Assessing Anthropogenic and Natural Contributions to Groundwater Salinization in the Coastal Area of Central Taiwan Using Transition Metal Signatures

Abstract

Taiwan's steep terrain and uneven precipitation patterns result in a strong dependence on groundwater resources, which play a critical role across various sectors of water use. In the coastal region of central Taiwan, particularly in the southern portion of the Jhuoshuei River alluvial fan, shallow aquifers are experiencing significant salinization. Sampling locations for this study were selected based on statistical analyses of historical groundwater quality data collected between 2015 and 2019. Multiple sets of groundwater samples were collected and analyzed to investigate their hydrochemical characteristics and seasonal variations. The results reveal a general trend of decreasing salinity with increasing depth. However, the elevated salinity in certain samples imposes limitations on conventional water quality analyses. Furthermore, basic hydrochemical parameters are insufficient to trace the origins of salinity, as they primarily reflect the presence rather than the source of saline intrusion. To address this limitation, further treatment of the samples was undertaken to differentiate between natural (e.g., lateral seawater intrusion) and anthropogenic (e.g., fish farming) sources of salinity. In this context, Chelex-100 resin was employed to purify and concentrate high-salinity samples, enabling the quantification of transition metal concentrations. As transition metals are typically present at low concentrations in seawater but are enriched in effluents from aquaculture activities, their levels in groundwater can serve as indicators of salinity sources. Preliminary results indicate that wells located near Taixi exhibit elevated concentrations of trace elements, particularly lead (Pb), suggesting the influence of anthropogenic inputs. In contrast, wells near Yiwu and Qiongpu display relatively low concentrations of trace elements, implicating lateral seawater intrusion as the dominant salinization mechanism in these areas. These findings highlight the utility of trace metal analysis in distinguishing between different salinity sources in coastal aquifers.