Past, Present, and Future Anatomy of an Oil Brine Plume Remediation near Poplar, Montana: A Case Study

The Biere #1-22 oil production well near Poplar, Montana leaked brine and light nonaqueous phase liquid (LNAPL) to the shallow alluvial aquifer for several years before final closure in 2002. Since 2008, 2.5 billion L of brine have been removed (~90% of the original Cl mass). However, Cl will not reach the original background levels due to the reservoir of solutes entrained in the Bearpaw bedrock remnant from the lateral dense aqueous phase liquid flow across the alluvial/bedrock interface. After removal of ~100,000 L of product since 2006, residual LNAPL is now confined to 2.2 ha (5.5 acres) a decrease from the original 2.7 ha (6.6 acres) areal extent by 17%. The initial ~7.5 m thick product in 2002 is stable at a maximum of ~1 m. However, LNAPL has infiltrated into fine-grained clay/silt units forming a smear zone in lenses 10 to 20 m bgs. Ongoing remediation has successfully mitigated benzene groundwater impacts over the last 14 years, with the benzene plume area having decreased by >99% (at the 5 μg/L) level from the maximum ~140 ha in 2002. This appears to be the first study evaluating the challenges to remediate a mixed LNAPL/dissolved organic/inorganic plume. Based on the mass removal to date, the asymptotic trends in solute concentrations, unpotable background groundwater quality, absence of a source/receptor pathway, lack of beneficial groundwater use, duration of mitigation with no obvious future accrual in benefit and the availability of institutional controls, it seems that the remedial strategies employed since 2006 have met their cost/benefit goals.