The Bohai Basin was transformed to an inner shelf sea hundreds of thousands years ago. This youngest land-sea transition participated in the significant modification of the distribution of fresh water, sediment fluxes and climate in East Asia and played an important role in the origin of the Asian marginal seas. Here we present the results of a magnetostratigraphic investigation and propose a conceptual model for the land-sea transition. Our findings indicate that the transition probably started several million years ago, from a fluvial system during the late Miocene and early Pliocene, to a lacustrine environment between the late Pliocene and Middle Pleistocene and finally to a marine system in the late Pleistocene. Comparison of our results with previous research suggests that the Bohai Paleolake was initiated from the late Pliocene, was fully developed prior to ~1.0 Ma and terminated around the late Middle Pleistocene. The Miaodao Islands formed the eastern “barrier” of the basin and since the Pliocene or earlier they played a significant role in blocking the lake water and sediments. They deformed from ~1.0 Ma, subsided significantly at ~0.3 Ma and completely by ~0.1 Ma, resulting in the maturation of the basin as an inner shelf sea.

Jinan is known as “Spring City,” because of its famous 72 artesian springs. Spring water plays an important role in the social and economic development of Jinan. However, the accelerating process of urbanization and more intensive human activities have significantly affected the Jinan springs. Based on the data from four spring groups (2015–2018), the hydrochemical characteristics of spring water were analyzed and 14 parameters were selected to evaluate the quality of spring water. In addition, the main ions variation characteristics of spring water in Jinan from 1958 to 2018 were analyzed, based on the previous studies. Subsequently, the spring flow dynamics of the Jinan spring area from 1958 to 2012 were also discussed. Overall, the water quality of the four spring groups is good. The chemical composition of the spring water is mainly controlled by the dissolution of calcite, followed by dolomite, and de-dolomitization. However, spring water is affected by human activities, and the hydrochemical type tends to evolve from HCO₃-Ca to HCO₃·SO₄-Ca or HCO₃-Ca·Mg. From 1958 to 2013, the main ions in spring water increased, while a relatively steady and certain fluctuation trend was observed from 2015 to 2018. Since 1958, the spring water flow in Jinan has experienced four stages, from decline to recovery. The exploitation of karst groundwater, change of land use type, and decrease of atmospheric precipitation are the three most important factors, affecting spring discharge dynamics. The slowdown of the increase of the main ion concentration in Jinan spring water and the resumption of spring water flowing indicate that remarkable achievements have been made to protect spring water, but there is still a long way to go to fully protect Jinan spring water.

Groundwater in coastal aquifers is often affected by seawater intrusion, resulting in water quality deterioration. Using groundwater influenced by seawater intrusion for irrigation can lead to crop failure, erosion of machinery and pipes, and adverse effects on farming. In this study, the results of water testing, methods of statistical analysis, ion ratios, a Piper diagram, and a variety of groundwater irrigation suitability models were used to analyze the chemical composition of groundwater and the influence of seawater intrusion. The result shows that the content of Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, and SO₄²⁻ in groundwater would increase due to seawater intrusion, and the increasing trend was consistent with the freshwater–seawater mixing line. With the deepening of seawater intrusion, the hydrochemical type gradually changes from Ca-HCO₃·Cl to Na·Mg-Cl·SO₄ and then to Na-Cl type, and the source of hydrochemical composition changes from “Rock Weathering Dominance” to “Evaporation Dominance”. When the Cl⁻ concentration is greater than 7.1 meq/L, groundwater will corrode pipelines and instruments; when greater than 28.2 meq/L, excessively high salinity of groundwater will have adverse effects on planting; and when greater than 14.1 meq/L, the groundwater hardness is too high, which may make the groundwater unsuitable for cultivation.

Seawater intrusion has a serious impact on industry, agriculture, and people’s daily life. Thus, the present study was designed to elucidate the pollution sources and groundwater evolution in typical intrusion areas of Shandong Province by hydrochemistry and environmental isotope techniques. The water samples were collected to analyze the groundwater evolution under different intrusion, and groundwater evolution in the south of Laizhou Bay from 2005 to 2019. The findings indicated that the groundwater level dropping funnel caused by overexploitation was the direct causation of seawater intrusion in the three typical intruded areas. The groundwater evolution paths demonstrated that the groundwater in the south of Laizhou Bay had the fastest evolution rate and the highest degree of evolution, followed by the Dagu River Basin. The groundwater evolution extent and fitting of mixing lines indicated that the groundwater in the south of Laizhou Bay, Longkou, and Dagu River Basin was dominated by palaeosaltwater intrusion, modern seawater intrusion, and sea-saltwater mixed intrusion, respectively. Palaeosaltwater mixing produces a more severe salinization effect compared to seawater mixing. Meanwhile, the isotopes are gradually enriched with the deepening of intrusion, while the decrease of isotopes is delayed compared with the saltwater retreat. This is caused by that the stable isotopes enriched in the aquiclude due to the chemical permeation effect will be released into the aquifer after the salinity attenuates in the aquifer. The palaeosaltwater intrusion caused by anthropogenic activities has promoted serious fluorine pollution in the south of Laizhou Bay, while the groundwater nitrate pollution in Longkou was the most serious, followed by the Dagu River Basin due to high-density agricultural and domestic activity.