The discharge of freshwater from the seafloor of the continental shelf is recognized as an important, direct transport pathway both for water and other materials between the land groundwater system and the marine environment. This submarine groundwater discharge (SGD) is possibly more important than its contribution to the water balance alone would suggest, because the concentration of dissolved material is greater than that of river water, and much of the riverine dissolved material is removed by colloids and/or uptaken by phytoplankton in the estuary region. In this special issue, 7 original papers and 1 short report are incorporated, focusing on recent SGD geochemistry research in Japan.

View full abstract

Submarine groundwater discharge (SGD) rates were measured continuously by automated seepage meters to evaluate the processes of groundwater discharge from land to the ocean in the coastal zone of Suruga Bay, Japan. Resistivity under the seabed and land surface were measured to evaluate the freshwater and saltwater interactions in the coastal aquifer. PSD analyses of SGD and groundwater level, show that dominant periods of SGD variations at near shore agree with those of groundwater level changes, and dominant periods of SGD variations at offshore agree with those of sea level changes. Resistivity measurements show that dynamics of fresh-salt water interface in the coastal aquifer changes corresponding to sea level changes. The ratio of terrestrial fresh-SGD to total-SGD was evaluated to be at most 9% by continuous measurements of SGD conductivity. SGD rate near the mouth of Abe River is smaller than those in the other area of the coast, which shows the effect of existence of the river on SGD or the effects of curvature of the bay on SGD. The role of SGD in the basin scale was evaluated from a waterbalance method in the Abe river basin. The SGD was estimated to be 39% of total discharge in the basin.

View full abstract

Stable isotopes (deuterium and oxygen-18), major inorganic ion chemistry, and tritium have been measured to understand the groundwater flow system in the Shizuoka plain, Japan. We found the following regional groundwater characteristics. (1) Most of shallow groundwaters in the Shizuoka plain is mainly classified as a calciumbicarbonate type, while deeper aquifers as a sodium-bicarbonate type. (2) The groundwater in this plain is recharged by either precipitation over the plain or Abe riverwater or both. The contribution of the riverwater in the groundwater decreases as the distance from the Abe River, although this contribution is different at eastern and western sides of river. In the western area, the riverwater contribution is smaller than local precipitation or Mariko riverwater. (3) The tritium concentration in the groundwaters shows that most groundwaters in the study area derive from rainwater after 1970 to present, although a part of deep groundwater indicates the residence time of 50 years or more. The groundwater flow characteristics in the studied plain are determined by the permeability of the aquifer materials rather than the depth. Thus, the deep groundwater collected near the Abe River where the alluvial gravel dominates has short residence time compared to that collected near the Oya River where the silt and clay dominate more. The submarine groundwater discharge at Motimune coast was confirmed the contribution from the land water. But their origin and flow paths are not yet identified.

View full abstract

Nitrite+nitrate, phosphate, silicic acid and dissolved organic carbon (DOC) in the submarine groundwater discharge (S.G.D.) and the bottom water (upper 10-50 cm from the bottom) in Toyama Bay were observed. The concentrations of nitrite+nitrate and silicic acid in S.G.D. were approximately 60 and 30 times, respectively, higher than those in the bottom water. Nitrite+nitrate, phosphate and silicic acid in groundwater under an alluvial fan of Katagai River and Kurobe River were also determined. The residence time for groundwater is estimated to be long due to the high silicic acid concentration. The ratio of silicic acid/nitrite+nitrate showed that the route and origin of S.G.D. from groundwater under an alluvial fan of Katagai River. The characteristics of DOC in S.G.D. were investigated by three-dimensional excitation emission matrix spectroscopy. The fluorescent peaks which derived from humic-like and tryptophan-like substances were recognized in the bottom water. The typical fluorescent peak could not be recognized in the S.G.D. have been reported. The flux of nitrite+nitrate, phosphate and silicic acid into Toyama Bay by S.G.D. were estimated to be 2,290±700, 22±20 and 22,900±10,400 ton/year, respectively.

View full abstract

We present here a geochemical study on the SGD in the Toyama Bay using CH₄ and its carbon isotope ratio as tracers. We found that all SGD fluids and many of land groundwaters contain less methane than natural water in equilibrium with atmospheric CH₄. The δ¹³C values of CH₄ in the SGD fluids range from -50 to -20‰ VPDB, almost comparable or slightly higher than that of atmospheric CH₄. The results suggest that most of methane in the SGD fluids had been derived from the atmosphere and a part of the CH₄ is oxidized in the course of groundwater flow through the aquifer, with no CH₄ production that is usual in highly anoxic groundwater .This is probably due to little organic matter content in the aquifer and/or a short residence time from recharge to discharge on seafloor. The low concentration and high carbon isotopic ratio of CH₄ are similar to those in some of the land groundwaters. The SGD fluid in Uozu should be flowing underground along the Katagai River.

View full abstract

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.

View full abstract

Recently, the flow rate of submarine groundwater discharge (SGD) has been measure around the world, in order to evaluate the effect of SGD on the marine environment. However, because the flux of SGD is spatially very inhomogeneous, it is difficult to extrapolate point measurements to estimate the spatial averages necessary to infer values relevant for large scale budgets. In this study, we developed a method for calculating the SGD flow rate by using sediment temperature. The method provides an estimate of the total flow rate, and also elucidates the flow mechanism of SGD. It has been recognized that sediment temperature is highly correlated with SGD flow rate (R²=0.94; correlation coefficient). We measured the sediment temperature (256 points, 1,024 km²) in the Katakai alluvial fan (Toyama Bay, western central Japan) in November 2003 and used the relation to infer a SGD distribution map. High flow areas are distributed as bands, with the total flow of SGD estimated 2.67×10⁵ml/min in the study area. The flow measured in August 2004 was smaller by 30 percent, with much of this reduction in the lower flow areas. It is shown that a permeable high aquifer reaches high flow locations directly, while the low flow locations are fed via higher resistance paths seeping out from the aquifer.

View full abstract

In Toyama prefecture, because of the steep slope from the mountains to the coast coupled with high precipitation, a large volume of river water flows into Toyama bay, and the submarine groundwater discharge (SGD) ranges widely on the steep continental shelf. However, the substance budget and its impacts to the coastal environment have not been previously studied. The purpose of this study is to clarify the water mass structure of Toyama bay and to estimate the fluxes of freshwater and its nutrients by using a box model. Using the T-S diagram it is clear that Toyama bay water is made up of coastal surface water, Tsushima Warm Current (TWO water and deep water (Japan Sea Proper Water). It demonstrates that the upper 200 m (Toyama Bay Shallow Water) is composed of high salinity water (TWC) and low salinity water derived from freshwater. The freshwater flux was calculated with a box model of the shallow water. The maximum SGD flux (2.3×10⁸ m³month^-1) was about 25% of the river water flux (9.1×10⁸ m³month^-1). And the nutrient fluxes via the SGD path were 55% (PO₄^3-) and 133% (NO₂^- + NO₃^-) of the river values. Furthermore the possibility was suggested from the N to P cocentration ratios that SGD entered the Toyama bay water colum primarily in the depth range of 〜100-180 m.

View full abstract