Impacts of Coastal Underground Brine Exploitation on the Depletion of Shallow Brine in Beach and Neritic Zones

Coastal underground brine (CUB) exploitation and groundwater discharge jointly drive the depletion of beach and neritic shallow underground brine (BNSUB). BNSUB is crucial for CUB exploitation, yet its reserve remain unquantified, and the impacts of different mining models on its evolution are poorly understood. This study investigates the northern beach and neritic zone of Yangkou salt farm on the southern coast of Laizhou Bay through in situ survey and numerical modeling. We estimate that the BNSUB distributed next to the Yangkou salt farm amounts to approximately 1.278 × 109 m3, with a total mineral resource of 7.577 × 107 tons. Brine in beach surface sediment (BBSS), phreatic and confined aquifer (BPCA), and aquitard (BA) contribute to the reserves in a ratio of 1:90:9 for both volume and mineral content. Under the normally operating salt farm CUB mining status, annual BNSUB mineral resource loss is approximately 1.705% of total reserves. If BNSUB salinity drops below the brine threshold, an estimated 1.189 × 107 tons of salt could be released to the sea. Continued mining accelerates BNSUB transport landward, while mining suspension promotes BNSUB discharge seaward. The salt input rate from BNSUB to the land is much lower than its rate of discharge to the sea, with only about 16% at most of the salt released by BNSUB migrating toward the coastal mining areas. These results not only reveal the huge reserves of BNSUB but also point out the current situation: BNSUB resources are continuously depleting, yet humans still cannot efficiently obtain BNSUB using the existing CUB mining mode.