Both natural and anthropogenic factors control surface water and groundwater chemistry and quality in the Ningtiaota coalfield of Ordos Basin, Northwestern China.

An understanding of the vertical variations in hydrogeochemical processes in various aquifers and quality suitability assessment is crucial for the utilization of groundwater in the Ningtiaota coalfield of Ordos Basin, Northwestern China. Based on 39 water samples collected from surface water (SW), Quaternary pore water (QW), weathered fissure water (WW), and mine water (MW), we conducted self-organizing maps (SOM) algorithm, multivariate statistical analysis (MSA), and classical graphical methods to elucidate the mechanisms controlling the vertical spatial variations in SW and groundwater chemistry and conducted a health risk assessment. The findings indicated that the hydrogeochemical type showed a transition from the HCO-Na type in SW to the HCO-Ca type in QW, then to the SO-Mg type in WW, and back to HCO-Na type in MW. Water-rock interaction, silicate dissolution, and cation exchange were the main hydrogeochemical processes in the study area. Additionally, groundwater residence time and mining operations were critical external factors that affect water chemistry. Contrary to phreatic aquifers, confined aquifers featured greater circulation depth, water-rock interactions, and external interventions leading to worse quality and higher health risks. Water quality surrounding the coalfield was poor, causing it to be undrinkable, with excessive SO, arsenic (As), and F, etc. Approximately 61.54% of SW, all of QW, 75% of WW, and 35.71% of MW can be used for irrigation.