Estimating the groundwater recharge rate is essential in all groundwater-related fields, including groundwater development, use, management, modeling, and contamination analysis. In this study, we proposed a combined method of water budget and climate change scenario for estimating the net groundwater recharge rate in the Nakdong River watershed (NRW), South Korea. For the climate change scenario method, the representative concentration pathway (RCP) 4.5 and 8.5 climate scenarios were adopted. First, using the water budget method from 2009 to 2018, the net groundwater recharge rate (NGRR) of 12.15–18.10% relative to annual precipitation (AP) was obtained, subtracting direct runoff (DR) of 21.18–25.32% relative to AP, evapotranspiration (EP) of 40.53–52.29% relative to AP, and baseflow of 12.42–17.84% relative to AP, from the AP (865–1494 mm). The average annual NGRR of the NRW was 200 mm (15.59%). Second, the mean NGRRs from 2009 to 2100 under the RCP 4.5 and RCP 8.5 scenarios were anticipated as 8.73% and 7.63%, respectively. The similarity between the water budget and climate change scenarios was confirmed using data from 2009 and 2018. According to the simple climate change scenario, it is predicted that annual precipitation will increase over the years while the groundwater level and net groundwater recharge rate will decrease. Nonetheless, the estimated NGRR by the water budget method in this study possesses uncertainty due to using potential ET instead of actual ET which should be estimated by considering soil water content.

An effective method, involving time and frequency domains was developed to interpret seismic precursors by comparing groundwater-level fluctuations recorded immediately and long before the occurrence of a known earthquake. The proposed method, consisting of the pre-processing (3-point filtering, band-pass filtering, and spectrum analysis) and post-processing (weighted moving average method and histogram and spectrum analyses) stages, was applied to the groundwater-level time series measured at three monitoring wells on Jeju Island, South Korea, from 00:00 on 8 September 2016 to 00:00 on 22 September 2016. The Gyeongju earthquake (Mw 5.4) occurred at 20:32 on 12 September2016. The histogram analysis exhibited an accentuating bellshape as the total number of waveforms, including those caused by the earthquake, of the groundwater-level fluctuations increased. The weighted moving average analysis indicated that various abnormal waveforms with different periods occurred in the fluctuations approaching the occurrence of the earthquake. The periods of seismic precursors in the groundwater-level fluctuations were determined by spectrum analysis and varied among the monitoring wells. Seismic precursor responses attributable to the Gyeongju earthquake were identified at least 8 hours before the earthquake, and the method used in this study indicates its good potential to predict an impending earthquake.

Approximately 40,000 chemical products are currently used in Korea; these products can contaminate the groundwater/soil, the surrounding environment, and organisms for extended periods of time. In this study, a hydrological field survey, a water quality analysis, and groundwater modeling were performed to identify the source and transport path of pollution that was caused by inorganic matter and artificial sweeteners, especially acesulfame, in the groundwater of an agricultural area in Chungnam Province, Korea. In the study area, a higher concentration of acesulfame displayed a spatial distribution similar to nitrate-nitrogen concentration. The characteristics of the groundwater flow and the distribution of the acesulfame were simulated using the Visual MODFLOW Classic Interface ver. 2014.1 and the MT3DMS module, respectively. The modeled area was divided into hilly (southern), residential (northwest), and agricultural (northeast) zones. The stream’s boundary was set to be the drainage channel in the southern hilly zone. From the simulation, we found that acesulfame spread actively from the source for 1–3 years before it reached equilibrium in the northern part of the model domain (the area downstream of the stream’s boundary). The concentration of acesulfame in the agricultural zone of the model domain decreased after five years, and it reached the steam boundary and residential zone within 10 years. After 10 years, most of the acesulfame was discharged from the agricultural zone and the hilly zone, while the concentration in the residential zone was approximately the same. Acesulfame is considered to be a potential indicator of man-made contamination for use in the management of groundwater quality.

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