Factors controlling groundwater chemistry in an agricultural area with complex topographic and land use patterns in mid‐western South Korea

Abstract

Abstract:Shallow and bedrock groundwater from granitic aquifers were investigated for the hydrogeochemistry of major and minor constituents in an agricultural area. Nitrate concentrations were observed up to 49 mg/l as NO 3 -N, with 22% of samples exceeding the drinking water standard, which could pose a significant threat because most residents rely on groundwater as their drinking water source. Principal component analysis revealed three principal components (PCs): (1) nitrate contamination, contributed by major cations, Cl , SO 2 4 and NO 3 , (2) reduction processes positively involving Fe, Mn and B, and negatively involving dissolved oxygen and NO 3 and (3) natural mineralization, involving HCO 3 and F . Cluster analysis, performed on the PC scores, resulted in seven sample groups, which were successfully identified by total depth, elevation and land use. The nitrate-contaminated groups had mixed land uses, with locally concentrated residential areas. Uncontaminated groundwater groups were found in the natural environment, including high-altitude spring water and bedrock groundwater with a higher degree of natural mineralization. Shallow groundwater groups in paddy fields in lowlands were affected by reducing environments, of which one group was characterized by high Fe, Mn and B, and negligible nitrate. Groundwater with intermediate nitrate and lower Cl and SO 2 4 was found primarily in hilly terrains with orchards and vegetable gardens, indicating lower contaminant loadings than lowland areas. Higher concentrations of F and nitrate were observed in the nitrate-contaminated water, which seemed unlikely to be explained by groundwater mixing. The strong acidity generated from nitrification may infiltrate deeper into the aquifer, induce accelerated weathering of bedrock and result in the coexistence of F and nitrate, which may be an evidence of intense nitrate loading, leading to soil acidification. Multivariate statistical analysis successfully delineated hydrochemical characteristics of groundwater attained by natural and anthropogenic processes in an agriculturally stressed area with complex topographic land use patterns.

Hydrogeochemical Characteristics of Groundwater Highly Polluted with Nitrate in an Agricultural Area of Hongseong, Korea

“…The concentrations of major cations were also the highest in Group 2 except for Mg, which was the second highest (Figure 5b-d). Although Mg concentrations were not the highest in Group 2, they were still higher than those reported in samples with high anthropogenic pollution in other areas in Chungnam province [14]. F and HCO concentrations were the lowest and the second lowest, respectively (Figure 5e,f).…”

“…The results in Figure 2 indicate that groundwater mineralization in the study area was driven by two factors, NO − 3 contamination and natural process of weathering [14]. Major cations showed high concentrations in many of the samples, which may be introduced to groundwater either through natural water-rock interaction or from contamination sources from the ground surface.…”

“…Many of the wells in Groups 3 and 5 were located in rice paddy, whose soil is mainly composed of clayey particles due to the need to retain water on the surface for the most of the growing season, while barley and vegetable field soils are composed of fine sand and silt particles for the efficient drainage of soil water. Retarded downward movement of stagnant water and small pore sizes in the soil make rice paddy an ideal location for depletion of DO and generation of anoxic condition, especially when a sufficient amount of dissolved organic matter is present [47][48][49], and denitrification in groundwater under paddy field was observed in other studies in Korea [14,50]. Although observed pH values were not much higher than ambient pH measured in the Korean National Groundwater Observation Network about 10 km away from the study area, groundwater buffer capacity is not anticipated to be particularly high in the study area with the absence of calcareous rocks, enabling any pH decrease from nitrification to be observed.…”

Temporal variability of nitrate concentration in groundwater affected by intensive agricultural activities in a rural area of Hongseong, South Korea

“…Chae et al (2004) found that thick silt layers with low permeability effectively limit nitrate levels in groundwater, and create favorable conditions for denitrification. The low nitrate loading of paddy fields was also noted by Koh et al (2009). Temporal variations in nitrate levels can be explained in terms of seasonally different patterns of rainfall and irrigation.…”

“…pH, HCO 3 , and F showed higher rankings in the BL group while DO, NO 3 -N, and Cl did in the EL group. The relationship between land use and nitrate concentrations in an agricultural area with crystalline bedrock was studied by Koh et al (2009) in Nonsan, South Korea. The study indicated that the nitrate concentration was strongly affected by land use and topography, in terms of sources and attenuation processes.…”