Nitrate vulnerability of groundwater in Jeju Volcanic Island, Korea.

Groundwater is the sole source of water for about 670,000 residents of Jeju Island, which is a volcanic Korean island. Since the 1990s, nitrate contamination of groundwater has emerged as a major environmental issue. To ensure the sustainability of water resources, this study aimed to develop a vulnerability model for nitrate contamination as a preventive measure. Based on intrinsic vulnerability determined using the DRASTIC model, the effects of anthropogenic parameters related to NO3 sources and groundwater use (land use and the hydraulic gradient, respectively) on contamination were tested using a geographic information system (GIS). The correlation between groundwater nitrate distribution and vulnerability was considerably stronger compared to the DRASTIC method, with the correlation coefficients (r) increasing from -0.048 to 0.562 and -0.069 to 0.481 in the western and eastern regions, respectively. However, in the southern and northern regions, nitrate concentrations in groundwater are low, likely due to the heavily paved land surface that resulted from urbanisation, such that groundwater vulnerability appeared negligible. To prevent further nitrate contamination in coastal groundwater, management policies for land use and groundwater exploitation should be enacted along with continuous groundwater monitoring at the regional scale.

Nitrate contamination of coastal groundwater: Sources and transport mechanisms along a volcanic aquifer.

Groundwater is the sole water supply source on Jeju volcanic island in Korea and increasing levels of nitrate contamination have raised serious public concerns. This study was objected to understand the nitrate contamination mechanisms of the coastal groundwater overlain by a low-permeability layer in the western part of the island. The 2-yrs of quarterly groundwater monitoring from Sep. 2016 to Oct. 2018 revealed that NO3-N concentrations of 60.7% of sampled wells exceeded 10 mg/L, and those of 17.8% in the range of 3-10 mg/L, indicating those wells are already contaminated or under influence of potential sources. The spatio-temporal variation of groundwater chemistry and stable isotopic signatures suggested that nitrate originated mainly from chemical fertilizers due to agricultural activities and in part from liquefied manure affected by local livestock industries. Two transport processes appeared to be responsible for the nitrate in the confined coastal aquifer: 1) the direct penetration from the land surface through the wellbore leakage at incompletely grouted wells, and 2) the addition of nitrate-contaminated groundwater from upgradient areas where the confining layer is pinched out. Presently the potential nitrate sources are distributed up to 600 m above mean sea level(amsl), and the isotopic signatures implies that the provenance of coastal groundwater could be upland areas of up to 1280 m amsl. Subsequently, nitrate sources at upland areas could be picked up and carried down to the coastal groundwater under the confined condition of the basaltic aquifer that the regional groundwater recharge occurs. Consequently, groundwater contamination in the coastal aquifer should be considered in the big picture of groundwater system accommodating both regional and local flows.

Hydrogeochemistry and isotopic tracing of nitrate contamination of two aquifer systems on Jeju Island, Korea.

The groundwater of Jeju Island (Republic of Korea) is vulnerable to contamination because its aquifers are mainly composed of highly permeable geological units and its agricultural fields are often exposed to excessive use of predominantly synthetic fertilizers. In the Gosan area of Jeju Island, we investigated nitrate contamination in both a perched aquifer above an impermeable clay bed and the regional groundwater beneath this aquitard. The δO and δD values indicate that the perched groundwater is recharged by local precipitation, whereas the regional groundwater is recharged mainly by regional flow from an adjacent mountainous region. The perched groundwater contained very high NO-N concentrations of up to 87 mg/L. The isotopic composition of nitrate in the perched groundwater showed that synthetic fertilizers applied in high excesses of crop N needs were the main cause of aquifer pollution. Elevated nitrate concentrations were also observed in the regional groundwater especially after precipitation events. Concentration and isotopic data revealed that the inflow of shallow perched groundwater along the poorly cemented or uncemented annulus of regional groundwater wells was one of the main reasons for the nitrate contamination observed in the regional groundwater. In both aquifers, δN and δO values showed that the sources of nitrate were derived from synthetic fertilizers that had been recycled in the soil zone by nitrification and in some portions of the perched aquifer (dissolved oxygen concentrations <2 mg/L) indicated that denitrification occurred locally.