Variations Of Hydrodynamics And Submarine Groundwater Discharge In The Yellow River Estuary Under The Influence Of The Water-Sediment Regulation Scheme, Evidence By Natural Radium And Radon Isotopes

Estuaries and subterranean estuaries are both important pathways for transport of continental weathering materials from land to the sea, and excert an influence on the biogeochemical cycling and coastal ecosystems. Currently, anthropogenic modifacations on estuaries have become increasingly evident. For example, large numbers of reservoirs have been constructed, which alter the natural allocation of river discharge into estuaries. The Yellow River estuary directly bears the load of water and sediment from the Yellow River. The "Water-Sediment Regulation Scheme" (WSRS) implemented annualy in the Yellow River leads a huge pulse of water and sediment to deliver into the Yellow River estuary during a short period. Such drastic conditions present during WSRS provide a natural laboratory to study hydrodynamics and submarine groundwater discharge (SGD). Natural radium and radon isotopes are important in tracing water transport process and SGD. This study focuses on the variations of hydrodynamics and SGD of the Yellow River estuary under the influence of WSRS. It will provide a new perspective to understand water renewal ability, chemical source and fate, and balance of materials in estuaries. Besides, it will provide methods to evaluate the anthropogenic impact of large-scale water conservancy project on estuaries, as well as lay scientific foundations to develop reasonable ecological and environmental protections.In this study, the temporal and spatial distributions of radium and radon isotopes were investigated in the Yellow River, the Yellow River estuary and the Yellow River Delta, both during the periods of the WSRS and non-WSRS in 2013 and 2014. At the same time, leaching experiments were conducted to simulate the process of radium desorption of groundwater in the Yellow River Delta. The variations of hydrodynamics of the Yellow River estuary under the influence of WSRS were evaluated qualitatively and quantitatively, on the basis of radium trends, some hydrological parameters such as salinity and turbidity, as well as the applications of radium water age model. The variations of SGD process under the influence of WSRS were also evaluated qualitatively and quantitatively, on the basis of the radium and radon trends, hydrological parameters of vertical profiles, as well as the applications of radium mass balance model, radium and radon time series model. Nutrient concentrations and compositions of the Yellow River, the estuary and groundwater of the Yellow River Delta were systematically contrasted in order to discuss the influence of WSRS on the nutrient input to the estuary. The main conclusions are as follows:(1) Despite the fact that there are drastic varations of water and sediment discharge between the WSRS and non-WSRS periods, the activities of disolved radium isotopes of the Yellow River have relatively small variations. The disolved radium is controlled by SPM concentration. The 222Rn activities of the Yellow River during the WSRS are significantly higher than that during non-WSRS, which probably results from increase of SGD under the influence of the WSRS.(2) The activities of radium isotopes of the Yellow River estuary are dramatically elevated during the WSRS than that during non-WSRS. The radium versus salinity indicates a non-conservative distribution pattern that is "high in the middle and low in both ends". The activities of radium isotopes of both surface and bottom water are relatively consistent. The 222Rn activities of the Yellow River estuary are also dramatically elevated during the WSRS than that during non-WSRS. The maximum of 222Rn activity appears at the estuarine entrance and the estuary with low salinity water. From intermediate to high salinity water, the 222Rn activity decreases rapidly to the level supported by seawater.(3) The activities of radium isotopes of groundwater along the coast of the Yellow River Delta are generally higher than those of waters in the Yellow River and the estuary. There are large spatial and temperal variations of radium activities of groundwater. The salinity and radium activites of groundwater during the WSRS are lower than those during non-WSRS, especially in the underlying acquifer which is near the river bank. There are also obvious spatial variations of 222Rn activities of groundwater, which probably results from different types of coastal sediments, as well as different impact of tidal water and sea level.(4) The results of leaching experiments indicate that the key factor on radium desorption in aquifer sediments lies in salinity of groundwater rather than SGD flow rate. The radium activity of groundwater increases with the salinity. The linear positive relationship between radium activity and salinity can provide reference information for quantifying SGD radium endmember.(5) The peak activity of radium isotopes occurs in fresher water of the estuary during the WSRS relative to that during non-WSRS. The diffusive scale of diluted water and sediment are also expanded during the WSRS. The radium and water-sediment distribution both indicate that WSRS accelerates the estuarines hydrodynamics. Based on the radium apparent age model, water transport rates were estimated at 2.4±0.6 km d-1 (2013 non-WSRS),5.9±2.2 km d-1 (2013 WSRS),2.7±0.7 km d-1 (2014 non-WSRS) and 5.5±1.2 km d-1 (2014 WSRS), respectively. As the Yellow River discharge increases from 400 m3 s-1 during non-WSRS to 3000 m3 s-1 during the WSRS, the water transport rate is double or triple.(6) The common high activities of 222Rn and 220Rn can be the proxy to indicate the locations with intensive SGD. The WSRS significantly increases the amount of SGD from these locations. The vertical profiles of hydrological parameters such as salinity, temperature, turbidity and DO verify that fresh SGD is significant during the WSRS. Based on the SGD models, the SGD fluxes were estimated at 11-26 cm d-during non-WSRS, and 62～122 cm d-1 during the WSRS. The SGD flux during the WSRS is conservatively estimated of 3 times of that during non-WSRS. During the WSRS, SGD tends to be more intense along the river channel direction with a large amount of fresh SGD which accounts for more than 24% of the total SGD.(7) The nutrients concentrations and compositions of the SGD is different from the river, demonstrating lower DIN/DIP and higher NH4+-N/DIN. The nutrients input into the estuary delivered by SGD may reverse the Yellow River in terms of the effect on the nutrient compositions of the estuary. Both the nutrient fluxes delivered by the river and SGD are intensified during the WSRS.