Geochemical investigation of groundwater in a Granitic Island: a case study from Kinmen Island,Taiwan

Kinmen Island is principally composed of low permeable granitoid and covered by a very thin sedimentary layer. Both surface and groundwater resources are limited and water demand is increasing with time. The groundwater in the granitoid has been surveyed as an alternative water source for daily use. Two to five highly fractured zones in the granitoid aquifer for each site were first determined by geochemical well logging. Accordingly, ten samples were collected from three sites. Using environmental isotopes and geochemical modeling, geochemical processes occurring due to water–rock interaction in the granitoid aquifer can be quantitatively interpreted. The stable isotopes of oxygen and hydrogen in groundwaters cluster along Taiwan’s local meteoric waterline, indicating evaporation does not have considerable effect on groundwaters. Given such a high evaporation rate for Kinmen Island, this result implies that infiltration rate of groundwater is high enough to reduce retention time through a well-developed fracture zone. NetpathXL is employed for inverse geochemical modeling. Results determine gypsum as being the major source of sulfate for deep groundwaters. The contribution from pyrite is minor. In addition, the weathering of albite to kaolinite is the dominant water–rock interaction characterizing geochemical compositions of deep groundwater in Kinmen Island.