The geology underlying the MARBO Annex consists of limestone reef deposits underlain by volcanic rocks. The volcanically derived Alutom formation consists of thick sequences of water-laid tuffaceous shales containing pyroclastic deposits of ash, dust, sandstones, and conglomerates. Interlayered within this formation are lava flows, breccia, and fragments of reef limestones. The MARBO Annex is underlain by the Barrigada and Mariana limestone formations.

Groundwater resources are primarily found in the northern half of the island in porous limestone deposits of the Barrigada and Mariana formations. The groundwater is encountered
approximately 300-500 feet bgs. The groundwater surface generally coincides with sea level and the depth to water depends on surface elevation. The thickness of the lens is generally around 90 to 120 feet.

Spatial variation in porosity of the Barrigada and Mariana limestone formations varies considerably depending on the depositional settings in the vicinity of the Northern Plateau.
Openings can range in size from microscopic to large, well-developed cavern systems, but are generally about 1/8 to 1/4 inches in diameter and are the result of dissolution of the
limestone.

The estimated sustainable yield of the aquifer is reported to be 59 mgd, and
an estimated 37 mgd is considered available for future development.

• Fractured Bedrock

Not identified in the documents reviewed.

• Tetrachloroethene (28 µg/L)
• Trichloroethene (210 µg/L)

• Tracer (dye) Test

• Pump and Treat
• Other (Monitored Natural Attenuation)

Cleanup goals are MCLs for TCE and PCE.

The treatment at the wellheads continues. However, the MNA aspect of the ROD is not working and a finding of technical impracticability will be made (2009 Five Year Review).

http://cfpub.epa.gov/supercpad/cursites/csitinfo.cfm?id=0902825