Hydrogeochemistry and Isotopic Tracing of Nitrate Contamination of Two Aquifer Systems on Jeju Island, Korea

Abstract

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.

Comparison of groundwater age models for assessing nitrate loading, transport pathways, and management options in a complex aquifer system

“…At the model top surface, a yearly varying recharge concentration boundary was applied. A previous study confirmed local denitrification at several perched wells (Koh et al, ). Therefore, the model included denitrification in the perched zone with a calibrated, first‐order reaction rate of 5.16E‐8 s −1 (Koh et al, ).…”

“…The NO 3 –N old concentration (2.2 mg/L) of the study area was taken from the measured values of the old groundwater‐dominated samples (i.e., 3 H‐free samples). The models did not consider biodegradation of nitrate because no appreciable reduction of nitrate (denitrification) was observed for the regional system (Koh et al, ). There could be some bias in the estimated nitrate loading history associated with local denitrification from the perched aquifer.…”

“…Geologically, the study area is composed of marine deposits (Seogwipo Formation [SGF]) that were formed approximately 1,880–500 Ka BP (kilo annum before present), volcanic rocks (500–17 Ka), interbedded clay (22–17 Ka), tuff (younger than 17 Ka), and alluvial silty loam (Koh, Park, Kim, & Moon, ; Lim, Lee, Kim, Hong, & Choi, ). Due to the existence of a locally distributed clay unit above the volcanic rock, the study area is characterized by two aquifer systems consisting of an upper perched zone and lower regional groundwater separated by the semi‐permeable clay layer (Koh et al, ; UNDP & FAO, ). The perched groundwater occurs at the top silt loam layer or tuff layer, and the regional groundwater is distributed in the volcanic rocks (Figure c).…”

“…Due to the existence of a locally distributed clay unit above the volcanic rock, the study area is characterized by two aquifer systems consisting of an upper perched zone and lower regional groundwater separated by the semi-permeable clay layer (Koh et al, 2012;UNDP & FAO, 1972). The perched groundwater occurs at the top silt loam layer or tuff layer, and the regional groundwater is distributed in the volcanic rocks ( Figure 1c).…”

Identifying the sources of nitrate contamination of groundwater in an agricultural area (Haean basin, Korea) using isotope and microbial community analyses

“…In many agricultural regions, groundwater is often the major source of water both for drinking water and irrigation purposes. Groundwater contamination by nitrate derived from agricultural activities is a serious problem in many countries including Korea (Ju et al, 2006;Kaown et al, 2009;Koh et al, 2012). Understanding the sources of nitrate pollution in aquifers is a prerequisite for taking measures designed to reduce nitrate pollution of aquatic systems.…”