The objective of the study is to investigate the effect of released silicon (Si) and aluminum (Al) during the formation of volcanic ash soil on the content of Si in groundwater on Jeju Island. Volcanic ash soils in Jeju Island were formed from pyroclastic materials that originated from basalt. We sampled four profiles, including basaltic bedrock of each soil series with large variations in elevation (160–320 m) and mean annual precipitation (MAP; 1,800–2,600 mm). The soil and bedrock minerals were analyzed for weathering of volcanic ash soils related to mineralogical transformations and mobility of Si and Al. Andisols (above 2,000 mm MAP) were dominantly composed of allophane and gibbsite. In Andisols used in the study, 10–70% of Si was lost, whereas the amount of Al was relatively conserved. This is because Al forms Al-humus complex and Andisols contain allophane. In contrast, non-Andisols located at low altitude with lower than 1,800 mm MAP were enriched with considerable amounts of Si and Al, because non-Andisols have crystalline clay minerals and quartz. These results indicate that Andisols, which are widely distributed in Jeju Island, may play an important role in contributing to the high concentration of dissolved Si in the groundwater.
On Jeju Island, where no perennial streams or rivers exist, Si released by weathering and dissolved in the soil solution is likely to be leached into groundwater [
In soils derived from pyroclastic materials, short-range-order minerals, including allophane, imogolite, and ferrihydrite, are developed. The common soil type are Andisols, which contain large proportions of Al-humus complexes and are greatly affected by climate in the process of soil formation [
The soils of Jeju Island are volcanic ash soils derived from basalt pyroclastic materials as the main parent material [
Previous studies on weathering and pedogenesis of Jeju Island soils described the conditions needed for allophane formation, mineral composition of soils on the island, and chemical components in some of the soil series present on the island [
In the present study, we investigated the changes in mineral composition of soil by sampling the soil profiles and basaltic bedrock collected from the lower part of the C-horizon of the soils in the study areas (Andisols in the eastern region and non-Andisols in the northern region). We compared the Si and Al mobility patterns in these soils according to the changes in their chemical composition. With regard to Si leaching, we sought to uncover the reason for the high Si concentration in groundwater by investigating the formation and transformation processes of clay minerals, including allophane and Al-humus complexes in volcanic ash soils derived from basaltic pyroclastic materials on Jeju Island.
Soil samples for four different soil types were collected in the eastern region (Pyeongdae and Namwon series) and the northern region of Jeju Island (Jeju and Gangjeong series) based on the laboratory characteristics data of the typifying pedon in accordance with the Taxonomical Classification of Korean Soils [
Site characteristics of the four investigated soil profiles derived from pyroclastic materials in Jeju Island, Korea.
Soil series | Elevation | MAP† | Vegetation | Coordinates | Soil taxonomy |
---|---|---|---|---|---|
(masl) | (mm·yr−1) | ||||
Pyeongdae | 320 | 2,660 | Wild grasses | N33°26′01.5″ | Acrudoxic Melanudands |
E126°43′30.4″ | |||||
Namwon | 165 | 2,290 | Pasture, wild grass | N33°22′03.5″ | Acrudoxic Melanudands |
E126°45′34.5″ | |||||
Jeju | 280 | 1,860 | N33°26′58.5″ | Andic Palehumults | |
E126°31′09.7″ | |||||
Gangjeong | 200 | 1,740 | N33°27′36.8″ | Mollic Paleudalfs | |
E126°29′07.7″ |
†Mean annual precipitation.
The mean annual precipitation of Jeju Island widely varies by region. The MAPs (in the period from 1998 to 2010) in the eastern, western, southern, and northern regions were 1,967 mm, 1,143 mm, 1,923 mm, and 1,498 mm, respectively. There is little regional difference in evapotranspiration, which was measured to be 1,229 mm and 1,207 mm in Western Gosan and Southeastern Seoguipo, respectively [
The MAPs of the study area were predicted in order to evaluate the mineral composition of the soil samples and the mobility of Si and Al according to the precipitation difference. The MAP was determined by substituting the altitude of the sampling points using the data from the equation representing the relationship between MAP and altitude, as analyzed by the Korean Institute of Geoscience and Mineral Resources (KIGAM) [
The collected soils were air-dried and sieved through a 2 mm sieve for the analysis of key soil properties (Table
Physical and chemical properties of the soils studied in Jeju Island.
Horizon | Depth | Munsell color | Bd | pH | OC | BS | Sio | Alp | Alo. | Ferr. | Al | MI |
---|---|---|---|---|---|---|---|---|---|---|---|---|
(cm) | (Moist) | (Mg·m−3) | (CaCl2) | (g·kg−1) | (%) | (%) | (%) | (%) | (%) | |||
Pyeongdae | ||||||||||||
A | 0–18 | 10 YR 2/1 | 0.51 | 4.87 | 120 | 2.74 | 0.77 | 1.88 | 7.27 | 4.10 | 0.44 | 1.49 |
AB | 18–44 | 10 YR 2/2 | 0.52 | 5.05 | 87.3 | 4.48 | 1.69 | 0.91 | 15.9 | 7.50 | 0.17 | 1.49 |
Bw1 | 44–73 | 10 YR 3/4 | 0.60 | 5.24 | 43.7 | 4.89 | 1.75 | 0.41 | 13.3 | 8.12 | 0.11 | 1.47 |
Bw2 | 73–100 | 10 YR 4/4 | 0.71 | 5.36 | 23.0 | 5.68 | 2.13 | 0.21 | 15.2 | 12.07 | 0.05 | 1.38 |
BC | 100–140 | 7.5 YR 4/4 | 0.72 | 5.59 | 15.4 | 5.98 | 3.03 | 0.17 | 19.5 | 12.48 | 0.04 | 1.26 |
Namwon | ||||||||||||
A1 | 0–26 | 10 YR 2/1 | 0.57 | 4.36 | 118 | 3.74 | 0.42 | 2.10 | 3.95 | 5.15 | 0.59 | 1.52 |
A2 | 26–47 | 10 YR 2/2 | 0.57 | 4.61 | 104 | 2.90 | 0.47 | 2.38 | 4.43 | 5.68 | 0.60 | 1.51 |
AB | 47–72 | 10 YR 3/2 | 0.59 | 4.69 | 91.5 | 3.48 | 0.69 | 2.28 | 6.52 | 6.30 | 0.52 | 1.52 |
Bw | 72–160 | 10 YR 4/4 | 0.74 | 4.94 | 30.6 | 7.95 | 1.06 | 0.50 | 9.95 | 9.07 | 0.16 | 1.43 |
Jeju | ||||||||||||
Ap | 0–20 | 10 YR 3/2 | 0.92 | 4.41 | 37.8 | 2.08 | 0.17 | 0.90 | — | 3.24 | 0.59 | — |
AB | 20–41 | 10 YR 3/3 | 1.03 | 4.36 | 19.9 | 1.73 | 0.19 | 0.69 | — | 2.94 | 0.55 | — |
Bt1 | 41–65 | 10 YR 4/3 | 1.29 | 4.29 | 9.80 | 3.81 | 0.14 | 0.47 | — | 2.37 | 0.63 | — |
Bt2 | 65–92 | 7.5 YR 4/4 | 1.48 | 4.19 | 7.12 | 6.32 | 0.13 | 0.36 | — | 2.08 | 0.56 | — |
Bt3 | 92–150 | 7.5 YR 4/4 | 1.48 | 4.12 | 6.40 | 10.1 | 0.16 | 0.28 | — | 2.06 | 0.46 | — |
Gangjeong | ||||||||||||
Ap | 0–24 | 10 YR 3/3 | 1.05 | 4.79 | 30.2 | 53.3 | 0.20 | 0.36 | — | 2.34 | 0.52 | — |
Bat | 24–38 | 10 YR 4/3 | 1.36 | 4.52 | 13.9 | 43.5 | 0.09 | 0.24 | — | 1.06 | 0.57 | — |
Bt1 | 38–53 | 10 YR 4/4 | 1.48 | 4.53 | 5.15 | 45.0 | 0.09 | 0.13 | — | 0.94 | 0.41 | — |
Bt2 | 53–85 | 10 YR 4/4 | 1.53 | 4.47 | 2.86 | 50.5 | 0.11 | 0.09 | — | 1.31 | 0.31 | — |
Bt3 | 85–135 | 10 YR 4/6 | 1.53 | 4.42 | 1.92 | 57.0 | 0.13 | 0.08 | — | 2.00 | 0.27 | — |
Bd: bulk density, OC: organic carbon, BS: base saturation, Sio: acid oxalate extractable Si, Alp: sodium pyrophosphate extractable Al, Alo: allophane, Ferr: ferrihydrite, MI: melanic index.
To determine the mineral compositions of the soil samples from the two different regions, we performed X-ray diffraction (XRD) and scanning electron microscope/energy-dispersive X-ray spectroscopy (SEM/EDS) analysis. To prepare bulk specimens for XRD analysis, we subjected the typifying pedon and rock powder sample of each layer to uniform crushing, sieving with a 0.015 mm sieve, and oven-drying at 105°C for 24 h. XRD analysis was performed with an X-ray powder diffractometer (Geigerflex 2013, Rigaku, Japan) that uses CuK
To compare the changes in Si and Al contents in the soils formed in two different regions, we prepared fine powders from bulk soil samples and basaltic bedrock collected from the lower part of the C-horizon of each soil profile. The samples were digested in a microwave oven using HNO3, HCl, HF, and H3BO3 solutions [
In the process of chemical weathering, mass balance calculation in a soil profile was used to estimate the mobility of elements by comparing the concentrations of these elements in bedrock [
A reference element was selected based on the bedrock type and climate with the consideration that the amount of element mass transfer can be overestimated or underestimated depending on the selected reference element [
The variation of mass transfer coefficients for Nb and Zr element from four soil profiles (■, Pyeongdae; ●, Namwon; □, Jeju; ○, Gangjeong) using Zr and Nb as immobile elements.
The main constituent minerals of basaltic bedrock collected from the lower part of the C-horizon of the eastern and northern soil series were plagioclase and pyroxene minerals, with a small amount of K-feldspar, mica, olivine, and magnetite (Table
Mineral compositions of basaltic bedrocks.
Soil series | Plagioclase | K-feldspars | Micas | Pyroxene | Olivine | Chlorite | Hematite | Magnetite |
---|---|---|---|---|---|---|---|---|
(wt. %) | ||||||||
Pyeongdae | 40.7 | 9.1 | 6.4 | 27.3 | 11.3 | 1.4 | — | 3.8 |
Namwon | 43.7 | 7.6 | 2.7 | 27.5 | 5.0 | 2.1 | 2.8 | 8.5 |
Jeju | 69.8 | 12.3 | 12.3 | 12.5 | 2.1 | — | 2.7 | 2.6 |
Gangjeong | 28.4 | 10.5 | 3.2 | 26.3 | 20.4 | 3.1 | — | 8.0 |
Regarding the mineral compositions of the sampled soils, all soil samples consisted of both primary and secondary minerals weathered from bedrocks (Table
Mineral compositions of soil samples.
Horizon | Depth | Q | Pl | K-F | Mi/Ill | Px | Ov | K | Ch | Ch/Sm | Ch/Vc | Gb | Hm/Gt | Mg |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
(cm) | (wt. %) | |||||||||||||
Pyeongdae | ||||||||||||||
A | 0–18 | 29.3 | 12.4 | 9.0 | 9.4 | 5.0 | 8.0 | 4.0 | — | 6.7 | — | 3.2 | 7.1 | 5.9 |
AB | 18–44 | 23.2 | 17.5 | 7.8 | 7.2 | 6.8 | 10.0 | 1.2 | — | 8.4 | — | 3.5 | 3.9 | 6.6 |
Bw1 | 44–73 | 19.4 | 11.8 | 7.9 | 9.7 | 8.5 | 4.1 | 3.7 | — | 6.3 | — | 17.4 | 5.4 | 5.9 |
Bw2 | 73–100 | 20.6 | 11.1 | 6.8 | 8.8 | 8.2 | 4.2 | 3.9 | — | 6.4 | — | 18.6 | 5.3 | 6.1 |
BC | 100–140 | 22.8 | 11.5 | 6.0 | 12.0 | 6.2 | 5.5 | 1.4 | — | 6.4 | — | 17.1 | 3.2 | 7.8 |
Namwon | ||||||||||||||
A | 0–26 | 38.5 | 14.8 | 3.5 | 12.9 | 6.0 | 1.0 | 3.7 | 7.0 | — | — | 3.7 | 5.4 | 3.2 |
A2 | 26–47 | 32.8 | 14.8 | 6.5 | 11.4 | 4.4 | 8.0 | 1.8 | 7.6 | — | — | 4.7 | 3.4 | 4.4 |
AB | 47–72 | 39.2 | 11.9 | 7.0 | 12.9 | 2.9 | 1.0 | 2.9 | 8.7 | — | — | 5.9 | 2.3 | 2.7 |
Bw | 72–160 | 33.4 | 16.2 | 5.7 | 9.1 | 5.6 | 2.9 | 2.1 | 9.4 | — | — | 11.2 | 2.8 | 1.7 |
Jeju | ||||||||||||||
Ap | 0–20 | 54.7 | 8.8 | 1.6 | 8.6 | 10.7 | — | 1.7 | — | — | 6.3 | 1.3 | 4.1 | 2.3 |
AB | 20–41 | 49.1 | 9.0 | 11.5 | 5.1 | 7.5 | — | 2.5 | — | — | 9.3 | 1.5 | 2.2 | 2.3 |
Bt1 | 41–65 | 51.2 | 7.9 | 4.9 | 7.5 | 6.8 | — | 2.6 | — | — | 12.3 | 2.2 | 3.3 | 1.4 |
Bt2 | 65–92 | 48.4 | 5.2 | 2.4 | 10.3 | 9.7 | — | 3.9 | — | — | 11.6 | 3.1 | 3.5 | 2.1 |
Bt3 | 92–150 | 48.3 | 5.4 | 7.0 | 11.1 | 5.2 | — | 3.8 | — | — | 12.8 | 3.2 | 2.4 | 1.0 |
Gangjeong | ||||||||||||||
Ap | 0–24 | 53.6 | 10.5 | — | 11.6 | 7.2 | — | 4.0 | — | — | 6.3 | 2.6 | 3.4 | 0.9 |
BAt | 24–38 | 45.5 | 16.6 | 7.0 | 7.4 | 8.9 | 0.4 | 3.1 | — | — | 8.2 | 1.9 | 1.0 | — |
Bt1 | 38–53 | 45.8 | 15.6 | 4.5 | 8.8 | 7.3 | 1.5 | 3.1 | — | — | 8.1 | 1.8 | 3.5 | — |
Bt2 | 53–85 | 51.7 | 10.8 | 4.5 | 9.3 | 5.5 | — | 4.7 | — | — | 9.9 | 1.0 | 2.5 | — |
Bt3 | 85–135 | 45.6 | 15.1 | 7.7 | 10.8 | 6.0 | — | 3.5 | — | — | 6.6 | 1.4 | 3.1 | — |
Q: quartz; Pl: plagioclase; K-F: K-feldspars; Mi/Ill: mica/illite; Hb: hornblende; Px: pyroxene; Ov: olivine; K: kaolinite; Ch/Sm: chlorite/smectite; Gb: gibbsite; Hm: hematite; Gt: goethite; Mg: magnetite.
Figure
X-ray diffraction patterns of the clay fraction of selected horizons. AD: air-dried; EG: ethylene glycolated; H: heated at 550°C; Q: quartz; Ch: chlorite; M: mica/illite; Sm: smectite; K: kaolinite; V: vermiculite; G: gibbsite; F: feldspar; Hm: hematite.
Figure
SEM/EDS of selected horizons of the profiles less than 2
The analysis using the selective dissolution method showed that the allophane content was highest in the overall soil profile of the Pyeongdae series (ranging from 7 to 20%), and the median allophane content in the AB and Bw horizons of the Namwon series (6.5% and 10%, respectively) were significantly lower than those in the Pyeongdae series (Table
In the Jeju and Gangjeong series from the northern region, the Si
Figure
Depth profiles of the percentage change of Si and Al for the soil series of Pyeongdae (■), Namwon (●), Jeju (□), and Gangjeong (○) calculated using Zr as the immobile element.
Figure
The percentage change of Si and Al in the soil series of Pyeongdae (■), Namwon (●), Jeju (□), and Gangjeong (○) from Jeju Island rainfall gradient as a function of mean annual rainfall. Each point was generated by depth integration to 1 m of soil horizon data.
Given that the Jeju and Gangjeong series developed into non-Andisols from parent materials derived from basaltic pyroclastic materials, highly soluble elements such as Ca and Na were leached in large quantities during pedogenesis. However, Si and Al fractions were high because most Si and Al remained in the soils as the main constituents of quartz or components of silicate clay minerals such as kaolinite, illite, and chlorite. In a previous study on the mineralogical evolution of non-Andisol soils in Dangsanbong in the western coastal area of Jeju Island, Ha et al. [
Quartz, which was not detected in basaltic bedrocks, was observed in all soil samples (Table
In the Pyeongdae and Namwon series characterized by Andisols, the ratio between allophane and Al-humus complexes present in soil profiles determines the Si and Al mobility. Al is well-reserved in the Namwon series, given the presence of Al-humus complexes down to the depth of 70 cm, which lowers the fluidity of Al, resulting in its continuous accumulation. In the Pyeongdae series, however, Al-humus complexes are formed only in the A horizon (18 cm), with allophane present in all horizons of the profile, indicating that formation of Al-humus complexes decreased the mobilization of Al. During pedogenesis, Si losses induce the formation of Al-humus complexes in the volcanic ash soil surface horizon, and the dissolved Si is mobilized downwards, leading to the formation of allophane with lower Si concentrations in the subsoil horizon, which in turn causes continuous leaching of dissolved Si. The subsoil horizons of the soils from the Pyeongdae and Namwon series had high allophane and gibbsite content (Table
Figure
Schematic illustration of the possible formation and transformation of clay minerals and humus complexes in volcanic ash soils derived from pyroclastic materials in a humid-temperate climate on Jeju Island.
With the increasing depth of the soil profile, the supply of organic matter decreases, thus hampering the formation of Al-humus complexes and decreasing the concentrations of organic acids, and the pH increases closer to or over 5. Because of this pH change, the concentration of Al in the soil solution increases, and Al is polymerized and exists in the form of oxide or hydroxide [
In the present study, we investigated the changes in the dynamics of minerals in the soils developed into Andisols (Pyeongdae and Namwon series) in the eastern region and those developed into non-Andisols (Jeju and Gangjeong series) in the northern region of Jeju Island. Si and Al mobility pattern variations were also investigated according to the chemical composition of the soil profiles. As a result, it was determined that plagioclase, K-feldspar, pyroxene, and magnetite are contained in the bedrocks as the main constituent minerals in all weathered soils. Illite and chlorite were found in most clay minerals, while kaolinite, smectite, and vermiculite were present in some soils in small quantities. In the non-Andisols of the northern region, quartz content ranged between 45 and 55%, which was twice as high as that in Andisols, and Allophane was not found in non-Andisols, which aligned with limited Si leaching. Al-humus complexes, allophane, and gibbsite were formed in Andisols, and gibbsite concentrations corresponded with high allophane concentrations. Andisols and non-Andisols were both developed from basaltic pyroclastic materials, but in non-Andisols with 1,800 mm MAP, quartz and layer silicate clay minerals were formed, with little Si and Al leaching observed. In Andisols with >2,000 mm MAP, Si leaching increased in the soils that formed large amounts of allophane and gibbsite. Al was preserved by forming Al-humus complexes or forming parts of silicate clay minerals. These results indicated that the high Si concentration in the groundwater in volcanic ash soils of Jeju Island can primarily be ascribed to the pedogenesis of Andisols. The amount of precipitation and the presence of materials such as allophane significantly impacted Si concentration and mobility in Andisols. Because Andisols are widely distributed across Jeju Island and allophane can be formed continuously in humid-temperate weather conditions, this will greatly contribute to the increase in Si concentrations in the groundwater of Jeju Island.
The research data used to support the findings of this study are included within the article.
The authors declare that they have no conflicts of interest.
The authors gratefully acknowledge Dr. Hae-Nam Hyun for his financial support through Jeju National University Research Fund in 2014 and 2017 and critical discussions and Dr. Hyomin Lee for his technical assistance.
Table S1: the content of elements on the fine earth fractions (2 mm) of soil samples and basaltic bedrocks.