Substrate Flood-Injection Approach to Treat the Source Area of a Thin Discontinuous Aquifer

摘要

An in situ groundwater remedial program is ongoing in the contaminant source area located beneath a currently inactive Titan missile launch complex known as Space Launch Complex 4 East (SLC-4E) at Vandenberg Air Force Base (VAFB), California. Here, trichloroethene (TCE) and ammonium perchlorate both source a contaminant plume extending over 1 mile (1.6 kilometers [km]) from the launch pad. Maximum concentrations of TCE and perchlorate detected in groundwater have ranged toward 2,500 and 900 micrograms per liter (μg/L), respectively. Unconfined groundwater lies on the upper weathered bedrock surface of a subhorizontal paleomarine terrace comprised of marine shale. The aquifer is thin, on the order of only 1 to 8 feet (ft) (0.3 to 2.4 meters [m]) thick and is discontinuous due to intermittent bedrock highs approaching or exceeding aquifer thickness; a result of historical erosion of the terrace surface. Groundwater is naturally aerobic with dissolved oxygen (DO) in the 5 to 7 milligrams per liter (mg/L) range, a generally positive oxidation-reduction potential (ORP), and sulfates in the 50 mg/L range. A recently completed groundwater pilot test program at SLC-4E featuring injections of an amended sodium lactate solution with bromide tracer followed by microbial inoculation, successfully transitioned the groundwater to strongly anaerobic, with documented destruction of perchlorate and sequential dechlorination of TCE and daughter products through to ethene within 4 to 6 months. Pilot test findings indicated that substrate distribution is controlled by an irregular bedrock topography, which, coupled with considerable access limitations, presents challenges towards addressing the entire source area. Based on pilot test findings, a subsequent site-wide well installation program was conducted in support of full-scale interim removal action (IRA) implementation. Wells were installed in the limited accessible areas overlying the groundwater source area, with planned injection and monitoring well pairs generally spaced 30 to 60 ft apart. Dilute batches of amended soluble sodium lactate solution were then mixed on site and gravityfed through manifolds into injection wells. This strategy was designed to create transient flooding of the aquifer around each injection well, allowing substrate to overcome bedrock highs, fill in bedrock lows, and move with the prevailing groundwater flow. Operation, maintenance, and reporting are ongoing. Use of the soluble substrate sodium lactate afforded effective transport with natural groundwater flow, reaching out well beyond injection wells to target treatment of the greater source area. Efforts are currently geared towards an expanded clean-up effort in the final quarter of the program, to treat plume margins not originally scoped in the IRA and ultimately exceed program objectives while meeting the project schedule and budgetary constraints.