抄録
Submarine groundwater discharge (SGD) is becoming recognized as a significant source of fresh water and chemical fluxes into the coastal oceans around the world. On the coast of eastern Toyama Bay (central Japan), land-based estimates of the water budget indicated the possible existence of SGD. The two-fold purpose of this study was to development a SGD flow measuring method with wide dynamic range and to demonstrate it by obtaining preliminary estimates of the SGD flux in Katakai River alluvial fan. A new flow rate meter was developed, named the Toyama University SGD-Flux Chamber (in collaboration with Nichiyu Giken Kogyo Co., Ltd.). Electrical conductivity and temperature sensors were mounted in a plastic chamber, conventionally used for gas flux measurements. The flow rate was calculated though the conductivity dilution velocity measured of the bottom seawater, diluted by spring groundwater from sea floor inside of the chamber (diameter: 40 cm; height: 15 cm). The initial field experiment was conducted to investigate the measurement sensitivity and accuracy of the SGD-Flux Chamber. Trial SGD water (tap water) was utilized, and two ranges of flow rate were measured: 10-50 mL/min withestimated uncertainty of±1mL (relative standard deviation: RSD=5%), and 50-400mL/min±2ml (RSD=2%). Therefore, the Flux Chamber is capable of measuring SGD flux within a large flow range, 10-400 mL/min (corresponding flow velocity values are: 5.7-458 cm/day). Flux measurements were then carried out from April to December, 2003 in the SGD area of Katakai River alluvial fan, 150-200 m seaward ofthe coastline in water depths of 8 m and 22 m. The average fluxes from April to December were 0.8-1.3 L/min/m2 at 8 m and 0.5-0.8 L/min/m2 at 22 m. On tidatime scales, the SGD flux increases with the dropping of the tide level with a time lag of 2 hours. There are indications the flux is controlled by changes in landward groundwater potential pressure, sea level at the SGD area, and the geotechnical condition of submarine sediment. Longer time series at multiple sites willbe required to discern the relations between the SGD flux and river flow rate, precipitation, and long-term (seasonal) changes of the landward unconfined aquifers.
