Study On The Flow And Origin Of The Submarine Spring In Qianhu Bay

The submarine spring is a special kind of spring,a groundwater flowing from the gap and sediment of the bottom of the sea.Compared with the spring on the land,the special feature of the submarine spring lies in its outcrop in the intertidal zone or the seabed.Submarine spring is a typical representative of submarine groundwater discharge(SGD).It is also an important channel of SGD.Because of the special formation conditions,the submarine spring has become the carrier of geological information,which is of great significance in the study of hydrogeology.As a freshwater resource,the submarine springs are found and used in ancient times,such as drinking water,agricultural irrigation and even industrial water use,and the submarine springs often form a special society in the local area because of special natural phenomena.Cultural and religious culture has the value of sightseeing.The number of submarine springs known in China is very small,and most of the relevant materials belong to news reports.Under the support of the National Natural Science Foundation,"the study of groundwater discharge and the process of water circulation in the Qianhu bay of Fujian(No.41672226)",the flow,recharge area and runoff route of the submarine spring in Qianhu Bay have been studied,and the following main understandings are obtained.(1)flow evaluation.The sea bottom spring flow is measured and calculated by weir method,seepage meters method and Darcy's law respectively.The mean seepage flow of the submarine spring is 19.4 m3/d by 5 tests by the weir method,and 12 measurements of the submarine spring at the outcrop of the submarine spring are measured by the seepage meter,and the seepage flow of the submarine spring is calculated to be 28.3 m3/d by using the Tessellation polygon method.Through the continuous measurement of water level meter in two tidal cycles and the measurement of 2 permeability coefficients,the results of Darcy's percolation velocity are calculated.It shows that intermittent seawater intrusion on the surface sediments of the beach during the flood tide,the overflow of the submarine spring is interrupted,and the effective time of the submarine spring on the sea beach is 7/9 of the whole tidal cycle.Based on the measurement and calculation results of the above three methods,the actual discharge of submarine springs is between 15.1 m3/d and 20.0 m3/d.(2)Chemical characteristics of regional groundwater.The results of hydrogen and oxygen stable isotopes show that the main source of groundwater recharge in the region is atmospheric precipitation,and the hydrochemical types of the 26 groundwater indicate that there is no abnormal change in the aquifer environment.The difference between the submarine spring and the other near shore groundwater and Chihu river shows that it has a special supply way,the 222Rn activity in the submarine spring is up to 86951 Bq/m3,and the mean value of the 222Rn activity far out of the coastal groundwater is 12438 Bq/m3,indicating that the submarine springs flow through the granite cracks in the formation process,which conforms to the submarine spring water.The characteristics of positive deviation of hydrogen and oxygen isotope values in coordinate systems.(3)The recharge area.The regional tectonic data show that the submarine spring is in the intersection point of the North East fault and the North West of the Chihu river,and the recharge area is the intersection area of the fault zone,that is,the inland area of the northern bank of Chihu river.By comparing the characteristics of hydrochemical types and stable hydrogen and oxygen isotopes,it is speculated that the main supply area is the near north west direction strip area represented by the sampling points G6,G9,G13 and G17,which is roughly the area of the northern bank of Chihu river and the area of Gushan-Fengshan under the northern bank of Chihu river,with the direct infiltration and recharge of atmospheric precipitation.(4)The route of runoff.The near North West fault zone in the study area and the overlying Quaternary loose sediments constitute a unified groundwater water bearing system.Due to the good water conductivity of the fault zone,it becomes an important water collection corridor and water guide channel.It is the main runoff route of the submarine spring,and the Quaternary loose sediments in the near shore have two layers of cohesive soil aquifers.The groundwater of the submarine spring is located at the bottom of the aquifer and bears pressure.In the intertidal zone,because of the partial loss of the top plate of the aquifers of the lowest aquifers,that is,the existence of the"skylight" and the uplift of the East fault zone of the Northeast fault zone,the pressure water flows from the sea to the beach to become the submarine spring.The results of high-density electrical prospecting show that most of the groundwater supplied to submarine springs is excreted from bedrock fissures and faults in the 8m?10m below the beach.