Ministry of Environment (MoE) and Korea Environment Corporation (K-eco) install and operate the National Groundwater Quality Monitoring Network all over the country for sustainable management of groundwater quality. Through this monitoring network, highest arsenic concentration ranged from 0.014 to 0.083mg/L has been detected consistently since 2011 in deepest well in Muan City three wells (each depth are 10, 30 and 83m) installed in to monitor background groundwater quality in 2010. The Mann-Kendall trend test with arsenic data in confidence levels of 90% showed increasing trend, which is expected to arsenic contamination of groundwater. In this area, bedrock is located under 6.5m from surface and consists of metamorphic rock (calc-schist and dolomitic limestone) mainly. To examine the effect of interaction between groundwater and rock in aquifer, geologic survey and groundwater quality analysis such as field survey, thin section analysis, X-Ray Diffraction (XRD), Inductively Coupled Plasma (ISP), Scanning Electron Microscope (SEM) and Electron Probe Micro-Analysis (EPMA) were conducted. Pyrites (FeS2) were found easily at overall depth of in-situ core. Pyrites were grown heavily in hydrothermal alteration zone of geologic boundary of metapsammite. Result of geologic investigation, it indicated that arsenic is observed on surface of some pyrite and weathered iron oxide. In case of groundwater, arsenic were detected 0.001~0.018 mg/L at 19 samples among totally 60 samples. Geologically, 12 samples located in metapsammitic rock area, 9 samples located in volcanic rock area, and 1 sample located in granitic rock area.
Result of geological investigation and groundwater analysis shows that groundwater quality is affected by geologic environment according to the depth and oxic/anoxic environment of groundwater. In case of study area of arsenic contaminated groundwater in hydrothermal alteration zone, it could be managed strictly by prohibiting utilization and groundwater development or installing water treatment facilities.