Preliminary Measurement of Submarine Groundwater Discharge in Taiwan

1. Yi-Jie Lin*,Chen-Tung Arthur Chen, Meng-Chia Chen Institute of Marine Geology and Chemistry, National Sun Yat-sen University Kaohsiung, 80424 Taiwan *E-mail：m935030011@student.nsysu.edu.tw Abstract A preliminaries study shows that Submarine Groundwater Discharge (SGD) exists around Taiwan even though groundwater overdrawing is serious. Only 4 of the 20 sites studied did not record any SGD signal. Two nearly fresh SGD samples were obtained, providing strong and direct evidence for the existence of SGD in Taiwan. Introduction SGD is the submarine seepage of all fluids from coastal sediments into the overlying coastal waters. There are many reports that SGD may contribute much nutrients to the coasts. Because of its difficulty in measurement, however, there are few reports on the characteristics of groundwater seepage, such as the flow rate and the water chemistry. In Taiwan, the only report was published in the Japanese journal Geochemistry (Chen et al. , 2005 ). Study Area and Methods Samples were collected monthly from May, 2004 to June, 2006 at Caishan in Kaohsiung (Fig. 1). A flux chamber was also used in the observation of the SGD seepage rates. Further, samples were collected from 20 different places around Taiwan. The SGD collecting device and the SGD-Flux chamber (Zhang and Satake, 2003) were used in this study (Fig. 2), the latter being the first time used to explore the SGD flux in Taiwan. Salinity, dissolved oxygen saturation (%), nutrients (NO3, NO2, PO4, SiO2, NH3), total alkalinity, pH and major ions were analyzed. Results and Discussion 1.Salinity Fig. 3 shows that the salinity of SGD at Caishan is lower than the seawater. Only 4 of the 20 sampling sites did not record any freshwater signal, and two of them were located in the groundwater overdrawing areas. At other stations, such as Fangsan in southwesternTaiwan (S=0.2) and Kaomay in central western Taiwan (S=1.4), fresh SGD samples were obtained. The above study provided direct evidence that some freshwater still seeps out in the coastal region, albeit at a low rate, even though seawater intrusion exists at many places in Taiwan. 2. Nutrients In the case of the nutrients in SGD at Caishan, Fig. 4 shows that the average concentrations of NO3 and NH3 are 42.4±59.3 μM and 9.17±13.0 μM, respectively. The major type of inorganic nitrogen exists in the form of NO3. The average concentrations of inorganic nitrogen (NO3+NO2+NH3), PO4 and SiO2 are, respectively, 54.6±64.4, 0.57±0.33 and 47.3±40.6 μM, all higher than the overlaying seawater. Around Taiwan, the average concentrations of inorganic nitrogen, PO4 and SiO2 are,20.9±26.8, 1.16±4.12, 76.5±64.5μM, respectively. 3.Flux This is a direct evidence for the existence of SGD. The average seepage rate is about 0.77 L/m2/hr. We assume that the SGD occurs within 1 km off the coastline in Taiwan, and the length of coastline in Taiwan is 1200 km. So the area is 1.2x109m2, and the fluxes of N and P are 166723 and 2332 mol/hr, respectively. These are 12.4 and 1.66 % of the river output, respectively. Conclusions Direct evidence is provided that freshwater still seeps out in the coastal regions of Taiwan, albeit at a low rate, even though seawater intrusions exist at many places. The N and P output amount of SGD is 12.4 and 1.66 % of the rivers’, respectively. Reference Chen, C. T. A., Zhang, J., Peng, T. R, and Hagiwara, T., 2005. Exploratory sampling of submarine groundwater discharge in Taiwan. Chikyukagaku, 39: 165-171. Zhang, J. and Satake, H., 2002. Submarine groundwater seepage in Toyama. Aquabiology, 24: 294-301. Acknowledgement The authors wish to thank professor J. Zhang for assistance. Preliminary Measurement of Submarine Groundwater Discharge in Taiwan Fig. 4 NO3 and NH3 vs. salinity at Caishan from May, 2004 to June, 2006. Fig. 5 shows the results of one flux chamber deployment at Caishan, and the salinity dropped by 0.2 unit within 15 minutes. The seepage rate was 2.76 L/m2/hr. Fig. 5. Salinity change of seawater within SGD-flux chamber. Fig. 2. SGD collecting device and SGD flux chamber Fig. 1. Sampling sites around Taiwan. Fig. 3 Salinity variation of SGD, seawater and spring water in Caishan.