Submarine Groundwater Discharge And Associated Nutrient Inputs Into Jiaozhou Bay And Their Environmental Effects

Submarine groundwater discharge?SGD?is an important hydrological process between groundwater-seawater interactions in coastal areas.Driven by both terrestrial and marine forcing components,SGD comprises fresh groundwater?SFGD?and re-circulated seawater?RSGD?.As a major part of global water cycle,SGD is an important pathway for transporting nutrients,heavy metals and carbon from aquifers into coastal waters,which influences biogeochemical cycle of various elements and ecological environment.In this study,an improved method is presented and then applied in Jiaozhou Bay?JZB?,a semi-enclosed bay,located in the south of the Shandong peninsula.The main contents are as follows.First,the eutrophication and pollution conditions of seawater are assessed and the major sources of nutrients and trace elements are identified for JZB.Secondly,the water age and SGD in different seasons are estimated in JZB,and the significance of improved tracer-based mass balance model is discussed.The third is to evaluate the primary production supported by nutrients from SGD and its influence on ecological environments.An improved method is presented which considers the losses of tracers caused by RSGD to enhance accuracy in estimating SGD.Theoretical analysis indicates that neglecting the losses of tracers induced by RSGD would underestimate the SGD by a percentage approximately equaling the tracer activity ratio of nearshore seawater to groundwater.In JZB,the eutrophication evaluation of nutrients shows that the nearshore seawater is classified as potential eutrophication with median phosphorous limiting,while the waters in other regions belong to poor nutrient.The analysis of the comprehensive index of trace elements indicates that the nearshore water is seriously polluted,while the other waters are slightly polluted.Radium isotopes are positively correlated with nutrients and alkalinity,indicating that SGD is their main source.The SGD fluxes in JZB estimated by the improved method are increased by 15-28%in spring and 28.8-43.3%in fall relative to those estimated by the old method for ²²⁶Ra and ²²⁸Ra models.Higher SGD flux is observed in fall and such seasonal variation pattern suggests a rapid response of local precipitation.In order to understand how the different factors such as sediments,RSGD,open sea water end-member,rivers and atmospheric deposition affect the water age estimation,we combine two radium mass balance models to assess the water age in JZB.The results show that the first three factors have strong influences on the water age estimations.Data analyses from previous studies in other coastal waters and JZB indicate that all the above-mentioned factors should be included in the radium or radon tracer-based models when estimating water age and SGD.The estimated fluxes of nutrients and alkalinity through SGD are significantly higher than those from local rivers.Compared with other sources of DIN,DIP and DSi,it is found that the nutrients from SGD account for more than 60%of total inputs,which contribute a significant fraction of primary production?>30%?.The high DIN/DIP ratio and eutrophication level in seawater are closely related to SGD.The unbalanced DIN/DIP in SGD and the large inputs of nutrients from SGD may change the nutrient structure of the water body,promote the primary productivity and affect the coastal ecosystem in JZB and other similar aquatic ecosystems.