学位論文の要旨
論 文 題 目

Evaluation of Human Impacts on Groundwater Resources inMountainous Catchments, Western Japan
広島大学大学院総合科学研究科総合科学専攻
学生番号 D196030
氏

名 SHARON BIH KIMBI

論文の要旨
The long-term cumulative effect of increasing urbanization, population andclimate change on watershed hydrology has altered the spatial availability ofgroundwater resources in many regions. In recent years, demographic trends have
led to concerns on how population growth will affect water resources. In many
Asian and Western countries, population densities are declining despite global
population growth with a significant increase in the proportion of the elderly. Withan increasing number of citizens over 65 and many cities experiencing
depopulation, Japan has experienced this demographic change faster than mostWestern and Asian countries. Despite these social changes, it still faces serious
environmental problems related to groundwater quality and quantity. Population
growth and increasing economic condition have led to the continuous expansion
and transformation of many Japanese’s mountainous rural catchments withassociated groundwater resource problems. Despite the significant impact ofurbanization (population growth) on groundwater resources, most studies havefocused on qualitative, regional impact, with little attention focused on thecombine effects of social and environmental issues on groundwater resources inmountainous rural catchments. Therefore, it is necessary to understand how
environmental and social aspects affects groundwater resources. This study
provides an insight into groundwater resources degradation resulting from anaging and depopulation society as well as urbanization and climate change effects.
The effects of population aging and depopulation on groundwater nitratecontamination in an island with intensive citrus cultivation, was examined bycomparing two neighbouring villages and watersheds with little different in theirsocial aspects. The northern one (Kubi) has a little higher farmer average age of76 years, and the decreasing rate of farmland for 10 years from 2005 to 2015 of46%, as compared with the eastern (Ocho) of 73 years and 37%, respectively. The
population in Ocho was 830, and twice of that in Kubi. Higher mean concentrations

of NO3−−N were recorded in Kubi village (6.55 mg/L) than in the Ocho area (4.75mg/L). NO3−−N contamination mainly was estimated to originate from chemicalfertilizer in Kubi and Organic fertilizer in Ocho, using stable nitrogen isotopes anda Bayesian isotope mixing model. These source distributions were closely
associated with a greater extent land use and social aspects such as aging anddepopulation. A substantial quantity of chemical fertilizer applied by the more
aged farmers in more depopulated Kubi was suggested to lead to inefficient
nitrogen uptake by plants, which results in higher leaching that pollutes its
groundwater more than those of Ocho.
Likewise, the hydrochemistry and dual isotope approach was used to evaluate
groundwater contamination status in Saijo watershed. Results showed that,calcium (Ca2+) and bicarbonate (HCO3−) were the most abundant cation and anion
respectively and the major water type in the study area was of Ca–Mg–HCO3 type.
Inferring from the contamination sources in the watershed using δ15N-NO3− andδ18O-NO3⁻ measurements indicated that two major contamination sources withvalues characteristic for nitrate derived from nitrification of soil organic nitrogen(+0.2 ‰ to +7.3 ‰ for δ15N) and nitrate derived from animal wastes or humansewage, e.g., via septic systems (+8.9 ‰ to + 12.3 ‰ for δ15N) in two distinct zones.
Despite agricultural activities in the study area, we found no evidence of nitrateoriginating from synthetic fertilizers in the groundwater.
Furthermore, the spatial variation in groundwater recharge under the effects ofurbanization and climate change, was investigated using SWAT over a period of
three decades. Results suggest, the water balance in the last 30yrs is greatly
influenced by land-use change with urbanization. The most significant change wasobserved in the decrease in the mean annual groundwater recharge byapproximately 25% (i.e., from 184.5 mm in the 1980s to 120.2 mm in the 2000s).
Rice paddy areas are observed to be the major recharge zones in the catchment,with the highest recharge compared to forest and residential areas. Higher rainfall
interception by forest canopy were responsible for the lower recharge values
observed at forested sites. Furthermore, the predicted scenario highlighted thatgroundwater recharge will be significantly affected by future climate change which
is projected to decrease groundwater recharge by 29% in the next 30years. Landand water resource management can be improved if the findings from the study
are used by decision-makers, planners, and managers the region. planners, andmanagers the region.