Kinmen Island is a small, tectonically stable, granitic island that has been suffering from a scarcity of fresh water resources due to excessive annual evapotranspiration over annual precipitation. Recent studies further indicate that shallow (0-70 m) sedimentary aquifers, the major sources of groundwater supply, have already been over-exploited. Therefore, this preliminary study is to investigate the existence of exploitable water resources that can balance the shortage of fresh water on this island. Site characterization data are obtained from island-wide geophysical surveys as well as small-scale tests performed in a study area formed by three deep (maximum depth to 560 m) vertical boreholes installed in mid-east Kinmen northeast to Taiwu Mountain. Vertical fracture frequency data indicate that the rock body is fractured with a spatially correlated pattern, from which three major fracture zones (depths 0-70, 330-360, and below 450 m) can be identified. Geologic investigations indicate that the deepest fracture zone is caused by the large-scale, steeply dipping Taiwushan fault. This fault may have caused a laterally extensive low-resistivity zone, a potential fractured aquifer, near Taiwu Mountain. The middle fracture zone is induced by the Taiwushan fault and intersects the fault approximately 21 m southeast of the study area below a depth of 350 m. Slug testing results yield fracture transmissivity varying from 4.8 9 10 -7 to 2.2 9 10 -4 m 2 /s. Cross-hole tests have confirmed that hydraulic connectivity of the deeper rock body is controlled by the Taiwushan fault and the middle fracture zone. This connectivity may extend vertically to the sedimentary aquifers through high-angle joint sets. Despite the presence of a flow barrier formed by doleritic dike at about 300 m depth, the existence of fresh as well as meteoric water in the deeper rock body manifests that certain flow paths must exist through which the deeper fractured aquifers can be connected to the upper rock body. Therefore, groundwater stored within the Taiwushan fault and the associated low-resistivity zone can be considered as additional fresh water resources for future exploitation.
This study is to develop an isotopic catchment-effect index (CEI) connecting the physiographic characteristics of stream catchments. A CEI, describing the extent of difference in stable water isotopic compositions (δ values) between stream water and local precipitation at any given sampling site, can help in judging whether water resource management should be focused on upstream regions of streams or local hydrology issues. To establish the isotopic CEI, this study measured δ values of stream water and derived δ O of local precipitation based on regional isotopic altitude gradient at montane catchments of various sizes. Results indicate that the CEI is strongly related to catchment physiographic characteristics, such as length of main stream, mean area, mean elevation, perimeter, and slope. These characteristics are considered important indices of streamflow. Based on mathematical regression modeling describing the relationships between CEI and respective physiographic factors, CEI values can predict respective physiographic factors and vice versa. Moreover, according to the multiple equations derived in this study, catchments of larger size and steeper slope give elevated CEI values while greater stream length reduces the CEI's value. A greater CEI value indicates that local stream water is principally sourced from upstream reaches rather than contributions from local precipitation. In addition, CEI values are greater in winter than in summer resulting from monsoon effect. Consequently, this study establishes CEI as a useful descriptor of the physiographic characteristics of catchments.
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