch PRBs for the treatment of explosives contamination in groundwater.

The groundwater plume selected for the field demonstration was the eastern-most explosives plume in the SWMU-17 area located at the Pueblo Chemical Depot (PCD) in Pueblo, Colorado. The State-mandated site-specific cleanup criteria of 0.55 ppb hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and 602 ppb octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) were targeted.

The overall objective of this project was to demonstrate and validate mulch PRB or mulch biowall technology in the field at the pilot scale. More specifically, the objectives included:

1. To test the efficacy of organic mulch as an electron donor that promotes the biological reduction of RDX and/or HMX-impacted groundwater by:

   a. Implementing a mulch/gravel PRB for the pilot test that meets the regulatory action levels for the demonstration site.

   b. Determining the extent of RDX and/or HMX removal across the mulch PRB.

   c. Monitoring the accumulation of any primary reduction intermediates (e.g., MNX, DNX, TNX) downgradient of the PRB.

2. To gather sufficient performance and cost data from the pilot test to estimate the cost of implementing the technology at full scale by:

   a. Monitoring the change in total dissolved organic carbon downgradient of the PRB during the demonstration to extrapolate the longevity of the implementation.

   b. Determining any fouling characteristics of the mulch/gravel PRB during the demonstration by conducting periodic slug tests in wells located within the PRB.

Early in the project, a bench-scale treatability study was conducted with contaminated groundwater from the site using pine mulch as the slow-release electron donor. Column tests were run at the average seepage velocity for the site using a 70%/30% (v/v) mulch/pea gravel packing to approach the formation’s permeability. Significant results included: (1) complete removal of 90 ppb of influent RDX and 8 ppb of influent HMX in steady-state mulch column effluent; (2) pseudo-first-order steady-state kinetic rate constant, k, of 0.20 to 0.27 hr^-1 based on RDX removal data, using triplicate column runs; (3) accumulation of reduced RDX intermediates in the steady-state column effluent at less than 2% of the influent RDX mass; and (4) no binding of RDX to the mulch in the batch and column tests. These successful results, together with groundwater flow modeling, were used to design the pilot-scale organic mulch/pea gravel biowall for the site.

A 100-ft long and 2-ft thick mulch PRB was installed at PCD using one-pass trenching. To discourage the occurrence of a bypass of groundwater flow around and under the PRB, a hydraulic control was installed and the PRB was keyed into the bedrock. The mulch PRB was in place by November 16, 2005, and became operational immediately upon installation.

Technology performance was monitored using a monitoring well network. Groundwater data collected from each monitoring event was compared to the base case (i.e., pre-PRB) and to itself (i.e., downgradient of PRB compared to upgradient). Performance objectives of the field demonstration were: (1) greater than 90% removal of RDX across the PRB and the treatment zone; (2) an RDX concentration of less than 0.55 ppb in the treatment zone; and (3) cumulative toxic intermediate concentration (i.e., MNX+DNX+TNX) of less than 20% of the upgradient RDX concentration. All performance objectives were met by June 2006, when the system appeared to have reached a pseudo-steady-state. By then, a sustained reducing/treatment zone had been created downgradient of the mulch PRB that showed greater than 93% RDX removal, RDX concentrations less than 0.55 ppb, and no accumulation of toxic intermediates.

Both ex-situ and in-situ processes have been reported in the literature for the remediation of RDX- and HMX-contaminated groundwater. Ex-situ processes include the treatment of pumped groundwater in granular activated carbon units, anaerobic bioreactors, electrochemical cells, and ultraviolet-oxidation reactors, all of which have the disadvantage of high pumping and reinjection costs. In-situ processes are generally cheaper and have fewer regulatory limitations. In-situ reduction processes using either zero-valent iron (ZVI) or anaerobic biodegradation have the potential to reduce RDX and HMX. For the purpose of cost comparison, the mulch PRB unit costs were compared to that of ZVI PRB technology over a 10-year life cycle. Unit costs of $0.08 and $0.11 were obtained for mulch PRB and ZVI PRB, respectively, for each gallon of contaminated groundwater treated over a 10-year period of technology operation. The unit cost differential between these two technologies is expected to be more dramatic over a shorter period of operation, primarily because of the high material cost of ZVI.

Mulch biowall technology is most cost-effective when implemented at shallow contaminated groundwater sites. In addition, cost advantages over other technologies can be further increased if a source of cheap and effective mulch can be identified in the vicinity of the site where the technology is to be implemented. Since mulch is created from naturally occurring flora, its supply is unlikely to be a problem in geographically non-arid regions. Operational costs associated with this technology are usually negligible. Post-treatment costs of the technology may include excavation and disposal of the spent mulch fill if binding of the target contaminant to the mulch is observed; however, TCLP testing results for the mulch fill in the site-specific treatability phase confirmed no leaching of the target contaminants of this demonstration, namely RDX, HMX, or any primary reduction intermediates. Therefore, post-treatment excavation and disposal are unlikely. (Project Completed - 2008)