Groundwater flow paths and groundwater ages at a radioactive waste repository located in a coastal area of South Korea were evaluated using the hydrochemical and hydrogeological characteristics of groundwater, surface water, rain water, and seawater, as well as by numerical modeling. The average gro...
Groundwater flow paths and groundwater ages at a radioactive waste repository located in a coastal area of South Korea were evaluated using the hydrochemical and hydrogeological characteristics of groundwater, surface water, rain water, and seawater, as well as by numerical modeling. The average groundwater travel time in the top layer of the model, evaluated by numerical modeling and groundwater age (34 years), approximately corresponds to the groundwater age obtained by chlorofluorocarbon (CFC)-12 analysis (26-34 years). The data suggest that the groundwater in wells in the study area originated up-gradient at distances of 140-230 m. Results of CFC analyses, along with seasonal variations in the δ18O and δD values of groundwater and the relationships between 222Rn concentrations and δ18O values and between 222Rn concentrations and δD values, indicate that groundwater recharge occurs in the summer rainy season and discharge occurs in the winter dry season. Additionally, a linear relationship between dissolved SiO2 concentrations and groundwater ages indicates that natural mineralization is affected by the dilution of groundwater recharge in the rainy summer season.
Groundwater flow paths and groundwater ages at a radioactive waste repository located in a coastal area of South Korea were evaluated using the hydrochemical and hydrogeological characteristics of groundwater, surface water, rain water, and seawater, as well as by numerical modeling. The average groundwater travel time in the top layer of the model, evaluated by numerical modeling and groundwater age (34 years), approximately corresponds to the groundwater age obtained by chlorofluorocarbon (CFC)-12 analysis (26-34 years). The data suggest that the groundwater in wells in the study area originated up-gradient at distances of 140-230 m. Results of CFC analyses, along with seasonal variations in the δ18O and δD values of groundwater and the relationships between 222Rn concentrations and δ18O values and between 222Rn concentrations and δD values, indicate that groundwater recharge occurs in the summer rainy season and discharge occurs in the winter dry season. Additionally, a linear relationship between dissolved SiO2 concentrations and groundwater ages indicates that natural mineralization is affected by the dilution of groundwater recharge in the rainy summer season.
Numerical modeling was conducted to evaluate the distribution of head, groundwater flow paths, and groundwater travel times in the study area, and to solve for the concentrations of solute in groundwater. The commonly used groundwater model MODFLOW (McDonald and Harbaugh, 1988) was incorporated into the Visual MODFLOW package or Groundwater Modeling System (GMS) software.
The distribution of hydraulic conductivity in bedrock was determined by a kriging technique, using an exponential semivariogram. Using this approach, the hydrogeological model was conceptualized in terms of water infiltration in the hydraulic soil domain (HSD), primary groundwater flow through pervious fractures in bedrock (HCD), slow groundwater flow through minor fractures in bedrock (HRD), and discharge to streams and to the sea (Fig. 1).
이론/모형
3). Average recharge values of groundwater, calculated using the groundwater level fluctuation method, ranged from 194 mm/a at HSD to 237 mm/a at HCD.
Concentrations of 222Rn in groundwater and seawater were measured using a radon detector, according to the method of Lee and Kim (2006). Water samples were collected in 1 L bottles for groundwater analysis and in 4 L bottles for seawater analyses, taking care to prevent leakage of 222Rn from the bottles and the generation of air bubbles.
Calibration of the model was performed by matching observed groundwater levels at monitoring wells (DB1-2, DB1-6, SS-5, RB-1, IJ, and KB-2) and simulated levels in the steady state condition. During model calibration, the boundary conditions inside the model area, hydraulic conductivity, and groundwater recharge were adjusted from their initial values so as to reach the error limits of actual values, using a trial-and-error method. The calibration between observed and calculated groundwater levels resulted in a simulation that matched observations at a 95% confidence level, represented by a correlation coefficient of 0.
MODFLOW can treat anisotropic and heterogeneous 3-D flows with constant density, using a finite difference method. The Visual MODFLOW package (version 4.2 by Schlumberger) was used to simulate groundwater flow paths and groundwater ages in the study area, assuming assumed piston flow (no dispersion).
성능/효과
However, the CFC-12 analysis indicated a delay of ca. 1 month for groundwater recharge by rain water.
Groundwater ages determined from five samples and using CFC-12, (more reliable than CFC-11 and CFC-113 for determining groundwater age) were 26–34 years. Surface water ages were determined to be 23–32 years (slightly younger than those of groundwater). Moreover, groundwater age showed significant temporal variations.
According to the CFC-12 analysis, groundwater ages were ca. 26–34 years and surface water ages were ca. 23–32 years.
According to the CFC-12 analysis, groundwater ages were ca. 26–34 years and surface water ages were ca.
The backward particle tracking of the numerical modeling indicated that the average groundwater travel time was in the range of 34 years (layer 1) to 3599 years (layer 6). Based on both groundwater travel times determined by backward particle tracking and groundwater ages determined by CFC-12 analysis, we concluded that in layer 1, groundwater ages in wells approximately matched groundwater travel times, and that groundwater in the study area was recharged from an estimated distance of 140-230 m up-gradient from the wells, given that water depths in the wells are 100-232 m.
Groundwater travel times using forward particle tracking were estimated from the half depths of the six layers at four imaginary wells along a ~20-m land surface interval, with an average lateral distance between imaginary wells of ~399 m (Fig. 11).
Recharge in the rainy summer season and discharge in the dry winter season were also indicated by a reasonably good linear relationship between oxygen and hydrogen isotope values, and between Rn-222 concentrations and isotopic compositions of groundwater. However, the CFC-12 analysis indicated a delay of ca. 1 month for groundwater recharge by rain water.
These groundwater types indicate the influence of rain water on groundwater in elevated land areas and the influence of seawater on groundwater in intermediate and lowland areas. However, the trends in chemical concentrations and EC values are indistinct in the higher-elevation lands and lowlands, possibly reflecting the variable chemical composition of groundwater obtained from wells of variable depths (100–232 m; Fig. 4).
It is thought that tunneling in the repository area during construction of National Road 31 disturbed the natural groundwater flow pattern. Lighter δ18O and δD values of groundwater in the rainy summer season suggest recharge from rain water.
It is thought that tunneling in the repository area during construction of National Road 31 disturbed the natural groundwater flow pattern. Lighter δ18O and δD values of groundwater in the rainy summer season suggest recharge from rain water. Recharge in the rainy summer season and discharge in the dry winter season were also indicated by a reasonably good linear relationship between oxygen and hydrogen isotope values, and between Rn-222 concentrations and isotopic compositions of groundwater.
Lighter δ18O and δD values of groundwater in the rainy summer season suggest recharge from rain water. Recharge in the rainy summer season and discharge in the dry winter season were also indicated by a reasonably good linear relationship between oxygen and hydrogen isotope values, and between Rn-222 concentrations and isotopic compositions of groundwater. However, the CFC-12 analysis indicated a delay of ca.
During the early stage of construction of the underground radioactive waste facility near the coastal city of Gyeongju, South Korea, groundwater flow paths and ages were determined from the hydrochemical and hydrogeological characteristics of groundwater, surface water, rain water, and seawater, as well as by numerical modeling. The backward particle tracking of the numerical modeling indicated that the average groundwater travel time was in the range of 34 years (layer 1) to 3599 years (layer 6). Based on both groundwater travel times determined by backward particle tracking and groundwater ages determined by CFC-12 analysis, we concluded that in layer 1, groundwater ages in wells approximately matched groundwater travel times, and that groundwater in the study area was recharged from an estimated distance of 140-230 m up-gradient from the wells, given that water depths in the wells are 100-232 m.
The variable groundwater ages determined by the CFC-12 analysis may represent the mixing of groundwater originating along different flow paths and at different depths, as well as of variable ages. The mixing of groundwater is supported by trends in chemical concentrations and EC values of groundwater, as distinct trends from higher to lower elevations are not observed.
23–32 years. The variable groundwater ages determined by the CFC-12 analysis may represent the mixing of groundwater originating along different flow paths and at different depths, as well as of variable ages. The mixing of groundwater is supported by trends in chemical concentrations and EC values of groundwater, as distinct trends from higher to lower elevations are not observed.
7‰), respectively. The δ18O and δD values of soil water were in the range of –9.67‰ to –6.86‰ (average, –8.37‰) and –72.0‰ to –50.3‰ (average, –60.7‰), respectively, and the δ18O and δD values of groundwater were in the range of –8.05‰ to –7.00‰ (average, –7.47‰) and –56.6‰ to –44.6‰ (average, –50.6‰), respectively. Hence, average values for groundwater are heaviest, followed by average values for soil water, surface water, and rain water.
11). Through the forward particle tracking, average groundwater travel times from the half-depth of each layer to layer 1 were estimated at 33, 66, 199, 266, 1278, and 8520 years for layers 1–6, respectively (Fig. 12; Table 5).
후속연구
The present results will contribute to the safety of radioactive waste disposal by providing insights into groundwater flow systems and groundwater ages affecting waste repository sites.
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