Simulation of nitrate nitrogen concentrations and DOM characteristics in groundwater from Southwest China’s Karst Wetlands using an improved GMS model

Abstract

Existing models often face limitations in the understanding and prediction of nitrate nitrogen (NO₃⁻-N) concentrations in karst groundwater. In this study, to tackle this issue, a Gaussian function model was coupled with the Groundwater Modeling System (GMS) to simulate NO₃⁻-N concentration changes in the southwest karst wetland of China. Additionally, fluorescence spectroscopy was employed to measure dissolved organic matter (DOM) components in the groundwater, providing insights into their variation and influence on NO₃⁻-N dynamics. The results demonstrated that coupling the Gaussian curve fitting method with the GMS model accurately simulated NO₃⁻-N concentration changes in the study area. The simulation revealed lower NO₃⁻-N levels in the northern region, with higher concentrations in the central area, peaking at 20.73 mg/L at lower elevations. NO₃⁻-N was primarily distributed in the southwestern region and upper Mudong Lake, exhibiting a diffusion trend from west to east. DOM analysis indicated significant autochthonous contributions, particularly microbial metabolic by-products. The total fluorescence intensity and DOM components increased downstream, with the lowest values at the source and the highest values at river confluences. The humification index (HIX) was correlated with NO₃⁻-N concentrations, where lower NO₃⁻-N levels corresponded to lower HIX values, and higher NO₃⁻-N levels corresponded to higher HIX values. In conclusion, this study provides valuable insights into NO₃⁻-N prediction in groundwater and the role of DOM, offering a reference for groundwater protection in the southwest China karst basin.