In 1991, an experiment was conducted in a very complex contaminant plume at the Rocky Mountain Arsenal (RMA) near Denver, Colorado. Extensive contamination of RMA groundwater had resulted from the Army’s manufacturing of chemical and incendiary weapons (e.g. mustard, lewisite, napalm) in the mid-1940s and the nerve agents VX and GB (Sarin) in the early 1950s. From 1952-1982, Shell and predecessors leased facilities at RMA and manufactured a range of pesticides. In the mid-1970s, contaminated groundwater was discovered flowing north and northwest beyond the base boundary and contaminants were found in groundwater production wells, surface water, and surface sediments leading to crop damage and livestock deaths. Since many of the contaminants would have been expected to move slowly compared to the groundwater flow due to their interactions with the soil, it had been speculated that those interactions were weakened and the transport of the contaminants “facilitated” by unknown processes within the plumes. The goal of the experiment was to determine if facilitated transport was occurring and, if so, to find out why. The field research included a two-well forced-gradient test involving flushing of the plume between injection and extraction wells with organic-free water and a subsequent period of monitoring of the flushed portion of the aquifer under natural-gradient conditions. The forced-gradient portion of the experiment clearly showed that all monitored organic contaminants were migrating at the same rate as groundwater, and related laboratory tests suggested that properties of the plume were preventing sorption of the contaminants by the soil.
Recently, the authors of this report (Myller et al., 1992), along with other colleagues, published a paper in Groundwater and Monitoring and Remediation (GWMR) that summarizes factors affecting transport of organic contaminants generally and specifically at RMA, including during the forced-gradient portion of the field research. The publication is online at this link: http://doi.org/10.1111/gwmr.70028. The GWMR publication notes that it “adds discussions of sorption and other studies by Shell Development Company (1991), analytical results from RMA monitoring, and laboratory sorption studies at the University of Waterloo, none of which were included or available when the report by Myller et al. (1992) was written. This paper also uses corrected or updated information and adds insights, calculations, figures, and discussion, including consideration of micro-and nano-plastic particles.” For these reasons, the GWMR publication is a more complete evaluation of the RMA field results. Where there are any discrepancies between Myller et al. (1992) and the GWMR publication, the GWMR publication should be taken as more accurate.