Assessment of groundwater pollution risks in a cold-temperate agricultural region and potential correlations with spectral indices of dissolved organic matter

Groundwater quality and dissolved organic matter (DOM) are important for water supply safety and human health. However, traditional monitoring is time consuming and costly, and the value of DOM spectral indices as rapid hydrogeochemical screening indicators in cold-temperate agricultural aquifers remains insufficiently understood. This study analyzed conventional water-quality indicators and DOM fluorescence components in 101 groundwater samples from Qitaihe City, northeastern China. Structural equation modeling (SEM) and redundancy analysis (RDA) were used to identify the spectral indices most strongly associated with conventional water-quality indicators. Zone-spectral indices based models were then developed to screen key groundwater-quality parameters, while random forest models compared relative groundwater-quality responses under different source-control scenarios. The results showed that ammonia nitrogen (NH 3 -N), nitrate (NO 3 − ), and iron (Fe) exceeded the relevant standards in 10, 16, and 11 samples, respectively, defining a compliant zone (S1), an NH 3 -N-NO 3 - co-exceedance zone (S2), and an NH 3 -N-NO 3 - -Fe co-exceedance zone (S3). Four fluorescent components were resolved in each hydrochemical zone, with DOM fluorescence dominated by terrestrial and microbial humic-like substances. The fluorescence index (FI) was closely associated with NO 3 − in S2 and with NH 3 -N and Fe in S3, indicating links among DOM transport, nitrogen transformation, and redox-sensitive Fe mobilization. Zone-specific FI-based models effectively captured variations in groundwater parameters in subsequent sampling campaigns, indicating strong validation performance. Random forest scenario assessment indicated that coordinated control of socioeconomic pressures, agricultural inputs, and wastewater discharge reduced relative groundwater-quality responses. This applied geochemical framework supports rapid screening, zoned monitoring, and source-control prioritization in cold-temperate aquifers.