Temporal variations of groundwater tables and implications for submarine groundwater discharge: a 3-decade case study in central Japan

Abstract

Abstract. Fresh submarine groundwater discharge (SGD) is the key pathway of flux and nutrients for the groundwater from land to the ocean. SGD flux is a current issue of discussion and a means to clarify the coastal marine system under climate change. SGD flux accounts for about one-quarter of the river runoff in the Katakai alluvial fan in Uozu, Toyama, Japan, which is an ideal area to study SGD flux considering the need for a rapid response to climate change and the prior research on SGD there. In this paper, the monthly groundwater table's condition over 30 years is analyzed using monthly rainfall, snowfall, and the climate change index. Rainfall has been on an upward trend, but the snowfall has decreased over 40 years. Furthermore, the groundwater table at monitoring wells in the coastal area increased, as a result of the increased rainfall. However, the relationship between snowfall and groundwater is negative. As expected by Darcy's law, SGD flux was controlled by the hydraulic gradient of the coastal groundwater. The estimated historic SGD flux by groundwater table variation shows an upward trend of SGD. Considering the increase in precipitation and the groundwater table, SGD flux may increase under climate change.

A Shift from Snow to Rain in Midlatitude Japan Increases Fresh Submarine Groundwater Discharge and Doubled Inorganic Carbon Flux over 20 Years

“…Mean concentrations of nitrate, phosphate, and silicate were statistically significantly lower in 2017–2021 compared to 2001–2003; the reduction percentages were 32 ± 5, 13 ± 2, and 20 ± 10%, respectively. These change rates were also similar to the increase in the FSGD volume (10–30%) in the previous studies . Anaerobic conditions in the aquifer can also cause a decrease in NO 3 concentration in terrestrial waters (e.g., denitrification).…”

“…The primary geology of the Katakai River alluvial fan is gravel/sand deposits (see Text S1), including the paleochannels. The fan’s shallow aquifer (∼60 m) is linked to the FSGD, where the increase in the water volume was observed with FSGD, and has no anthropogenic influence (e.g., agriculture or urban area) on groundwater quality from origins deeper than 20 m . In this study, we define this FSGD-connected shallow aquifer (20–60 m) as the FSGD system (FSGDS).…”

“…This site also has the best suitable locations and water depths for effective scuba diving operations. Previous studies have also indicated that all samples collected from the FSGD monitoring area (∼22 m water depth; 1000 m 2 ) are similar in all hydrological properties, dissolved constituent concentrations, and isotopic properties throughout the four seasons with negligible tidal influence. − , The total amount of FSGD into Toyama Bay was estimated to be 8.1 ± 1.0 × 10 7 m 3 /year in 2001–2003 and 10.9 ± 1.3× 10 7 m 3 /year in 2017–2021 based on Zhang et al (Figure e and Text S2).…”

“…The midlatitude region (30–60°N), where this research area is located, accounts for 31% of the global carbon load but has experienced a significant decrease in snow cover over the last 40 years because of global warming . Zhang et al evaluated the long-term trends of the FSGD volume and groundwater level in a land facing the central-western coast of Japan, which experienced a significant decrease in snowfall and an increase in winter precipitation over the last 30 years. − The study indicated two critical scientific facts: (1) The amount of water that changed from snowfall to rainfall in winter increased the infiltration from higher catchments into shallow groundwater aquifers (<60 m) while decreasing the infiltration into deeper groundwater aquifers, and (2) the FSGD volume has increased by 20–30% during at least 2002–2015, coinciding with a rise in shallow groundwater levels. Similar model simulations have been verified for the North Atlantic land facing at midlatitude and showed that the warming-related change from snowfall to rainfall would accelerate the water outflow from higher catchments into coasts.…”

“…As an appropriate area where the effects of global warming are more directly reflected in the FSGD, this study chose the freshwater-rich Katakai River alluvial fan and the coast of Toyama Bay in west-central Japan. A series of observation studies on FSGD and groundwater have been conducted since 2000. ,− Those dense studies presented that the FSGD in this area is directly connected to the terrestrial freshwater system and has quite similar hydrological and geochemical properties to shallow groundwater. The runoff system associated with FSGD is simple: the average residence time among the fans estimated from tritium concentrations is relatively short (10–20 years), and the tidal influence on FSGD is negligible (the tide level changes only 15–30 cm the entire Toyama Bay) .…”