Numerical investigation of offshore freshened groundwater reservoirs in the East China Sea

Offshore freshened groundwater (OFG) has been proven to exist in continental margins around the world and has been identified as a potential unconventional water resource. In China, freshwater resources are very limited in the developed coastal areas and islands. Buried paleo-channels associated with the ancient Changjiang (Yangtze) river are suspected to be viable hosts of a offshore freshened groundwater reservoir system in the East China Sea, North of Chengsi island. In this study, we used an integrated modeling approach to predict OFG potential and optimize the design of a controlled source electromagnetic survey to image the reservoir. We develop a conceptual 2D geological model of the Quarternary sediments in the region. Porosity and permeability values were assigned based on borehole observations to produce a hydrogeological model. We present the results of a numerical modeling study of groundwater transport and variable density flow as a result of sea-level fluctuation over the past 200,000 years. Based on the bathymetry, the present-day shelf was sub-aerially exposed to meteoric recharge for most of that period. We considered a range of recharge scenarios and the simulation results indicate a high likelihood that freshwater reservoirs would be preserved until present day. Two freshwater intervals were observed between 80 m – 100 m, and 200 m – 300 m below the seafloor. A layered resistivity model was designed based on the observation of two primary freshwater reservoir layers in the flow model. Numerical tests were carried out on the feasibility of controlled- source electromagnetic method in the study area to optimize the survey setup. This approach can be adapted for offshore freshened groundwater prospecting in other siliciclastic shelf environments.