The hydrogeochemical characteristics of groundwater, in an area where widespread contamination by nitrate ( ${\text{NO}}_3^{-}$ ) was anticipated, were studied using traditional geochemical investigation and multivariate statistical analysis. Widespread ${\text{NO}}_3^{-}$ contamination as high as 67.2 mg/L as NO₃–N was observed, and positively correlated with that for chemicals ( ${\text{Cl}}^{-}$ , major cations) with surface origin. Principal component analysis (PCA) revealed that three processes affected groundwater chemistry of the area: (1) leaching of pollutants from the ground surface; (2) reduction of ${\text{NO}}_3^{-}$ in areas with low dissolved oxygen (DO); and (3) ingress of low ${\text{NO}}_3^{-}$ deep groundwater. Five sample groups were identified from cluster analysis, and analysis of land use patterns around each group showed that fate and distribution of ${\text{NO}}_3^{-}$ contamination were mainly controlled by surface topography and predominant land use type. The highest ${\text{NO}}_3^{-}$ concentrations were associated with confined livestock feeding operations in hilly terrain areas, where infiltrating water also had high DO. Lower ${\text{NO}}_3^{-}$ concentrations found in the lowland flat areas were thought to be due to either reducing conditions in rice paddies leading to N attenuation or drawing in of deep groundwater by pumping to meet agricultural needs during periods of low rainfall.

Long-term evaluation from 2000 to 2020 of a temperate reservoir indicated that water quality and trophic status were not critically controlled by the inter-annual dynamics of monsoon precipitation. The fluctuation of annual concentrations of TP, TSS, and EC did not always correlate with the variation of precipitation. BOD and COD demonstrated monotonically increasing trends with Sen’s slope of 0.02 and 0.01, respectively, while Chl-a manifested a decreasing trend (slope = −0.23). The increases of different magnitudes in the levels of TP, TSS, and Chl-a in the monsoon and the early post-monsoon periods were observed in the drought, flood, and normal years. The drought years showed distinct seasonal variations in many parameters, while those in the flood and the normal years were very similar. Pearson correlation and empirical regression analyses resulted in weaker correlation between water quality and hydrological parameters than other reservoirs, which, along with low impact of precipitation, was attributed to the low drainage ratio (28.8) of the study area. BOD/COD ratios were higher than 0.5 in the reservoir, indicating the presence of a significant fraction of biodegradable organic matter. The ratio increased in the longitudinal flow direction (>50% in the drought years), implying the possibility of autochthonous sources of organic matter. TSID evaluation resulted in variation of limiting factors not related to the fluctuation of annual precipitation. The loadings of the significant principal components were very similar among the different precipitation groups, supporting the other findings that annual precipitation governed by monsoon intensity would not solely control the water quality dynamics of the study area.

The variations in water quality parameters and trophic status of a multipurpose reservoir in response to changing intensity of monsoon rain was investigated by applying a trophic state index deviation (TSID) analysis and an empirical regression model to the data collected in two periods from 2014 to 2017. The reservoir in general maintained mesotrophic conditions, and Carlson’s trophic state index (TSIc) was affected most by TSI_TP. Nutrient concentrations, particularly phosphorus, did not show strong correlations with precipitation, particularly in the period with weak monsoon, and a significant increase in total phosphorus (TP) was observed in Spring 2015, indicating the possibility of internal phosphorus loading under decreased depth and stability of water body due to a lack of precipitation. TSI_Chl was higher than TSI_SD in most data in period 1 when a negligible increase in precipitation was observed in the monsoon season while a significant fraction in period 2 showed the opposite trend. Phytoplankton growth was not limited by nutrient limitation although nutrient ratios (N/P) of most samples were significantly higher than 20, indicating phosphorus-limited condition. TSID and regression analysis indicated that phytoplankton growth was limited by zooplankton grazing in the Spring, and that cell concentrations and community structure in the monsoon and post-monsoon season were controlled by the changing intensity of the monsoon, as evidenced by the positive and negative relationships between community size and cyanobacterial population with the amount of precipitation in the Summer, respectively. The possibility of contribution from internal loading and an increase in cyanobacterial population associated with weak monsoon, in addition to potential for nutrient enrichment in the post-monsoon season, implies a need for the application of more stringent water quality management in the reservoir that can handle all potential scenarios of eutrophication.

Long-term groundwater monitoring is essential for assessing nitrate contamination, yet regional concentration patterns may obscure changes at individual wells. This study examined groundwater NO₃-N concentrations and temporal trends using 15 years (2010–2024) of data from 57 wells at 38 monitoring sites in Jeonbuk Province, South Korea. Mann–Kendall tests, Sen’s slope estimates, and Spearman correlations were used to evaluate temporal changes and their associations with land use and environmental characteristics. Regional nitrate concentrations fluctuated without a consistent directional pattern, while 11 wells showed increasing trends, 11 decreasing trends, and 35 no significant trend. Wells with decreasing trends had a higher median long-term NO₃-N concentration (7.55 mg/L) than those with increasing trends (3.45 mg/L), although the difference was not statistically significant. Long-term nitrate concentrations were generally higher in western agricultural lowlands and associated with geographic gradients and selected agricultural land uses, whereas temporal trends showed limited associations with the examined environmental variables. These findings indicate that existing nitrate contamination levels and temporal changes provide distinct information. Integrating long-term concentrations with well-specific trends can help identify persistent contamination and emerging deterioration, supporting more targeted groundwater monitoring and management.