[A-HW22] 流域生態系における物質輸送と循環：源流から沿岸まで

[AHW22-11] Impact of brackish-water aquaculture on groundwater resources in a coastal alluvial aquifer

*Anna Fadliah Rusydi^1,5、Shin-ichi Onodera²、Mitsuyo Saito³、Seiichiro Ioka⁴、Rizka Maria^5,6 (1.Graduate School of Integrated Arts and Sciences, Hiroshima University, Japan、2.Graduate School of Advanced Science and Engineering, Hiroshima University, Japan、3.Graduate School of Environmental and Life Science, Okayama University, Japan、4.Institute of Regional Innovation, Hirosaki University, Japan、5.Research Center for Geotechnology, Indonesian Institute of Sciences, Indonesia、6.Faculty of Geological Engineering, University of Padjajaran, Indonesia)

キーワード：Salinity, Trace metals, Ammonium, Groundwater loss

Groundwater in coastal Southeast Asia is reported experiencing quality decrease because of human activities, e.g., urbanization and groundwater extraction. However, research on brackish-water aquaculture is a well-known Southeast Asia activity to be further investigated. Integrated physio-chemical and stable isotope analysis in one of the most extensive brackish-water aquacultures in Indonesia shows that this activity's impacts on groundwater conditions are no less severe than other economic activities. The comparison between two dominant land-uses in the study area shows that brackish-water aquaculture groundwater significantly has higher saline water content than agriculture groundwater. The Cl⁻ contents in brackish-water aquaculture groundwater range from 1,660 to 16,100 mg/L, which is averagely more than 10-fold higher than agriculture groundwater. The highest Cl⁻ content is observed in confined groundwater within the depth of 20–30 m. The hydrochemistry data and isotopes ratios of δ²H and δ¹⁸O in water confirm enrichment of seawater which is intentionally introduced to the fish pond. Our results also highlight a significant correlation (p-value < 0.1) of salinity vs trace metals, and salinity vs ammonium-nitrogen. Whilst high salinity is potentially categorized as “anthropogenic” contaminant; the trace metals are conceivably a “natural contaminant” that experience dissolution process supported by high salinity and reductive aquifer.

In respect to the reductive aquifer, the groundwater in both brackish-water aquaculture and agriculture sites NH₄⁺–N as predominant dissolved organic nitrogen, which is one indication of low-redox condition. However, brackish-water aquaculture groundwater has higher NH₄⁺–N concentrations, from 2.4 to 13.0 mg/L or about 7-fold higher than NH₄⁺–N concentrations in agriculture groundwater. The δ¹⁵N_NH4 values suggest mineralization of organic material in the sediments as the main source of NH₄⁺–N. Furthermore, NH₄⁺–N is significantly correlated with Na^+, which implies that mobilization of ammonium sediment to groundwater is promoted by cation exchange. Finally, nearly 2 x 10⁶ m³ confined groundwater in brackish-water aquaculture region is contaminated by Cl⁻ and trace metals contaminations, and vulnerable to elevated NH₄⁺–N contents.

This study is supported by The Asia-Pacific Network for Global Change Research (APN) under Grant No. CRRP2019-09 MY-Onodera (funder ID: http://dx.doi.org/10.13039/100005536).