The Hydrochemistry of ChusanYongchulso Spring, Cheonbu-ri, Buk-myeon, Northern Ulleung Island

울릉도 북면 천부리 추산 용출소의 수질화학적 특성

Abstract

We investigated the hydrochemical properties of ChusanYongchulso Spring located in Buk-myeon, Ulleung Island, focusing on the formation and characteristics of aquifers in and around the Nari caldera. Abundant pumice with high permeability and numerous fractures (including faults and joints) that formed as a result of caldera subsidence are widely distributed in the subsurface, favoring the formation of aquifers. Because of the presence of porous pyroclastic rocks with a high internal surface area, the water type of the springs is characterized by $NaHCO_3$, with upper stream waters and the upper spring being characterized by $NaHCO_3$ and NaCl, respectively. Components with a high coefficient of determination with EC are $HCO_3$, Na, F, Ca, Mg, Cl, $SiO_2$, and $SO_4$. The high concentrations of Na and Cl might be attributable to the main lithologies in the area, given that alkaline volcanic rocks are distributed extensively across Ulleung Island. Eh and pH, which are considered to be important indicators of water-rock interaction, are unrelated to most components. According to the results obtained from factor analysis, the variance explained by factor 1 is 54% and by factor 2 is 25.8%. Components with a high loading on factor 1 are F, Na, EC, Cl, $HCO_3$, $SO_4$, $SiO_2$, Ca, $NO_3$, and Mg, whereas components with a high loading on factor 2 are Mg and Ca, along with K, $NO_3$, and DO with negative loadings. It is suggested that the high concentrations of Na, Cl, F, and $SO_4$ are closely related to the presence of fine-grained alkaline pyroclastic rocks with high permeability and porosity, which favorintensewater-rock interaction. However, a wide-ranging investigation that encompasses methods such as geophysical prospecting and geochemical analysis (including isotope, trace-element, and tracer techniques) will be necessary to gain a better understanding of the groundwater chemistry, aquifer distribution, and water cycling of Ulleung Island.

울릉도 북면 천부리에 소재하는 자분샘인 추산용출소 지하수의 수질특성을 규명하고, 이를 통하여 대수층의 발달특징을 고찰하였다. 상류구배인 나리분지 일대에 광범위하게 분포하는 부석층과 화산쇄설층, 칼데라 함몰과 관련된 단층과 절리와 같은 파쇄대는 투수계수가 높아 용출소의 지하수함양에 유리한 조건을 제공한다. 특히 다공성이며 표면적이 넓은 알칼리성 화산쇄설물로 구성된 화산암류로 인해 상류하천수와 용출소는 독특한 수질특성을 보인다. 용출소의 수질유형은 $Na-HCO_3$형이며, 상류의 샘과 하천수는 $Na-HCO_3$형과 Na-Cl형의 경계에 놓인다. EC과 상관성이 높은 성분으로는 $HCO_3$와 Na를 비롯하여 F, Ca, Mg, Cl, $SiO_2$, $SO_4$ 등이 있다. 높은 Na, Cl 함량은 울릉도 지질 전반에 알칼리계열의 화산암이 분포하며, 고기 화산활동에 영향을 받은 것으로 판단된다. 물-암석반응을 잘 반영하는 요소인 Eh와 pH는 어떤 수질성분과도 상관성이 거의 없는 것으로 나타났다. 요인분석결과에 의하면, 요인 1의 영향력은 54%로 나타났으며, 요인 2의 경우 25.8%이다. 요인 1에 높은 적재량을 가지는 성분은 F, Na, EC, Cl, $HCO_3$, $SO_4$, $SiO_2$, Ca, $NO_3$, Mg 등이다. 요인 2에 대해 적재량이 높은 성분은 Mg, Ca이며, 음의 적재값이 높은 성분은 K, $NO_3$, DO 등이다. 이 지역의 독특한 수질특성 즉 높은 Na, Cl, F, $SO_4$ 등은 알칼리계열의 화산쇄설암류의 세립질 입자, 높은 공극률 등이 물-암석반응을 가속화시킨 결과로 해석된다. 그러나 울릉도의 수질특성과 대수층의 발달, 수리순환을 규명하기 위하여서는 광범위한 지구물리탐사와 더불어, 동위원소, 미량원소, 추적자 등을 통한 추가연구가 필요하다.

Fig. 1. Shaded relief topographic map showing the study area location (rectangular box) in the northwestern part for he Nari caldera (modified after Kim and Lee, 2008)

Fig. 2. Topographic map of the Nari caldera area, northern Ulleung Island, showing the sampling sites.

Fig. 3. The dam constructed at Yongchulso Spring.

Fig. 4. Water types plotted on a Piper diagram.

Fig. 5. Stiff diagrams of Sungin Valley and Sillyung Spring. (a) Sungin Valley 1 (October 2010). (b) Sungin Valley 2 (October 2010). (c) Sillyung spring (August 2010).

Fig. 6. Stiff diagrams for Yungchulso Spring 2 by season.

Fig. 7. Stiff diagrams forYungchulso Spring 3(discharge water) by season.

Fig. 8. Chemical relationships of HCO₃ and Na with EC.

Fig. 9. Chemical relationships of Na and Ca with HCO₃

Fig. 10. Chemical relationships of Ca and K with Na.

Fig. 11. Chemical relationships of Cl, Ca, and SO₄ with F.

Fig. 12. Chemical relationships of Eh with Do and of Na with Cl.

Fig. 13. Factor plot of the hydrochemical characteristics of Ulleung water.

Table 1. Spring water and stream water of northern Ulleung Island. CS: Chusan Spring, SY: Silyungsu, SV: Sungin Valley stream(*). The unit of chemical component concentrations is mg/L unless otherwise noted.

Table 2. Correlation matrix of physicochemical components

Table 3. Factor matrix extracted using principal component analysis