Trends in agricultural groundwater quality in Chungcheongnam-do, South Korea from 2021 to 2024

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Abstract
This study investigates seasonal and interannual variations in agricultural groundwater quality in Chungcheongnam-do, South Korea, from 2021 to 2024 across 20 monitoring sites, including rice paddies (n = 5), upland fields (n = 5), and greenhouse cultivation areas (n = 10). Samples were collected in April and July each year and analyzed for pH, electrical conductivity (EC), NO 3-N, Cl-, major cations (Na + , K + , Ca 2+ , Mg 2+), and heavy metals (Cd, As, Pb). The mean pH values ranged from 6.4 (paddies) to 6.9 (uplands), generally within the acceptable range (6.0-8.5). EC was highest in paddies (0.45 dS m-1), followed by greenhouses (0.41 dS m-1) and uplands (0.28 dS m-1). NO 3-N concentrations were 10.2 mg L-1 in paddies, 9.1 mg L-1 in uplands, and 6.3 mg L-1 in greenhouses-exceeding the national average (5.30-6.68 mg L-1), with 10-20% of paddy and upland sites surpassing the regulatory threshold (20 mg L-1). Cl-concentrations were also elevated-2.3 mg L-1 (paddies), 28.2 mg L-1 (uplands), and 47.0 mg L-1 (greenhouses)-yet remained below the permissible limit. All samples complied with heavy metal standards. Seasonal differences in water quality were statistically insignificant, reflecting the relative stability of groundwater systems. However, declining EC and ionic concentrations in greenhouse areas appear to result from a shift toward hydroponic cultivation. These findings underscore growing concerns about nitrate contamination in paddy regions and highlight the need for targeted nutrient management and region-specific groundwater protection policies.

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