# Characteristics of Deep Groundwater Flow and Temperature in the Tertiary Pohang Area, South Korea
## 6 Conclusions  
The lithological and geophysical logs of the four deep boreholes could identify the deep groundwater flows in the Pohang area that experienced the youngest tectonic movements in the Korean peninsula. The groundwater flow directions and \(\beta\) values of the boreholes were estimated by using a 1D steady-state heat transfer model. The geological stratigraphy of the four boreholes primarily contained Tertiary semiconsolidated mudstone, dacite, tuffs (crystal tuff, lapilli tuff, and tuffaceous sediment), basic dyke, sandstone/mudstone, andesitic breccia, and granodiorite. BH-1, BH-2, and BH-3 had similar stratigraphy overall, and no rhyolite was found in BH-4 at depths greater than 600 m. The semiconsolidated mudstone was located at a greater depth in BH-1 and BH-2 than in BH-3 and BH-4.  
In the interpretation of the groundwater flow, the fractures and faults induced nonlinear temperature changes and upward and downward flows. Moreover, several aquifer zones were identified at depths of 350-410, 510-590, 600-650, 730-760, 870-890, 950-1000, and 1150-1360 m, as well as hydraulically active fractures intersecting the deep boreholes. In BH-1, upward flows occurred at four depths, at 358-410, 488-514, 540-566, and 890-998 m, whereas a downward flow occurred at 586-646 m. Similarly, in BH-2, upward flows occurred at four depths, at 618-652, 736-757, 1178-1189, and 1287-1356 m, whereas a downward flow occurred at 580-610 and 955-976 m. In BH-3, an upward flow occurred at 138-381 m, with a linear temperature change. In BH-4, an upward flow occurred at 567-600 m, whereas downward flows occurred at 303-350, 394-480, and 873-967 m.  
In conclusion, the upward and downward flows were identified by solving the 1D steady-state heat transfer equation, in combination with the lithological log and geophysical logs. However, owing to the loss of the core recovery, slight discrepancies were observed between the geophysical logs and lithology log. The groundwater flow occurred mostly along the fault and fracture zones in the rhyolite, basic dyke, and sandstone/mudstone. In addition, by using the lithological log and geophysical logs, several aquifer zones and hydraulically active fractures intersecting the deep boreholes were identified.  
Upward or downward flows based on the 1D steady-state heat transfer model can be imprecisely identified, owing to the deviations in the centering of the loggers in the boreholes. There also exists uncertainty regarding groundwater flow along depth, since more complex groundwater flow patterns can be detected through detailed investigations of fractures and faults, and these could not be considered in the deep boreholes. Despite the uncertainty, analyses combining geophysical logs and a lithological log, as well as a 1D steady-state heat transfer model, can contribute in understanding the deep fractures/fault distributions/thermal and flow characteristics of boreholes in relation to hydrogeological activities, through the analysis of the deep fractures/faults and geothermal potential around the study area.  
Conceptualization, W.-R.L. and S.-Y.H.; methodology, S.-Y.H. and W.-R.L.; validation, W.-R.L., S.-Y.H., C.L., S.H., I.-H.P., and H.-C.K.; investigation, W.-R.L., C.L., S.H., I.-H.P., and H.-C.K.; data curation, W.-R.L., C.L., S.H., I.-H.P., and H.-C.K.; writing--original draft preparation, W.-R.L. and S.-Y.H.; writing--review and editing, S.-Y.H., W.-R.L., C.L., S.H., I.-H.P., and H.-C.K.; visualization, W.-R.L.; supervision, S.-Y.H.; project administration, S.-Y.H.; and funding acquisition, S.-Y.H. and C.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by grants from the Principal Research Fund of the Korea Institute of Geoscience and Mineral Resources (KIGAM), grant number 16-3411, and Basic Science Research Program