Geochemistry of ground and surface waters in Pocheon area, South Korea – mixing and inverse modeling VeselinaValkov PhD student Environmental and Conservation Science Program Civil Engineering Department NDSU Geol 628 Geochemistry 2010
Study area the Pocheon area is located about 50 km norteast of Seoul, South Korea. the geothermal gradient in the Pocheon area (37.7°- 45.7°C km-1) is higher than the average value (25.3 8C km-1) in the southern part of the Korean Peninsula Several mostly >500 m deep boreholes have been drilled in the central parts of the area in order to exploit thermal groundwater (outflowing temperature 25-44 °C). The geology of the study area mainly consists of Precambrian gneiss and Jurassic granite Quartz, feldspar (average Ab0.85An0.15) and biotite are the predominant minerals in the granite. Calcite and minor amounts of secondary silicates such as montmorillonite commonly occur in the fractures, which act as conduits for groundwater flow in the area.
Sampling and analysis For this study, thirty water samples (4 surface waters, 15 shallow groundwaters, and 11 deep groundwaters) were collected in 1998 (Fig. 2). At some sites, samples were collected seasonally. Hydrogeochemical and environmental isotope data for groundwater in the study area suggested the occurrence of two distinct water types, a Ca-HCO3 type and a Na-HCO3 type. The former water type is characterized by relatively high concentrations of Ca, SO4 and NO3. • surface water (PCs-2, 3) and most of the shallow groundwater (PCw-1, 3, 4, 5, 7) belong to the Ca-HCO3 type with relatively low pH = 5.4–7.2. • all the deep groundwaters (spa waters; PCd-1, 1–2, 2, 3) and some of the shallow groundwaters(PCw-2, 6) belong to the Na-HCO3 type with higher pH = 7.0–8.8.
Some results Based on Q-mode factor analysis is assumed • a dominant reaction forming Na-HCO3 type waters was the hydrolysis of Na-silicates such as sodic plagioclase. • For Ca-HCO3 type waters suggested that their hydrochemistry was controlled by calcite dissolution (for Ca and HCO3) and near surface condition resulting in NO3 contamination and lower temperatures. • Considering the geologic and hydrogeologic conditions of the study area, Na-HCO3 type waters possibly have evolved from Ca-HCO3 type waters • In the Piper diagram, PCw-2 and PCw-6 waters plotted between Ca-HCO3 type and Na-HCO3 type waters , suggesting a mixture of Ca-HCO3 type and Na-HCO3 type groundwaters.
. The results of Q-mode factor analysis indicate that there are three end members of water in the Pocheon area and relevant mixing ratios: 1. PCs-2, a typical surface water, • PCw-1, a typical shallow well groundwater • PCd-1, a typical spa water at depth.
pH-Eh diagram shows that the most of the ground and surface waters are inside the hatched-line loop and in the stability limits of water. Just one of the shallow groundwater is out of the range. Ground waters usually tend to be moderately reduced, because they are not in contact with atmospheric oxygen. The surface waters are out of the groundwater loop on the transitional environment. Fig 4 Range of Eh-pH conditions in natural environments based on data of Baas-Becking et al. (1960).
Mixing simulationMix model is used if two or more aqueous solutions are to be mixed together. Each solution is multiplied by its mixing fraction and a new solution is calculated by summing over all of the fractional solutions. The PCw-1 shallow and PCd-1 deep ground waters are mixed in fractions 0.15 and 0.85 respectively.
Inverse modeling • Inverse modeling is used to reduce the geochemical reactions that account for the change in chemical composition of water along a flow path. At least two chemical analyses of water at different points along the flow path are needed, as well as a set of phases that are potentially reactive along the flow path. From the analyses and phases, mole-balance models are calculated. • Inverse modeling attempts to determine sets of mole transfers of phases that account for changes in water chemistry between one or a mixture of initial water compositions and a final water composition.
Inverse modeling. Pathway from is Na-HCO3 (30 m) shallow groundwater goes into the Ca-HCO3 (80m) type shallow groundwater at depth. Phases – Albite, Halite, Gypsum, Kaolinite, Ca-montmorillonite, Calcite, K-feldspar and Gibbsite. All values are in mol/kgw Phase mole transfers Halite NaCl -2.534e-003 Gibbsite Al(OH)3 -1.544e-003 Na-HCO3 (30m) Ca-HCO3 (80m) Halite -2.534e-003 NaCl Kaolinite -2.056e-003 Al2Si2O5(OH)4 Ca-Montmorillon 1.120e-003 Ca0.165Al2.33Si3.67O10(OH)2 Albite 9.262e-004 NaAlSi3O8 Halite -2.534e-003 NaCl Kaolinite -1.389e-003 Al2Si2O5(OH)4 Albite 2.335e-003 NaAlSi3O8 Halite -2.629e-003 NaCl Ca-Montmorillon -1.908e-003 Ca0.165Al2.33Si3.67O10(OH)2
Inverse modeling. Pathway is from Ca-HCO3 (depth 80 m) shallow groundwater to Na-HCO3 (depth 500 m) type deep groundwater. Phases – Albite, Halite, Gypsum, Kaolinite, Ca-montmorillonite, Calcite, K-feldspar and Gibbsite. All values are in mol/kgw Phase mole transfers Gibbsite Al(OH)3 1.593e-003 Calcite 2.338e-004 CaCO3l Kaolinite 1.961e-003 Al2Si2O5(OH)4 Ca-Montmorillon -1.069e-003 Ca0.165Al2.33Si3.67O10(OH)2 Ca-HCO3 (30m) Na-HCO3 (500m) Halite 2.319e-004 NaCl Kaolinite 2.107e-003 Al2Si2O5(OH)4 Ca-Montmorillon -1.148e-003 Ca0.165Al2.33Si3.67O10(OH)2 Albite 2.319e-004 NaAlSi3O8 Kaolinite 2.345e-003 Al2Si2O5(OH)4 Ca-Montmorillon -1.463e-003 Ca0.165Al2.33Si3.67O10(OH)2