Evidence for submarine groundwater discharge on the Southwestern shelf of Taiwan

Abstract

This study was aimed at identifying the locations of the submarine groundwater discharge (SGD) on the shelf of Southwestern Taiwan by means of oceanographic measurements, quantifying its influence on the hydrographic conditions in the area, and estimating the volume rates of the discharge. Two high resolution hydrographic surveys of the region, including water and bottom sediment sampling campaigns, were completed in February and October of 2009. Water samples were also collected from the neighboring on-land groundwater wells. At some locations in the study regions, the vertical profiles exhibited a slight but detectable (0.009 to 0.105 psu) decrease of salinity manifested in the near-bottom portion of the water column. Although convectively unstable, this feature appeared robust and persisted for the eight months between the surveys. The salinity anomalies in the near-bottom layer were often accompanied by the maxima of fluorescence, chlorophyll, silica, nitrate, and iron concentrations, as well as the minima of turbidity. In February, 2009, the n-alkane composition of organic matter in the water collected from an on-land groundwater well exhibited high content of C 24 alkane. A similar anomalously high concentration of C 24 alkane was encountered in the bottom sediment samples from the suspected SGD sites. In October, 2009, the dominant marker of SGD signature was the C 16 alkane. Based on these data, we specified the likely locations of the SGD sources in the study area, all of which were restricted to the inner shelf at the depths less than 8 m. We argue that the influence of the SGD on oceanographic regime in the region is small but observable. Its signature is confined to the lowermost 0.1–2.1 m layer of the water column. The groundwater seepage rates roughly estimated under the assumption of the advection–diffusion balance based on the eddy diffusivity values typical for the bottom layer, are of the order of 0.1 to 1 g m −2 s −1.