This article aims to overview the previous research trends and find out the future perspective about the relationship between groundwater and ecosystems. Especially, we focused on the dynamics of microbes in groundwater which involve any material cycles and ecosystems, and seagrass/seaweed meadows and coral reefs which are categorized as Groundwater Dependent Ecosystems (GDEs) in the coastal groundwater discharge area and play important roles in the conservation of material cycles and biodiversity. Regarding the microbes, most of the studies were focused on the application of groundwater as drinking water and purification of pollutants in groundwater in the past, however, the relationship between groundwater flow and the dynamics of microbes has been studied recently. Regarding the seagrass/seaweed meadows and coral reefs, most of the research was targeted the area with a significant impact of Submarine Groundwater Discharge (SGD). Some previous studies demonstrated that SGD tends to increase the biomass of seagrass/seaweed as a nutrient source, however, it leads to a lack of their biodiversity. Some other studies reported that SGD may intensify the coral reef vulnerability due to the growth of specific algae and degradation of skeletal density and breeding potential of coral reefs if SGD-derived nutrients cause eutrophication in seawater. However, the relationship between groundwater and these ecosystems are still not fully understood in the previous research. It will be important to accumulate the results in various observation sites, apply the new techniques, and cooperate the researchers with different backgrounds (e.g. biogeochemistry, microbiology, ecology) based on groundwater hydrology for further understanding.

Keywords were extracted from 45 original papers and 19 rapid communications published in the Journal of Groundwater Hydrology during 10 years from 2010 to 2019. Based on the extracted keywords, original papers and rapid communications were categorized by research themes, and it was found that many researches on dating, isotope ratio, nitrate nitrogen, denitrification, etc. were reported. Based on these results, this paper describes a research concept on hardware and software that looks 10 to 20 years into the near future, focusing on “Visualization of uncertainties in groundwater modeling technologies”, “Real-time data analysis and information dissemination”, “Estimation and management of groundwater resources for sustainable use”, “Drilling, verification and maintenance of wells”, etc., which have hardly been reported in recent years.

For the purpose of developing a simplified soil pollution evaluation method using luminous bacteria for petroleum hydrocarbons, in the previous study, the acute toxic effects of n-alkanes which are the main components of gasoline were evaluated. On the other hand, many petroleum hydrocarbon products such as gasoline also contain many branched alkanes. In this study, to investigate the differences in toxic effects due to differences in molecular structure, acute toxicity tests using luminescent bacteria was conducted with structural isomers of chain alkanes (2 types of pentane and 5 types of hexane) and a cyclic alkane (cyclohexane). The alkane structural isomers dissolved in methanol were used as test solutions. Then magnitude relationship of the acute toxicity were 2-methylbutane ＜ n-pentane ＜ 2,2-dimethylbutane ＜ Cyclohexane ＜ 3-methylpentan ≦2,3-dimethylbutane≦2-methylpentane ＜ n-hexane, based on the estimated value of the alkane concentration at which the relative luminescence intensity ratio at the reaction time of 60 minutes becomes 50%. Comparison of the test results for the structural isomers indicated that the acute toxicity of the straight chain alkanes is higher than that of the branched chain ones. In addition, this study suggested that the acute toxicity might increase with increasing number of carbon atom in the straight chain.

To understand the dynamics of transient tracers in the spring with small catchment area, a small spring in suburban area, namely Satoyama, nearby Nagoya city was sampled weekly for 1 year and the major dissolved chemical components concentrations, stable isotope ratios, CFCs and SF₆ concentrations were determined. The water chemistry and stable isotope ratio results suggest that the spring water seems to be sourced by the young groundwater with a residence time less than few years. The CFC-12 and SF₆ concentrations show a clear seasonal trend, higher in summer and lower in winter, and the variation range of concentration corresponds to the residence time of 10 years in the spring water. The 10 years variation of residence time is not interpreted reasonably by the hydro-geochemical data. The concentrations of CFC-12 and SF₆ in the spring were negatively correlated with the water temperature, indicating that the spring water is in contact with ambient air through the discharge process. These results suggest that careful consideration should be necessary when applying gaseous tracer method to small-scale springs with fluctuations in water temperature.