Assessment of terrestrial factors controlling the submarine groundwater discharge in water shortage and highly deformed Island of Taiwan, Western Pacific Ocean

Abstract

Submarine groundwater discharge (SGD) from seventeen sites was investigated on the coasts around Taiwan in order to verify its occurrences and to understand its characteristics. The fresh water fraction (FWF) in most pore water samples, except the Fangsan sampling site, range from 0 to 59% based on the salinity evidence. The relative low FWF evidence implies that the recirculated saline groundwater discharge (RSGD) is more important than the submarine fresh groundwater discharge (SFGD) in most sampling sites. Fangsan, which is located at the southwestern coast of Taiwan, has nearly 100% FWF in SGD. Evaluation of the relationship between the magnitude of SGD flux and geological type of the Taiwanese coast proves to be difficult because complex hydrological factors affect the SGD, rather than the coastal topography. According to the hydrological evidence, the modeled-SGD discharge is enhanced by high precipitation but is generally reduced by severe groundwater pumping. In addition, modeled-SGD has an inverse relationship with river base flow, indicating the river effect. The almost fresh SGD with flow rates ranged from 34 to 42 m/year at Fangsan deserves attention because a seven-year-long groundwater budget calculation implies that the aquifer could not supply so much fresh SGD over a large area. This suggests that a significant amount of fresh water SGD at Fangsan is derived from a point source via a fault passage, considering its geological background. Although the rudimentary salinity and stable isotope results indicate that RSGD plays an important role in SGD, the type of submarine spring discharge via fault zones may very well be the most prominent in highly deformed areas elsewhere in the world, too.