Ca/Na concentration-constrained variations of dissolved organic matter leaching from groundwater-irrigation area soil in North China Plain.

This study investigates the quantity and quality variations of dissolved organic matter (DOM) leaching from the soil in groundwater irrigation area of the North China Plain, constrained by the concentration of Ca/Na. Soil samples with dominant humic-like (HLC) and protein-like (PLC) components were paired with parallel concentration gradients of Ca/Na extractants for equilibrium experiments. Fluorescence-PARAFAC, UV-visible spectroscopy, and multiple statistical analyses were combined for data analysis and interpretation. The results reveal that the primary DOM components remained dominant for specific soil sample, with a higher relative abundance of PLC (HLC) in Ca (Na) extract. HLC preferentially binds to soil phase in all extractions, while PLC is readily released into the solution. However, Ca inhibits HLC desorption and promotes PLC release more significantly than Na, as indicated by stronger ion/proton reaction (IPR) and electrostatic effect (ESE). The strongest IPR and ESE are seen in the HLC-dominated DOM extracted with Ca, suggesting a condition where Ca bridges to HLC and forms total dissolved organic carbon (DOC) that decreases. In contrast, Na extraction exhibits only a weaker ESE that is offset by soil-contained HLC and exchangeable Ca, resulting in subtle DOC decrease. The trends in leaching of HLC and PLC are self-dependent, and the level of variation in either component correlates with the increasing concentration of specific cations present. These findings underscore the crucial role of soil organic matter (SOM) composition and its interaction with leaching cations in soil management in large-scale groundwater irrigation areas, where SOM quality and groundwater chemistry vary.

Quantity and quality variations of dissolved organic matter (DOM) in column leaching process from agricultural soil: Hydrochemical effects and DOM fractionation.

Leaching dynamics of dissolved organic matter (DOM) from agricultural soil are influenced by the chemistry of irrigation water. This study used multiple spectroscopy methods to investigate the dynamics of DOM in column leaching process by utilizing selected solutions. DOC was leached out by Na solutions more effectively than by Ca, despite the additional contributions of the ligand exchange of SO42- and the hydrolysis of HCO3-. The concentration ranking corresponding to solutions is Na2SO4 > NaHCO3 > NaCl > CaCl2. Furthermore, the aromaticity and molecular weights of DOM leached by Na solutions increased with the leaching, whereas contrary trends were reared by Ca. Thus, specific cation substantially affects DOM concentration and composition. Parallel factor analysis of the three-dimensional fluorescence spectra (EEM-PARAFAC) determined the characteristics of DOM components. (1) The long-wavelength, humic-like components (HLC, i.e., C1, C3, and C5) were largely accumulated at the early water saturating stage but attenuated toward dynamic stability in the salt-solution leaching (SSL) stage. (2) Reactive functional group-related HLC (i.e., C2, and C4) remarkably increased in response to the solutions chemistry. (3) The protein-like component (PLC, i.e., C6) was released with insignificant dynamic differences between the solutions. Despite approximately stable DOC concentration at the late stage of each SSL, HLC exhibited accelerating release in sodium salts but declining trend in calcium salt whereas PLC showed a roughly constant dynamic in the both. Ca-bridging reactions with the detachment of OM-Ca-clay complex prompting HLC release and its formation to stabilize the HLC well explain the contrast dynamics of HLC with Na or Ca leaching. The diffusion process that was defined for the slow, lasting and small concentration of leaching of inactive components could contribute to the steady dynamic of PLC, given its insensitivity to either the salt cation. This study identified distinct release kinetics of DOM compositions from those of DOC by EEM-PARAFAC and correspondingly the potential mechanisms, which offers promising advantages in monitoring DOM transport and fractionation in dynamic leaching process.

Distribution of antibiotics in alluvial sediment near animal breeding areas at the Jianghan Plain, Central China.

Antibiotics are increasingly detected in groundwater, but little is known about their presence in deep underground sediments. In this study, underground sediment samples were collected from pig farms, chicken houses, fishponds and riverbanks in a small region of the Jianghan Plain. Sixteen antibiotics were detected in different layers of sediments in various animal-breeding areas, with the riverbank containing the most pollution, followed by the fishpond, then the pig farm and the least being the chicken house. Samples taken from different sections of the collection sites and tested for each antibiotic compound revealed significant pollution, with the riverbank containing the most pollution, followed by the fishpond, then the pig farm and the least being the chicken house. The concentrations of the targets did not decrease with depth, but increased between 0.6 m and 1.0 m, which was a significant fluctuation. The aquifer sediment analysis indicated that the greatest antibiotic retention was within 8 m, with a small increase between 12 and 16 m, consistent with the depth of sandy aquifer layers. None of the compounds were detected in the deep layer at 20 m but for sulfadiazine and chlorotetracycline. Tetracyclines and Fluoroquinolones were the two groups observed at higher concentrations in most sediment layers, although their residual levels no more than 20 ng g-1. This study revealed that antibiotics generally exist in the underground environment, along with groundwater migration and particle adsorption. The pollution of antibiotics in alluvial sediments is an immense challenge to groundwater remediation, and its environmental effects should be studied.

Nitrogen loss by anaerobic ammonium oxidation in unconfined aquifer soils.

Anaerobic ammonium oxidation (anammox) is recognized as an important process for nitrogen cycling, yet little is known about its role in the subsurface biosphere. In this study, we investigated the presence, abundance, and role of anammox bacteria in upland soil cores from Tianjin, China (20 m depth) and Basel, Switzerland (10 m depth), using isotope-tracing techniques, (q)PCR assays, and 16 S rRNA &hzsB gene clone libraries, along with nutrient profiles of soil core samples. Anammox in the phreatic (water-saturated) zone contributed to 37.5-67.6% of the N-loss (up to 0.675 gN m-2 d-1), with anammox activities of 0.005-0.74 nmolN g-1 soil h-1, which were even higher than the denitrification rates. By contrast, no significant anammox was measured in the vadose zone. Higher anammox bacterial cell densities were observed (0.75-1.4 × 107 copies g-1 soil) in the phreatic zone, where ammonia-oxidizing bacteria (AOB) maybe the major source of nitrite for anammox bacteria. The anammox bacterial cells in soils of the vadose zone were all <103 copies g-1 soil. We suggest that the subsurface provides a favorable niche for anammox bacteria whose contribution to N cycling and groundwater nitrate removal seems considerably larger than previously known.

Fluorescence spectroscopy reveals accompanying occurrence of ammonium with fulvic acid-like organic matter in a fluvio-lacustrine aquifer of Jianhan Plain.

This study is the first to investigate the simultaneous presence of NH4 (+) and fluorescent organic matter components (FOCs) from a fluvio-lacustrine aquifer in Central Jianghan Plain. Sediment, groundwater, and surface water samples were collected for the sediment organic matter extraction, 3D fluorescence spectroscopy characterization, and/or hydrochemical analysis. NH4 (+) and dissolved organic carbon was ubiquitous in the groundwater. The fluorescence spectroscopy revealed good relationships between NH4 (+) and fulvic acid-like components (FALCs) in the groundwater and sediment-extracted organic matter (SEOM) solutions. NH4 (+) also exhibited significant positive correlation with protein-like component (PLC) (p < 0.001), with the stronger in the SEOM solutions than that in groundwater. Comparisons of spectroscopic indices [e.g., humification index (HIX), biological index (BIX), spectra slope (S275-295), and specific UV absorbance (SUVA254)] between the groundwater and SEOM solutions revealed more labile properties of SEOM. This result indicates that the decreasing NH4 (+)-FOCs correlations of groundwater relative to sediments may be attributed to microbial degradation. Factor analysis identifies important factors that cause NH4 (+) occurrence in the groundwater. The accompanying increase of FALC (C1) and NH4-N with the mole concentration of the normalized HCO3 (-)/(Ca(2+)+Mg(2+)) and [H(+)] suggests that couple effects of various biodegradations simultaneously occur in the aquifer, promoting the occurrence of NH4-DOMs.

Linking groundwater dissolved organic matter to sedimentary organic matter from a fluvio-lacustrine aquifer at Jianghan Plain, China by EEM-PARAFAC and hydrochemical analyses.

The sources of dissolved organic matter (DOM) in groundwater are important to groundwater chemistry and quality. This study examined similarities in the nature of DOM and investigated the link between groundwater DOM (GDOM) and sedimentary organic matter (SOM) from a lacustrine-alluvial aquifer at Jianghan Plain. Sediment, groundwater and surface water samples were employed for SOM extraction, optical and/or chemical characterization, and subsequent fluorescence excitation-emission matrix (EEM) and parallel factor analyses (PARAFAC). Spectroscopic properties of bulk DOM pools showed that indices indicative of GDOM (e.g., biological source properties, humification level, aromaticity and molecule mobility) varied within the ranges of those of two extracted end-members of SOM: humic-like materials and microbe-associated materials. The coexistence of PARAFAC compositions and the sustaining internal relationship between GDOM and extracted SOM indicate a similar source. The results from principal component analyses with selected spectroscopic indices showed that GDOM exhibited a transition trend regarding its nature: from refractory high-humification DOM to intermediate humification DOM and then to microbe-associated DOM, with decreasing molecular weight. Correlations of spectroscopic indices with physicochemical parameters of the groundwater suggested that GDOM was released from SOM and was modified by microbial diagenetic processes. The current study demonstrated the associations of GDOM with SOM from a spectroscopic viewpoint and provided new evidence supporting SOM as the source of GDOM.

Hydrochemistry indicating groundwater contamination and the potential fate of chlorohydrocarbons in combined polluted groundwater: a case study at a contamination site in North China.

Groundwater contamination characteristics and the potential fate of chlorohydrocarbons were investigated at a combined polluted groundwater site in North China. Groundwater chemistry and (2)D and (18)O isotope compositions indicated that high salination of groundwater was related with chemical pollution. The elevated salinity plume was consistent with the domain where typical chlorohydrocarbon contaminants occurred. The concentrations of heavy metals, oxidation-reduction potential, and pH in organic polluted areas significantly differed from those in peripheral (background) areas, indicating modified hydrochemistry possibly resulting from organic pollution. Under the presented redox conditions of groundwater, monochlorobenzene oxidation may have occurred when the trichlorohydrocarbons underwent reductive dechlorination. These findings suggested that inorganic hydrochemistry effectively indicated the occurrence of chemical contamination in groundwater and the potential fate of chlorohydrocarbons.

Occurrence of antibiotics in the aquatic environment of Jianghan Plain, central China.

The occurrence of 19 antibiotics (sulfonamide, fluoroquinolone, tetracycline and macrolide) was studied in surface water and groundwater samples collected from Shahu County of Jianghan Plain, central China, in autumn (dry season) and spring (wet season). In autumn, chlorotetracycline, doxycycline and enrofloxacin were the three antibiotics with the highest concentrations and high relevance ratios in all of the water samples. The concentration of chlorotetracycline was greatest in surface water at 122.3 n gL(-1) and was as high as 86.6 ng L(-1) in groundwater, which are among the highest values reported worldwide. In spring, tetracycline was found to be more than 100 ng L(-1) in groundwater and surface water, which also contained high concentrations of ofloxacin (135.1 ng L(-1)), norfloxacin (134.2 ng L(-1)) and erythromycin dehydrate (381.5 ng L(-1)). Most of the SMs were observed at higher detection frequencies in spring than in autumn, which can be ascribed to surface runoff by rain water during the wet season (spring). The average concentrations of compounds in the fluoroquinolone and tetracycline categories were far higher than those in the sulfonamide and macrolide categories, which had concentrations of less than 16 ng L(-1) in groundwater (except erythromycin dehydrate), while macrolides were found in all samples, except erythromycin dehydrate. The main antibiotics present in groundwater were also the dominant compounds found in surface water, with correlation coefficients of 0.93 and 0.97 in autumn and spring, respectively, indicating the potential contamination of groundwater by the infiltration of contaminated surface water.

Multivariate analysis of the heterogeneous geochemical processes controlling arsenic enrichment in a shallow groundwater system.

The effects of various geochemical processes on arsenic enrichment in a high-arsenic aquifer at Jianghan Plain in Central China were investigated using multivariate models developed from combined adaptive neuro-fuzzy inference system (ANFIS) and multiple linear regression (MLR). The results indicated that the optimum variable group for the AFNIS model consisted of bicarbonate, ammonium, phosphorus, iron, manganese, fluorescence index, pH, and siderite saturation. These data suggest that reductive dissolution of iron/manganese oxides, phosphate-competitive adsorption, pH-dependent desorption, and siderite precipitation could integrally affect arsenic concentration. Analysis of the MLR models indicated that reductive dissolution of iron(III) was primarily responsible for arsenic mobilization in groundwaters with low arsenic concentration. By contrast, for groundwaters with high arsenic concentration (i.e., > 170 µg/L), reductive dissolution of iron oxides approached a dynamic equilibrium. The desorption effects from phosphate-competitive adsorption and the increase in pH exhibited arsenic enrichment superior to that caused by iron(III) reductive dissolution as the groundwater chemistry evolved. The inhibition effect of siderite precipitation on arsenic mobilization was expected to exist in groundwater that was highly saturated with siderite. The results suggest an evolutionary dominance of specific geochemical process over other factors controlling arsenic concentration, which presented a heterogeneous distribution in aquifers. Supplemental materials are available for this article. Go to the publisher's online edition of the Journal of Environmental Science and Health, Part A, to view the supplemental file.

Multidimensional spectrofluorometry characterization of dissolved organic matter in arsenic-contaminated shallow groundwater.

Multidimensional spectrofluorometry was employed to characterize dissolved organic matter (DOM) in arsenic-contaminated shallow aquifers at Jianghan Plain (JHP), central China, to better understand the effect of biogeochemical processes on arsenic mobilization. The microbial humic quinone and amino acid components identified indicate the importance of microbially mediated processes in the reduction of iron oxyhydroxides. The relationship of quinone and amino acid-like components with Fe(2+) and S(2-) helps us understand the sequential redox transformation (SRT) in the high arsenic aquifer system and the origin of bicarbonate in groundwater. Correlation between DOM components and dissolved arsenic and Fe suggests that arsenic mobilization could be linked to the microbial reduction of iron oxyhydroxides with liable DOM as electron donors as well as to the electron shuttling function served by humic quinones.