# Spatial Variations in Microbial Compositions in a
## 2 Materials and Methods
### Study Area
#### 2.1.2 Hydrogeology  
Samcheok City has an area of 1184.8 km\({}^{2}\) and a population density of 60.3 persons/km\({}^{2}\)(Samcheok City, 2020). In Samcheok City, 7,418,510 m\({}^{3}\) of groundwater, from 2201 wells, is used annually. Of this amount, 3,076,220 m\({}^{3}\) (41.47%) is used for living, 2,903,970 m\({}^{3}\) (39.14%) for agriculture and fisheries, 1,427,860 m\({}^{3}\) (19.25%) for industrial use, and 10,460 m\({}^{3}\) is for other uses (0.14%) [7]. The annual mean groundwater recharge rate for the study area was calculated using the NRCS-CN (Natural Resources Conservation Service-Curve Number) method at 13.31%, and the groundwater development capacity is estimated at 148,024,000 m\({}^{3}\)/year, which is 10.15% of the total water resources (1,458,617,500 m\({}^{3}\)/year). Groundwater use per unit area is 6261.40 m\({}^{3}\)/year/km\({}^{2}\), and the density of groundwater facility development is 1.86 locations/km\({}^{2}\)[7]. The groundwater recharge area in Samcheok City is 891.05 km\({}^{2}\) (65.3%), the transit area is 290.83 km\({}^{2}\) (21.3%), and the discharge area is 183.30 km\({}^{2}\) (13.4%) [7].  
There are 13 local streams in the Samcheok area [7]. The mean transmissivity values per hydrogeological unit in the Samcheok area are 8.73 m\({}^{2}\)/day, 7.10 m\({}^{2}\)/day, 1.57 m\({}^{2}\)/day, 6.80 m\({}^{2}\)/day, 1.90 m\({}^{2}\)/day, 5.61 m\({}^{2}\)/day and 2.64 m\({}^{2}\)/day for unconsolidated clastic deposits, semi-consolidated clastic sedimentary rocks, intrusive igneous rocks, non-porous volcanic rocks, clastic sedimentary rocks, carbonate rocks and metamorphic rocks, respectively. The corresponding mean hydraulic conductivity values are 0.1890 m/day, 0.0982 m/day, 0.0442 m/day, 0.1011 m/day, 0.0276 m/day, 0.1487 m/day and 0.1054 m/day, respectively [7]. Among the seven hydrogeological units mentioned above, metamorphic rocks and carbonate rocks account for 65% of the total area.  
Figure 2: Geology of the study area.  
The rivers in the study area are characterized by abundant water flowing upstream of the Cheonggyecheon River; however, all of the water seeps into the ground in the vicinity of Hawolsan, leaving only gravel occupying the riverbed with no stream flow (Figure 3). The stream is therefore a dry valley in which water flows only when it rains. Approximately 1 km downstream, water starts to emerge gradually and overland flow resumes. However, the flow amount is relatively small compared to the amount of water that seeped into the ground upstream. In addition, the study area includes a limestone cave called Sohangul, located in Geundeok-myeon, Samcheok City, which discharges the largest amount of groundwater in Korea. The measured flow rate of groundwater discharged from Sohangul spring to the stream is 28,477.4 m\({}^{2}\)/day [7].  
From 2000 to 2019, the average annual precipitation for the period was 1302 mm. The maximum annual precipitation from 2000 to 2019 was 2000 mm in 2006, and the minimum was 900 mm in 2015 (Figure 4). In addition, precipitation has generally decreased over the years (see declining trendline). The precipitation data for the past two decades indicates that more than 60-70% of the total annual precipitation occurs in the wet season due to the monsoon weather, which is characteristic of the climate of the Korean Peninsula [37; 38]. The annual average air temperature has demonstrated a general increase from 2000 to 2019.