Geochemical monitoring of deionized seawater injected underground during construction of an LPG rock cavern in Namikata, Japan, for the safety water curtain system

An underground liquified petroleum gas (LPG) storage facility was constructed between 2003 and 2013 in Namikata, Imabari City, Ehime Prefecture, Japan, to increase domestic LPG stockpiles. The most important issue during construction and operation of this facility is gas leakage prevention. To thwart water leakage, the water curtain system was constructed according to design standards, and a large amount of deionized seawater (seal water) was continuously injected into the rock mass around the cavern to keep the water level constant during both construction and operation. It is possible to distinguish three end member waters (existing groundwater, seawater or fossil seawater, and seal water) using the salinity and isotope (δ¹⁸O) difference because seal water injected underground has almost the same δ¹⁸O value as seawater. In this study, continuous observation is carried out using the geochemical techniques for flow analysis with a mixing proportion of three end-members in the initial construction period (April 2005 to March 2006) of the LPG underground storage facility. It is determined that existing groundwater and fossil seawater originally distributed in this region are partly replaced by seal water in the cavern.