Alternative water resources in granitic rock: a case study from Kinmen Island, Taiwan

Abstract

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.

Generic workflow of groundwater flow simulations in geologically complicated domains with millions of unstructured grids

“…It assumes that hydrogeological parameters in Huayu's regolith are the same as those in the regolith of Kinman, a granitic island 210 𝑛𝑛𝑚𝑚 to the west of Taiwan (Liou et al 2010). Note that this paper does not investigate the effects of heterogeneity and anisotropy on the resulting groundwater flow field.…”

“…The density of DDAXX is set as 2,650 𝑛𝑛𝑔𝑔/𝑚𝑚 3 (del Potro et al 2013). A low net infiltration rate of 35 mm/year is assumed because Taiwan's offshore islands generally have higher evapotranspiration than precipitation (Liou et al 2010). Detailed hydrogeologic parameters of the eight rock units are listed in Table 1.…”

Generic workflow of groundwater flow simulations in geologically complicated domains with millions of unstructured grids

“…Note that the fifty geologic structures are still considered in the final HCM, but each rock unit contains at least one geologic structure; for example, DR2XX represents the 17 DR2s below the regolith. It assumes that hydrogeological parameters in Huayu's regolith are the same as those in the regolith of Kinman, a granitic island 210 km to the west of Taiwan (Liou et al 2010). Note that this paper does not investigate the effects of heterogeneity and anisotropy on the resulting groundwater flow field.…”

“…The density of DDAXX is set as 2650 kg/m 3 (del Potro et al 2013). A low net infiltration rate of 35 mm/year is assumed because Taiwan's offshore islands generally have higher evapotranspiration than precipitation (Liou et al 2010). Detailed hydrogeologic parameters of the eight rock units are listed in Table 1.…”

A fracture study by using bipole–bipole cross-well logging

“…The rock matrix is mainly composed of gneiss, gabbro, granite, and basalt. For detailed geological and hydrogeological information regarding Kinmen please refer to Liou et al (2009). The OYO Profiler-4, which is incorporated with the electrode switching function for 32 electrodes and 4-channel simultaneous measurement, was used.…”