High–arsenic (As) groundwater poses a serious threat to human health. The upper and middle reaches of the Yellow River are well–known areas for the enrichment of high–arsenic groundwater. However, little is known about the distribution characteristics and formation mechanism of high-As groundwater in the lower reach of the Yellow River. There were 203 groundwater samples collected in different groundwater systems of the lower Yellow River for the exploration of its hydrogeochemical characteristics. Results showed that more than 20% of the samples have arsenic concentrations exceeding 10 μg/L. The high-As groundwater was mainly distributed in Late Pleistocene–Holocene aquifers, and the As concentrations in the paleochannels systems (C2 and C4) were significantly higher than that of the paleointerfluve system (C3) and modern Yellow River affected system (C5). The high-As groundwater is characterized by high Fe²⁺ and NH₄⁺ and low Eh and NO₃⁻, indicating that reductive dissolution of the As–bearing iron oxides is probably the main cause of As release. The arsenic concentrations strikingly showed an increasing tendency as the HCO₃⁻ proportion increases, suggesting that HCO₃⁻ competitive adsorption may facilitate As mobilization, too. In addition, a Gibbs diagram showed that the evaporation of groundwater could be another significant hydrogeochemical processes, except for the water–rock interaction in the study area. Different sources of aquifer medium and sedimentary structure may be the main reasons for the significant zonation of the As spatial distribution in the lower Yellow River.

High-fluoride (F⁻) groundwater is a widespread environmental problem that poses significant risks to human health in many regions worldwide. Understanding the origin, circulation, and evolution of fluoride-rich groundwater is therefore essential for effective groundwater management and mitigation strategies. In recent years, stable isotope techniques have helped to address key gaps in understanding the hydrogeochemical processes governing F⁻ enrichment, particularly regarding the source identification and water-rock interaction mechanisms that remain poorly constrained. This study reviews the applications of hydrogen–oxygen, strontium–calcium, and lithium–boron isotopes in research on high-F⁻ groundwater systems. Hydrogen and oxygen isotopes (δ²H and δ¹⁸O) are widely used to identify groundwater recharge sources, mixing processes, and evaporative effects, thereby providing key constraints on the origin of fluoride-rich groundwater. Strontium and calcium isotopes (⁸⁷Sr/⁸⁶Sr and δ^44/40Ca) serve as effective tracers of water-rock interactions and associated hydrogeochemical processes, including mineral weathering and dissolution, cation exchange, and secondary mineral precipitation, which play critical roles in fluoride mobilization and enrichment. In addition, lithium, and boron isotopes (δ⁷Li and δ¹¹B) provide valuable insights into the influence of geothermal fluids and deep hydrothermal processes on fluoride accumulation in groundwater systems. Overall, the integrated application of these stable isotope systems offers a robust framework for elucidating the formation mechanisms and evolutionary pathways of high-F⁻ groundwater. Moving beyond qualitative source identification, future research should prioritize the development of Bayesian isotope mixing models that explicitly quantify uncertainty in fluoride source apportionment and utilize sensitivity analysis to test competing hydrogeochemical mechanisms.

Groundwater nitrate pollution in freshwater lake basins threatens ecosystems and human health, yet its sources and drivers remain poorly quantified. This study investigates the nitrate sources variation, geochemical characteristics of nitrate-contaminated groundwater, and its associated human health risks in the sub-basins of five rivers draining the Poyang Lake, China, via hydrogeochemical analysis and integrated assessment approach. Results showed that 52% of groundwater samples exceeded the background value of nitrate. High-nitrate groundwater primarily occurred as Cl-Ca and Cl-Na water types, while low-nitrate waters were of HCO3-Ca and HCO3-Na. Nitrate sources and transformations were strongly affected by nitrification, were closely associated with artificial surfaces and farmland Manure/sewage (M&S, 46.9%) and soil nitrogen (SN, 31.4%) were quantified as the dominant nitrate sources, while nitrogen fertilizers (NF, 17%) and atmospheric deposition (AD, 4.8%) made relatively minor contributions. Reconstructedinitial isotopic signatures showed lower δ15N and δ18O than measured values, yet still were within the manure and sewage isotopic ranges, reinforcing identified sources. Variations of uncertainty in nitrate sources tracing process were revealed by UI90, with SN (average 0.67), M&S (average 0.62), CF (average 0.54), and AD (average 0.11), respectively. Infants and children were predicted to be more vulnerable to nitrate exposure, with hazard quotient (HQ) values exceeding 1 in 54% and 46% of cases, respectively. Spatially, groundwater from the sub-basins of Rao and Xin rivers exhibited the highest contamination levels, while the sub-basin of Fu River showed the lowest. These findings underscore a need for targeted mitigation strategies to protect freshwater lake basins globally.