No abstract

Reduction in river discharge and sediment load has left deltaic lobes in the world’s many river deltas starving, but knowledge of how the subaqueous topography of these abandoned subdeltas responds to environmental changes is limited. In this study, we aimed to determine the long-term dynamics of the subaqueous seabed of abandoned delta lobes to advance current knowledge. As a case study, we selected an abandoned subdelta on the Yellow River Delta of the Bohai Sea, China, for which three-decade long (1984–2017) bathymetric data and long-term river discharge and sediment load records are available. We analyzed the seafloor surface change and quantified the void space from the sea water surface to the seafloor. In addition, we surveyed the seafloor surface with an M80 unmanned surface vehicle carrying a multibeam echo sounder system (MBES) in 2019 to obtain high-resolution microtopography information. We found that a net volume of 5.3 × 108 m3 of sediment was eroded from the study seabed within an area of 3.6 × 108 m2 during 1984–2017. This volumetric quantity is equivalent to 6.89 billion metric tons of sediment, assuming a bulk density of 1.3 t/m3 for the seabed sediment. The seabed erosion from 0 to −5 m, from −5 to −10 m, and below −10 m has showed a similar increasing trend over the past 33 years. These findings suggest that seabed erosion in this abandoned subdelta will very likely continue, and that other abandoned delta lobes in the world may have been experiencing similar seabed erosion due to the interruption of the sediment supply and sea level rise. It is not clear if the seabed erosion of abandoned delta lobes would have any effect on the stability of the coastal shoreline and continental shelf.

The coastal area is a major area of socio-economic development and the most active zone for human activities. With the rapid development of the economy and the strengthening of urban construction, the groundwater environmental problems in coastal areas are increasingly prominent. It is significant to investigate the groundwater chemical characteristics, hydrochemical types, and the factors that influence groundwater chemistry for water resources protection and groundwater resources development. In this paper, 48 groundwater samples and 5 surface water samples from the study area were analyzed with statistical analysis, Piper diagram, Gibbs diagram, mineral saturation index method, and the ionic scale factor, and explored the factors that influence groundwater chemistry. The modified Nemerow index method was also applied to evaluate the groundwater. The results show that the groundwater in the study area is neutral to weakly alkaline (average pH = 7.0~8.0). The groundwater chemical types are mainly Cl·SO4-Na and SO4·Cl-Ca·Mg. Hydrochemistry is mainly influenced by rock weathering and evaporative concentration. TDS was strongly correlated with TDS, Na+, Mg2+, K+, Ca2+, Cl−, SO42−, and the saturation index showed a gradual increase along the groundwater drainage flow path, it indicates that the main groundwater ions originate from the dissolution of halite, sulfate, and carbonate. Combining GIS technology and the kriging spatial interpolation method, we obtained the current situation map of groundwater quality in Laoshan District. The groundwater quality is mainly III water, and the overall water quality is good; IV and V water is mainly distributed in the middle and lower reaches of rivers, distributed in a belt pattern along the coastline. It is mainly influenced by both the human factor and seawater intrusion. It is significant for the utilization of groundwater resources and the management of seawater intrusion in the Laoshan District. In addition, the proposed research ideas and methods provide a reference for the study of groundwater genesis in other coastal areas in the world.

The Yellow River Delta is considered one of the most water-stressed regions in China as the competition for water between various industrial sectors and agriculture is increasing. Since 2007, the regional governments have taken a leading role in agricultural water-saving planning by upgrading irrigation systems and constructing water-saving facilities. In this study, the effects of water-saving irrigation strategies on the groundwater table and quality, including both phreatic surface and deep confined aquifers were investigated in the Yellow River basin during the last two decades. The average phreatic water level of bore-wells was monitored as 13.47 m during 2011–2020 in comparison with the level of 12.09 m during 2011–2020. Besides, the depth of the deep confined aquifer dropped dramatically during 2001–2013 with a declining rate of 2.55 m/a compared to the value of 0.385 m/a after 2014, suggesting the reduced exploitation of deep groundwater and increased availability of groundwater after the water-saving campaign. In addition, the physicochemical parameters of both phreatic water and deep confined aquifer indicated improved water quality. Consequently, understanding how the water-saving planning influences the groundwater availability is important, which would provide an insightful guidance for the water managers to protect the groundwater environment in the Yellow River Delta.