Yiqun Gan

Seasonal variation of antibiotics concentration in the aquatic environment: a case study at Jianghan Plain, central China

25 antibiotics (macrolides, tetracyclines, fluoroquinolones and sulfonamides) were detected in swine wastewater, river water, rivulet water and in groundwater samples from multi-level monitoring boreholes (with sampling ports, respectively, at 10, 25 and 50 m below the land surface) at Jianghan Plain, central China. Except swine wastewater, the antibiotic concentrations in groundwater, river and rivulet water were higher in spring than those in winter. Nineteen antibiotics were detected at 100% frequencies in all kinds of water samples. In groundwater, fluoroquinolones and tetracyclines were the predominant antibiotics and the total concentrations of 25 antibiotics commonly decreased with the aquifer depth. Most groundwater samples collected in spring had high concentrations of norfloxacin, with average values of 65.27 ng · L(-1), 37.28 ng · L(-1) and 46.83 ng · L(-1), respectively, at 10, 25 and 50 m deep boreholes. By contrast, the concentrations of sulfamethazine and erythromycin were rather low in groundwater, but high in surface water. Groundwater samples collected from sites close to rivers or rivulets had much higher contents of antibiotics than those from other sites, indicating that the dominant source of antibiotics in groundwater should be the contaminated rivers or rivulets, rather than the scattered pig and poultry farms in the study area.

Occurrence and risk assessment of antibiotics in surface water and groundwater from different depths of aquifers: A case study at Jianghan Plain, central China

The occurrence of 14 antibiotics (fluoroquinolones, tetracyclines, macrolides and sulfonamides) in groundwater and surface water at Jianghan Plain was investigated during three seasons. The total concentrations of target compounds in the water samples were higher in spring than those in summer and winter. Erythromycin was the predominant antibiotic in surface water samples with an average value of 1.60μg/L, 0.772μg/L and 0.546μg/L respectively in spring, summer and winter. In groundwater samples, fluoroquinolones and tetracyclines accounted for the dominant proportion of total antibiotic residues. The vertical distributions of total antibiotics in groundwater samples from three different depths boreholes (10m, 25m, and 50m) exhibited irregular fluctuations. Consistently decreasing of antibiotic residues with increasing of depth was observed in four (G01, G02, G03 and G05) groundwater sampling sites over three seasons. However, at the sampling sites G07 and G08, the pronounced high concentrations of total antibiotic residues were detected in water samples from 50m deep boreholes instead of those at upper aquifer in winter sampling campaign, with the total concentrations of 0.201μg/L and 0.100μg/L respectively. The environmental risks posed by the 14 antibiotics were assessed by using the methods of risk quotient and mixture risk quotient for algae, daphnids and fish in surface water and groundwater. The results suggested that algae might be the aquatic organism most sensitive to the antibiotics, with the highest risk levels posed by erythromycin in surface water and by ciprofloxacin in groundwater among the 14 antibiotics. In addition, the comparison between detected antibiotics in groundwater samples and the reported effective concentrations of antibiotics on denitrification by denitrifying bacteria, indicating this biogeochemical process driven by microorganisms wont be inhibitory influenced by the antibiotic residues in groundwater.

Production of Abundant Hydroxyl Radicals from Oxygenation of Subsurface Sediments

Hydroxyl radicals (•OH) play a crucial role in the fate of redox-active substances in the environment. Studies of the •OH production in nature has been constrained to surface environments exposed to light irradiation, but is overlooked in the subsurface under dark. Results of this study demonstrate that abundant •OH is produced when subsurface sediments are oxygenated under fluctuating redox conditions at neutral pH values. The cumulative concentrations of •OH produced within 24 h upon oxygenation of 33 sediments sampled from different redox conditions are 2-670 μmol •OH per kg dry sediment or 6.7-2521 μM •OH in sediment pore water. Fe(II)-containing minerals, particularly phyllosilicates, are the predominant contributor to •OH production. This production could be sustainable when sediment Fe(II) is regenerated by the biological reduction of Fe(III) during redox cycles. Production of •OH is further evident in a field injection-extraction test through injecting oxygenated water into a 23-m depth aquifer. The •OH produced can oxidize pollutants such as arsenic and tetracycline and contribute to CO2 emissions at levels that are comparable with soil respiration. These findings indicate that oxygenation of subsurface sediments is an important source of •OH in nature that has not been previously identified, and •OH-mediated oxidation represents an overlooked process for substance transformations at the oxic/anoxic interface.

Iron-Anode Enhanced Sand Filter for Arsenic Removal from Tube Well Water

Sand filters are widely used for well water purification in endemic arsenicosis areas, but arsenic (As) removal is difficult at low intrinsic iron concentrations. This work developed an enhanced sand filter by electrochemically generated Fe(II) from an iron anode. The efficiency of As removal was tested in an arsenic burdened region in the Jianghan Plain, central China. By controlling a current of 0.6 A and a flow rate of about 12 L/h, the filter removed total As in the tube well water from 196 to 472 μg/L to below 10 μg/L, whereas the residual As was about 110 μg/L without electricity. Adsorption and subsequent oxidation on the surface of Fe(III) precipitates are the main processes controlling the removals of As and Fe. During a 30-day intermittent operation, both effluent As concentration and electrical energy consumption decreased progressively. Although filter clogging was observed, it can be alleviated by replacing the top layer of sand. Our findings suggest that dosing Fe(II) by an iron anode is an effective means to enhance As removal in a sand filter.

Arsenic speciation in aquifer sediment under varying groundwater regime and redox conditions at Jianghan Plain of Central China

At Jianghan Plain of central Yangtze basins where the health of >73, 000 people has been affected by long term intake of high arsenic groundwater, over 100 sediment samples from four boreholes at the field monitoring sites were collected and analyzed to delineate the distribution and speciation of As in the shallow aquifer sediment. Results showed that sediment As concentration is generally dependent on the lithological conditions, with the higher As concentration present in fine particle sediment, especially in the silty sand layers underlying clay or silty clay layers. High As concentration in the sediment mainly occurred in three different depth ranges: <5m, 15-35m, and >35m. Both the groundwater regime and redox conditions played important roles in controlling sediment As speciation. Arsenate (86%) was the dominated As species in the near surface sediment. As the redox turned to be reducing, arsenite (64%) became the dominant species in the underlying clay and silty clay layers. But in the silty sand aquifer near the boundary of unconfined aquifer and confined aquifer, arsenate (85%) became the dominant species again as results of redox potential elevation. In the deep medium to coarse sand aquifers (>35m deep), As-sulfides (49%-63%) were the main species of As. The speciation and reactivity of sediment As strongly controlled the spatial distribution of groundwater As concentration, while seasonal variation in groundwater As concentration and speciation affected the content and speciation of sediment As.

Variability in carbon isotope fractionation of trichloroethene during degradation by persulfate activated with zero-valent iron: Effects of inorganic anions.

Stable carbon isotope analysis has the potential to be used for assessing the performance of in situ remediation of organic contaminants. Successful application of this isotope technique requires understanding the magnitude and variability in carbon isotope fractionation associated with the reactions under consideration. This study investigated the influence of inorganic anions (sulfate, bicarbonate, and chloride) on carbon isotope fractionation of trichloroethene (TCE) during its degradation by persulfate activated with zero-valent iron. The results demonstrated that the significant carbon isotope fractionation (enrichment factors ε ranging from -3.4±0.3 to -4.3±0.3‰) was independent on the zero-iron dosage, sulfate concentration, and bicarbonate concentration. However, the ε values (ranging from -7.0±0.4 to -13.6±1.2‰) were dependent on the chloride concentration, indicating that chloride could significantly affect carbon isotope fractionation during TCE degradation by persulfate activated with zero-valent iron. The dependence of ε values on chloride concentration, indicated that TCE degradation mechanisms may be different from the degradation mechanism caused by sulfate radical (SO4(-)). Ignoring the effect of chloride on ε value may cause numerous uncertainties in quantitative assessment of the performance of the in situ chemical oxidation (ISCO).

Effects of inorganic anions on carbon isotope fractionation during Fenton-like degradation of trichloroethene

Understanding the magnitude and variability in isotope fractionation with respect to specific processes is crucial to the application of stable isotopic analysis as a tool to infer and quantify transformation processes. The variability of carbon isotope fractionation during Fenton-like degradation of trichloroethene (TCE) in the presence of different inorganic ions (nitrate, sulfate, and chloride), was investigated to evaluate the potential effects of inorganic anions on carbon isotope enrichment factor (ε value). A comparison of ε values obtained in deionized water, nitrate solution, and sulfate solution demonstrated that the ε values were identical and not affected by the presence of nitrate and sulfate. In the presence of chloride, however, the ε values (ranging from -6.3±0.8 to 10±1.3‰) were variable and depended on the chloride concentration, indicating that chloride could significantly affect carbon isotope fractionation during Fenton-like degradation of TCE. Thus, caution should be exercised in selecting appropriate ε values for the field application of stable isotope analysis, as various chloride concentrations may be present due to naturally present or introduced with pH adjustment and iron salts during Fenton-like remediation. Furthermore, the effects of chloride on carbon isotope fractionation may be able to provide new insights about reaction mechanisms of Fenton-like processes.

Roles of hydroxyl and sulfate radicals in degradation of trichloroethene by persulfate activated with Fe2+ and zero-valent iron: Insights from carbon isotope fractionation

Active species including hydroxyl (HO) and sulfate radicals (SO4-) play important roles in contaminant degradation during the persulfate based in-situ chemical oxidation (ISCO) process. The generation and contribution of active species are critical and can potentially be evaluated using compound specific isotope analysis (CSIA). However, the evaluation of stable isotope fractionation (or isotope enrichment factor ε values) for contaminants degraded by individual active species of concern is required but lacking. This study firstly determined the carbon isotope fractionation of trichloroethene (TCE) degradation by SO4·- with chemical probe methods to obtain ε values from -6.4±0.7 to -6.9±0.5‰. The ε values were significantly different from those reported for TCE degradation by HO, which could be used to identify the competing TCE degradation by HO and SO4-. Relying on the observed ε values and the extended Rayleigh-type equation, the contributions of SO4- and HO to TCE degradation were evaluated in persulfate activated by Fe0 or Fe(II). This study provides an illuminating idea to determine stable isotope fractionation for contaminant degradation by individual active species, which is crucial for the application of CSIA in relevant environments.

Groundwater flow and hydrogeochemical evolution in the Jianghan Plain, central China

Hydrogeochemical analysis and multivariate statistics were applied to identify flow patterns and major processes controlling the hydrogeochemistry of groundwater in the Jianghan Plain, which is located in central Yangtze River Basin (central China) and characterized by intensive surface-water/groundwater interaction. Although HCO3-Ca-(Mg) type water predominated in the study area, the 457 (21 surface water and 436 groundwater) samples were effectively classified into five clusters by hierarchical cluster analysis. The hydrochemical variations among these clusters were governed by three factors from factor analysis. Major components (e.g., Ca, Mg and HCO3) in surface water and groundwater originated from carbonate and silicate weathering (factor 1). Redox conditions (factor 2) influenced the geogenic Fe and As contamination in shallow confined groundwater. Anthropogenic activities (factor 3) primarily caused high levels of Cl and SO4 in surface water and phreatic groundwater. Furthermore, the factor score 1 of samples in the shallow confined aquifer gradually increased along the flow paths. This study demonstrates that enhanced information on hydrochemistry in complex groundwater flow systems, by multivariate statistical methods, improves the understanding of groundwater flow and hydrogeochemical evolution due to natural and anthropogenic impacts.RésuméDes analyses hydrogéologiques et statistiques multivariées ont été utilisées pour identifier les schémas de flux et les processus principaux contrôlant l’hydrogéochimie des eaux souterraines de la plaine de Jianghan, qui est. localisée dans la partie centrale du bassin de la rivière Yangtze (Centre de la Chine) et caractérisée par des interactions importantes entre les eaux de surface et les eaux souterraines. Bien que les eaux de type HCO3-Ca-(Mg) prédominent dans le secteur d’étude, 457 échantillons (21 d’eaux de surface et 436 d’eaux souterraines) ont été classés selon 5 grappes à l’aide de l’analyze hiérarchique par grappe. Selon l’analyze factorielle, les variations hydrochimiques de ces grappes sont gouvernées par trois facteurs. Les composants principaux (ex. Ca, Mg et HCO3) des eaux de surface et eaux souterraines proviennent du lessivage des carbonates et silicates (facteur 1). Les conditions Redox (facteur 2) influencent la contamination par les éléments géogéniques Fe et As dans les parties captives peu profondes des aquifères. Les activités anthropiques (facteur 3) sont responsables principalement des fortes concentrations en Cl et SO4 dans les eaux de surface et des nappes souterraines. De plus, le score du facteur 1 des échantillons de l’aquifère captif peu profond croit le long voies d’écoulement. Cette étude démontre que l’information renforcée sur l’hydrochimie d’un système aquifère complexe, par des méthodes statistiques multivariées, permet d’améliorer la compréhension des flux souterrains et l’évolution hydrogéochimique du fait des impacts naturels et anthropiques.ResumenSe aplicaron análisis hidrogeoquímico y estadísticas multivariantes para identificar patrones de flujo y los procesos principales que controlan la hidrogeoquímica del agua subterránea en la llanura de Jianghan, ubicada en la cuenca central del río Yangtze (China central) y caracterizada por una intensa interacción agua superficial/agua subterránea. Aunque el agua de tipo HCO3-Ca-(Mg) predominó en el área de estudio, las 457 muestras (21 de aguas superficiales y 436 de aguas subterráneas) se clasificaron efectivamente en cinco grupos por análisis de agrupamiento jerárquico. Las variaciones hidroquímicas entre estos grupos se rigen por tres factores del análisis factorial. Los principales componentes (por ejemplo, Ca, Mg y HCO3) en las aguas superficiales y subterráneas se originaron a partir de la meteorización del silicato y del carbonato (factor 1). Las condiciones redox (factor 2) influyeron en la contaminación geológica de Fe y As en agua subterránea confinada poco profundas. Las actividades antropogénicas (factor 3) causaron principalmente altos niveles de Cl y SO4 en aguas superficiales y subterráneas freáticas. Además, la puntuación del factor 1 de las muestras en el acuífero confinado poco profundo aumentó gradualmente a lo largo de las vías de flujo. Este estudio demuestra que la información mejorada sobre hidroquímica en sistemas complejos de flujo de aguas subterráneas, mediante métodos estadísticos multivariantes, mejora la comprensión del flujo de aguas subterráneas y la evolución hidrogeoquímica debido a los impactos naturales y antropogénicos.摘要江汉平原位于中国中部的长江中游地区,区内地表水-地下水相互作用强烈。本文结合水文地球化学分析和多元统计方法识别江汉平原的地下水流模式,并分析影响地下水水文地球化学特征的主控因素。结果显示457个水样(包括21个地表水和436个地下水样)的水化学类型差异不明显,主要为HCO3-Ca-(Mg)型,聚类分析根据水化学特征有效地将这些样品划分为五类。通过因子分析,共提取了三个造成水化学特征差异的公因子:第一公因子为水-岩相互作用,包括碳酸盐溶解和硅酸盐风化过程,控制地表水和地下水中的主要组分(如Ca、Mg和HCO3);第二公因子为氧化还原条件,影响浅层承压水中原生Fe和As的富集;第三公因子为人为活动,常导致地表水和浅层潜水中的Cl和SO4污染。此外,在浅层承压含水层中,水样的第一公因子得分沿地下水流向逐渐增大。本文的研究表明多元统计方法可有效地识别复杂地下水流系统的水化学信息,从而有助于更好地掌握受自然和人为因素双重影响下的地下水径流和水文地球化学演化规律。ResumoAnálises hidrogeoquímicas e estatística multivariada foram aplicadas para identificar os padrões de fluxo e principais processos que controlam a hidrogeoquímica das águas subterrâneas na planície de Jianghan, que está localizado na bacia central do rio Yangtze (centro da China) e é caracterizada pela ocorrência de intensa interação entre águas superficiais e subterrâneas. Apesar do predomínio do tipo de água HCO3-Ca-(Mg) na área de estudo, as 457 amostras (21 de águas superficiais e 436 subterrâneas) foram efetivamente classificados em cinco grupos por análise de agrupamento hierárquico. As variações hidroquímicas entre esses grupos foram governadas por três fatores da análise fatorial. Os componentes principais (p.ex. Ca, Mg e HCO3) da água superficial e subterrânea originaram-se do intemperismo de carbonato e silicato (fator 1). Condições redox (fator 2) influenciaram o Fe gênico e a contaminação por As em águas subterrâneas confinadas rasas. Atividades antropogênicas (fator 3) foram as principais causas dos altos níveis de Cl e SO4 em águas superficiais e subterrâneas freáticas. Além disso, o valor do fator 1 de amostras no aquífero confinado raso aumentou gradualmente ao longo dos caminhos de fluxo. Este estudo demonstra que informações aprimoradas sobre a hidroquímica em sistemas complexos de fluxo de águas subterrâneas, por métodos estatísticos multivariados, melhoram a compreensão do fluxo de águas subterrâneas e a evolução hidrogeoquímica devido a impactos naturais e antropogênicos.

Experimental constraints on redox-induced arsenic release and retention from aquifer sediments in the central Yangtze River Basin

The consumption of arsenic (As) contaminated groundwater affects the health of almost 20 million people in China. Unlike the preponderance of observations within the deltas of South and Southeast Asia, groundwater As concentrations in the central Yangtze River Basin, China, vary by up to an order of magnitude seasonally. In order to decipher the cause of seasonal release and retention of As between sediments and groundwater, we conducted batch sediment incubations under varying (imposed) redox conditions. Incubations were conducted under both N2 and O2 gas purges to simulate conditions observed within the field. In all cases, anoxic conditions resulted in As release to solution while As was removed from solution under oxic conditions. These experiments confirm that anoxia is a prerequisite for As mobilization into groundwater from Yangtze River Basin sediments. Alternating redox conditions resulted in Fe minerals dissolution, transformation, crystallization, and precipitation, and subsequent As release and retention in the system. More importantly, aquifer sediments at depths >15u202fm release As through multiple redox cycles without an exogenous electron donor (carbon source), organic matter in the sediments is sufficiently reactive to support microbial reduction of As(V) and Fe(III). These results provide direct evidence for previously described mechanisms explaining the observed seasonal variation of groundwater As concentrations in the central Yangtze River Basin, where seasonal changes in surface and groundwater levels drive changes in redox conditions and thus As concentrations.