EA conducted a quantitative assessment of the Army’s Fort Irwin operational range complex after an updated conceptual site model identified the potential for an on-range groundwater pathway for MC. Numerous downgradient monitoring and production wells were sampled for MC using low-flow sampling methods. During the Investigation, groundwater samples from the wells were analyzed for total metals, dissolved metals, explosives, perchlorate, and speciation of chlorine and oxygen isotopes (perchlorate speciation).
EA designed a unique low-flow perchlorate speciation sampling method using a perchlorate-selective bifunctional anion exchange resin to help discriminate between naturally occurring and anthropogenic sources of perchlorate. The perchlorate speciation data were collected over 30 hours of low-flow sampling through a specially engineered branched resin tube structure designed by EA that allowed proper contact between the aqueous medium and multiple resin tubes while cutting sampling time down from 6 days. The data indicated perchlorate detected was similar in isotopic composition to natural perchlorate extracted from within pristine groundwater from the western United States. The specialized technical approach and sampling methods developed by EA made it possible to conclude that on-range perchlorate was of natural origin and not attributable to synthetic perchlorate associated with pyrotechnics used on the range’s complex. The Phase I Assessment categorized six operational ranges as Inconclusive.
The results of Fort Irwin’s Phase II show that MCOC from the operational ranges are not migrating at levels that pose an unacceptable risk to off-range human receptors. Therefore, the Inconclusive ranges should be re-categorized as Unlikely and placed into a periodic review program under the Operational Range Assessment Program.