Enrichment of geogenic phosphorus in a coastal groundwater system: New insights from dissolved organic matter characterization

Impacts of nanobubbles on fouling control in phosphorus water distribution systems: Performance and mechanism investigation

Phosphorus water distribution system (PWDS) serves as an efficient carrier for phosphorus utilization across various fields. However, fouling formation commonly occurs in PWDS, leading to a range of technical and economic issues. This study evaluates the potential of nanobubbles (NBs) for controlling fouling, particularly phosphorus fouling, in PWDS. The results indicate that NBs inhibited fouling accumulation, resulting in an overall reduction of 8.7-42.9 %. Specifically, NBs appeared to adsorb cations (e.g., calcium ions) in the water due to their negative charged, resulting in a reduction of hydroxyapatite precipitation in fouling by 35.7-84.0 %. Meanwhile, NBs facilitated the transformation of calcite into loosely structured aragonite, which reduced the calcium precipitation content by 7.3-54.7 %. NBs can also induce particle aggregation and settling through enhanced coagulation, leading to a reduction in humic acid phosphate content by 31.5-40.7 % and silicate particle content by 10.8-47.9 %, respectively. In summary, this study elucidates how NBs influence and control phosphorus fouling as well as other types of fouling, providing valuable insights for the development of environmentally friendly and effective fouling control strategies.

Unraveling the fate of phosphorus in alluvial aquifers of the middle-lower Yellow River: Coupled natural and anthropogenic impacts

In recent years, groundwater phosphorus (P) contamination has received increasing attention, yet most studies focus solely on either anthropogenic or geogenic influences. This research addressed the combined effects of human activities and natural processes on P enrichment in the middle-lower Yellow River basin, where dissolved inorganic phosphorus (DIP) concentrations reached 0.59 mg/L. Hydrogeochemical analysis, along with multiple statistical methods and the Redfield ratio, revealed that geogenic processes were the dominant drivers of groundwater P enrichment, accounting for 77.5 % of the samples, while anthropogenic activities, particularly intensive agriculture, densely residential area and industrial development, contributed to P inputs in 22.5 % of the samples. Further analysis using dual isotopes (δ13C-DIC and δ56Fe) demonstrated that OP mineralization was the dominant geogenic P enrichment process, with the reductive dissolution of P-rich iron minerals serving as a secondary contributor. A comparative analysis between the middle-lower Yellow River basin and the central Yangtze River basin highlighted that the abundance of natural P-containing carriers and the closed or open nature of the groundwater environment jointly determined the extent of geogenic and anthropogenic P enrichment. This study provides valuable insights into the coupled impacts of natural and anthropogenic factors, enhancing our understanding of groundwater P dynamics.

Spatio-temporal dynamics variation of dissolved organic matter and water quality parameters in Giheung Reservoir: A vertical perspective

Water reservoirs in densely populated areas play a crucial role in water supply, irrigation, and flood regulation. This study investigated the spatiotemporal dynamics of dissolved organic matter (DOM) and its interactions with water quality parameters in Giheung Reservoir, South Korea. Monthly water samples were collected at three depths, measuring fifteen physicochemical properties alongside DOM characterization using Excitation-Emission Matrix Parallel Factor Analysis (EEM-PARAFAC), identifying four DOM components: C1 (microbial protein-like), C2 (humic-like), C3 (terrestrial humic-like), and C4 (tyrosine/tryptophan-like). Seasonal stratification influenced DOM distribution, with humic-like DOM (C2, C3) accumulating in deeper layers during summer, while protein-like DOM (C1, C4) dominated surface waters due to enhanced biological productivity, and mixing periods homogenizing DOM across depths. The dimictic thermal regime, with moderate summer stratification and full mixing in spring and autumn, governed these patterns. 2D correlation spectroscopy (2D-COS) identified sequential DOM variation across layers, with humic-like DOM increasing at depth during stratification. Fluorescence indices-Fluorescence Index (FI), Humification Index (HIX), and Biological Index (BIX)-were integrated into structural equation modeling (SEM), showing that temperature and dissolved oxygen strongly drive protein-like DOM, while nitrogen and phosphorus compounds shape DOM reactivity. These findings enhance understanding of DOM biogeochemical processes and provide valuable insights for reservoir management to maintain water quality.

Marine Recalcitrant Dissolved Organic Matter Gained by Processing at Sandy Subterranean Estuaries

The sandy subterranean estuary (STE) connecting fresh groundwater to saline sea water is characterized by strong geochemical (salinity, redox, and pH) gradients, with evidence emerging for its role as a hot spot for consumption of labile substrates. This inspired us to conduct a study to evaluate whether this holds true for dissolved organic matter (DOM), especially given the still mysterious origin of marine recalcitrant DOM. Here, characterization of DOM of 21 groundwater samples (depth 1-13 m, salinity 3.9‰ to 32.4‰) across a 65 m transect of an STE located in coastal Guangdong, China, has found systematic biotransformation toward "recalcitrant" carboxyl-rich alicyclic molecules (CRAM). The fraction of CRAM (%CRAM) increases from 33.1% to 76.7% with an increasing degree of DOM degradation and increasing salinity. Further, processing of DOM, including the more "biolabile" DOM with lower %CRAM released from aquitard, is more active under oxic conditions than under reducing conditions. Given the large quantities of sea water that recirculates through the sandy STEs globally, the amount of "recalcitrant" DOM (RDOM) entering the ocean after processing is likely to be considerable. While more studies are needed, the ocean can gain "recalcitrant" CRAM-like compounds in this way.

Long-term and seasonal evaluation on environmental microbiology and water quality of Shanmei reservoir in southeast China

As a crucial source of potable water, the quality of water in Shanmei reservoir strongly and directly impacts the safety and well-being of downstream residents. Microorganisms play a pivotal role in the reservoir's resource and energy cycle. However, ecological protection efforts for the Shanmei reservoir have encountered numerous challenges in recent years. This study conducted an extensive visual analysis of microbial communities in sediment from the Shanmei reservoir between 2022 and 2024 using amplicon sequencing technology targeting 16S rRNA gene. The results showed that the microbial diversity of sediment showed an obvious seasonal pattern. At the same time, microbial composition also changes with the long-term evolution of time, which may be closely related to the change in environmental conditions. In addition, we have also carried out long-term multi-dimensional monitoring of the water quality of the Shanmei reservoir, and the results show that the water quality has reached the national drinking water grade. In conclusion, this study not only unveiled the interseasonal dynamics and long-term evolutionary characteristics of sediment microbial communities but also elucidated the significant influence of environmental factors on their composition, structure and function. These findings offer a fresh perspective for understanding the freshwater ecosystem microbial dynamics and provide a scientific foundation for reservoir management and water quality protection.

Potential influence mechanism of mineral-organic matter (OM) interactions on the mobility of toxic elements in Pb/Zn smelter contaminated soils

To date, how complex mineral-organic matter (OM) interactions affect the migration and mobility of potentially toxic elements (PTEs) in soils is highly understudied. This work mainly focused on the occurrence characteristics of PTEs and their close association with the composition characterization of mineral elements and dissolved OM (DOM) molecules. The results revealed that quartz (20.20%), albite (15.60%) and biotite (14.37%) were the dominant minerals in soils. CHO molecules were most abundant, accounting for 58.41%. The unsaturated hydrocarbons with both low and high O/C ratios were the dominant organic compounds, accounting for 21.56% and 36.73%, respectively. Sequential extraction results indicated that most Cd was hosted in carbonate minerals, while considerable amounts of As, Cu, Mn, Pb and Zn were bound to Fe/Mn oxyhydroxides. The elemental distribution characteristics displayed the coexistence of As, Cd, Cu, Mn, Pb and Zn with O, S, Al, Si, Ca and Fe. Fe oxyhydroxides might preferentially retain the unsaturated hydrocarbons with low O/C ratio and phenols. Furthermore, Fe oxide-organic composites had more significant impacts on Mn than As, Cd, Cu, Pb and Zn mobility. Overall, these findings would provide important insights into how mineral-OM interactions played the key roles on PTEs mobility in soils.

Molecular characteristics of dissolved organic phosphorus in watershed runoff: Coupled influences of land use and precipitation

Land use and precipitation are two major factors affecting phosphorus (P) pollution of watershed runoff. However, molecular characterization of dissolved organic phosphorus (DOP) in runoff under the joint influences of land use and precipitation remains limited. This study used Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) to study the molecular characteristics of DOP in a typical P-polluted watershed with spatially variable land use and precipitation. The results showed that low precipitation and intense human activity, including phosphate mining and associated industries, resulted in the accumulation of aliphatic DOP compounds in the upper reaches, characterized by low aromaticity and low biological stability. Higher precipitation and widespread agriculture in the middle and lower reaches resulted in highly unsaturated DOP compounds with high biological stability constituting a higher proportion, compared to in the upper reaches. While, under similar precipitation, more aliphatic DOP compounds characterized by lower aromaticity and higher saturation were enriched in the lower reaches due to more influence from urban runoff relative to the middle reaches. Photochemical and/or microbial processes did result in changes in the characteristics of DOP compounds during runoff processes due to the prevalence of low molecular weight and low O/C bioavailable aliphatic DOP molecules in the upper reaches, which were increasingly transformed into refractory compounds from the upper to middle reaches. The results of this study can increase the understanding of the joint impacts of land use and precipitation on DOP compounds in watershed runoff.

Predicting groundwater phosphate levels in coastal multi-aquifers: A geostatistical and data-driven approach

The groundwater (GW) resource plays a central role in securing water supply in the coastal region of Bangladesh and therefore the future sustainability of this valuable resource is crucial for the area. However, there is limited research on the driving factors and prediction of phosphate concentration in groundwater. In this work, geostatistical modeling, self-organizing maps (SOM) and data-driven algorithms were combined to determine the driving factors and predict GW phosphate content in coastal multi-aquifers in southern Bangladesh. The SOM analysis identified three distinct spatial patterns: K+Na+pH, Ca2+Mg2+NO₃-, and HCO₃-SO₄2-PO43-F-. Four data-driven algorithms, including CatBoost, Gradient Boosting Machine (GBM), Long Short-Term Memory (LSTM), and Support Vector Regression (SVR) were used to predict phosphate concentration in GW using 380 samples and 15 prediction parameters. Forecasting accuracy was evaluated using RMSE, R2, RAE, CC, and MAE. Phosphate dissolution and saltwater intrusion, along with phosphorus fertilizers, increase PO43- content in GW. Using input parameters selected by multicollinearity and SOM, the CatBoost model showed exceptional performance in both training (RMSE = 0.002, MAE = 0.001, R2 = 0.999, RAE = 0.057, CC = 1.00) and testing (RMSE = 0.001, MAE = 0.002, R2 = 0.989, RAE = 0.057, CC = 0.998). Na+, K+, and Mg2+ significantly influenced prediction accuracy. The uncertainty study revealed a low standard error for the CatBoost model, indicating robustness and consistency. Semi-variogram models confirmed that the most influential attributes showed weak dependence, suggesting that agricultural runoff increases the heterogeneity of PO43- distribution in GW. These findings are crucial for developing conservation and strategic plans for sustainable utilization of coastal GW resources.

Mobilization and enrichment of geogenic iodine in a floodplain groundwater system: New insights from sources and characterization of dissolved organic matter

High iodine groundwater occurs widely in the lower reaches of Yellow River floodplain, which has aroused public concern. The biogeochemical behavior of dissolved organic matter (DOM) plays a crucial role in the mobilizing iodine from aquifer media. In this study, the molecular composition of DOM in groundwater characterized by FT-ICR-MS, and the optical properties of organic matter obtained by combining three-dimensional fluorescence spectroscopy and parallel factor analysis (EEM ⁃ PARAFAC), were used to elucidate the effect of DOM on the migration and enrichment of iodine in groundwater in the eastern Henan Plain, which is located in the lower reaches of Yellow River floodplain, Northern China. The results show that，the total iodine concentration in groundwater in the study area is ranged from 4.68 to 1598 μg/L, and the average value was 216.4 μg/L. High iodine groundwater shows a distribution pattern along the Paleochannels of Yellow River, which is closely related to the richness of organic matter in the buried sediments of the Paleochannels of Yellow River. Organic matter in the sedimentary aquifers plays an important role in regulating the mobilization and enrichment of iodine, and its degradation process is conducive to the release of iodine. DOM components in high iodine groundwater are more homogeneous, more unsaturated, and has more aromatic molecules than those in low iodine groundwater. The activation of organic iodine in groundwater system may be accompanied by the degradation of N+ aliphatic compounds (CHON, CHONSP and CHON) and the formation of oxygen-poor highly unsaturated phenols (CHOSP, CHOP and CHOS) organic compounds. In addition to biodegradation, the adsorption of iron oxide rich in sedimentary aquifers can partially remove the high AI and O/C components of DOM in groundwater and enrich the remaining OPHUP components. The findings provide new insights into the coupling mechanism between iodine release and DOM in aquifers.

Seasonal Dynamics of Methane Fluxes from Groundwater to Lakes:Hydrological and Biogeochemical Controls

Methane (CH4) inputs to lakes through lacustrine groundwater discharge (LGD-derived CH4) represent a potentially important but often overlooked source of lake methane emissions. Although great efforts have been made to quantify LGD-derived CH4 fluxes and their spatial variablity, the underlying mechanisms controlling seasonal LGD-derived CH4 fluxes and their influence on lake CH4 emissions remain poorly understood, particularly in humid inland areas. To address this gap, we applied the 222Rn mass balance model, as well as hydrological, isotopic and microbial methods to assess seasonal LGD-derived CH4 fluxes and their influence on the seasonal variability of lake methane emissions in a typical oxbow lake, central Yangtze River. The results revealed wide seasonal differences in LGD-derived CH4 fluxes, which were controlled by hydrological and biogeochemical processes. During the dry season, although more intense methane oxidation and weaker methanogenesis occurred in groundwater, the much higher LGD rate (51.71 mm/d) produced a higher LGD-derived CH4 flux (16.41 mmol/m2/d). During the wet season, methanogenesis was more active and methane oxidation was weaker, but a lower LGD rate (12.16 mm/d) led to a lower LGD-derived CH4 flux (5.33 mmol/m2/d). Furthermore, higher LGD-derived CH4 flux in the dry season resulted in higher CH4 emissions from the lake and diminished the extent of methane oxidation in the lake. In comparison to other regions, the differences in LGD-derived CH4 fluxes and their seasonal variations were found to be controlled by climatic conditions and lake types in different global regions. Higher LGD-derived CH4 fluxes and more pronounced seasonal variations could be associated with higher temperature, larger water depth and more intense water level fluctuations. This study provides a novel perspective and broader implications for the comprehension and evaluation of seasonal methane emissions and understanding the carbon cycle in global lake ecosystems in humid areas with intense water level fluctuations.

Identification of methane cycling pathways in Quaternary alluvial-lacustrine aquifers using multiple isotope and microbial indicators

Groundwater rich in dissolved methane is often overlooked in the global or regional carbon cycle. Considering the knowledge gap in understanding the biogeochemical behavior of methane in shallow aquifers, particularly those in humid alluvial-lacustrine plains with high organic carbon content, we investigated methane sources and cycling pathways in groundwater systems at the central Yangtze River basins. Composition of multiple stable isotopes (2H/18O in water, 13C in dissolved inorganic carbon, 13C/2H in methane, and 13C in carbon dioxide) was combined with the characteristics of microbes and dissolved organic matter (DOM) in the study. The results revealed significant concentrations of biogenic methane reaching up to 13.05 mg/L in anaerobic groundwater environments with abundant organic matter. Different pathways for methane cycling (methanogenic CO2-reduction and acetate-fermentation, and methane oxidation) were identified. CO2-reduction dominated acetate-fermentation in the two methanogenic pathways primarily associated with humic DOM, while methane oxidation was more closely associated with microbially derived DOM. The abundance of obligate CO2-reduction microorganisms (Methanobacterium and Methanoregula) was higher in samples with substantial CO2-reduction, as indicated by isotopic composition. The obligate acetate-fermentation microorganism (Methanosaeta) was more abundant in samples exhibiting evident acetate-fermentation. Additionally, a high abundance of Candidatus Methanoperedens was identified in samples with apparent methane oxidation. Comparing our findings with those in other areas, we found that various factors, such as groundwater temperature, DOM abundance and types, and hydrogeological conditions, may lead to differences in groundwater methane cycling. This study offered a new perspective and understanding of methane cycling in worldwide shallow alluvial-lacustrine aquifer systems without geothermal disturbance.

Effects of reservoir construction on optical and molecular characteristics of dissolved organic matter in a typical P-contaminated river

The source and composition characteristics of dissolved organic matter (DOM) are crucial to identify and evaluate the sources of pollution in the watershed. The construction of reservoirs changes the hydrological condition and pollutant fate of the river. However, the effects of reservoirs' construction on DOM in the watershed and the underlying mechanisms are still unclear. This study aims to examine and compare the characteristics of DOM in reservoirs and streams in the Huangbai River, a typical reservoir-affected and P-contaminated river within the Yangtze River catchment. The results showed that DOM in reservoirs was characterized by more contribution from autochthonous source, under the influence of reservoirs' construction; while, DOM in rivers was mainly originated from terrestrial input. Reservoirs had more lipid-like and protein-like compounds, while rivers contained more oxy-aromatic-like compounds. The percentage of CHOP molecules in reservoirs was significantly higher than that in rivers. The underlying mechanism is that more suitable conditions were created for plankton to grow after constructing reservoirs, which converted inorganic orthophosphate into organic phosphorus, and over time, organic phosphorus was gradually enriched in reservoirs, which exacerbated the risk of eutrophication in the reservoir water body. This study can provide theoretical support for monitoring and evaluation of water quality in reservoir-affected rivers.

Characteristics and mechanism of phosphate removal by lanthanum modified bentonite in the presence of dissolved organic matter

Lanthanum modified bentonite (LMB) is a widely used adsorbent for removing inorganic phosphorus from polluted water to prevent eutrophication. However, its efficiency can be affected by various environmental factors, including dissolved organic matter (DOM), which is still unclear. In this study, we systematically explored the influence of model DOMs, including HA, bovine serum albumin (BSA), and sodium alginate (SA), on phosphate adsorption by LMB, as well as to elucidate the underlying adsorption mechanisms. Our results showed that only HA had a significant effect on phosphate adsorption by LMB, causing inhibition. When three DOMs were mixed with phosphate in different proportions and DOM was mainly HA, the performance of phosphate adsorption on LMB became worse, while BSA can slightly offset this impact. The kinetics of HA and phosphate adsorption on LMB followed the pseudo-second-order kinetic model, and isotherms fitted the Langmuir model, with a maximum adsorption capacity of 5.7 mg g-1 for P and 12 mg g-1 for HA. However, when HA and phosphate were mixed based on their Qm, a C/P molar ratio of 5.35, LMB preferentially adsorbed phosphate. HA invasion was also disadvantageous for phosphate removal by LMB, in which P adsorption was less efficient at low-concentrations. However, during co-adsorption the adsorption capacity for HA was higher. With a secondary addition of higher levels of P, both pollutants were adsorbed more effectively. In the natural water experiment, phosphate concentration decreased with increasing shaking time, while the UV254 value showed a downward trend, indicating that LMB also adsorbed HA. Characterization results showed that La and phosphate formed LaPO4 precipitation, forming La-O-P inner-sphere complexes as the main mechanism of phosphate removal by LMB. La and HA formed La-HA complexes, with O-CO bonds from HA competing for lanthanum with phosphate. Despite HA obstructing pores from adsorbent, LMB still maintained a good binding ability with phosphate. It may form La-P-HA ternary complexes during adsorption to keep HA adsorption amount.