Korea has been operating groundwater monitoring systems since 1996 as the Groundwater Act enacted in 1994 enforces nationwide monitoring. Currently, there are six main groundwater monitoring networks operated by different government ministries with different purposes: National Groundwater Monitoring Network (NGMN), Groundwater Quality Monitoring Network (GQMN), Seawater Intrusion Monitoring Network (SIMN), Rural Groundwater Monitoring Network (RGMN), Subsidiary Groundwater Monitoring Network (SGMN), and Drinking Water Monitoring Network (DWMN). The Networks have a total of over 3500 monitoring wells and the majority of them are now equipped with automatic data loggers and remote terminal units. Most of the monitoring data are available to the public through internet websites. These Networks have provided scientific data for designing groundwater management plans and contributed to securing the groundwater resource particularly for recent prolonged drought seasons. Each Network, however, utilizes its own well-specifications, probes, and telecommunication protocols with minimal communication with other Networks, and thus duplicate installations of monitoring wells are not uncommon among different Networks. This mini-review introduces the current regulations and the Groundwater Monitoring Networks operated in Korea and provides some suggestions to improve the sustainability of the current groundwater monitoring system in Korea.

Classical molecular dynamics (MD) simulations have been performed to investigate the effects of substitutions in the octahedral sheet (Mg for Al) and layer charge on an atomistic model of the montmorillonite edge. The edge models considered substitutions in both the solvent accessible and inaccessible octahedral positions of the edge bond chain for a representative edge surface. The MD simulations based on CLAYFF, a fully-flexible forcefield widely used in the MD simulations of bulk clay minerals, predicted Mg–O bond distances at the edge and in bulk that agreed with those of the density functional theory (DFT) geometry optimizations and available experimental data. The DFT results for the edge surfaces indicated that substitutions in the solvent inaccessible positions of the edge bond chain are energetically favorable and an increase in layer charge and local substitution density coincided with the occurrence of five-coordinate, square pyramidal Mg and Al edge structures. Both computational methods predicted these square pyramidal structures, which are stabilized by water bridging H-bonds between the unsaturated bridging oxygen [(Al or Mg)–O–Si] and other surface O atoms. The MD simulations predict that the presence of Mg substitutions in the edge bond chain results in increased disorder of the edge Al polyhedra relative to the unsubstituted edge. In addition to the square pyramidal Al, these disordered structures include trigonal bipyramidal and tetrahedral Al at the edge and inverted Si tetrahedra. These simulation results represent the first test of the fully-flexible CLAYFF forcefield for classical MD simulations of the Na-monmorillonite edge and demonstrate the potential of combined classical MD simulations and DFT geometry-optimizations to elucidate the edge structure of 2:1 phyllosilicate minerals.

Naturally occurring mackinawite (tetragonal FeS) with incorporated transition metals is an important precursor to the formation of metal sulfides in ore deposits, but experimental results have not been sufficient to establish clear trends in the structure and stability of the transition-metal-enriched mineral. Using density functional theory with dispersion corrections, we report the first systematic examination of the relationship between composition and structure for FeS incorporating bivalent transition metals. Our method was validated by successful calculations of the structures of FeS, FeSe, FeSe_1−xS_x, Fe_1−x Me_xSe (Me = Co, Ni, Cu), and FeNi_xTe. Two classes of transition-metal-incorporated FeS structures then were investigated: Fe_1−xMe_xS (metal-substituted FeS) and FeMe_xS (FeS intercalated by a metal at either a tetrahedral or square-pyramidal interstitial site), where Me = Co, Ni, and Cu, and x ≤ 0.25. We find that incorporated transition metals can both increase and decrease lattice parameters, depending on the metal and how it is incorporated into the FeS structure. As the mole fraction of substituting metal increases, the FeS unit-cell volume decreases for Co, is nearly constant for Ni, but increases for Cu. Metal substitution changes the c lattice parameter, which is sensitive to interactions between the mackinawite sheets, as much as it does the a and b lattice parameters. Upon intercalation, the unit-cell volume and c parameter increase but the a and b parameters decrease. Experimental structural data are consistent with our results for metal-substituted FeS. We determined the thermodynamic stability of metal-incorporated FeS by computing the free energy involved in the metal incorporation reactions as a function of chemical potential of sulfur. The thermodynamic results lead to the general conclusions that metal incorporation into mackinawite most likely occurs via substitution, which may be important to influence phase transformation pathways of mackinawite.

The edges of montmorillonite (MMT) react strongly with metals and organic matter, but the atomic structure of the edge and its surface complexes are not unambiguous since the experimental isolation of the edge is challenging. In this study, we introduce an atomistic model of a Na MMT edge that is suitable for classical molecular dynamics (MD) simulations, in particular for the B edge, a representative edge surface of 2:1 phyllosilicates. Our model possesses the surface groups identified through density functional theory (DFT) geometry optimizations performed with variation in the structural charge deficit and Mg substitution sites. The edge structure of the classical MD simulations agreed well with previous DFT-based MD simulation results. Our MD simulations revealed an extensive H-bond network stabilizing the Na-MMT edge surface, which required an extensive simulation trajectory. Some Na counter ions formed inner-sphere complexes at two edge sites. The stronger edge site coincided with the exposed vacancy in the dioctahedral sheet; a weaker site was associated with the cleaved hexagonal cavity of the tetrahedral sheet. The six-coordinate Na complexes were not directly associated with the Mg edge site. Our simulations have demonstrated the heterogeneous surface structures, the distribution of edge surface groups, and the reactivity of the MMT edge.