Treatment of an explosives plume in groundwater using an organic mulch biowall

A field demonstration of a mulch permeable reactive barrier (PRB), or “biowall,” as an in situ treatment technology for explosives in groundwater is summarized. Organic mulch consists of insoluble carbon biopolymers that are enzymatically hydrolyzed during decomposition to release aqueous total organic carbon (TOC). The released TOC is then available for microorganisms to use as an electron donor to transform electrophilic contaminants via reductive pathways. A 100-foot-long and 2-foot-thick mulch biowall was installed at the Pueblo Chemical Army Depot in Colorado to treat a shallow groundwater plume containing hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). To discourage groundwater flow bypassing around and under the biowall in this highly permeable formation, a hydraulic control was installed and the PRB was keyed into the bedrock. Technology performance was monitored using a monitoring well network to establish the development and extent of the downgradient treatment zone. Performance objectives of the field demonstration were: (1) greater than 90 percent removal of RDX across the PRB and the treatment zone; (2) an RDX concentration of less than 0.55 μg/L in the treatment zone; and (3) cumulative toxic intermediate concentration (nitroso intermediates of RDX, MNX, DNX, and TNX) of less than 20 percent of the upgradient RDX concentration. All performance objectives were met within seven months after installation once the system reached a pseudo-steady state. By this point, a sustained reducing/treatment zone had been created downgradient of the mulch PRB that showed greater than 93 percent RDX removal, RDX concentrations less than 0.55 μg/L, and no accumulation of toxic intermediates. The mulch biowall implemented during this demonstration was successful at meeting performance objectives while addressing the majority of potential concerns of the technology.