The Michoud Assembly facility is an active plant operated by Lockheed Martin for NASA. The facility is engaged in the manufacture and assembly of the external tank for the space shuttle. In 1966, about 100 tons of TCE were released from a large tank in the Building 190 Tank Farm Area. Subsurface investigations revealed the presence of chlorinated ethenes, primarily TCE and its degradation products, in soil and groundwater samples. A groundwater extraction and treatment system was installed to address shallow groundwater contamination. However, the storm drainage system within the area has been found to be leaking and therefore acts as a groundwater "sink" in the main area of contamination. This leakage to the storm sewer system resulted in unacceptable concentrations of VOCs in the storm sewer water.
Geosyntec's Scope of Services
Geosyntec designed an innovative application of permeable reactive wall technology that takes advantage of the hydraulic gradient resulting from the leaking storm sewer system. We identified an existing manhole within the vicinity of the spill area where there was significant inward leakage of contaminated groundwater. Our design involved the replacement of this manhole with a pre-fabricated structure (sump) that incorporates zero-valent iron backfill in an annulus around the structure. An internal valving arrangement allows control of the hydraulic gradient across the zero-valent iron reaction zone. This allows control of the flow rate and residence time of the groundwater as it flows through the reaction zone. Under highly reducing conditions and in the presence of metallic iron surfaces, certain dissolved chlorinated organic compounds in groundwater will degrade to non-toxic compounds such as ethene, ethane, and chloride. Geosyntec developed this innovative configuration that takes advantage of this process to treat the dissolved-phase TCE and its degradation products. Geosyntec also served as the construction manager and resident engineer for installation of the system described above.
Geosyntec's permeable reactive barrier system provides cost-effective, passive treatment of the impacted groundwater that discharges to the sewer, and it significantly reduces the concentration of VOCs in the storm sewer discharge.