Responses of submarine groundwater to silty-sand coast reclamation: A case study in south of Laizhou Bay, China

• This study is focused on responses of silty submarine groundwater to coastal reclamation.
• Groundwater in Holocene strata was influenced more obviously than that in Late Pleistocene strata.
• 38m depth is the boundary, groundwater was influenced by freshwater recharge that has a freshening influence above that level after reclamation.
• Above the 8m depth, saline water has been changed to fresh water.

Coastal reclamation can result in considerable changes in the quality and quantity of submarine groundwater at the land/sea interface. In this study, submarine groundwater monitoring wells and water samples were designed and implemented to get data of groundwater level, electrical conductivity, temperature, and hydrochemistry data to examine the responses of silty-sand submarine groundwater in different sedimentary strata to reclamation in south of Laizhou Bay. The submarine groundwater is mainly saline water and its salinity in the deep aquifer is higher than that of seawater and close to brine. It was formed in the Late Pleistocene and Holocene. Drilling core data indicates that there is a stratigraphic boundary at a depth of 18.58 m, with Holocene strata above, and Late Pleistocene strata below, this level. Continuous electrical conductivity data indicates that the submarine groundwater properties are stratigraphically distributed in this study area. And there is an interface at a depth of 38 m. Above the 38 m depth, the water quantity of submarine ground-saline water is freshening. The major ions showed a tendency to change continually above 25 m, but the tended to stabilize below 25 m depth. Freshwater is the major recharge source in the upper section of the Holocene strata, between the surface and 8.00 m depth, and the hydrochemical type has changed from ClNa to Cl·HCO3Na. In the lower section of Holocene strata (8.00–23.00 m) and upper section of late Late Pleistocene strata (23.00–38.00 m), groundwater is influenced by seawater and groundwater of upper aquifer. The freshwater, seawater, and groundwater recharge in the upper aquifer has no influence on the groundwater in the section below the late Late Pleistocene (between 38.00 and 49.15 m) and the early Late Pleistocene strata (between 49.15 and 75.00 m). The filling layer, added in the coastal reclamation project, is comprised of clayey silt and fine sand, and its high porosity means that it is suitable for water storage. After the reclamation, meteoric water was stored in the filling layer that became the stable recharge resource. The recharge resource and the filling layer can improve the properties of the confined aquifer to increase the groundwater level. They may also influence the groundwater quantity and the flow direction. The groundwater in the Holocene strata is clearly influenced by recharge and the previously saline water aquifers receive freshwater on an on-going basis, such that the water is now becoming fresh.

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