Ryota Fujinaga’s research while affiliated with Hokkaido University and other places

Water-rock interaction and groundwater mixing are important phenomena in understanding hydrogeological systems and the stability of rock slopes especially those consisting largely of moderately water-soluble minerals like calcite. In this study, the hydrogeological and geochemical evolutions of groundwater in a limestone quarry composed of three strata: limestone layer (covering), interbedded layer under the covering layer, and slaty greenstone layer (basement) were investigated. Water-rock interaction in the open-pit limestone quarry was evaluated using PHREEQC, while hierarchical cluster analysis (HCA) and principal component analysis (PCA) were used to classify and identify water sources responsible for possible groundwater mixing within rock layers. In addition, Geochemist's Workbench was applied to estimate the mixing fractions to clarify sensitive zones that may affect rock slope stability. The results showed that the changes in Ca²⁺ and HCO3⁻ concentrations of several groundwater samples along the interbedded layer could be attributed to mixing groundwater from the limestone layer and that from slaty greenstone layer. Based on the HCA and PCA results, groundwaters were classified into several types depending on their origin: (1) groundwater from the limestone layer (LO), (2) mixed groundwater flowing along the interbedded layer (e.g., groundwater samples L-7, L-11, S-3 and S-4), and (3) groundwater originating from the slaty greenstone layer (SO). The mixing fractions of 41% LO: 59% SO, 64% LO: 36% SO, 43% LO: 57% SO and 25% LO: 75% SO on the normal days corresponded to groundwaters L-7, L-11, S-3 and S-4, respectively, while the mixing fractions of groundwaters L-7 and L-11 (61% LO: 39% SO and 93% LO: 7% SO, respectively) on rainy days became the majority of groundwater originating from the limestone layer. These indicate that groundwater along the interbedded layer significantly affected the stability of rock slopes by enlarging multi-breaking zones in the layer through calcite dissolution and inducing high water pressure, tension cracks and potential sliding plane along this layer particularly during intense rainfall episodes.

Groundwater flow and its geochemical evolution in mines are important not only in the study of contaminant migration but also in the effective planning of excavation. The effects of groundwater on the stability of rock slopes and other mine constructions especially in limestone quarries are crucial because calcite, the major mineral component of limestone, is moderately soluble in water. In this study, evolution of groundwater in a limestone quarry located in Chichibu city was monitored to understand the geochemical processes occurring within the rock strata of the quarry and changes in the chemistry of groundwater, which suggests zones of deformations that may affect the stability of rock slopes. There are three distinct geological formations in the quarry: limestone layer, interbedded layer of limestone and slaty greenstone, and slaty greenstone layer as basement rock. Although the hydrochemical facies of all groundwater samples were Ca-HCO3 type water, changes in the geochemical properties of groundwater from the three geological formations were observed. In particular, significant changes in the chemical properties of several groundwater samples along the interbedded layer were observed, which could be attributed to the mixing of groundwater from the limestone and slaty greenstone layers. On the rainy day, the concentrations of Ca²⁺ and HCO3⁻ in the groundwater fluctuated notably, and the groundwater flowing along the interbedded layer was dominated by groundwater from the limestone layer. These suggest that groundwater along the interbedded layer may affect the stability of rock slopes.

Geochemical evolution of groundwater and groundwater mixing are useful in understanding the hydrogeologic systems especially for the moderately dissolved mineral like calcite and the stability of rock slopes. In this study, a quarry composed of limestone layer (covering), interbedded layer under the covering layer, and slaty greenstone layer (basement) was selected. The geochemical evolution of groundwater in the open-pit limestone quarry was evaluated using PHREEQC and possible mixing between two different groundwaters was applied by Geochemist’s Workbench for identifying sensitive zones affecting rock slopes. The results showed that the geochemical trend of groundwater was distinguished by the layer. The changes in Ca2+ and HCO3 concentrations of several groundwater samples along the interbedded layer resulted from the mixing between groundwater from the limestone and slaty greenstone layers. The mixing fractions were significantly high and became the majority of groundwater flowing through the limestone layer on the rainy day. These indicate that groundwater along the interbedded layer may affect the stability of rock slopes by enlarging multi-breaking zones in the layer through calcite dissolution and generating high water pressure, tension cracks and potential sliding plane along this layer particularly when the groundwater is significantly mixed between the two layers through heavy rain.

Groundwater chemistry of a mine is important in the chemical evolution of water flowing through the geological formation. Groundwater could influence the stability of rock slopes and other mine constructions especially in limestone quarries since calcium carbonate as a major component is easily dissolved in water. Groundwater samples in a limestone quarry were analyzed and their geochemical properties were evaluated by using PHREEQC for characterization of groundwater flow and its evolution. The results showed that the hardness of samples varied from 55 to 133 mg/L, which is located between soft and hard character. The ionic strength of the samples ranged from 2.510-3 to 4.010-3 in the limestone layer and from 1.910-3 to 2.810-3 in the schalstein layer of the basement rock. The hydrochemical facies of all samples were Ca-HCO3. The groundwater samples collected in the limestone layer contained higher concentrations of Ca2+ than those collected in the schalstein layer except two sample locations (L-7 and L-11), which were influenced by the groundwater in the alternate layer between two layers. The logPCO2(g) values of groundwater in the limestone layer were mostly higher than the logPCO2(g) of atmospheric carbon dioxide. This could be attributed to the dissolution of the limestone in groundwater, which produced excessive aqueous carbon dioxide. The logPCO2(g) values of groundwater in the limestone layer decreased in the period of lower groundwater levels and then increased in the period of higher groundwater levels of the rainy season. The logPCO2(g) values of groundwater in the schalstein layer ranged from -3.2 to -4.0. The logPCO2(g) values of two sampling locations (S-3 and S-4) in the schalstein layer exceeded -3.4 only in the rainy season. This indicates that in the rainy season, the groundwater in the schalstein layer may be mixed with the groundwater flowing through the limestone layer. These results imply that the groundwater flow pathways between the two layers exist, which may affect the stability of rock slopes.

Groundwater in a mine or quarry is one of the important factors that need to be well understood. It could influence other relevant factors such as the stability of rock slope and other mine constructions (e.g. tunnel, access road, etc.). The objectives of this study are to analyze groundwater samples in and around a limestone quarry, to characterize their geochemical properties by using PHREEQC, and to evaluate the groundwater flow based on the results. The results showed that water hardness varies from 55 to 121 mg/L, which stands between soft and hard character. The ionic strength of sample ranged from 2.4×10-3 to 3.8×10-3 for the limestone and 1.9×10-3 to 2.7×10-3 for the schalstein. According to Piper diagrams, the type of groundwater was Ca-HCO3. The groundwater samples collected in the limestone layer contained higher concentrations of Ca2+ than those collected in the schalstein layer. The logPCO2(g) values of groundwater in the limestone layer were higher than the logPCO2(g) of atmospheric carbon dioxide. This could be attributed to the reaction between the limestone and groundwater, which produces excessive aqueous carbon dioxide. The logPCO2(g) values of groundwater in the limestone layer decreased in the period of lower groundwater levels (May 2015) and then increased in the period of higher groundwater levels of the rainy season (July 2015). The logPCO2(g) values of groundwater in the schalstein layer ranged from -3.2 to -3.8. The logPCO2(g) in S-1 and S-2 were always below -3.4, but those in S-3 and S-4 exceeded -3.4 in the rainy season. This indicates that in the rainy season, groundwater samples in S-3 and S-4 may be mixed with the groundwater flowing through the limestone layer.