学位論文の要旨（英文）Impact of brackish-water aquaculture activity on groundwater
vulnerability in coastal alluvial plain: an evaluation to reachsustainability（和文）沿岸沖積平野の地下水脆弱性に対する汽水養殖活動の影響：持続可能性に到達するための評価

広島大学大学院総合科学研究科総合科学専攻
学生番号 D183751
氏
名 Anna Fadliah Rusydi
論文の要旨 (Abstract)
Brackish-water aquaculture, also known as coastal aquaculture, is an aquaticorganisms’ cultivation in brackish-water. It is well-known in Southeast Asia
countries, particularly Indonesia. Although the activity gives a positive contribution
to food production and economic growth, its affects may cause groundwater quality
deterioration. The chemicals from the ponds, such as salt and nutrients, may
infiltrate to subsurface and contaminate groundwater systems. Additionally, coastal
alluvial plains generally have fertile soil and large groundwater storage capacity,
making this location a target of urbanization. Meanwhile, coastal groundwater also
is facing twofold problems: (i) it is naturally vulnerable to contamination (i.e.,seawater intrusion) and (ii) urbanization potentially trigger groundwater quality
deterioration (i.e., due to over-exploitation, leakage of industrial and household
wastewater` network).
Previous studies have confirmed that geological settling, excessive
groundwater pumping, wastes from industrial, agricultural, and household activities
are the leading causes of contamination. Research on the environmental effect ofbrackish-water aquaculture activity, in the meantime, is still very limited to date,
and the information about the depreciation of groundwater quality is not welladdressed yet even from recent studies.
This thesis provides a comprehensive evaluation of brackish-wateraquaculture impact on the vulnerability of coastal alluvial groundwater inIndonesia, which has not been studied comprehensively to date. By understanding
the groundwater vulnerability and actual data from the fields, it is expected that theIndonesian government can take proper strategies to maintain sustainability.
The focus is on three crucial groundwater quality aspects, which aresuspected to derive from brackish-water aquaculture activities, namely: (1) saline
water contamination, (2) metals contamination, and (3) nitrogen contamination.
Water quality analysis is carried out for major cations (consist of Na+, K+, Ca2+, andMg2+) and anions (consist of Cl−, SO42−, and HCO3−); bromide ion (Br−); dissolved
arsenic (As), iron (Fe2+) and manganese (Mn2+); dissolved inorganic nitrogen(comprise of NHO4+–N, NO3—N, and NO2−–N); stable isotopes in water (δ2H andδ18O), sulfate (δ34S and δ18O), and ammonium (δ15N); and coliform bacterial. The
land-uses and geology features are also evaluated to understand environmental

factors that potentially contributed to high saline water, metals, and nitrogenconcentrations.
As the representative, the area under study is Indramayu, located in thenorth of West Java, Java Island, Indonesia. The fisheries production from brackishwater culture in Indramayu is the highest in Java Island and the second highest inIndonesia. Despite the groundwater in this region is essential to support
livelihood andaquaculture activities,
comprehensive
information
ongroundwater condition is limited. The groundwater samples are collected from two
categorizes of land-uses: brackish-water aquaculture region, hereinafter written as
a lower coastal region (LC), and agriculture and residential areas, hereinafterwritten as an upper coastal region (UC).
Samples are collected for sediments and groundwater. The sediments samples
are collected at different depths from two boreholes in UC and LC areas tounderstand the geological conditions of the aquifer system. Groundwater samples
are collected two times, in August 2017 (dry) and November 2019 (very dry). There
are 18 groundwater samples (5 from LC; 13 from UC) and 28 groundwater samples(10 from brackish-water aquaculture area; 18 from agriculture and settlement areas)collected in the first and second sampling periods.
The examination of the geological profile demonstrates that LC sedimentcontains thicker marine clay than UC sediment. Both LC and UC sediment comprisehigh exchangeable cations and cation exchange capacity on average. Thegroundwater quality analysis results show that the LC site has been contaminatedwith saline water and dissolved iron and manganese. In the meantime, although noindication of dissolved inorganic nitrogen, LC groundwater is potentially vulnerable
to elevated concentration of ammonium-nitrogen. Moreover, the contents ofchemicals parameters are generally higher in the very dry period than in the dryperiod. However, the characteristics of groundwater do not significantly change.
The evaluation from hydrochemistry characteristics and stable isotopes inwater (δ2H and δ18O) and sulfate (δ34S and δ18O) confirm a strong impact of saline
water in the LC region. The groundwater is characterized by dominant contents ofNa+ and Cl−, strong correlation between Na+ and Cl−, relatively high ratio of Mg2+/Ca2+and significantly low ratio of HCO3−/Cl−, and the ratios of Br− and Cl− in LC
groundwater indicates similar trend with seawater. Meanwhile, evidence ofgroundwater salinization in the UC area is not widely found. Furthermore, δ2H andδ18O values tell that both LC and UC groundwater are originally from rainwater.
However, the low values of δ2H and δ18O in LC region imply that groundwater
experiences saline water enrichment. In agreement with this finding, δ34S and δ18O
values indicate the seawater presence in LC groundwater. The high salinity possibly
arises from the brackish-water pond and marine clay.
An evaluation of As, Fe2+, and Mn2+ shows that groundwater in LC and UC
sites is naturally vulnerable to Fe2+ and Mn2+ contamination. As concentrations,meanwhile, are very low or under the threshold of drinking-water regulation. There
are two most critical environmental factors that are responsible for elevatedconcentrations of Fe2+ and Mn2+ in the study site: high salinity and low redox
environment. High salinity, in this case described by high Cl− and SO4−, is suspectedof supporting Fe2+ and Mn2+ leaching from minerals and soils through ion-exchange
processes. Then, the reduced environment, indicated by low ORP, and DO and NO3−–
N reduction, promotes the dissolution of Fe2+ and Mn2+ from soils and minerals togroundwater. In line with salinity conditions, the most vulnerable area with tracemetals contamination is recognized in the LC region.
Analysis of dissolved inorganic nitrogen analysis shows that ammoniumnitrogen (NH4+–N) is the predominant dissolved inorganic nitrogen species, compare
to nitrate-nitrogen (NO3−–N) and nitrite-nitrogen (NO2−–N) in almost groundwater
samples, particularly in LC site. The combined parameters of δ15NNH4, coliform
bacteria, land use, and geology successfully depict that the sources of NH4+–N in LC
and UC groundwater are different. The LC groundwater has a narrow range ofδ15NNH4 values over a wide range of NH4+–N concentrations. The δ15NNH4 values
indicate that NH4+–N potentially derived from mineralization of organic nitrogen toammonium. Furthermore, ammonium has a significantly positive relationship withsodium indicating the exchangeable ammonium is mobilized to groundwater via
cation exchange. On the contrary, UC groundwater has a wide range of δ15NNH4values over a narrow range of NH4+–N concentrations. The isotopic ratios of δ15NNH4
signify NH4+–N attenuation from several sources namely manure, mineral fertilizer,sewage, and pit latrines. Also, the presence of E. coli confirms the indication ofhuman and animal waste contamination. However, since ammonium has norelationship with sodium, cation exchange is not feasible and NH4+–N flows into thegroundwater from anthropogenic sources along with liquid wastes. The comparisonof NH4+–N concentrations and δ15NNH4 compositions between LC and UC
groundwater showed that the LC groundwater is potentially more vulnerable toNH4+–N contamination. Interestingly, the relatively high NH4+–N in LC
groundwater is from natural sources and attributable to high salinity.
From a drinking-water point of view, the LC groundwater is notrecommended to directly use as clean water. The concentrations of Cl−, SO42−, Na+,
Fe2+, and Mn2+ are significantly higher than the guidelines regulated by the World
Health organization and Indonesian Government. Therefore, Indonesian
government should consider groundwater resources carefully in developing
brackish-water aquaculture activity because the sustainability of this resource ispossibly problematic in the future.