Offshore Porewater and Flux Chamber Sampling of San Diego Bay Sediments at Site 9, Naval Air Station, North Island

Abstract : Porewater measurements at 20 offshore stations at North Island, IR Site 9, indicate that elevated levels of VOCs are present in a tightly restricted area near the southern reach of Bravo Pier. The highest levels were always observed at the deepest measurement points, indicating that the concentrations are significantly attenuated within the sediment before entering the bay. In addition, the primary VOCs observed were 1 ,1-DCE, 1,2-DCE, and VC, indicating that degradation of the TCE source product is occurring before the material reaches the bay. Measurement of water seepage and VOC fluxes at six offshore stations where elevated VOCs were measured indicate that fluxes of VOCs to the bay are limited primarily to the areas around PW02 and PW03 between Bravo Pier and the shore. Seepage measurements show that the flux is driven largely by a tidal pumping of the porewater in which porewater with elevated VOC levels migrates out during low water, and bay water with undetectable VOC levels returns to the sediment during high water. The flux measurements indicate that overall, the largest VOC flux to the bay is for 1,2-DCE with an estimated total annual input of about 4.6 kg, followed by 1,1-DCE with 1.4 kg, VC with 1.3 kg, and TCE with 0.001 kg (1.2 g). Based on the low/undetectable levels of VOCs in ambient bay water, the relatively high solubility of the target VOCs, and the close proximity to the mouth of the bay, it appears likely that any materials entering the bay at this site are rapidly flushed to the ocean. However, the potential for localized effects on infaunal organisms near PWO2 and PW03 does exist and should be evaluated.